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Avocet Pde OPW - App E Slope · OPW20/0092
Appendix E Slope Stability Report Application for a Development Permit for Operational Works (Earthworks, Road Work, Drainage Work, Landscaping and Driveway) 38 Avocet Parade, Peregian Beach 4573 QLD Lot 270 on P93113 HARBOURAGE PTY LTD
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37 636 783 527
PO BOX 851
Moffat Beach, QLD, 4551
[email protected]
www.geotechengineering.com.au
RG Strategic 29 Apr 2020
By Email Letter 01 Revision 00
Project: GE200132
[email protected]
Dear Maree,
Report on Geotechnical Investigation
Proposed Residnetial Dwelling
38 Avocet Parade, Peregian Beach
At the request of RG Strategic, Geotech Engineering Pty Ltd (GE) undertook a geotechnical
investigation for the above project. The summary of the investigation is detailed in Table 1.
Table 1: Summary of Investigation Results
Relative Susceptibility in accordance with AGS Guidelines Susceptibility Rating
0.33 Low
Notes: The above is a summary of the detailed investigation results discussed within this report. The entire report
must be read, understood, and kept in entirety.
Senior Geotechnical Engineer
Billy Blake B.Eng(Civ)(Hons), CPEng, RPEQ, NER, MIEAust
Attachments: Test Location Plan, Bores, Labs, Home Owner’s Guide to Footing Maintenance
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1.0 Introduction
Geotech Engineering Pty Ltd (GE) was commissioned to undertake a geotechnical investigation
for the Proposed Residnetial Dwelling located at 38 Avocet Parade, Peregian Beach.
This report outlines the results of the investigation, laboratory testing, analysis and interpretive
reporting on the following items:
• All factual information resulting from the investigation (investigation methodologies,
detailed desktop review, test location plans, bore logs);
• Summary of subsurface conditions and adopted subsurface conditions including
groundwater;
• Stability assessment in accordance with AGS guidelines;
• Earthworks and site preparation comments;
• Site movements;
• Foundation recommendations and allowable bearing pressures;
• Hillside construction comments
This report must be kept in entirety.
2.0 Site Details
2.1 Detailed Description
The site can be described as typical for a residential allotment that has triggered slope stability
requirements. The site has varying grades falling away to the east at relatively moderate grades
broadly speaking initially and then becoming gently graded to the limit of the allotment at the
eastern boundary. The photograph shown on the title page of this report gives a good
indication of site conditions at the time of the walkover assessment .
2.2 Geology
Referencing available geology datasets available from QLD Globe, the site is in an area
underlain by Quartenarly aged alluvial deposits which typically comprises sands, clays, gravel,
and silt. The encountered subsurface conditions broadly agree with the expected geology.
2.3 Groundwater
During the walkover assessment, no groundwater was observed across the entire site. It must
be noted that groundwater is transient and seasonal in nature. Given this, the time at which
the walkover assessment was undertaken may not indicate worst case conditions.
2.4 Vegetation
During the walkover assessment, a variety of vegetation ranging from small shrubs to mature
trees were observed. The mature trees were all observed to be typically vertical with no
misshapen or curved trunks. Misshapen or curved trunks are usually good indicators for
previous slope instability.
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2.5 Structures
At the time of the inspection, several structures were onsite that could be observed externally
without intrusive investigation. The existing structures and retaining walls typically appeared
to be performing in accordance with a structure under normal conditions suggesting no
shallow soil creep occurring. A timber fence along the northern boundary of the property has
tilted; however, it appears to be due to insufficient lateral resistance to for wind loading.
2.6 Fieldwork & Results
The fieldwork was undertaken on 23 Apr 2020 and included 3 bores drilled to to a maximum
depth of 3.5 m using a 4WD mounted auger drill rig. Any exposed faces were also inspected
to inform the results of the investigation. The subsurface conditions were logged through
tactile and visual assessment of returned spoil by an experienced geo-technician from Global
Testing.
The encountered subsurface conditions are described in detail on the attached bore logs;
however, in summary comprise minor fill over sand of loose density grading medium dense
typically below 1.0 m depth as described in the Global Testing investigation report 200309..
Groundwater measurements or the lack of groundwater are made within the bore logs
attached; however, it must be noted that groundwater is transient and will fluctuate with
seasons and time.
3.0 Land Stability
3.1 Explanatory Notes
Local authorities prepare mapping tools to aid in assessing potential slope stability
requirements. It is important to note that where a site specific investigation and slope stability
assessment are undertaken, they take precedence.
Landslides are caused where a soil or rock mass has a decrease in strength, usually attributed
to groundwater, resulting in the material not being able to maintain its own self-weight
resulting in a slope failure.
The Australian Geomechanics Society (AGS) developed a set of tools and procedures for
assessing the stability of a slope. The tools and procedures presented with the AGS
documentation form the basis of the assessment undertaken herein and their documentation
is attached for reference.
The guidelines also identify that the regulator typically sets the standard for levels of tolerable
risk. AGS defines tolerable risk as “risks within a range that society can live with so as to secure
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certain benefits. It is a range of risk regarded as non-negligible and needing to be kept under
review and reduced further if practicable”.
The guides suggest that areas of moderate or greater risk should not be developed unless a
rigorous remediation plan be implemented to reduce the area to a level of Acceptable Risk.
AGS defines acceptable risk as “risk which everyone affected is prepared to accept. Action to further
reduce such risk is usually not required unless reasonably practicable measures are available at low
cost in terms of money, time and effort.”.
AGS and regulators generally accept risk levels of “low” or lower as a level of acceptable risk.
Typically where risk levels equate to a level of “low” or lower, this would be comparable to a
global stability factor of safety in excess of 1.5. Where the results of the investigation have
indicated a risk level of “low” or better, it is considered that the factor of safety against global
instability for the area of assessment is greater than 1.5.
3.2 Quantitative Landslide Stability Analysis
AGS prepared a quantitative assessment tool for determining the level of risk for a site. The
tool separates the potential hazards for a site, from a stability context, and rates them with a
level of risk frequency. The multiplication of these individual risk frequency summates as the
risk frequency rating which is then measured against the levels of risk nominated by AGS.
The results of this analysis are shown in Table 2.
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Table 2: Results of AGS Quantitative Risk Assessment
Item Description Risk Level Factor
Natural Surface Slope Between 5 and 15 degrees Moderate 0.5
Slope Shape/Appearance Concave High 1.5
Site Geology Sedimentary rock Moderate 1.0
Soil Profile Residual soil >3m deep High 1.5
Regional Position on Hill Lower Slope High 1.5
Groundwater Evidence None apparent Low 0.7
Slope Instability Evidence No sign of instability Low 0.8
Proposed Cut Depths Cuts 1 m to 3 m in height Moderate 1.5
Proposed Cut Angles 45 to 60 High 1.5
Fill Depths Fill <1 m in depth Moderate 1.1
Fill Depth Angles 45 to 60 High 1.5
Battered Support Engineered - Other Very High 1.5
Type of Fill Track-rolled fill (sand) Very High 1.5
Services Fully sewered Moderate 0.8
Stormwater Disposal To council system Moderate 0.7
Connectors Not used Very High 2.0
Footing Types AS2870 fit for purpose High 1.2
Founding Strata Transported Soils High 1.2
Inground Tanks Unknown Very High 4.0
Landscaping/Drainage Unknown Very High 4.0
Upslope Failure (if needed) No action Very High 4.0
Site Frequency ( 𝟎. 𝟎𝟎𝟎𝟒 × ∑ 𝑰𝒕𝒆𝒎 𝑹𝒊𝒔𝒌 𝑭𝒂𝒄𝒕𝒐𝒓𝒔 ) 0.33
Site Relative Susceptibility Risk to Instability Low
Table 3: Results of AGS Qualitative Risk Assessment
Hazard Likelihood Consequence Risk Level
Global Failure Barely Credible Catastrophic Low
Shallow slumping of surface soils Unlikely Minor Low
4.0 Geotechnical Comments
Geotechnical comments are based on the factual findings of the investigation, best practice,
local experience, published correlations; however, are fundamentally founded in opinion and
this should be considered.
5.1 Earthworks Comments
The following comments are provided in relation to site preparation for earthworks.
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www.geotechengineering.com.au
• The area should be stripped of uncontrolled fill, soft, overly wet, foreign, or otherwise
deemed unsuitable material (material that is potentially compressible) down to a relatively
stiff dry base ensuring no organics material is present.
• Stripped areas must be inspected by the geotechnical testing authority (GTA) to ensure no
soft spots or loose zones are present. This should be done with the use of a loaded body
truck or 12 tonne roller as a minimum. Areas showing signs of movement under the action
of the testing equipment should be either over excavated and replaced with select fill or
conditioned onsite through tyning, blending or other suitable methods.
• Although no standard exists for the moisture content of soils, it is recommended that the
site soils be placed at or near optimum moisture content (OMC) for general earthworks
operations. This increases efficiency during earthworks operations. A range of +/- 2% of
OMC is recommend for general earthworks.
• Where site soils are proposed for use as fill in other areas, won material will need to have
a maximum particle size of 75 mm or be observed to break down under the energy of
compactive equipment.
• Fill materials should be placed in relatively horizontal layers with a maximum placement
thickness of 200 mm. The materials should be compacted to a minimum dry density ratio
of 95% relative to Standard compactive effort.
• Highly plastic clays at this site will be very sensitive to the presence or lack of moisture and
therefore care should be used to place the material within the range nominated and to a
maximum dry density of 102% relative to Standard compactive effort. Where this does not
happen, the material could readily swell, soften, or be subject to significant trafficability
issues.
• Fill material will be subject to potential settlement post placement. Well compacted fill
placed in accordance with AS3798 and otherwise, good earthworks procedure should
typically be subject to settlement in the order of 0.5% to 1.0% of the fill thickness over a
log cycle of time.
5.2 Site Movement
The results of the investigation, previous experience, and published correlations were used to
estimate the maximum characteristic ground surface movement (ys) value in accordance with
AS 2870.
The results of the calculations indicate that at the time of the investigation, a ys value
approximately equal to less than <20 mm is considered suitable; however, the site must be
classified as Class P given the requirement for slope stability assessment.
It is important to understand that numerous things can affect and consequently increase or
decrease the ys value for a site.
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The following are items that may affect the site and as a result change the site classification to
a “Class P”.
• Excessive moisture as a result of poor drainage, maintenance to drainage structures,
excessive watering of gardens;
• Placement or removal of vegetation that draws moisture from the soil surrounding the
structure; and
• Additional earthworks outside the limitations set out in AS2870.
Long term maintenance is typically required for sites for maintenance. Useful information is
attached which should be read and understood by both the builder and property owner.
5.3 Foundations
It is recommended that all high level footings have perimeter, column and load bearing
footings found a minimum 0.3 m into medium dense sands. Where footings found into at this
depth and into this material they can be sized and dimensioned for 150 kPa. All foundation
excavations should be inspected by a suitably experienced and qualified geotechnical engineer
prior to the placement of concrete to confirm ground conditions. Further to this, all footings
should found in similar strata to limit differential settlement.
5.4 Hillside Construction
The following precautionary works should be implemented in additional to other items
highlighted within this document. These points are considered to be typical good hillside
construction practice as explained in attached documentation:
• Foundations for lightly loaded or non-liveable structures should be founded on or in
medium dense sands; Heavy structures should be supported on either piles founded into
medium dense sands or high level footings founding into the same material.
• Where slopes are not supported by an engineer designed support system, natural slopes
within or near building envelope or movement sensitive areas should be no steeper than
3.5H:1V.
• Building platform fill should be limited to a maximum height of 1 m. The filling should be
step keyed into the natural slope and battered no steeper than 3H:1V unless an engineer
designed retention system is in place.
• Unsupported cuts steeper than 2.5H:1V will require engineer designed retention systems.
• All stormwater and surface water is to be discharged by an approved stormwater drainage
system. The use of subsurface drainage systems are not suitable unless greater than 50
m downslope from the envelope at this site.
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• Ongoing maintenance of the effluent wastewater area is also considered mandatory. The
location of the application area should be located below the proposed building. Plumbing
and services must also not be allowed to leak and regular maintenance and inspections
should be undertaken.
• Erosion protection of cut and fill batters will be required. Batters should incorporate crest
and toe drainage. Batters should be vegetated appropriately to minimise erosion.
• Careful construction practices should be undertaken to ensure undermining of adjoining
property structures does not occur.
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6.0 Limitations
Geotech Engineering Pty Ltd (GE) has prepared this document for 38 Avocet Parade, Peregian Beach
for RG Strategic. The document is specific to this particular project and for the purposes as described
within the document. The document cannot be used for any other project on the same site or other
sites or relied upon by a third party. Any party relying upon this document beyond its exclusive use
and purpose as stated above, and without the express written consent of GE, does so entirely at its
own risk and without recourse to GE for any loss or damage. GE has necessarily relied upon
information provided by the client and/or their agents.
It is important to understand that ground conditions can be spatially variable over short distances as
a result of geological processes or human interference. These changes can occur after the time of
which field work was undertaken. Given this, the assumed subsurface conditions within this document
are only indicative of the conditions at the testing locations to the depths investigated at the time of
the fieldwork.
The advice given in this document is based upon the assumed subsurface conditions. The relevance
and accuracy of advice given is directly affected by variations in ground conditions across the site
between testing locations that has not been captured and time elapsed since investigation works.
This document must be kept in entirety, read in full and understood. Interpretations or conclusions
made by others unless supported by an expressed written statement, interpretation, outcome or
stated within this document cannot be the responsibility of GE.
This document or parts of it should not be used as part of a specification or design for a project without
prior review and agreement by GE.
__________________________________________________________________________________
We trust this information is satisfactory for your current needs. Should you have any questions, please
feel free to contact the signed authors.
