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Slope Stability Report - Site 2
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David Rogalsky 16 December 2020
Letter 02 Revision 0
Arcos Project: 200729
By Email
Dear Team
Report on Geotechnical Investigation
Proposed Residential Dwellings
142 Playfords Road, Kin Kin Site 2
At the request of the above, Arcos Unit Group Pty Ltd (AG) undertook a geotechnical
investigation for the above project. The summary of the investigation and subsequent analysis
is detailed in Table 1. The value presented in Table 1 is condition upon the recommendations
within this report being implemented and is described in accordance with the Australian
Geomechanics Society “Guidelines for Landslide Risk Management”.
Table 1: Summary of Investigation Results
Result of AGS Slope Instability Assessment
low
Notes: The above is a summary of the detailed investigation results discussed within this report. The entire report
Error! No document variable supplied.
must be read, understood, and kept in entirety.
Arcos Unit Group Pty Ltd
Billy Blake B.Eng(Civ)(Hons), CPEng, RPEQ, NER, MIEAust
30 Nov 2020 Geotechnical Investigation
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1.0 Introduction
Arcos Unit Group Pty Ltd (AG) was commissioned to undertake a geotechnical investigation
for the proposed residential dwellings located at 142 Playfords Road, Kin Kin.
This report outlines the results of the investigation, laboratory testing, analysis and interpretive
reporting on the following items:
• Review of the existing slope stability hazards, their attributed risks and impacts of the
proposed development in alignment with the practices set out within the Australian
Geomechanics Society guidelines; and,
• Provide recommendations from a geotechnical context that will avoid or reduce any
potential instability.
This report must be kept in entirety. The use of this report is undertaken in accordance with
our Company’s terms and conditions.
2.0 Regional Geology
The site is an area underlain by the Late Triassic to Early Jurassic aged Myrtle Creek Sandstone
which typically comprises “Quartzose sandstone, orthoquartzite, sublabile to labile sandstone,
siltstone, shale”. The encountered conditions agree with anticipated geological units.
3.0 Existing Stability Risk
Local authorities prepare mapping tools to aid in assessing potential slope stability
requirements. The mapping tools are a “high level” approach where the site specific
investigations are specific to a particular site/allotment. It is important to note that where a
site specific investigation and slope stability assessment are undertaken, they take precedence.
Reference to the available overlays for the area show the allotment has been initially mapped
as “High” to “Very High” in terms of landslide hazard.
4.0 Previous Geotechnical Investigation
A geotechnical investigation for the proposed site 2 was previously undertaken by Global
Testing. This report is attached and the encountered conditions in this investigation have
informed the analysis undertaken by this office. Given this, where discrepancies exist between
the encountered conditions in the referenced report and the encountered conditions, our
analysis cannot be considered to be accurate and reassessment will be required.
The referenced report encountered uncontrolled fill overlying natural sandy clay, which is
inferred to be weathered material from the predicted geological unit at the site. No
30 Nov 2020 Geotechnical Investigation
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groundwater seepage was noted in the borehole logs, however a dam is present directly
adjacent to the site.
5.0 Walkover Assessment
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.
An experienced geotechnical engineer undertook the walkover assessment on 15 December
2020. As part of the walkover assessment, 'spot' measures were taken of the slope angles and
observations pertaining the any obvious previous slope instability on the allotment and
immediately adjoining areas that were readily accessible at the time of the inspection was
undertaken. The house site has been cut and filled in parts to create two relatively flat pads,
with batters of between 33 and 50° to tie into the surrounding ground levels. Directly
encompassing the pad the site has a relatively uniform grade, falling away to the west at
approximately 15°. No obvious signs of disturbed ground, tension cracks or hummocks were
noted in the natural ground surrounding the site, however tension cracks were evident in the
uncontrolled fill pad. 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. The photographs attached to this report show the conditions of the site at the time
of the inspection and assessment undertaken.
6.0 AGS Assessment
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
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
30 Nov 2020 Geotechnical Investigation
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reduce such risk is usually not required unless reasonably practicable measures are available at low
cost in terms of money, time and effort.”.
AGS suggest that regulators generally accept risk levels of “low” or lower as a level of
acceptable risk.
AGS prepared a semi-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 1.
Table 1: Results of AGS Qualitative Risk Assessment
Hazard Likelihood Consequence Risk Level
Global Failure Barely Credible Catastrophic Low
Shallow slumping of surface soils LIkely Minor Low to moderate
Based on the results of the AGS assessment, the perceived risk based on the conditions
encountered and information available at the time of the assessment is “low”.