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[email protected]
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GEOTECH REPORT
Geotech Engineering Pty Ltd undertakes quality
These notes have been provided to clarify and assurance checks in accordance with its’ quality
enhance the reader’s understanding of the terminology system requirements; however, restrictions on the
used throughout the report, and also to identify where project outside of Geotech Engineering Pty Ltd’s
limitations exist. All investigations are undertaken control can result in unanticipated variations, changes
based on budgetary constraints which impart a level of in policy or interpretation of said policies by the relevant
uncertainty as a result of limiting the amount of authority, or the actions of contractors subject to
investigation undertaken. commercial pressure.
Copyright Site Variations
This report remains the property of Geotech If site variations become evident at the time of
Engineering Pty Ltd at all times unless ownership is construction or after the release of a report, Geotech
transferred in writing. It cannot be included in any Engineering Pty Ltd requests immediately notification.
documentation without the express written permission This notification is proposed in order to assist in
of Geotech Engineering Australia Pty remediation prior to any permanent structures or
design procedures are undertaken.
Subsurface Report Sheets (Bores etc)
Collaboration
The provided subsurface sheets are inherently an
interpretation undertaken by a suitably qualified and Geotech Engineering Pty Ltd strongly recommends
experienced person; however, the reliability of the open and collaborative communication about the use
provided information will rely on the method of and inherent interpretation of Geotech Engineering Pty
investigation. It is prudent to state that these sheets Ltd’s report. Please contact the author of the report to
only represent a very small percentage of the arrange for an obligation free discussion regarding the
subsurface conditions across a particular project. potential benefits of having geotechnical consultants.
It reasons that given the relatively small representation
of the subsurface conditions for a project, spacial
variability between testing locations must be taken into
consideration including the orientation of strata
intersections.
The presence or lack of groundwater is usually stated
on the subsurface sheets; however, it must be noted
that the measurements undertaken onsite are typically
very inaccurate and if the presence or lack of
groundwater will have any degree of affect on the
project, additional groundwater monitoring techniques
should be employed such as groundwater monitoring
wells.
Reporting
The report has been prepared and reviewed by suitably
qualified and experienced persons. The persons draw
on previous experience, local knowledge, published
literature, and otherwise standard engineering
approaches and principals. The context of which the
report has been prepared for must be considered and
if the context changes, due consideration should be
given to the possibility that the report is no longer
considered relevant and review by Geotech
Engineering Pty Ltd must be undertaken.
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Interpretation of the
Subsurface Sheets
It is important to note that the point load strength index
The terminology included on Geotech Engineering Pty
does not take into consideration the degree of fracturing of
Ltd’s has been evolved from AS1726:2016. Typically a
a rock mass which can both lower and increase the overall
soil’s strength, colour, plasticity, grain size, other
rock strength. The strength of rock is broken down as
inclusions, and moisture will be described. A rock’s
follows:
strength, weathering, colour, grain, and other inclusions
are typically described.
Very Low Strength (VLS) < 2 MPa (UCS)
Low Strength (LS) 2 to 6 MPa (UCS)
Soil Medium Strength (MS) 6 to 20 MPa (UCS)
High Strength (HS) 20 to 60 MPa (UCS)
Soil is typically described by the components that make it Very High Strength (VHS) 60 to 200 MPa (UCS)
up by percentage or whichever has a governing behaviour. Extremely High Strength (EHS) > 200 MPa (UCS)
The types and sizes reproduced from AS1726 are:
Boulder >200 mm
Cobble 63 to 200 mm Very Low Strength (VLS) < 0.1 Is(50)
Gravel 2.36 to 63 mm Low Strength (LS) 0.1 to 0.3 Is(50)
Sand 0.075 to 2.36 mm Medium Strength (MS) 0.30 to 1.0 Is(50)
Silty 0.002 to 0.075 mm High Strength (HS) 1.0 to 3.0 Is(50)
Clay < 0.002 mm Very High Strength (VHS) 3.0 to 10.0 Is(50)
Extremely High Strength (EHS) > 10.0 Is(50)
The strength of soil is usually assessed by suitably
experienced persons through the use of laboratory test The Is(50) values are based on a correlation between the
results, published literature, field assessments or a unconfined compressive strength (UCS) of 20. This value
combination of the above. The strength of soil is varies significantly and site specific testing must take
characterised into either cohesive (clays and silts) or precedence.
cohesionless (sands and gravels) as shown below:
Rock weathering is broken down as follows:
Cohessive Soil Strength Term
Very Soft < 12.5 kPa Extremely weathered Behaves as soil but rock
Soft 12.5 to 25 kPa (EW) structure evident
Firm 25 to 50 kPa Highly weathered Colour and strength of original
Stiff 50 to 100 kPa (HW) fresh rock is not recognisable.
Very Stiff 100 to 200 kPa Decomposition of rock evident
Hard > 200 kPa Moderately Defects are heavily stained with
weathered (MW) rock mass showing signs of
Cohesionless Soils Strength Term (uncorrected Qc) discolouration
Very Loose <2 MPa Slightly weathered Some staining but minimal to no
Loose 2 to 5 MPa (SW) change in strength
Medium Dense 5 to 15 MPa Fresh stained Minor staining evident
Dense 15 to 25 MPa (FS)
Very Dense > 25 MPa Fresh rock No change
(FR)
Rock
Rock fracturing is broken down as follows and can only be
Rock is typically described by the components that make accurately described where undamaged continuous
it up by percentage or whichever has a governing sampling has been undertaken:
behaviour. The strength of rock is typically the most
important component. Typically, the strength of rock is Fragmented (Fg) Defect spacings < 20 mm
based on laboratory testing, tactile assessment and other Highly Fractured (HF) Defect spacings 20 to 50 mm
observations as relevant. The industry standard method Fractured (Fr) Defect spacings 50 to 200 mm
for assessing the strength of rock in the field is to use the Slightly Fractured (SF) Defect spacings 200 to 1000 mm
point load strength index (AS4133.4.1). Unbroken (Ub) Defect spacings > 1000 mm
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Foundation Maintenance and Footing Performance: A Homeowner’s Guide cairo SIF 18 replaces Information Sheet 10/91 Bulidings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause of movement in Dulicings can usually be related to one or more problems in the foundation sow. & is important for the homeowner to identify the soil type in order to ascertain the measures that should be put in place in order to ensure that problems In the foundation soil can be prevented, thus protecting against Duliding movement. This Building Technology File is Gesigned to Identify causes of soil-related buliding movement, and to suggest methods of prevention of resultant cracking in buildings. ‘The rypes of sotls usually presepr under the topsall in land saned for residential batidings can be split Into Two approximare groups — granidar and clay, Quite often, foundation sof Is a milsvure of borh types. The general problems assoclaned with sells having grarailar contenr are usually caused by erosion Clay souks are qubject to saturation and swedll/shrink problenes: Classifications for a given area can generally be obralmed by sppiicarion to the local authortry, bur these are sornetines unreliable and if there ls dowhrt, a georechnlcal report should he commissioned. As mos bulldings suffering movement problems are founded on chy sol, there & 2n emphasis on classification of snlls according to the aenount of swell and shrinkage they experience whh warlations of wares content. The table below ts Table 2.1 from AS 2870, the Residermiat Stab and Footing Code. Settlement due to construction Thore are two types of sertiement that occur as a resulr of construction: * Imediare settiement occurs when a bulding & first placed on tts foundation soll, 25 a result of compaction af the soll under the weight of the structure, The cohestve qualiry of clay sou mitigates agatast this, bur granular (particularly sandy) sall ts suxscepribie. © Consotidation setriement Is a fearure of clay satl and may rake pilace becaise of the expulsion of motsture froen the soll or because of the poll's lack of restetance to local compressive or shear stresses. This will usually take plane during the firs few months after construction, bat has been known to take macy years tn exceptional cases. These problems are the province of the bullder and should be raken Erosion AI sols are prone ta erosion, bur sandy sot) ts particularly suscepethie to belng washed away. Even clay with a sand component of sty 10% of more can suffers from erosion, Saturation ‘This ts particularly a problem tn clay safls. Senurarion creates a bog Dike suspension of the soll that causes ir to ioae virmually af] of ts hearing capacity. To a lesser degren, sand 1s affected by saruration Decente saturated sand may undergo a reduction in volume particularly Imported sand fil for bedding and blinding layers. However, this usually occurs as Immediate setriement and should formally be the prowlnce of the tullder. ‘Seasonal swelling and shrinkage of soli All clays react to the presence of water by sowty absorbing ir, making the soil Increase In volune (see table below). The degree of Increase varles considerably hetwnen different clays, 25 dees the degree af decrease during the swhsequent drying out caused by fair weather periods. Because of the low absorprion and expulsion rate, rhis phenomenon will not wsually he norkceable unless there are Prolonged rainy or dry periods, usually of weeks or months, depending on the land and sol) charactertsrics. ‘The swelling of so8 creates an upward force on the foatings of the building, and shrinkage creares subeidence that takes away the support needed by the fooring to retain equilibrium. Shear fathare Tht phenomenon occurs when the foundation soll does not have suffictenr strengrh to support the weight of the fooring, There are Two major past-comstruction causes * Significart load increase. * Reduction of lateral support of rhe soll under the fporing due to fon, BuBting Tecbactagy Fae I J 10) cosie vech =e . Guan eee Racin be caused by saturation of the soll problems. adjacene to or under the toortng. GENERAL DEFINITIONS OF SITE CLASSES Chass Foundation A Most sand and rock sites with little or na ground movement fram moisture changes s Stightly reactive clay sites with only sight ground movement from moisture changes M Moxtoratety reactive clay or stk sites, which can experience moderame ground movement from maisture changes H Highly reactive clay sites, which can experience high ground movement from motsture changes E Extremely reactive sites, which can experience extreme ground movement from matsture changes AwP Filed sites + || Sete cee teeta eee eee eee owe
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Tree root growth Trees and shrubs that are allowed te grow in the vicinity of footings can cause foundation soil movement in two ways: © Roots that prow under footings may increase in cross-sectional size, exerting upward pressure on footings. * Roots in the vicinity of footings will absorb mach of the moisture in the foundation soil, causing shrinkage or subsidence. Unevenness of Movement The types of ground movernent described above usually occur unevenly throughout the building's foundation soil. Settlement due to construction tends to be uneven because of- © Differing compaction of foundation soil prior to comstruction_ © Differing moisture content of foundation soil prior to construction. Movement due to non-constraction causes is usually more uneven still Erosion can undermine a footing that traverses the flow or can create the conditions for shear Gilure by eroding soil adjacent to 2 footing that rans in the same direction as the flow. Saturation of clay foundation soil may occur where subfloor walls create adam thar makes water pond. ft can also ocour wherever there isa source af water near footings in clay soil. This leads to a severe reduction in the strength of the soil which may create local shear failure. Seasonal swelling and shrinkage of clay soil affects the perimeter af the building first, then gradually spreads eo the interior. The swelling process will usually begin at the uphill extreme of the building, or on the weather side where the land is flat. Swelling gradually reaches the interior soil as absorption continues. Shrinkage usually begins where the sun's heat is greatest_ Effects of Uneven Soll Movement on Structures Erosion and saturation Erosion removes the support from under footings, tending to create subsidence of the part of the structure under which it occurs. Brickwork walls will resist the stress created by this removal of support by bridging the pap or cantilevering until the bricks or the mortar bedding fail. Older masonry has little resistance. Evidence of failure varies according tw circumstances and symptoms may include © Step cracking im the mortar beds in the body of the wall ar above! below openings such as doors or windows. © Vertical cracking in the bricks (usually but not necessarily in line with the vertical beds or perpends). Lsolated piers affected by erosion or saturation of foundations will eventually lose contact with the bearers they support and may tilt or fall ower. The floors that have los this support will become bouncy, sometimes rattling omaments etc Seasonal swelling/shrinkage in day Swelling foundation soil due te rainy periods first lifts the most exposed extremities of the footing system, then the remainder of the perimeter footings while gradually permeating inside the building footprin w lift internal footings. This swelling first tends to create a dish effect, because the external footings are pushed higher than the internal ones. The first noticeable symptom may be that the floor appears slightly dished. This is often accompanied by some doors binding on the floor or the door head, together with some cracking of cornice mitres. In buildings with timber flooring supported by bearers and joists, the floor can be bouncy. Externally there may be visible dishing of the hip or ridge lines. As the moisture absorption process completes its journey to the innermost areas of the building, the internal footings will rise. If the spread of moisture is roughly even, it may be that the symptoxns will temporarily disappear, but it is more likely that swelling will be uneven, creating a difference rather than a disappearance in symptoms. In buildings with timber flooring supported by bearers and joists, the isolated piers will rise more easily than the strip footings or piers under walls, creating noticeable doming of Mooring. As the weather pattern changes and the soil begins to dry out, the external footings will be first affected, beginning with the locations where the sun's effect is strongest. This has the effect af lowering the external footings. The doming is accentuated and cracking reduces or disappears where it ocaurred because of dishing, but other cracks open up. The roof lines may become convex. Daring and dishing az also affected by weather in other ways. In areas where warm, wet summers and cooler dry winters prevail, water migration tends to be toward the interior and doming will be accentuated, whereas where summers are dry and winters are cold and wet, migration tends to be toward the exterior and the underlying propensity i toward dishing. Movement caused by tree roots In general, growing roots will exert an upward pressure on footings, whereas soil subject to drying because of tree or sirub roots will rend to remove support from under footings by inducing shrinkage. Complications caused by the structure itself Most forces that the soil causes to be exerted on structures are vertical — i.e. either up or down. However, because these forces are seldom spread evenly around the footings, and because the building resists uneven movement because of its rigidity, forces are exerted from one part of the building to another. The net result of all these forces is usually rotational. This resultant force often complicates the diagnosis because the visible sympeoms do not simply reflect the original cause. A common symptom is binding of doors on the vertical member of the frame. Effects on full masonry structures Brickwork will resist cracking where it can. Ir will atrempt mm span areas that lose support because of subsided foundations or raised points. It is therefore usual to see cracking at weak points, such as openings for windows or doors. In the event of construction settlement, cracking will usually remain unchanged after the process of settlement has ceased. With local shear or erosion, cracking will usually continue to develop until che original cause has been remedied, or until the subsidence has completely neutralised the affected portion of footing and the structure has stabilised on ceher footings that remain effective. In the case of swell/shrink effects, the brickwork will in some cases return to its original position after completion of a cycle, however it is more likely that the rotational effect will mvt be exactly reversed, and it is also usual that brickwork will settle in its new position and will resist the forces trying to return it w its original position. This means that in a case where swelling takes place after construction and cracking occurs, the cracking is likely to at least partly remain after the shrink segment of the cycle is complete. Thus, each time the cycle is repeated, the likelihood is chat the cracking will become wider until the sections of brickwork became virtually independent. With repeated cydes, once the cracking is eszablished, if there is no other complication, it is normal for the incidence of cracking stabilise, as the building has the articulation it needs to cope with the problem. This is by no means always the case, however, and monigoring of cracks in walls and floors should always be treated seriously. Upheaval caused by growth of tree roots under foceings is not a simple vertical shear stress. There is a tendency for the mot to also exert lateral forces that attempt to separate sections of brickwork after initial cracking has occurred.
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The normal structural arrangement is that the inner leaf of
brickwork in the external walls and at least some of the internal walls
(depending on the roof type) comprise the load-bearing structure on
which any upper floors, ceilings and the roof are supported. In these
cases, it is internally visible cracking thar should be the main focus of
attention, however there are a few examples of dwellings whose
external leaf of masonry plays some supporting role, so this should be
checked if there is any doubt. In amy case, externally visible cracking
is important as a puide to stresses on the structure penerally, and ie
should also be remembered that the external walls must be capable of
supporting themselves.
Effects on framed structures
Timber or steel framed buildings are less likely to exhibit cracking due
to swellishrink than masonry buildings because of their flexibility.
Also, the doming/dishing effects tend to be lower because of the
lighter weight of walls. The main risks to framed buildings are
encountered because of the isolated pier footings used under walls.
Where erosion or saturation causes a footing to fall away, this can
double the span which 2 wall must bridge. This additional stress can
create cracking in wall linings, particularly where there is 2 weak
point in the structure caused by a door or window opening. It is,
however, unlikely that framed structures will be so stressed as to suffer
serious damage without first exhibiting some ar all of the above
symptoms for a considerable period. The same warning period should
zpply in the case of upheaval. [t should be noted, however, that where
Framed buildings are supported by strip footings there is only ome leaf
of brickwork and therefore the externally visible walls are the
supporting structure for the building. In this case, the subfloor
masonry walls. can be expected to behave as full brickwork walls.
Effects on brick veneer structures
Because the load-bearing structure of a brick veneer building is the
frame that makes up the interior leaf of the external walls plus
perhaps the internal walls, depending on the type af roof, the
building can be expected to behave as a framed structure, except that
the external masonry will behave in a similar way ew the external leaf
of a full masonry structure.
Water Service and Drainage
Where a water service pipe, a sewer or stormwater drainage pipe is in
the vicinity of a building, a water leak can cause erosion, swelling or
saturation of susceptible soil. Even a minuscule leak can be enough to
saturate 2 clay foundation. A leaking tap near a building can have the
same effect. In addition, trenches containing pipes can become
watercourses even though backfilled, particularly where broken
rubble is used as Fill Water that runs along these trenches can be
responsible for serious erosion, interstrata seepage into subfloor areas
and saturation.
Pipe leakage and trench water flows also encourage tree and shrub
roots to the source of water, complicating and exacerbating the
problem. Poor roof plumbing can result in Large volumes of rainwater
being concentrated in a small area of soil:
© Incorrect falls in roof guttering may result in overflows, as may
gutters blocked with leaves ete
* Corroded guttering or downpipes can spill water to ground.
* Downpipes not positively connected to a proper stormwater
collection system will direct a concentration of water to soil thar is
directly adjacent to footings, sometimes causing large-scale
problems such as erosion, saturation and migration of water under
the building.
Seriousness of Cracking
In general, most cracking found in masonry walls & a cosmetic
nuisance only and can be kept in repair of even ignored. The table
below is 2 reproduction of Table C1 of AS 2870-2011.
AS 2870-2011 also publishes figures relating to cracking in concrete
floors, however because wall cracking will usually reach the critical
point significantly earlier than cracking in slabs, this table is not
reproduced here_
Prevention /Cure
Plumbing
Where building movernent is caused by water service, roof
plumbing, sewer or stormwater failure, the remedy is to repair the
problem. It is prudent, however, to consider also rerouting pipes
away from the building where possible, and relocating taps to
positions where any leakage will not direct water to the building
vicinity. Even where pully traps are present, there is sometimes
sufficient spill to create erosion or saturation, particularly in modern
installations using smaller diameter PVC fixtures. Indeed, some
gully traps are not situated directly under the taps that are installed
to charge them, with the resulr that water from the tap may enter
the backfilled trench that houses the sewer piping. If the trench has
been poorly backfilled, the water will either pond or flow along the
bottom of the trench. As these trenches usually ran alongside the
footings and can be at a similar depth, it is not hard to see how any
water that is thus directed into a crench can easily affect the
foundation's ability to support footings or even gain entry to the
subfloor area.
Ground drainage
In all soils there is the capacity for water to travel on the surface and
below it. Surface water flows can be established by inspection during
and after heavy or prolonged rain. If necessary, a grated drain system
connected to the stormwater callection system is usually an easy
solution.
{ce is, however, sometimes necessary when attempting to prevent water
migration that testing be carried out to establish watertable height
and subsoil water flows. This subject is referred to in BTF 19 and
may properly be regarded as an area for an expert consultant.
Protection of the building perimeter
[eis essential so remember that the soil that affects footings extends
well beyond the actual building line. Watering of garden plants,
shrubs and trees causes some of the mast serious water problems.
For this reason, particularly where problems exist or are likely to
occur, it is recommended that an apron of paving be installed around
as much of the building perimeter as necessary. This paving tiould
CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS
Approximate crack width Damage
Description of typical damage and required repair limit (see Note 3) category
Hairline cracks <0.1 mm o
Fine cracks which do not need repair <l mm 1
(Cracks noticeable but easily filled. Doors and windows stick slightly. Simm 2
(Cracks can be repaired and possibly a small amount of wall will need to be 5-15 mm (or a number of cracks 3
replaced. Doors and windows stick. Service pipes can fracture. Weathertightness 3 mm or more in one group)
often impaired.
Extensive repair work involving breaking-out and replacing sections af walls, 15—25 mm bur also depends on 4
especially over doors and windows. Window and door frames distort. Walls lean number af cracks
or bulge noticeably, some loss of bearing in beams. Service pipes disrupted.
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exend ourwards a minimum of 900 mum (more in highly reactive soil) and should have a minimum fall away from the building of 1:60. The finished poving should be no les than 100 mm below brick vent bases. Tis prudent ro relocate dminage pipes away from this paving, If possible, to avold complications from future leakage If this is nor practical, earthenware pipes should be rephiced by PVC and backfilling should be of the same soil rype as the surrounding soil and compacted to the ame density. Except in areas where freezing of water is an Issue, it is wise to” remove taps in the baikting area and relocate chem well away from the building — preferably moe uphill from ir (see BTF 19). Tc may he desirable to install a grated drain at the curside edge of the paving on the uphill side of the building. Ifsubsall drainage is needed rhis can be installed under the surface drain, Condensation In buildings with a subfloor void such as where bearers and joists support flooring, insufficient ventilation creates ideal conditions for condensation, particularly where there Is little clearance between the floor and the ground. Condensation adds to the moisture already present in the subfloorand significantly slows the process of drying Warning: Although this Building Technology Pile deals with cracking in buildings, it should be eid thar aubfloor moisture can result in the development of other problems, notably: © Wacerthar is tearumiteed into masonry, metal or timber building elements causes damage and/or decay two those elements. * High subfloor humidity and moisture content create an ideal, * Where high moisture levels are transmitted co the flooring and walls, an increase in the dust mite count can ensue within the living areas. Dust mites, as well as dampness in general, can be a health hazard two inhabitants, particularly those who are abnormally aasceptible to respimrory ailments: The garden The ideal vegetation layour is to have lawn or plants thar require only light watering immedianely adjacere to the drainage or paving edge, then moce demanding planes, shrubs and trees spread our in thar order. Overwarering due to misuse of avomaric watering symems iso common canse of sarusarion and water migration under footings. If ic is necematy to use these systems, it is important w mmove garden beds to a completely safe distance from buildings. Existing trees Where a tree is canting a pooblem of soil drying or there is the existence or threat of upbeaval of footings, if the offending mow are subsidiary and their removal will not significantly damage the tree, they should be severed and a conceete or metal harrier placed vertically in the soil w prevent fucure coor growth in the direction of the building, If it is not possible to remove the relevant mors without damage tw the tree, an application ro remove the ree should be made to the local authority, A prudent plan is wo transplan likely offenders before they become a problem. regarding root pacterns, volume of water needed and safe distance from buildings of most species. Boranic gardens ace also sources of information. For informarion on plant mow and drains, see Building Technology Pile 17. Excavation Exravation around footings must be properly engineered. Soil supporting foorings can only be safely excavared aran angle thar allows the soil under the footing to remain stable. This angle is called Deano ween fe Biren sat eclos between soil ype and conditions Removal of sail within che angle of repose will cause subsidence. Where erosion has occurred that has washed away soil adjacent ro footings, soll of the same classification should be ionroduced and compacted to the same density, Where footings have been undermined, augmentation or other specialist work may be required. Remediation of footings and foundations is generally the realm of a specialie consulrant. Where isolated footings rise and fall because of swell/shrink effect, the homeowner may be remperd to alleviate floor bounce by filling the gap thar has appeared between the bearer and the pier with blocking. The danger hese is thar when the next swell segment of the cycle occurs, the ertra blocking will pash the floor up inno an accentuated dome and may also cause local shear failure in the soil. If I is necessary to use blodeing, Ic should be by a pairof fine wedges and monitoring should be carried our fortnightly, This BYF was prepared by John Lewer FAIB, MIAMA, Partner, Construction Diagnosis. ‘The infermation in this and other Issues in the series was derived from various scuross and was beteved to be correct when pubished. The Information ts advisory. it is provided In good falth and not clained io be an exhaustive treatment of the relevant subject. Further proteasional odvice needs to be obtained before tahing any action based on the information provided. ‘Disiibuted by ‘ CSIRO PUBLISHING PO Bex 1139, Collingwood 3066, Australia Tal (03) 9562 7666 Fax (03) 96627555 werw.publish.csiro.au Email: [email protected] © CSIRO 2098. Unauthorised copying of this Bullding Technology File ts prohibited
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AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE) Sensible development practices are required when building on hillsides, particularly the hillside has more than a low risk of instability (GeoGuide LR7). Only building techniques intended to maintain, or reduce. the overall level of landslide risk should be considered. Examples of good hillside construction practice are illustrated below. EXAMPLES OF GOOD HILLSIDE CONSTRUCTION PRACTICE =a Vegetation reamed = Surface water interception drainage d a Waleright adequately sted anc founded root water storage tanks (with due regard for impact of polentcal leakage) Fimoble structure Root water piped of site or stored — On-site delention tanks, waterlignt and adequately foundes. Potential leakage managed by subsoil drains - — ~ Piar footings into rock Subsoll drainage may be required in slope Cutting and fling minimised in develooment ~ Sewage ¢ffuent pumped out or connected io sewer Tanks adequataly foundad and watertght. Potential leakage managed by sub-soll drains ~____ Engineered retaineig walls with both surface and ‘subsurface drainage (constructad before dwelling) (p) Aa (pei) See 280 AGS [TON Opperie 1 WHY ARE THESE PRACTICES GOOD? Roadways and parking areas - are paved and incorporate kerbs which prevent water discharging straight into the hillside (GeoGuide LR5). Cuttings - are supported by retaining walls (GeoGuide LR6). Retaining walls - are engimeer designed to withstand the lateral earth pressures and surcharges expected, and include drains to prevent water pressures developing in the backfill. Where the ground slopes steeply down towards the high side of 2 retaining wall, the disturbing force (see GeoGuide LRG) can be two or more times that in level ground. Retaining walls must be designed taking these forces into account. Sewage - whether treated or not is either taken away in pipes or contained in properly founded tanks so it cannot soak into the ground. Surface water - from roofs and other hard surfaces is piped away to a suitable discharge point rather than being allowed to infiltrate into the ground. Preferably, the discharge point will be in a natural creek where ground water exits, rather than enters, the ground. Shallow, lined, drains on the surface can fulfil the same purpose (GeoGuide LR5). Surface loads - are minimised. No fill embankments have been built. The house is a lightweight structure Foundation loads have been taken down below the level at which a landslide is likely to occur and, preferably, to rock. This sort of construction is probably not applicable to sol slopes (GeoGuide LR3). If you are uncertain whether your site has rock near the surface, or is essentially a sol slope, you should engage a geotechnical practitioner to find out. Flexible structures - have been used because they can tolerate a certain amount of movement with minimal signs of distress and maintain their functionality. Vegetation clearance - on soil slopes has been kept to a reasonable minimum. Trees, and to a lesser extent smaller vegetation, take large quantities of water out of the ground every day. This lowers the ground water table, which in tum helps to maintain the stability of the slope. Large scale clearing can result in a rise in water table with a consequent increase in the likelihood of a landslide (GeoGuide LR5). An exception may have to be made to this rule on steep rock slopes where trees have little effect on the water table. but their roots pose a landslide hazard by dislodging boulders. Possible effects of ignoring good construction practices are illustrated on page 2. Unfortunately, these poor construction practices are not as unusual as you might think and are often chosen because, on the face of it, they will save the developer, or owner, money. You