7.0 Hillside Construction
The following 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 and keyed
into stiff (or stronger) natural clays; Heavy structures should be supported on either piles
founded into weathered sandstone 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
2H: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.
• Plumbing and services must also not be allowed to leak and regular maintenance and
inspections should be undertaken.
30 Nov 2020 Geotechnical Investigation
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• 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.
8.0 Geotechnical Recommendations
The following sections details specific geotechnical constraints or recommendations in
addition to general hillside construction notes.
8.1 Site Preparation and Earthworks
The following comments are provided in relation to site preparation for earthworks.
• Fill at this site must be limited to not more than 1 m unless supported by an engineer
designed and certified retaining structure
• Areas where fill is proposed 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.
• Areas sloping in excess of approximately 1 vertical unit to 10 horizontal units (1V:10H),
the surface will need to be benched at a minimum vertical bench height of 0.3 m to enable
keying in of the new fill and existing soil. The benches will also need to be shaped
adequately to promote drainage.
• Edge batters should be formed by over filling an area and ‘trimming’ back to the desired
profile.
• 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 98% relative to Standard compactive effort.
• Highly plastic natural clays are 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.
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• Earthworks quality testing will be required on this site. The GTA will need to provide
supervision in accordance with AS3798 and undertake field density tests to monitor the
actual dry density and moisture content of placed materials.
• Completed platforms should be sealed with a minimum 250 mm layer of relatively
granular material (CBR15% or greater) to minimise the effects of relatively clayey
subgrades drying out leading to excessive cracking.
• Earthworks activities should be undertaken in relatively homogenous sequences where
achievable (ie. do not place “pockets” of gravel material next to impermeable clays as this
will lead to zones where water could become trapped).
• 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.
Arcos can assist in preparing earthworks documentation or guidelines for contractors to
adhere to in order to achieve an efficient and “fit for purpose” end product.
8.2 Retaining Structures
Retaining walls must be designed by a suitably qualified and experienced engineer and
preferably in accordance with AS 4678. The geotechnical parameters for use in design of either
flexible or rigid wall systems are presented in Table 2.
Passive forces should be ignored in areas where disturbance may occur (ie. future trenching
or earthworks processes). Compaction for retaining wall backfill should be undertaken with
due care. Layers should be placed thinner than for normal earthworks processes (say a
maximum layer thickness of 100 mm) and compacted using light weight equipment such as
hand controlled compactive equipment to minimise the stresses on the wall. If hand controlled
equipment is not practical, temporary wall propping will be required.
Walls will need to incorporate full height drainage in accordance with good design practice to
minimise hydrostatic pressure. The below parameters do not include surcharge loadings.
Table 2: Retaining Wall Parameters (Unfactored)
Unit Weight Friction Angle Drained Cohesion
Material
(kN/m3) (Ø) (c’)
Loose Sands 18 28 0
Very Stiff Clays 19 26 3
Weathered Rock 21 32 10
Notes: *- Compacted under Level 1 conditions and behaves as clayey material
30 Nov 2020 Geotechnical Investigation
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8.3 Footings
In accordance with industry standards, a geotechnical engineer from Arcos should undertake
the testing within the footings to confirm that the encountered material is in alignment with
the adopted parameters and assumptions provided within this report. Where this does not
occur, Arcos cannot be held liable for any of the information presented within this report.
All footings will need to be designed to accommodate potential ground surface movement,
settlement values and any additional settlement values associated with the placement of new
fill.
Settlement for suitably sized and designed high level footings should be in the order of 1-2%
of the width of the footing.
If footings are located adjacent to an underground service or other obstruction that is not
homogenous with the founding strata, the footing should be extended a minimum 0.3 m
below an imaginary line projected at a 45 degree angle from the lowest point of the
service/obstruction. This is depicted graphically for reference:
Where footings found into very stiff or stronger clays, they may be dimensioned for a
maximum allowable bearing pressure of 150 kPa. Based on the bore logs, the above material
is encountered at approximately 0.5 m depth below existing surface levels at the time of the
investigation. Alternatively, founding into the weathered sandstone could achieve 250 kPa.
The ultimate geotechnical end bearing and shaft adhesion parameter are presented in Table
3. The values given will need to reduced by applying a suitable geotechnical strength reduction
factor (Φg) for limit state or divided by a suitable factor of safety for working stress methods.