should not lose sight of the fact that the cost and anguish associated with any one of the disasters Hlustrated, is likely to more than wipe out any apparent savings at the outset. ADOPT GOOD PRACTICE ON HILLSIDE SITES 74 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE) EXAMPLES OF POOR HILLSIDE CONSTRUCTION PRACTICE Unstabllised rock topples and Iravets downsiope Vegelation removed Steep unsupported out fils Discharges of rootwater soak away ralher than conducted offsite of to secure storage for re-use Structure unable te tolerate ae ecitement and cracks oo ™~ Poory compacted 4 setties «’ * A unevenly and cracks pool tf inadequate walling unable i fo support fH Ir inadequately Lose. sabaatad fill slides avd possitty News downslope Ponded water enters stope and actvates landskde 1 vie yay = ~Prestte travel ccrensiope which impacts other development dawnt! San wea ASE OPO Ascordm | WHY ARE THESE PRACTICES POOR? Roadways and parking areas - are unsurfaced and lack proper table drains (gutters) causing surface water to pond and soak into the ground. Cut and fill - has been used to balance earthworks quantites and level the site leaving unstable cut faces and added large surface loads to the ground. Fadure to compact the fill properly has led to settlement, which will probably continue for several years after completion. The house and poo! have been built on the fill and have settled with it and cracked. Leakage from the cracked pool and the applied surface loads from the fill have combined to cause landslides. Retaining walls - have been avoided, to minimise cost, and hand placed rock walls used instead. Without applying engineering design principles, the walls have failed to provide the required support to the ground and have fated. creating a very Situation. A heavy, rigid. house - has been built on shallow, conventional, footings. Not only has the brickwork cracked because of the resulting ground movements, but it has also become involved in a man-made landslide. Soak-away drainage - has been used for sewage and surface water run-off from roofs and pavements. This water soaks inte the ground and raises the water table (GeoGuide LR'5}. Subsoil drains that run along the contours should be avoided for the same reason. If felt necessary. subsoil drains should run steeply downhill in a chevron, or herring bone. pattem. This may conflict with the requirements for effuent and surface water disposal (GeoGuide LRQ) and if so, you will need to seek professional advice. Rock debris - from landslides higher up on the slope seems likely to pass through the site. Such locations are often referred to by geotechnical practitioners as “debris flow paths”. Rock is normally even denser than ordinary fill, so even quite modest boulders are likely to weigh many tonnes and do 3 lot of damage once they start to roll. Boulders have been known to travel hundreds of metres downhill leaving behind a trail of destruction. Vegetation - has been completely cleared. leading to a possible rise in the water table and increased landslide risk (GeoGuide LR5). DON'T CUT CORNERS ON HILLSIDE SITES - OBTAIN ADVICE FROM A GEOTECHNICAL PRACTITIONER More information relevant to your particular situation may be found In other Australian GeoGuldes: « GeoGuide LR1 - introduction « GeoGuide LR6 - Retaining Wals * GecoGuide LR2 -Landsides « GeoGuide LR7 - Landslide Risk « GeoGuide LR3 -Landsides In Soll « GeoGuile LRS -EfMivent & Surface Water Disposal * GeoGuide LR4 -Landsides In Rock GeoGuite LR10 - Coastal Landslides « _GeoGuide LAS -Water & Drainage « GeoGuide LR11 - Record Keeping The Australian GeoGuides (LR series) ae a set of publications Intended for property owners; local councils; planning autnodtes; Gevelopers; insurers: (awyers and, in fact, anyone who lives with, or has an interest in, a natural of engineered stope, 3 cutting or an excavaton. They ae intended to help you understand why slopes and retaining structures can be 3 Nazard and what can be done wtn appropriate advice and local counc! approval (if required) to remove, reduce, of minimise ine risk they represent The GeoGukles have been prepared oy the Ausirailan Geomechanics Society, a specialist technica society wifin Engineers Australia, the ational peak body for ail engineering disciplines in Australia, wnose members are professiona geotechnica’ engineers and engineering geologists with 3 particular interest n ground engineering. Tne GeoGuites nave Deen funded under tne Australian governments’ Nationa Disaster Misgation Program. Australian Geomechanics Vol 42 No 1 March 2007 75
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AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK) LANDSLIDE RISK Concept of Risk Risk is a familiar term, but what does it really mean? It can be defined as “a measure of the probability and severty of an adverse effect to health, property, or the environment.” This definition may seem a _ bit complicated. In relation to landslides, geotechnical Practitioners (GeoGuide LR1) are required to assess risk in terms of the likelihood that a particular landslide will occur and the possible consequences. This is called landslide msk assessment. The consequences of a landslide are many and varied, but our concems normally focus on loss of, or damage to. property and loss of life. Landslide Risk Assessment Some local councils in Australia are aware of the potential for landslides within their jurisdiction and have responded by designating specific ‘landslide hazard zones”. Development in these areas is often covered by special regulations. If you are contemplating building, or buying an existing house, particularly in a hilly area, or near cliffs, go first for information to your local council. Landslide risk assessment must be undertaken by a _geotechnical practitioner. It may involve visual inspection, geological mapping, geotechnical investigation and monitoring to identify: * potential landslides (there may be more than one that could impact on your site) the likelihood that they will occur the damage that could result the cost of disruption and repairs and the extent to which lives could be lost. Risk assessment is a predictive exercise, but since the ground and the processes involved are complex, prediction tends to lack precision. If you commission a landslide risk assessment for a particular site you should expect to receive a report prepared in accordance with current professional guidelines and in a form that is acceptable to your local council, or planning authority. Risk to Property Table 1 indicates the terms used to describe risk to property. Each risk level depends on an assessment of how likely a landslide is to occur and its consequences in dollar terms. “Likelihood” is the chance of it happening in any one year, as indicated in Table 2. “Consequences” are related to the cost of repairs and temporary loss of use if a landslide occurs. These two factors are combined by the geotechnical practitioner to determine the Qualitative Risk. TABLE 2: LIKELIHOOD Rare 1-100,000 The terms “unacceptable”. “may be tolerated”, etc. in Table 1 indicate how most people react to an assessed risk level. However, some people will always be more prepared. or better able. to tolerate a higher risk level than others. Some local councils and planning authorities stipulate a maximum tolerable level of risk to property for developments within their jurisdictions. in these Situations the risk must be assessed by a geotechnical practitioner. |f stabilisation works are needed to meet the stipulated requirements these will normally have to be carried out as part of the development, or consent will be withheld. TABLE 1: RISK TO PROPERTY Qualitative Risk | Significance - Geotechnical engineering requirements Very high | VH | Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment options essential to reduce risk to Low. May be too expensive and not practical. Work likely to cost more than the value of the property. High H | Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce risk to acceptable level. Work would cost a substantial sum in relation to the value of the property. May be tolerated in certain circumstances (subject to regulator's approval) but requires investigation, planning and implementation of treatment options to reduce the nsk to Low. Treatment options to reduce to Low risk should be implemented as soon as possible. Moderate M Usually acceptable to regulators. Where treatment has been needed to reduce the risk to this Low | | Jevel, ongoing maintenance is required. Very Low | VL | Acceptable. Manage by normal slope maintenance procedures. 172 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK) Risk to Life Most of us have some difficulty grappling with the concept of risk and deciding whether, or not, we are prepared to accept it. However, without doing any sort of analysis, or commissioning a report from an “expert”, we all take risks every day. One of them is the risk of being killed in an accident. This is worth thinking about, because it tells us a lot about ourselves and can help to put an assessed risk into a meaningful context. By identifying activities that we either are, or are not, prepared to engage in we can get some indication of the maximum level of risk that we are prepared to take. This knowledge can help us to decide whether we really are able to accept a particular nsk, or to tolerate a particular likelihood of loss, or damage. to our property (Table 2). In Table 3, data from NSW for the years 1928 to 2002, and other sources, is presented. A risk of 1 in 100.000 means that, in any one year, 1 person is killed for every 100,000 people undertaking that particular activity. The NSW data assumes that the whole population undertakes the activity. That is, we are all at risk of being killed in a fire, or of choking on our food, but it is reasonable to assume that only people who go deep sea fishing run a nsk of being killed while doing it It can be seen that the risks of dying as a result of falling, using a motor vehicle. or engaging in water- related activities (including bathing) are all greater than 1:100,000 and yet few people actively avoid situations where these risks are present. Some people are averse to flying and yet it represents a lower risk than choking to death on food. Importantly, the data also indicate that, even when the risk of dying as a consequence of a particular event is very small, it could still happen to any one of us any day. If this were not so, no one would ever be struck by lightning. Most local councils and planning authorities that stipulate a tolerable msk to property also stipulate a tolerable risk to life. The AGS Practice Note Guideline recommends that 1:100,000 is tolerable in newly developed areas, where works can be carned out as part of the development to limit nsk. The tolerable level is raised to 1:10,000 in established areas, where specific landslide hazards may have existed for many years. The distinction is deliberate and intended to prevent the concept of landslide risk management, for its own sake, becoming an unreasonable financial burden on existing communities. Acceptable risk is usually taken to be one tenth of the tolerable risk (1:1,000,000 for new developments and 1:100,000 for established areas) and efforts should be made to attain these where it is practicable and financially realistic to do so. TABLE 3: RISK TO LIFE Risk (deaths per Activity/Event Leading to participant per Death year) (NSW data unless noted) 1:1,000 Deep sea fishing (UK) 110.000 | Motor_cycing. horse riding . ultra-light fiying (Canada) 1-23,000 Motor vehicle use 1-30,000 Fall 1:70,000 Drowning 1:180,000 Fire/burn 1:660,000 Choking on food 1:1,000,000 Scheduled airlines (Canada) 1-2,300,000 Train travel 1:32,000,000 Lightning strike More information relevant to your particular situation may be found In other AUSTRALIAN GEOGUIDES: GeoGuide LR1 - Introduction GeoGuide LR2 -Landsides GeoGuide LR3 - Landsiides In Soll GeoGuide LR4 = - Landsiides In Rock GeoGuide LRS - Water & Drainage GeoGuide LR5 - Retaining Walls GeoGuide LR8 - Hillside Construction GeoGuide LR9 - Effluent & Surface Water Disposal GeoGulde LR10 - Coastal Landslides GeoGuide LR11 - Record Keeping The Australian GeoGuides (LR series) are a set of publications Intended for property owners; local councils; planning authorities; developers; Insurers; lawyers and, in fact, anyone who lives with, or has an Interest in, a natural or engineered slope, a cutting. or an excavation. They are intended to help you understand why slopes and retaining structures can be a hazard and what can be done with appropriate professional advice and local council approval (If required) to remove. reduce, or minimise the risk they represent. The GeoGuides Nave been prepared by the Australian Geomechanics Society, a specialist technical society within Engineers Austraila, the national peak body for all engineering disciplines in Australla, whose members are professional geotechnical engineers and engineering with a particular interest In ground engineering. The GeoGuides have Deen funded under the Ausirallan governments” geologists National Disaster Mitigation Program. Australian Geomechanics Vol 42 No 1 March 2007 173
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AUSTRALIAN GEOGUIDE LRG (RETAINING WALLS)
RETAINING WALLS
Retaining walls are used to support cuts and fills. Some are built in the open and back®ll is placed behind them (gravity
walls). Others are inserted into the ground (cast in situ or driven piles) and the ground is subsequently excavated on one
side. Retaining walls, like all man-made structures, have a finite life. Properly engineered walls should last 50 years, or
more, without needing significant repairs. However, not all walls fit this category. Some. particularly those built by
inexperienced tradesmen without engineering input, can deflect and even fad because they are unable to withstand the
eee ae eee eae eee ae Be eee ee ees cet een oe
time. Design of retaining walls more than 900mm high should be undertaken by a geotechnical practitioner or
structural ineer and normal ire local council ‘oval.
Retaining walls have to withstand the weight of the ground on the high side. any water pressure forces that develop, any
additional load (surcharge) on the ground surface and sometimes swelling pressures from expansive clays. These
forces are resisted by the wall itself and the ground on the low side. Engineers calculate the forces that the retained
ground, the water, and the surcharge impose on a wall (the disturbing force) as well a5 the maximum force that the wall
and ground on the low side can provide to resist them (the restoring force). The ratio of the restoring force to the
disturbing force is called the “factor of safety” (GeoGuide LR1). Permanent retaining walls designed in accordance with
accepted engineering standards will normaly have a factor of safety in the range 1.5 to 2.
Never ado surcharge to the high side of a wall (e.g. place fil, erect a structure, stockpile bulk materials, or park vehicles)
unless you know the wall has been designed with that purpose in mind.
Never more than lightly water plants on the high side of a retaining wall. es ee
Never excavate at the toe of a retaining wall.
Any of these actions will reduce the factor of safety of the wall and could
lead to failure. fin doubt about any aspect of an existing retaining wall, or
changes you would like to make near one, seek advice from a
geotechnical practitioner, or a structural engineer. This GeoGuide sets out
basic inspection requirements for retaining walls and identifies some
common signs that might mdicate all is not well. GeoGuide LR11
provides information about records that should be kept
GRAVITY WALLS
Gravity walls are so called because they rely on ther own weight (the
force of gravity) to hold the ground behind in place.
Formed concrete and reinforced blockwork wails (Figure 1) - should
be built so the backfill can drain. They should be inspected at least once
@ year. Look for signs of tilting, bulging. cracking, or a drop in ground
level on the high side. as any of these may indicate that the wall has j— tnekeod draivage layer
Started to fal. Look for rust staining, which may indicate that the steel tented a
reinforcement is deteriorating and the wall is losing structural strength
(concrete cancer”). Ensure that weep holes are clear and that water is St, $$ Fras dvadeing rater
able to drain at all times, as high water pressures behind the wall can lead Seana on Peaea
to sudden and catastrophic failure. a]
Concrete “crib” walls (Figure 2) - should be filled with clean gravel, or S aa
“plue metal with a nominated grading. Sometimes soil is used to reduce Ie
cost, but this is undesirable, from an engineering perspective. unless a
intemal drainage is incorporated in the wall's construction. Without “= erateedina nee
backfill drainage, a soil filled crib wall is likely to have a lower factor of Al
safety than is required. Crib walls should be inspected as for formed Figure 2 -Typical crib
concrete walls. In addition, you should check that material is not being lost
through the structure of the wall, which has large gaps through it [— Orinerage tiryier
|
Timber “crib” walls - should be checked as for concrete crib walls. in {
addition, check the condition of the timber. Once individual e@ements
show signs of rotting it is necessary to have the wall replaced. If you are
uncertain seek advice from a geotechnical practitioner, or a structural
engineer.
Masonry walls: natural stone, brick, or interlocking blocks (Figure 3) -
more than about 1m high, should be wider at the bottom than at the top
and include specific measures to permit drainage of the backfill. They
should be checked as for formed concrete walls. Natural stone walls
should be inspected for signs of deterioration of the individual blocks:
strength loss, comers becoming rounded, cracks appearing, or debris
from the blocks collecting at the foot of the wail.
{Cary saat are Baca
Li a6 for concrete wal
Figure 3 -Typical masonry wall
170 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR6 (RETAINING WALLS) Old Masonry walls (Figure 4) - Many old masonry retaming walls have not been built in accordance with modern design standards and often have a low “factor of safety” (GeoGuide LR1). They may therefore be =~=—__ thickrees close to failure and a minor change in thew condition, or loading, could =>, initiate collapse. You need to take particular care with such structures \ and seek professional advice sooner rather than later. Although masonry walls sometimes deflect significantly over long periods of time collapse. ie when it occurs, is usually sudden and can be catastrophic. Familiarity with a particular situation can instil a false sense of confidence. Reinforced soil walls (Figure 5) - are made of compacted select fill in Sere which layers of reinforcement are buried to form a “reinforced soil zone”. _ The reinforcement is all important, because it holds the soil “wail” "7 7 together. Reinforcement may be steel strip, or mesh, or a variety of SS 22 geosynthetic (‘plastic’) products. The facing panels are there to protect the soil “wall” from erosion and give it a finished appearance. Most reinforced soil walls are proprietary products. Construction should be carried out strictly in accordance with the manufacturer's instructions. inspection and maintenance should be the same as for formed concrete and concrete block walls. If unusual materials such as timber, or used tyres, are used as a facing it should be checked to see that @ is not rotting, or penshing. OTHER WALLS Cantilevered and anchored walls (Figure 6) - rely on earth pressure on the low side, rather than self-weight, to provided the restoring force and an adequate factor of safety. These walls may comprise: « a line of touching bored piers (contiguous bored pile wall) or * sprayed concrete panels between bored piers (shotcrete wall) or * honzontal timber or concrete planks spanning between upright timber Figure 5 - Typical reinforced soil wall or steel soldier piles or « steel sheet piles. Depending on the form of construction and ground conditions, walls in excess of 3 m height normally require at least one row of permanent = Ground anchor —— (nee requared ter ground anchors. J caminvered wal) INSPECTION me - . 4 Al walls should be inspected at least once a year, looking for tiltmg and >_< Retwinirg wall other signs of deterioration. Concrete walls should be inspected for Ca cracking and rust stains as for formed concrete gravity walls. Contiguous bored pie walls can have gaps between the piles - look for loss of soil from behind which can become a major difficulty if it is not corrected. : Timber walls should be inspected for rot. as for timber crib walls. Steel sheet piles should be inspected for signs of rusting. In addition, you = = should make sure that ground anchors are maintained as described in = 7 . GeoGuide LR4 under the heading "Rock bolts and rock anchors”. Pee. eres One of the most important issues for walls is that their internal draimage systems are operational. Frequenty verity that intemal drainage pipes and surface interception drains around the wall are not blocked nor have become inoperative. More information relevant to your particutar situation may be found In other Australian GeoGuldes: GeoGuide LR7 - Landslide Risk GeoGuide LRA = - Hillside Construction GeoGuide LR3 - Effluent & Surface Water Disposal GeoGuide LR4 -Landsides In Rock GeoGuide LR10 - Coastal Landsiides GeoGuide LRS - Water & Drainage GeoGuide LR11 - Record Keeping The Austafan GeoGuides (LR series) are a set of publications Intended for property owners; local councis; planning authontes: Gevelopers; Insurers; lawyers and, In fact, anyone who lives with, or has an interest in, a natural or engineered slope, a cutting, or an excavaton. They are intended to help you understand why slopes and retaining stuctures can be a nazard and what can be done with appropiate professiona! advice and local council approval (if required) to remove, reduce, or minimise the risk they represent. The GeoGuides Nave been prepared by tne Austaian Geomechanics Society a specialist technical society within Engineers Austrata, the natonal peak body for al engineering disciplines in Australia, whose members are professiona@ geotechnical engineers and engineering geologists with 2 particular Interest in ground engineering. The GeoGuldes have been funded under the Australian govemments' National Olsaster MMigation Program GeoGuide LR1 - Introduction GeoGuide LR2 -Landsides GeoGuide LR3 - Landsides In Soll Australian Geomechanics Vol 42 No 1 March 2007 73
Page 38
AUSTRALIAN GEOGUIDE LR5 (WATER & DRAINAGE)
One way or another, water usually plays a critical part in initiating a landslide (GeoGuide LR2). For this reason, tis a
key factor to be controlled on sites with more than a low landslide risk (GeoGuide LR7).
Groundwater and Groundwater Flow
The ground is permeable and water flows through it as illustrated in Figure 1. When rain falls on the ground, some of it
runs along the surface ("surface water run-off) and some soaks in, becoming groundwater. Groundwater seeps
downwards along any path it can find until t meets the water table: the local level below which the ground is saturated. If
itreaches the water table. groundwater either comes to 2 halt in what is effectively underground storage, or it continues
to flow downwards. often towards a spring where it can seep out and become surface water again. Above the water
table the ground is said to be “partially saturated”, because it contains both water and air. Suctions can develop in the
partially saturated zone which have the effect of holding the ground together and reducing the risk of a landslide.
Vegetation and trees in particular draw large quantities of water out of the ground on a daily basis from the partially
saturated zone. This lowers the water table and increases suctions, both of which reduce the likelihood of a landslide
occurring.
AO
nn % Waste water and effluent disposal
o “augment natural inflows
Vegetation extracts water
and lowers water tate
Figure 1 - Groundwater flow
Groundwater Flow and Landslides
The landslide risk in a hillside can be affected by increase in soak-away drainage or the construction of retaining walls
which inhibit groundwater fiow. The groundwater is fikely to rise after heavy rain, but it can also nse when human
interference upsets the delicate natural balance. Activities such as felling trees and earthworks can lead to:
a reduction in the beneficial suctions in the partially saturated zone above the water table.
increased static water pressures below the water table.
increased hydraulic pressures due to groundwater flow.
loss of strength, or softening. of clay rich strata,
loss of natural cementing in some strata,
transportation of soil particles.
Any of these effects. or a combination of them, can lead to landslides like those illustrated in GeoGuides LR2, LR3 and
LR¢.
Limiting the Effect of Water
Site clearance and construction must be carefully considered ¢ changes in groundwater conditions are to be limited.
GeoGuide LR8 considers good and poor development practices. Not surprisingly much of the advice relates to sensible
treatment of water and is not repeated here. Adoption of appropriate techniques should make it possible to either
maintain the current ground water table, or even cause jt to drop, by limiting inflow to the ground.
if drainage measures and surface protection are relied on to keep the risk of a landslide to a tolerable level, it is important
that they are inspected routinely and maintained (GeoGuide LR11).
The following techniques may be considered to limit the destabilising effects of rising groundwater due to development
and are flustrated in Figure 2.
168 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR5 (WATER & DRAINAGE)
Figure 2 - Techniques used to control groundwater flow
Surface water drains (dish drains, or table drains) - are often used to prevent scour and limit inflow to a slope. Other
than in rock, they are relatively ineffective unless they have an impermeable lining. You should clear them regularly, and
@s required, anc not less than once a year. ff you live in an area with seasonal rainfall, it is best to do this near the end
of the dry season. If you notice that soil or rock debris is falling from the slope above, determime the source and take
a@ppropnate action. This may mean you have to seek advice from a geotechnical practitioner.
Surface protection - is sometimes used in addition to surface water drainage to prevent scour and minimise water
inflow to a slope. You should inspect concrete, shotcrete or stone pitching for cracking and other signs of deterioration at
least once a year. Make sure that weepholes are free of obstructions and able to drain. If the protection is deteriorating.
you should seek advice from a geotechnical practitioner.
Sub-soil drains - are often constructed behind retaining walls and on hillsides to intercept groundwater. Their function is
to remove water from the ground through an appropriate outlet. {t is important that subsoil drains are designed to
complement other measures being used. They should be laid in a sand, or gravel, bed and protected with a graded
stone or geotextile filter to reduce the chance of clogging. Sub-sol drains should always be laid to a fall of at least 1
vertical on 100 horizontal. Ideally the high end should be brought to the surface, so it can be flushed with water from
time to time as part of routine maintenance procedures.
Deep, underground drains - are usually only used in extreme circumstances, where the landslide nsk is assessed as
not being tolerable and other stabilisation measures are considered to be impractical. They work by permanently
lowering the water table in a slope. They are not often used in domestic scale developments, but if you have any on your
site be aware that professional maintenance is essential. {f they are not maintained and stop working, the water table will
rise and a landslide may even occur during normal weather conditions. Both an increase or a reduction in the normal
flow from deep drains could indicate a problem if it appears to be unrelated to recent rainfall. If changes of this sort are
observed, you should have the draims and your site checked by a geotechnical practitioner.
Documentation - design drawings and specifications for geotechnical measures intended to minimise landslide nsk can
be of great assistance to a geotechnical specialist, or structural engineer, called in to inspect and report on them. Copies
of available documentation should be retained and passed to the new owner when the property is sold (GeoGuide
LR11). You should also request details of an appropnate maintenance program for drainage works from the designer
and keep that information with other relevant documentation and maintenance records.
More information relevant to your particular situation may De found In other Austrailan GeoGuides:
© GeoGuide LRi - Introduction GeoGuide LR7 - Landslide Risk
GeoGuide LR2 -Landsides GeoGuide LR& = - Hillside Construction
GeoGuide LR3 - Landsides in Soll GeoGuide LRS -EMuent 4 Surface Water Disposal
GeoGuide LR4 -Landsides In Rock GeoGuide LR10 - Coastal Landsiides
GeoGuide LR6 - Retaining Was * GeoGuide LR1I1 - Record Keeping
The Australian GeoGuides (LA serles) ae a set of publications intended for propeny owners, local councils, planning authorities,
Gevelopers: Insurers: lawyers and. im fact, anyone who live with, of has an interest in, a natural or engineered slope. a cutting. or an
excavaton. They are intended to help you understanc why slopes and retaining structures can be a hazard and what can be done with
appropriate professional advice and local council aporoval (f required) to remove. reduce, or minimise the ns& they represent. Tne
GeoGuides Nave deen prepared by the Australian Geomechanics Society a specialist technica society within Engineers Australia, tne
national peak body for all engineering aiscpiines in Australia, whos members are professiona’ geotechnical engineers and engneering
a wth 3 particular interest in ground engineering. The GeoGuides nave been funded under the Austraian govemments’
Nationa’ Disaster Miagation Program.
Australian Geomechanics Vol 42 No 1 March 2007 168
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AUSTRALIAN GEOGUIDE LR4 (LANDSLIDES IN ROCK)
Rocks have been formed by many different geological processes and may have been subjected to intense pressure,
large scale distortion, extreme temperature and chemical change. As a result there are many oifferent rock types and
their condition vanes enormously. Rock strength varies and is often significantly reduced by the presence of
discontinuities (GeoGuide LR1). You may think that rock lasts forever, but in reality it weathers under the combined
effects of water, wind, chemical change, temperature variation, plant growth and animal activity and erodes with time.
Rock is often the parent maternal that ends up forming soil slopes (GeoGuide LR3). Inevitably different rocks have
different physical and chemical characteristics and they weather and erode to form different types of soil.
Weathering can lead to landslides (GeoGuide LR2) on rock slopes. The type of landslide depends on the nature of rock,
the way @ has weathered and the presence or absence of discontinuities. It is hard to generalise. though normally a
specific combination of discontinuities and material types will be the determining factor and these are often underground
and out of sight. Typical examples are provided in the figures 1 to 4. A geotechnical practitioner can assess the
landslide nsk and propose appropriate maintenance measures. This often entails making geological observations over
an area significantly larger than the site and a review of available background information, including records of known
landslides and aerial photographs. Depending on the amount of information available, geotechnical investigation may or
may not be needed. Every site is different and every site has to be assessed individually.
It is impossible to icte when a landslide will occur on a rock s but failure is normally sudden and
the coi wences can be catastrophic.
ar !
Figure 3 - Block slide on wok layer SS Wolte failure along discontinuities
if the landslide risk is assessed as being anything other that Low, or Very Low, (GeoGuide LR7) @ may be possible to
carry out work aimed at reducing the level of risk.
The most common options are:
1) Trimming the slope to remove hazardous blocks of rock.
2) Bolting. or anchoring. to fix hazardous blocks in position and prevent movement.
3) Installation of catch fences and other rockfall protection measures to limit the impact of rockfalis.