The Φg should be selected by the designer.
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Table 3: Deep Level Footings – Ultimate Geotechnical Parameters (Long Pile)
Bored
Material
Fb (kPa) Fs (kPa)
Weathered rock 1800 60
Notes: * - Fb = Ultimate End Bearing Pressure (unfactored) for long piles, Fs = Ultimate Shaft Adhesion (unfactored).
The upper 1 m of soil profile should be ignored in shaft calculations to allow for shrinkage
cracks. The values presented in Table 3 assume that the material equal to four pile diameters
below the base of any footings is as strong or stronger.
Settlement values are subjected to a variety of factors; however, a properly designed and
constructed pile should have a serviceability settlement between 1% and 2% of its’ diameter.
30 Nov 2020 Geotechnical Investigation
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9.0 Limitations
Arcos Unit Group Pty Ltd (AG) has prepared this document for project and client described on
the front page of this report. 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 AG, does so entirely at its own risk and without recourse to AG for any loss
or damage. AG 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 AG.
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 AG.
__________________________________________________________________________________
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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GEOTECH REPORT
These notes have been provided to clarify and Arcos Unit Group Pty Ltd undertakes quality assurance
enhance the reader’s understanding of the terminology checks in accordance with its’ quality system
used throughout the report, and also to identify where requirements; however, restrictions on the project
limitations exist. All investigations are undertaken outside of Arcos Unit Group Pty Ltd’s control can result
based on budgetary constraints which impart a level of in unanticipated variations, changes in policy or
uncertainty as a result of limiting the amount of interpretation of said policies by the relevant authority,
investigation undertaken. or the actions of contractors subject to commercial
pressure.
Copyright
Site Variations
This report remains the property of Arcos Unit Group
Pty Ltd at all times unless ownership is transferred in If site variations become evident at the time of
writing. It cannot be included in any documentation construction or after the release of a report, Arcos Unit
without the express written permission of Arcos Unit Group Pty Ltd requests immediately notification. This
Group Pty Ltd notification is proposed in order to assist in remediation
prior to any permanent structures or design procedures
Subsurface Report Sheets (Bores etc) are undertaken.
The provided subsurface sheets are inherently an Collaboration
interpretation undertaken by a suitably qualified and
experienced person; however, the reliability of the Arcos Unit Group Pty Ltd strongly recommends open
provided information will rely on the method of and collaborative communication about the use and
investigation. It is prudent to state that these sheets inherent interpretation of Arcos Unit Group Pty Ltd’s
only represent a very small percentage of the report. Please contact the author of the report to
subsurface conditions across a particular project. arrange for an obligation free discussion regarding the
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 Arcos Unit Group
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 Arcos Unit Group Pty Ltd’s
does not take into consideration the degree of fracturing of
has been evolved from AS1726:2016. Typically a soil’s
a rock mass which can both lower and increase the overall
strength, colour, plasticity, grain size, other inclusions, and
rock strength. The strength of rock is broken down as
moisture will be described. A rock’s strength, weathering,
follows:
colour, grain, and other inclusions are typically described.
Very Low Strength (VLS) < 2 MPa (UCS)
Soil Low Strength (LS) 2 to 6 MPa (UCS)
Medium Strength (MS) 6 to 20 MPa (UCS)
Soil is typically described by the components that make it High Strength (HS) 20 to 60 MPa (UCS)
up by percentage or whichever has a governing behaviour. Very High Strength (VHS) 60 to 200 MPa (UCS)
The types and sizes reproduced from AS1726 are: Extremely High Strength (EHS) > 200 MPa (UCS)
Boulder >200 mm
Cobble 63 to 200 mm
Gravel 2.36 to 63 mm Very Low Strength (VLS) < 0.1 Is(50)
Sand 0.075 to 2.36 mm Low Strength (LS) 0.1 to 0.3 Is(50)
Silty 0.002 to 0.075 mm Medium Strength (MS) 0.30 to 1.0 Is(50)
Clay < 0.002 mm High Strength (HS) 1.0 to 3.0 Is(50)
Very High Strength (VHS) 3.0 to 10.0 Is(50)
The strength of soil is usually assessed by suitably Extremely High Strength (EHS) > 10.0 Is(50)
experienced persons through the use of laboratory test
results, published literature, field assessments or a The Is(50) values are based on a correlation between the
combination of the above. The strength of soil is unconfined compressive strength (UCS) of 20. This value
characterised into either cohesive (clays and silts) or varies significantly and site specific testing must take
cohesionless (sands and gravels) as shown below: precedence.