4) Deep drainage designed to limit changes in the ground water table (GeoGuide LR5).
Although such measures can be effective, they need inspection and on-going maintenance (GeoGuide LR11) if they are
to be fective for periods equivalent to the life of a house. Design should be undertaken by a geotechnical
ctitioner and will normal! wire local council approval. !t should be appreciated that it may not be viable to
Carry out remedial works in all circumstances: for example where the landslide is on someone else's property. where the
cost is out of proportion to the value of the property, or where the risk inherent in carying out the work is actually greater
than the risk of leaving things as they are. In situations such as these, development may be considered inappropmate.
166 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR4 (LANDSLIDES IN ROCK) ROCK SLOPE HAZARD REDUCTION MEASURES Removal of loose blocks - may be effective but, depending on rock type, ongoing erosion can result in more blocks becoming unstable within a matter of years. Routine inspection. every 5 or so years, may be required to detect this. Rock bolts and rock anchors (Figure 5) - can be installed in the ground to improve its strength and prevent individual blocks from falling. Rock bolts are usually tightened using a torque wrench, whilst rock anchors carry higher loads and require jacking. Both can be designed to be “permanent” using stainless steel, or sheathing. to inhibit corrosion, but the cost can be up to 10 times that of the “temporary” alternative. You should inspect rock bolts and rock anchors for signs of water seepage, rusting and deterioration around the heads at least once every 5 years. If you notice any of these warning signs, have them checked by a geotechnical practitioner. It is recommended that you keep copies of design drawings and maintenance records (GeoGuide LR11) for the anchors on your site and pass them on to the new owner should you sell. Rock fall netting, catch fences and catch pits (Figure 6) - are designed to catch or control falling rocks and prevent them from damaging nearby property. You should inspect them at least once every 5 years, and after major falls, and arrange for fallen and trapped rocks to be removed if they appear to be filling up. Check for signs of corrosion and replace steel elements and fixings before they lose significant strength. Cut-off drains (Figure 7) - can be used to intercept surface water run-off and reduce flows down the cliff face. Suitable drains are often excavated into the rock, or constructed from mounds of concrete, or stabilised soil, depending on conditions. Drains must be laid to a fall of at least 1% so they drain adequately. Frequent inspection is needed to ensure they are not blocked and continue to function as intended. Clear trees and large bushes (Figure 7) - from slopes since roots can prize boulders from the face increasing the landslide hazard. Figure 7 Natural cliffs and bluffs - often present the greatest hazard and yet are easily overlooked, because they have “been there forever". They can exist dove 3 bulding, road. or Déach, presenting the risk of a rock falling onto whatever Is Delow. They aiso sometimes Support bulidings with a fine view to the Moron. Cil's should be observed frequently to ensure that they are not deteriorating. You may fing f convenient to use binoculars to jook for signs of exposed “fresh” rock on the face, where a recent fail has occurred, or to go to the foot of the cit? from time to time to see If debris is collecting. A thorough inspection of 2 cliff face Is often a major task requiring the use Of rope access methods and should only De undertaken by an appropriately q mera meagan if tension cracks are observed In the ground ane top 3. tate melas acto, ence Mey cous aa miner fare If you have any concems at ail about the More information relevant to your particular stuation may pe found in other Australian GeoGuiies: GeoGuide LR1 - introduction GeoGuide LR? - Landslide Risk GeoGuide LR2 -Landsides GeoGuide LRS - Hillside Construction GeoGulde LR3 - Landslides In Soll GeoGuide LRS - EMuent 4 Surface Water Disposal GeoGuide LRS - Water & Drainage GeoGuide LR10 - Coastal Landsiides « GeoGuide LR6 -Retamingwais GeoGuide LR11 - Record Keeping The Austratan GeoGuides (LR series) are a set of pubéications intenoed for property owners; local councils; planning authorties; developers; Insurers; lawyers and, In fact, anyone who Eves with, or has an Interest in, a natural or engmesred slope. a cufing, or an excavation. They are intended to help you understand why slopes and retaining structures can be 2 nazard and what can be done with appropriate professional advice and local council approval (If required) to remove, reduce, or minimise the ns& they represent. The GeoGuides Nave Deen prepared by the Australian Geomechanics Society, 2 specialist technica society within Engineers Austrata, the nabonal peak body for af engineering disciplines in Australia, whose members are professional gectechnical engineers and engineering ts wih a particutar interest in ground engineering. The GeoGuides have deen tunded under the Australian govemments’ Nationa’ Disaster Miigation Program. Australian Geomechanics Vol 42 No 1 March 2007 167
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AUSTRALIAN GEOGUIDE LR3 (LANDSLIDES IN SOIL) Landslides occur on soil slopes and the consequences can include damage to property and loss of life. Soll slopes exist in all parts of Australia and can even occur in places where rock outcrops can be seen on the surface. If you live on, or below, a soil slope # is portant to understand why a landslide might occur and what you can do to reduce the risk it presents. it is always worth asking the question “why is this slope here?”, because the answer often leads to an understanding of what might happen in the future. Slopes are usually formed by weathering (breakdown) and erosion (physical movement) of the natural ground - the "parent material". Many factors are involved including rain, wind, chemical change, temperature vanation, plant growth, animal activity and our own human enthusiasm for development The general process ss outlined in Figure 1 The upper levels of the parent material progressively weather over thousands, or millions, of years, losing strength. This can result in a surface layer which looks similar to the parent material (although its colour has probably changed) but has the strength of a soil - this is called “residual soil”. At some stage the weathered surface layer is exposed to the elements and fragments are transported down the slope. In this context 3 fragment could be a single sand grain, a boulder, or a landslide. The time scale could be anything from a few seconds to many thousands of years. The transported fragments often collect on the lower slopes and form a mew soil layer that blankets the original slope - “cofuvum”. If material reaches a nver or the sea it is deposited as “alluvium” or as a “marine deposit”. With appropriate changes iin nver and sea level this material can again find itself on the surface to commence another cycle of weathering and erosion. In places often, but not only, near the coast. this can include sand sized fragments which form beaches and are sometimes blown back onto the land to form dunes. a wy ? = = Weathered parent material (residual sail) Fragments of parent material Landslides can occur almost anywhere on a sol slope. Slides can be rotational, translational, or debris flows (see GeoGuide LR2) and may have a number of causes. - ay or Landslide from parent material or residual soil further uphill be due to tree Gearance, installing . soakaway drains, or & period of heavy rain Figure 2 164 Australian Geomechanics Vol 42 No f March 2007
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AUSTRALIAN GEOGUIDE LR3 (LANDSLIDES IN SOIL) Landslide due to poorly supported excavation, excassive fill depth, steep excavated Figure 3 Some of the more common causes of landslides in soil are: 1) Falls of the parent material or residual sol from above, due to natural weathering processes (Figure 2). 2) Increased moisture content and consequent softening of the soil, or a rise in the water table. These can be due to excessive tree clearance, ill-considered soak-away drainage or septic systems, or heavy rainfall (Figure 2). 3) Excavation without adequate support, increased surface load from fill placement, or inadequately designed shallow foundations (Figure 3). 4) Natural erosion at the toe of the slope due to scour by a river or the sea (Figure 3). 5) Reactivation of an ancient landslide (Figure 3). Most soil slopes appear stable, but they af achieved their present shape through 3 process of weathering and erosion and are often sensitive to minor changes in the factors that affect their stability. As a general rule, human activities only improve the situation if they have been designed to do so. Once this idea is understood, @ is probably easy to see why the following basic mules are so important anc should not be ignored without seeking site specific advice from a geotechnical practiboner: . Do not clear trees unnecessarily. . Do not cut into a slope without supporting the excavated face with an engineer designed structure. . Do not ade weight to a slope by placing earth fil or constructing buildings with madequately designed shallow foundations (Note: in certain circumstances weight is added to the toe of 3 slope to inhibit landslide movement. but this must be carried out in accordance with a proper engineering design). - Deo not allow water from storm water drains, or Tom septic waste or effluent disposal systems to soak into the ground where it could trigger a landside. More information in relation to good and poor hillside constructon practice is given in GeoGuiee LR8. With appropriate engineering input it is often possible to reduce the likelihood. or consequences, of a landslide and so reduce the risk to Property and to life. Such measures can include the construction of properly designed storm water and sub-sol drains, surface protection (GeoGuide LR5) and retaining walls (GeoGuide LR6). Design should be undertaken by a nical itioner and will normal ire local council wal More information relevant to your particular situation may be found In other Austrailan GeoGuides: GeoGuide LR1 - Introduction GeoGuide LR2 - Landsides GeoGuide LR4 - Landslides In Rock GeoGuide LRS - Water & Drainage GeoGuide LR - Retaining Walls GeoGuile LR7 - Landslide Risk GeoGuie LRS = - Hillside Construction GeoGuide LRS - Effiuent & Surface Water Disposal GeoGuide LR10 - Coastal Landsiides. GeoGuide LR11 - Record Keeping The Ausvallan GeoGuides (LR series) we a set of publications intended for propeny owners; loca? councils, planning authorities; Developers; insurers; lawyers and. in fact, anyone whe llvee with, or has an interest in, a natural or engmeeres slope, a cutting of an excavation. They are intended to ne!p you understand why slopes and retaining structures can be a hazard and what can be cone with aporoprate professional advice and Iocal counc? aporoval (fF required) fo remove, reduce, or minimise the ne& they represent. Tne GeoGuiles have deen prepared by ine Australian Geomechanics Society. a speciatsi tecnnkca’ society within Engineers Ausiralla, the natonal peak body for all engineering @scpiines in Australia, whose members are professiona’ gectecnnica engmeers and engineering geologisis with 3 particular interest in ground engineering. The GeoGuides have been funded under the Australian governments’ Natona Disaster Mitigation Program. Australian Geomechanics Vol 42 No 1 March 2007 165
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AUSTRALIAN GEOGUIDE LR2 (LANDSLIDES) What is a Landslide? Any movement of a mass of rock, debris, or earth, down a slope, constitutes a “landslide”. Landslides take many forms. some of which are @ustrated. More information can be obtained from Geoscience Australia, or by visiting its Australian Landslide Database at www 93 gov au/urban/factsheets/landslide jsp Aspects of the impact of landslides on buildings are dealt with in the book “Guideline Document Landslide Hazards” published by the Australian Building Codes Board and referenced in the Building Code of Australia. This document can be purchased over the internet at the Australian Building Codes Board's website www_abcb.gov.au - Landslides vary in size. They can be small and localised or very large. sometimes extending for kilometres and involving millions of tonnes of soil or rock. It is important to realise that even a 1 cubic metre boulder of soll, or rock, weighs at least 2 tonnes. If it falls, or slides, it is large enough to kill a person, crush a car, or cause serious structural damage to a house. The maternal in a landslide may travel downhill well beyond the point where the failure first occurred, leaving destruction in its wake. It may also leave an unstable slope in the ground behind @. which has the potential to fall again, causing the landslide to extend (regress) uphill, or expand sideways. For all these reasons, both “potential” and “actual” landslides must be taken very seriously. They present a real threat to life and property and require proper management. identification of landslide risk is a complex task and must be undertaken by a geotechnical practitioner (GeoGuide LR 1) with specialist experience in slope stability assessment and slope stabilisation. What Causes a Landslide? Landslides occur as a result of local geological and groundwater conditions, but can be exacerbated by inappropriate development (GeoGuide LR8), exceptional weather, earthquakes and other factors. Some slopes and cliffs never seem to change, but are actually on the verge of failing. Others, often moderate slopes (Table 1), move continuously, but so slowly that it is not apparent to a casual observer. In both cases, small changes in conditions can trigger a landslide with serious consequences. Wetting up of the ground (which may involve a rise in ground water table) is the single most important cause of landslides (GeoGuide LR5). This is why they often occur during. or soon after, heavy rain. inappropriate development often results in small scale landslides which are very expensive in human terms because of the proximity of housing and people. Does a Landslide Affect You? Any slope, ciiff, cutting, or fill embankment may be a hazard which has the potential to impact on people, property. roads and services. Some tell-tale signs that might indicate that a landslide is occurring are listed below: trees leaning down slope, or with exposed roots debris/fallen rocks at the foot of a cliff open cracks, or steps, along contours ground water seepage, or springs bulging in the lower part of the slope tilted power poles, or fences hummocky ground cracked or distorted structures These indications of instability may be seen on almost any slope and are not necessarily confined to the steeper ones (Table 1). Advice should be sought from a geotechnical practitioner if any of them are observed. Landslides do not respect property boundaries. As mentioned above they can “run-out” from above, “regress” from below, or expand sideways, so 3 landslide hazard affecting your property may actually exist on someone else's land. Local councils are usually aware of slope instability problems within their jurisdiction and often have specific development eee eee and maintenance requirements. Your local council is the first place to make enquiries if you are responsible for any sort of it or own or oc: on or near sloping land or a cliff. TABLE 1 - Slope Descriptions Appearance a ‘enum Slope Characteristics Gentle O° - 10° 1on6 Easy walking. Moderate 10°- 18° 1on3 Walkable. Can drive and manoeuvre a car on driveway Walkable with effort. Possible to dnve straight up or down Steep 18°- 27° 1on2 roughened concrete driveway, but cannot practically manoeuvre a car. Very Steep 27°- 45° toni Can only climb slope by clutching at vegetation, rocks etc. Extreme 45°- 64° 10n 0.5 Need rope access to climb slope Cliff 64°- 84° 10n 0.1 Appears vertical. Can absed down. Vertical or Overhang | 84° - 902° infinite Appears to overhang. Abseller likely to lose contact with the face. Some typical landslides which could affect residential housing are Hlustrated below: 162 Australian Geomechanics Vol 42 No 1 March 2007
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AUSTRALIAN GEOGUIDE LR2 (LANDSLIDES) Rotational or circular slip failures (Figure 1) - can occur on Small scale moderate to very steep soil and weathered rock slopes (Table fandsiide: 1). The sliding surface of the mowing mass tends to be deep seated. Tension cracks may open at the top of the slope and bulging may occur at the toe. The ground may move in discrete "steps" separated by long periods without movement. More rapid movement may occur after heavy rain. Translational slip failures (Figure 2) - tenc to occur on moderate to very steep slopes (Table 1) where sod, or weak rock, overlies stronger strata. The sliding mass is often relatively shallow. It can move, or deform slowly (creep) over long periods of time. Extensive linear cracks and hummocks sometimes form along the contours. The siding mass may accelerate after heavy rain. Wedge failures (Figure 3) - normally only occur on extreme slopes. or cliffs (Table 1). where discontinuities in the rock are inclined steeply downwards out of the face. Rock falls (Figure 3) - tend to occur from cliffs and overhangs (Table 1). Clifis may remain apparently unchanged for hundreds of years. Collections of boulders at the foot of a cliff may indicate that rock falls are ongoing. Wedge fadures and rock falls do not “creep”. Familiarity with a particular local situation can instil a false sense of security since failure, when @ occurs, is usually sudden and catastrophic. Debris flows and mud slides (Figure 4} - may occur in the foothills of ranges. where erosion has formed valleys which slope down to the plains below. The valley bottoms are often lined with loose eroded material (debris) which can “flow” if it becomes saturated during and after heavy rain. Debris fows are likely to occur with little warning: they travel a long way and often involve large volumes of soll. The consequences can be devastating. Figure 4 More information relevant to your particular sttuation may be found In other Austrailan GeoGuldes: GeoGuide LR1 - Introduction GeoGuide LR3 - Soll Slopes GeoGuide LR4 - Rock Slopes GeoGuie LR? - Landslide Risk GeoGuile LRS - Hillside Construction GeoGuiie LAS - EfMfivent & Surface Water Disposal GeoGuide LRS - Water & Drainage GeoGuie LR10 - Coastal Landslides GeoGuide LR6 - Retahing Walls GeoGuile LR11 - Record Keeping The Australian GeoGuides (LR series) we a set of pubications Intended for propery owners; loca’ councils: planréng authorities; Gevelopers; Insuress; lawyers and. in fact, anyone who fives with, or has an interest in, a Natura! or engineered slope. a cutting. or an excavation. They are intended to help you understand why slopes and retaining structures can be a hazaré and what can be done with appropriate professional advice and local counc!) approval (9 required) to remove, reduce, or minimise the ns& they represemt. The GeoGuides Nave Deen prepared by the Australian Geomechanics Society, a speciaiist technica society within Engineers Austrata, the national peak body for ali engineering Gecpiines in Australla, whose members are professiona geotectiinica’ engineers and engineering gedlogisis with a particular interest in ground engineering. The GeoGuides have been funded under the Australian govemments’ Nationa! Disaster Mitgation Program. Australian Geomechanics Vol 42 No 1 March 2007 163
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Prctogupnacatmey of Geg Kote aid Tow Pilie
AUSTRALIAN GEOGUIDE LR1 (INTRODUCTION)
lINTRODUCTION TO LANDSLIDE RISK
AUSTRALIAN GEOGUIDES
The Australian GeoGuides (LR series) are a set of information sheets on the subject of landslide nsk management and
maintenance, published by the Australian Geomechanics Society (AGS). They provide background information intended to
help people without specialist technical knowledge understand the basic issues involved. Topics covered include:
LR1 - Introduction LR2 - Landslides LR3 - Landslides in Soil
LR4 - Landslides in Rock LR5 - Water & Drainage LR6 - Retaining Walls
LR7 - Landslide Risk LR& - Hillside Construction LR® - Effiuent & Surface Water Disposal
LR10 - Coastal Landslides LR11 - Record Keeping
The GeoGuides explain why slopes and retaining structures can be a hazard and what can be done with appropnate
professional advice and local authority approval (if required) to remove. or reduce, the nsk they represent.
Preparation of the GeoGuides has been funded by Australian governments through the National Disaster Mitigation Program
(NDMP). This is a national program aimed at identifying and addressing natural disaster risk priorities across Australia.
Technical input has been provided by experienced geotechnical engineers, engineering geologists and local government and
goverment agency representatives from around Australia.
BACKGROUND
A number of landslides and cliff collapses occurred in Ausiralia in the 1980's and 1990's in which fives were lost. Of these the
Thredbo landslide probably received the most publicity, but there were several others. During this period the AGS issued a
number of advisory notes to practitioners in relation te the assessment of landstide risk and its reduction. Building on these
Notes, and responding to changes in technology. a technical paper known as AGS2000 was prepared. It was followed in 2002
by an intensive nation-wide educational campaign attended by a lange number of interested professionals from government
departments and prvate industry. This resulted in an increased awareness of the risks associated with unstable slopes and 3
changed approach im many government departments responsible for regional planning, domestic development, roads, ralways
and the maintenance of natural features such as cliffs.
STATUS OF THE GEOQGUIDES
The GeoGuides reflect the essence of good practice as perceived by a large number of geotechnical engineers, engineering
ee ee other practitioners such as local government a The sacGites are generic and do not, and a eanecs.
b st be it
hand — of the site. {ft is expected that some local councils will Sinan ete ee
publications in planning and building legislation. Check with your local council to see how it regards these documents.
Companion publications to the GeoGuides are:
»* AGS (2007a) Guideline for Landslide Susceptibility, Hazard and Risk Zoning for Land Use Management Australian
Geomechanics Society, Australian Geomechanics, Vol 42, Not and its associated commentary (AGS 2007b).
* AGS (2007c). Practice Note Guidelines for Landslide Risk Management. Australian Geomechanics Society
Australian Geomechanics, Vol 42, No1 2007. and its associated "Commentary" (AGS 2007¢).
Copies of the above documents are available on the AGS website wew.australiangeomechanics org
160 Australian Geomechanics Vol 42 No 1 March 2007
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PReb gen conway of Pl Pete
TERMINOLOGY
AUSTRALIAN GEOGUIDE LR1 (INTRODUCTION)
Terminology tends to change with time and place and with the context in which it is used. The terms listed below have
the following meanings in the GeoGuides:
Consequence the outcome, or potential outcome. arising from the occurrence of a landslide expressed quantitatively, or
qualitatively, In terms of loss, disadvantage, damage. injury, of 105s of life.
Discontinuny in relation to the ground is a crack, 2 bedding plane {a DoUndary Delween siraa) of taunt (a plane along
which the ground has smeared) which forms a plane of weakness and reduces the overall strength of the
ground.
Equiiprium the condition when the forces on a mass of soll or rock in the ground, or on a retaining structure, are equat
and opposite.
Factor of safety (FOS)
theoreticaly Me forces avalable to prevent a part of Me ground, of a reaming structure, trom moving
divided by those trying to move It A FOS of one or less Indicates that failure Is likely to occur, but not how
likely It i. To allow for unknowns and to limit movements engineers always am to achieve a FOS
significantly larger than one.
Failure
when part of the ground experiences movement a5 2 result of the out of balance forces on It Fafure of a
retaining structure means It is no longer adie to full fs intended function.
Genechnical practivoner
when referred to In the Australian GeoGuides (LR series), Is a professional geciecnnka engineer, or
engineering geotogist, with chartered status in a ed national professional Institution and relevant
training, experience and core competencies im landslide risk assessment and management In some
government departments, technical officers are specticaily trained to undertake some of the functions of a
geotechnical practitioner.
Hazard 3 condition wih the potentiat for causing an undesirable consequence. in relation to landslices ths
includes the location, size, speed, distance of travel and the Mkellhood of fs occurrence within a given
period of time.
Landside the movement or the potential movement, of a mass of rock, debris. or earth down a siope.
Likeinood a guaitiative Description of prooapill ty. or trequenc’ ry, Of Occurrence.
Partial saturation the condRion In the ground above the water table where ooth alr and water are present as well as soll, or
Tock.
Perched water tare 3 wale table above ihe irue waler table suppomed by 3 low permeability stratum.
Permeabulty a measure of the abliky of ihe ground to alow water to flow through It
Risk a measure of the probability and severity of an adverse effect to life. health, propery or ihe environment
Sip faliure landslide.
Staole the condition when fafure wil not occur. Over geotogical time no pan of the ground can be considered
stable. Over short periods (eq the Ife of a structure} stabilty Imples a very low fitellhoos of tabure.
Retaning siucture anything bulit by humans which is Mtended fo suppor the ground and inhibit tabure
Structure in relation to rock, of Sol, means the spacing, extent, orlentaton and type o discontinuites found In ihe
ground 3t 3 particutar location.
Tension crack ‘a distinct open crack that normally Gevelops in the ground around a landside and indicaies actual, or
imminert , failure.
Water table the level in he Ground below which Ais saturated and ine volds are filed with water.
Australian Geomechanics Vol 42 No 1 March 2007
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 APPENDIX C: LANDSLIDE RISK ASSESSMENT QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY QUALITATIVE MEASURES OF LIKELIHOOD Approximate Annual Probability Implied Indicative Landslide ae em jad Indicative Notional Recurrence Interval hina ais ae S Value Boundary 107 5x102 10 years The event is expected to occur over the design life. ALMOST CERTAIN A 107 100 years 20 years The event will probably occur under adverse conditions over the LIKELY B 3 - 4 design life. 107 azig 4 1000 years aa er: The event could occur under adverse conditions over the design life. | POSSIBLE Cc 107 3x10 10,000 years i The event might occur under very adverse circumstances over the UNLIKELY D Noe design life. 5 10° Sxl 100,000 years 2G years The event is conceivable but only under exceptional circumstances RARE = 6 : over the design life. 10° ae 1,000,000 years | 200,000 ears [Tie event is inconceivable or fanciful over the design Life. BARELY CREDIBLE F Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor. not vice versa. QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY Approximate Cost of Damage Indicative Notional Description Descriptor Level Value Boundary - Structure(s) completely destroyed and/or large scale damage requiring major engineering works for 200° & iS iS 20% 100% stabilisation. Could cause at least one adjacent property major consequence damage. CAUSES ! 60% 3 Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant MAJOR > 40° stabilisation works. Could cause at least one adjacent property medium consequence damage. ~ 20% 7 Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works. MEDIUM 3 ease 10% Could cause at least one adjacent property minor consequence damage. = = = 5% 1% Limited damage to part of structure. and/or part of site requiring some reinstatement stabilisation works. MINOR 4 ‘ Little damage. (Note for high probability event (Almost Certain). this category may be subdivided at a 0; = 4 AN T ca notional boundary of 0.1%. See Risk Matrix.) INSEE ‘ Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the unaffected structures. GB) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of restatement of the damaged portion of the property (land plus structures), stabilisation works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees. temporary accommodation. It does not include additional stabilisation works to address other landslides which may affect the property. (4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor. not vice versa Australian Geomechanics Vol 42 No 1 March 2007 Form No. 68039 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 APPENDIX C: —- QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED) QUALITATIVE RISK ANALYSIS MATRIX - LEVEL OF RISK TO PROPERTY LIKELIHOOD CON: SEQUENCE ES TO PROPERTY (With Indicative Approximate Cost of Demsge)_ Indicative Value of 1: CATASTROPHIC : MAJOR 3: MEDIUM 4: MINOR Approximate Annual 200% 60% 20% 5% IN! SIGNIFIC ANT Probability 0.5% A - ALMOST CERTAIN 107 H MorL (5) B - LIKELY 107 M L C - POSSIBLE 107 M VL D - UNLIKELY 107 L VL E - RARE 107 M L L VL VL F - BARELY CREDIBLE 10% L VL VL VL VL Notes: (5) For Cell AS, may be subdivided such that a consequence of less than 0.1% is Low Risk. (6) When considenng 2 nsk assessment it must be clearly stated whether it 1s for existing conditions or with nsk control measures which may not be implemented at the current time. RISK LEVEL IMPLICATIONS Risk Level Example Implications (7) Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the property. H HIGH RISK Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce nisk to Low. Work would cost a substantial sum m relation to the value of the property. May be tolerated m certain circumstances (subject to regulator's approval) but requires investigation, planning and M MODERATE RISK umplementation of treatment options to reduce the nsk to Low. Treatment options to reduce to Low nsk should be implemented as soon as practicable. - Usually acceptable to regulators. Where treatment has been required to reduce the nsk to this level, ongomg maintenance is L LOW RISK required VL VERY LOW RISK Acceptable. Manage by normal slope maintenance procedures. Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk: these are only given as a general euide. Form No. 68039 RL2 August 2010 Australian Geomechanics Vol 42 No 1 March 2007
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE ADVICE GEOTECHNICAL Obtain advice from 2 quabfied experianced geotechnical practioner at early | Prepare detailed plan and start vite works before ASSESSMENT stage of planing and before ws works. geotechnical adrice PLANNING SITE PLANNING Having obnmed advice, plan the development with ths nak | Plan development without rosard for the Risk ising from the identified harards and consequences in mind DESIGN AND CONSTRUCTION Use flexible stuctmes which incorporam property desimed brickwork, timber | Fleer plans which require extents coming and HOUSE DESIGN or steel frames, umber or pamel cladding i Consider use of split levels. Movement intolerant suctares. Use decks for recreational areas where sppropciste. SITE CLEARING ee Tndtecrminately clear the site. ACCESS = ‘beuow for cut, Ds, recaumng walls anc dramage. Excavate and £0 for site access beiore DRIVEWAYS Goal apecihcateas Sc pais anty ened we toneaiied gectechnical advice Drivewsys and wom cary Deed to be fully supported on pears. EARTHWORKS Rewin netral coutour wherever possible. Tadescriminatory balk saxthworks. Septh. ‘Large scale cuts and benching. Cots a ee eee Unseppored cats ‘Mimumise baight Loose or poorly compacted £0 wiach ifit Subs, Strip vegemtion and topsoil and key into natural slopes price to filling may flow a considerable Gctamcs incindas ‘Uve clean SIL carecials and compact to expoeenng smadards. onto below. Fis Barat to spprogeias wope or support with engmmedred retaining wall Bloch samral drainage lines Tecinds stumps, trees, vegetation, topsod, boulders, brelding rubble otc im SE Racy, Ouicrcrs Remove ox stabilise boulder: which may ave macceptable nuk Disturb ct undeccot detached blocks or & Bou sexs Support rock Sces where necessary boulders. desis to resist applied sod and water forces. Constract a structerally inadequate wall wach as RETADNING Found cn rock where practicable. sandstone «flagsing brick oo =mremiorced WALLS Provide subsurface draimags within wall backSill and surface draimags om shops | blockwork. - shove Lack of subsurdace drains and weepbolss Constract wall a5 s00n 35 possible after cut/Sl! operation. Found withm rock where “Found om topscal, Toow SIL demched Sonidars FOOTINGS ‘Use roms of piers or strip footings arented =p and down slope. or undercet cif. Design for lacaral cosp pressures if necossary. Back footing excavations to exclude ingress of surface water. Support on piers to rock where practicable. Design for high sod] pressures ae develop on aphall cide wicket thare aaphs aseae Eeatenena enon DRAINAGE Provide at tops of cut and fill slopes. Discharge at top of Sik and cuts. Descharge to stect drainage of nataral water courses. Allow weter to pond on bench areas. Sumacn Provide general falls to prevent bicckage by siltation and incorporate silt traps. ‘Lins to mininess mGltraton and maks flauble wher posible —__ Special souctures to dissipate anergy 21 changes of slope andor direction. Provide filter around sebsurface drain. Discharge roof runof inte abscrpnom mmmches. RNIN Provide drain bebind retaining vals, ‘Use flexible papslines with access for maintenance. Prevent inflow of surface water. Sure ‘Oseally requires pump-out or mums semer systeas, absorpoce menches may | Discharge Groctly caro and aso slopes. ee be possible m scans areas if mak is acceptable. Use absorpeion tranche: wi consderica a Storage tanks thomld be water-tight and adeqeamly founded of tandilide risk. EROSION Control erosion as this my lead to mstabiliry. Faire to cbhsare cathwork: and dame CONTROL & Ene cleared area Teccommandanon: when ms eects sesepoa DRAWINGS AND SITE VISITS DURING CONSTRUCTION DRAWINGS Application dawmas should bs viewed by aectechmical comsnizant SITE VISITS Sits Visits by consultant may be appropnate during constuction INSPECTION AND MAINTENANCE BY OWNER OWNER'S Clean cratmage sytem: reper broken join = dram and leaks in supply RESPONSIBILITY | pipes. Where stmcteral distress is ovidant sos advice. E seepage observed determine cme: or soak advice on consequences Australian Geomechanics Vol 42 No 1 March 2007
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THE ONLY NAME IN SOILS
QBCC Licence No. 1018403
PH: 07 54 740 900
3/53 Gateway Drive, Noosaville 4566
Email: [email protected]
JOB NUMBER 20/0309 SITE CLASSIFICATION REPORT
CLIENT RG STRATEGIC AUSTRALIA.