Cohessive Soil Strength Term Rock weathering is broken down as follows:
Very Soft < 12.5 kPa
Soft 12.5 to 25 kPa Extremely weathered Behaves as soil but rock
Firm 25 to 50 kPa (EW) structure evident
Stiff 50 to 100 kPa Highly weathered Colour and strength of original
Very Stiff 100 to 200 kPa (HW) fresh rock is not recognisable.
Hard > 200 kPa Decomposition of rock evident
Moderately Defects are heavily stained with
Cohesionless Soils Strength Term (uncorrected Qc) weathered (MW) rock mass showing signs of
Very Loose <2 MPa discolouration
Loose 2 to 5 MPa Slightly weathered Some staining but minimal to no
Medium Dense 5 to 15 MPa (SW) change in strength
Dense 15 to 25 MPa Fresh stained Minor staining evident
Very Dense > 25 MPa (FS)
Fresh rock No change
Rock (FR)
Rock is typically described by the components that make Rock fracturing is broken down as follows and can only be
it up by percentage or whichever has a governing accurately described where undamaged continuous
behaviour. The strength of rock is typically the most sampling has been undertaken:
important component. Typically, the strength of rock is
based on laboratory testing, tactile assessment and other Fragmented (Fg) Defect spacings < 20 mm
observations as relevant. The industry standard method Highly Fractured (HF) Defect spacings 20 to 50 mm
for assessing the strength of rock in the field is to use the Fractured (Fr) Defect spacings 50 to 200 mm
point load strength index (AS4133.4.1). Slightly Fractured (SF) Defect spacings 200 to 1000 mm
Unbroken (Ub) Defect spacings > 1000 mm
Page 12
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
EXAMPLES OF GOOD HILLSIDE ee
Surface water interception drainage
Waterton, edequately sited and founded
rood water storage tartks (with due regard for
Impact of potertial ipakage)
Fleaib le structure
Roed water piped aff ate or stored
Onsite detention tanks, walertight ard
od fourdied. Poterlial teakage
\— Cutting and filing minimised in development
Sewage aftuant pumped cut or connected to seinne:
Tanks adequately teunded and watertight. Potential
legiage managed by suh-scil draing
‘
“_. Engingered retaining walls with both surface and
subsurface drainage (constructed before dwelling) 2
© AGE (2006)
EXAMPLES OF POOR HILLSIDE PRACTICE
Unstabfised rock topples
Bnd terete doensiope
Vegatation removed ——
Discharges ot rootwater soak ‘Staee unsupported \ }
meny rather than conducted off out fails 1
ule & to secure storage for re-use \
s '
‘Sruciure unatie jo tolerate —, ~
tattorrent and cracks inary = » \
a“ a ~ — \
Poorly compacted fil cotties a 6 . »,
uneverty and cracks pool 2a 5 \ LD,
- \ x
inadequate walling unatie LY >
bo support i y \
ry
Loose. saturated A cides
and posaily flows doweslape
Incetoquataby supported cut tale rc BS seat) Roohwaner introduced into slope
Sarabed
‘Dwelling met founded in bedrock
\___-Aptence of subeod drminage within 4
—— Pondect water enters shape and activates landside nia bins
'—Rossitte tavel downslope which Impacts other davalagment down Se stan ASS (2009) Appencis J
114 Australian Geomechanics Vol 42 No 1 March 2007
Page 13
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 Deceripei Deccripe Level Indicative Notional Recurrence Interval escripaon ‘seriptor ve Value Boundary 107 5x10? 10 years 5 The event is expected to occur over the design life. ALMOST CERTAIN A 10? . 100 years 20 years ae a will probably occur under adverse conditions over the LIKELY B y 200 7 , —— 10° 3x10 4 1000 years > 000. soars The event could occur under adverse conditions over the design life. | POSSIBLE Cc 107 5x10 10,000 years ~ pa a might occur under very adverse circumstances over the UNLIKELY D 5 2 - 10° 5x10 20,000 years The event is conceivable but only under exceptional circumstances sx10* 100,000 years over the design life RARE > . 10° * 1,000,000 years | 700-000 vears [The 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% ae ie ~ © © © 200% 100% stabilisation. Could cause at least one adjacent property major consequence damage. CATASTROPHIC 1 60% ° 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% “° Moderate damage to some of structure. and/or significant part of site requiring large stabilisation works. MEDIUM 3 “~ 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, 2 N FI! A N 0.5% notional boundary of 0.1%. See Risk Matrix.) INSIG Cc 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. @G) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement 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 night; 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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APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION Fas vide dramage measures and erosion contol. oe Strip vegetation and topscdl and key into aamral slopes price to filling ‘Use clean 40] matecials and compact to engimecring standards. Batter to appropriate slope or support with engineered retaining wall. GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE ADVICE GEOTECHNICAL Obnim advice Som a expensnced geotechnical practioner at early | Prepare detailed plan and stact site works before ASSESSMENT sage of plannms and before site works geotechnical advice. PLANNING SITE PLANNING Having obtained geotechnical advice, plan the development with the mek | Plan development without regard for the Risk. from the identified hazard: and consequences in mind DESIGN AND CONSTRUCTION Use flexible structures which incorporate properly designed brickwork, timber | Floor plans which require extensive cutting and HOUSE DESIGN or steel Tames. imber or panel cladding i : Consider use of split level: Movement intolerant structures. Use docks for recreational areas whers appropriate. SITE CLEARING Retain natural vegetation wherever practicable. Indjscriminately clear the site. ACCESS & Satisfy requirements below for cats, fills, retaining walls and dramage. Excavate and fill for site access before DRIVEWAYS Council specifications for grades may need to be modified geotechnical advice. Driveways and parking areas may need to be fully supported on piers. EARTHWORKS Retain natural contours wherever possible. Indtscximinatory bulk oarthworks. Manzzise dopa. Large scale cuts and benching Curs Pr we ened eens we ee eee they Unsupported cuts | Imors Gramazerequements Loose or poorly compacted S10, which if it fails, may flow a considerable distance including omto propecty below. Block natural drainage lines. Fill over existing vegetation and topsoil. Inchnde stumps, tress, vegetation, topsoil i rubble stc in £0. Rack OUTCROPS Remove or stabilise boulders which may have unacceptable nak. Disturb or undercut detached blocks or & BOULDERS Support rock faces where necessary. boulders Engineer design to resist applied soil and water forces. Construct a stractazally inadequate wall such 2: RETAINING Found on rock where practicable. sandstons flagging, brick or umreinforced WALLS Provide subsurface drainage within wall backiill and surface drainage on slope | blockwork. above. Lack of sabsurface drains and weephoks. Construct wall as scon a5 possible after cat/fill operation. Found within rock where practicable. Found om topsoil, loose SI detached boulders FOOTINGS ‘Use rows of piers or strip footings oriented up and down slope. or undercut clift. Desise for lateral cosep pressures if necessary. BackSll footing excavations to axchads imaress of surdace water. Engineer desimed. Support on piers to rock where practicable. SWIMMING POOLS | Provide with under-~drainage and sravity drain outlet where practicable. Desise for high soil pressures which may develop on uphill side whilst there zazy be Little or no Lateral sapport on downhill sids. DRAINAGE Provide at tops of cat and fill slopes. Discharge at top of fills and cats. Discharge to street drainage or natural water courses. Allow water to pond on beach areas. SURFACE Provide gemeral falls to prevent blockage by siltation and imcorporate silt traps. Lime to minimise infiltration and make flexible where possible. tal stractures to dissipate emergy at changes of slope and/or direction. Provide Slter around subvarface drain. Discharges rood ranoff into absorpticn trenches omens Provide drain behind retains walls. - ‘Use flexible pipelines with access for maintenance Prevent inflow of surface water. Soar & Uvaally requizes Or mains sewer systems; absorption teaches may | Discharge sullage directly onto and imto slopes. SULLAGE be possible in some areas if rick is acceptable. Use absorption tenchss without consideration — Storage tanks should be water-tight and adequately founded. of landslide misk. EROSION Control erosion as this may lead to instability. Failure to observe carthworks and dramago CONTROL & Revegetate cleared area recommendations when landscaping. LANDSCAPING DRAWINGS AND SITE VISITS DURING CONSTRUCTION DRAWINGS Building Application drawings should be viewed by geotechnical consukant SITE VISITS Site Visits by consultant be iate darms comstruction’ INSPECTION AND MAINTENANCE BY OWNER OWNER'S Clean dramage systems; repair broken joints in drm: and leaks m supply RESPONSIBLITY i Pupes. Where structural distress is evident see advice Tf seepage observed, determines causes or sesk advice om consequences.