SITE PROPOSED ALTERATIONS & ADDITIONS TO EXISTING UNITS.
LOCATION 38 AVOCET PARADE, PEREGIAN BEACH.
REPORT DATE 24/04/2020
1. INTRODUCTION
The purpose of this report is to classify the subject allotment in accordance with Australian
Standard 2870 - 2011 'Residential Slabs and Footings'. From this classification, a footing
system can be recommended for the structure so as to allow adequate performance of the
footings under the soil conditions determined at the site.
Trees: There are several medium sized trees on & adjacent to the proposed site. Global
Testing recommends an experienced arborist be commissioned to quantify the existing
size, location, predicted maximum height and type of these trees to aid in the design
process. It is the responsibility of the designing engineer to apply the principals of
AS2870-2011 Appendix H "Guide to Design of Footings for Trees". The classification
herein excludes the effect of trees on the site.
2. SITE DESCRIPTION
Type of Allotment Residential
Vegetation (Building Envelope) Sparsely grassed/ Scattered medium sized
mature trees
Vegetation (Allotment) Sparsely grassed/ Scattered medium sized
mature trees
Road Status Sealed
Slope 13% to the East
Surface Water will Drain Towards The East
Will Surface Water from adjoining Yes
allotments traverse this site?
Does the site have any watercourses No
Surrounding Allotments
Left Existing Residence
Right Existing Residence
Rear Existing Residence
Depth to Weathered Rock N/a
Depth to Water Table N/a
3. SITE CLASSIFICATION AND TARGET STRATA
Based on the findings of the site investigation, the classification of the site is considered to
conform to:
CLASS “ P ” ( Due to poor bearing capacity & the presence of
trees-See Target Stratum).
PLEASE NOTE: If not for the poor bearing capacity encountered & presence of trees,
this site would have the characteristics of an “S” Site.
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in accordance with Australian Standard 2870 - 2011, 'Residential Slabs and Footings,
Construction'.
The Target Stratum for the foundation of the structure at locations BH1 & BH2 is the
SAND at a depth of 1900mm (min.150kPa) below existing ground level.
The Target Stratum for the foundation of the structure at location BH3 is the SAND at a
depth of 1800mm (min.150kPa) below existing ground level.
It is the designing engineer’s responsibility to address the potential influence of any trees
present on or around the proposed building site.
If any further fill is placed over the existing ground, the structure shall be piered through
the fill into the existing material. Further note that the placement of reactive material as
fill or cutting of the site may change the site’s classification.
PLEASE NOTE: This report does not address slope stability. If a slope stability analysis
is required, the services of a suitably qualified geotechnical engineer should be sought.
4. SOIL PROFILE
Refer to Soil Profile Report for borelog information
Whilst every effort has been made to ensure that the borelogs carried out at the subject allotment
are indicative of the soil profile over the site any discrepancy between the profile detailed in
the borelogs and that observed during construction shall be referred to Global Testing for
immediate attention.
5. MATERIALS LABORATORY TEST RESULTS
Soil Laboratory testing was not carried out on samples of the soil taken from the site because
the clay content of the material was too low for Shrink-Swell movements to be critical in the
classification of the soil profile. Determination of the classification for the site in this situation
is based upon results from Dynamic Cone Penetrometer testing performed over the site.
BOREHOLE N/a
DEPTH RANGE OF SAMPLE N/a
LIQUID LIMIT (%) N/a
PLASTIC LIMIT (%) N/a
PLASTIC INDEX (%) N/a
LINEAR SHRINKAGE (%) N/a
INSTABILITY INDEX (%) N/a
SHRINK SWELL INDEX, Iss (%) N/a
PREDICTED SURFACE MOVEMENT N/a
Please note that the Plasticity Index (PI) determined by the Atterberg Test may be correlated to
an Instability Index (Ips)with reference to the results of extensive testing carried out by others,
notably, Dr P Mitchell, ‘Footings for Small Scale Domestic Structures,’ March 1990.
Yours Sincerely,
MICHAEL PINKNEY. (Laboratory Manager)
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LIMITATIONS
Recommendations given in this report are based on the information supplied by the client
regarding the proposed building construction in conjunction with the findings of the
investigation. Any change in construction type, building location or omission in the client
supplied information, may require additional testing and/or make the recommendations
invalid.
The recommendations herein identify a target soil stratum into which the footings are to be
founded. The target stratum has been located by the depth in mm of the target stratum's upper
horizon boundary below the existing ground surface level at the time of the site investigation.
Any cutting or filling works and any surface erosion or deposits subsequent to the site
investigation, will alter the measured location of the stratum relative to the surface. Where
required, the author should be notified in such cases to confirm the location of the target
stratum.
The description of the soil given in the Soil Profile section of this report is intended as a brief
overview of the soil's primary constituents. For a detailed classification of the soil, the reader
should refer to the Soil Profile Reports and/or Borehole Reports attached to this report.
Every reasonable effort has been made to locate the test sites so that the borehole profiles are
representative of the soil conditions within the area investigated. The client should be made
aware however, that exploration is limited by time available and economic restraints. In some
cases, soil conditions can change dramatically over short distances, therefore, even careful
exploration programs may not locate all the variations.
If soil conditions different from those shown in this report are encountered or are inferred
from other sources, then the author must be notified immediately.
This report may not be reproduced except in full. The information and drawings provided
with this report shall only be used and will only be applicable for the development shown on
the client supplied information provided for this site.
Any dimensions, contours, slope directions and magnitudes shown on the Test Locality Plan
shall not be used for any building construction or costing calculations. The purpose of the
plan is to show approximate location of field tests only.
Any changes made to these recommendations by persons unauthorised by the author will
legally be interpreted as that person assuming the responsibility for the long-term
performance of the footing system.
The recommendations contained in this report have not taken into consideration the long-term
effects of any previous, current or potential subsurface work by mining companies or
potential slope instability problems. At the time of writing this report neither our client (nor
his agent) nor the local authority had made the author aware that these problems may be
affecting this allotment. If a mining subsidence or slope stability assessment is required for
this allotment, the recommendations of a suitably qualified geotechnical engineer should be
sought.
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THE ONLY NAME IN SOILS
SOIL PROFILE REPORT
CLIENT: RG Strategic Australia. AUTHORISED RG Strategic Australia.
BY:
PROJECT: 38 Avocet Parade, DATE TESTED: 23/04/2020
Peregian Beach. JOB NO: 20/0309
Drilling Method: Jacro 105 100mm. Borehole
No.1
Depth Description PENET. Est. Bearing
(blows per 100mm)
(mm) Capacity
(kPa)
1 2 3 4
0 BITUMEN.
50 (FILL) SANDY CLAYEY GRAVEL (GC)- Fine - -
to coarse grained, pale brown, low plasticity fines,
fine to coarse sand, moist, medium-dense.
100 -
200 -
300 SAND (SP)- Fine to coarse grained, pale grey/ 2 50
brown, moist, loose.
400 2
500 2
600 2 50
700 2
800 2
900 2 50
1000 2
1100 2
1200 SAND (SP)- Fine to coarse grained, pale grey/ 3 100
brown, moist, medium-dense.
1300 3
1400 3
1500 3 100
1600 3
1700 4
1800 4
1900 5 150
2000 5
2100 6
2200 6 200
2300 6
2400 6
2500 6 200
2600 6
2700 6
2800 7 225
2900 7
3000 7
-
3500 Hole Terminated.
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THE ONLY NAME IN SOILS
SOIL PROFILE REPORT
CLIENT: RG Strategic Australia. AUTHORISED RG Strategic Australia.
BY:
PROJECT: 38 Avocet Parade, DATE TESTED: 23/04/2020
Peregian Beach. JOB NO: 20/0309
Drilling Method: Hand Auger 100mm. Borehole
No.2
Depth Description PENET. Est. Bearing
(blows per 100mm)
(mm) Capacity
(kPa)
1 2 3 4
0 SAND (SP)- Fine to coarse grained, pale grey/
brown, moist, loose.
100 2 50
200 1
300 2
400 1 30
500 2
600 2
700 2 50
800 2
900 2
1000 2 50
1100 2
1200 2
1300 SAND (SP)- Fine to coarse grained, pale grey/ 3 100
brown, moist, medium-dense.
1400 3
1500 3
1600 Hole Collapsing-Hole Terminated. 4 125
1700 3
1800 4
1900 5 150
2000 5
2100 5
2200 5 150
2300 6
2400 6
2500 6 200
2600 6
2700 6
2800 7 >200
2900 6
3000 6
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THE ONLY NAME IN SOILS
SOIL PROFILE REPORT
CLIENT: RG Strategic Australia. AUTHORISED RG Strategic Australia.
BY:
PROJECT: 38 Avocet Parade, DATE TESTED: 23/04/2020
Peregian Beach. JOB NO: 20/0309
Drilling Method: Hand Auger 100mm. Borehole
No.3
Depth Description PENET. Est. Bearing
(blows per 100mm)
(mm) Capacity
(kPa)
1 2 3 4
0 SAND (SP)- Fine to coarse grained, pale grey/
brown, moist, loose.
100 1 30
200 1
300 1
400 2 50
500 1
600 2
700 1 30
800 2
900 1
1000 2 50
1100 2
1200 SAND (SP)- Fine to coarse grained, pale grey/ 3 100
brown, moist, medium-dense.
1300 3
1400 3
1500 3 100
1600 Hole Collapsing-Hole Terminated. 3
1700 3
1800 5 150
1900 6
2000 6
2100 6 200
2200 6
2300 7
2400 7 >200
2500 8
2600 7
2700 7 >200
2800 7
2900 7
3000 7
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THE ONLY NAME IN SOILS
Site- 38 Avocet Parade,
Peregian Beach.
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THE ONLY NAME IN SOILS
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The only name in soils
Test Locality Plan
LEGEND
N BH - Borehole Location
PN - Dynamic Cone Penetrometer Location
PT - Percolation Test
-Trees
BH2
PN2
2m 5m
12m
BH1
4.5m PN1
13% Slope
BH3 7m
PN3
10m
Avocet Parade
Not to Scale
Note: Any dimensions, contours, slope directions and magnitudes shown on the plan shall not be used for any building
construction or costing calculations. The purpose of the plan is to show the approximate location of the field tests only. All
dimensions in metres UNO .
Client: RG Strategic Australia. Job No. 20/0309
Address: 38 Avocet Parade, Date Tested: 23/04/2020
Peregian Beach. Authorised:RG Strategic Australia.
Page 60
~ Ci STRATEGIC STRATEGIC SOLUTIONS, CREATIVE THINKING rgstrategic.com.au
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