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Approved Plan - EXE21-0015
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APPROVED PLAN
EXE21/0015
NOOSA COUNCIL
19 JUNE 2021
-
28 May 2021 Project No. 21182-001-Rev0
Mr Dean Frith
Email: [email protected]
SLOPE STABILITY ASSESSMENT
PROPOSED SHED
210 LAWNVILLE RD, BLACK MOUNTAIN
Dear Dean,
1.0 INTRODUCTION
Tectonic has undertaken a slope stability assessment for a proposed shed at 210 Lawnville Rd, Black
Mountain. This report presents the results of our slope stability assessment, together with geotechnical
advice for the development. In summary, subject to implementation of the recommendations made herein,
it is assessed that there would be a Low Risk of slope instability affecting the proposed shed in accordance
with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007
(AGS 2007).
1.1 Details of Site and Development
The subject site is described as Lot 22 on SP236310 and covers a total area of 16,879 m2. The site is
accessed via a bitumen driveway from Lawnville Road to the south. The surrounds feature similar rural
residential blocks. Text Figure 1 below illustrates the site position. A detailed description of the site features
is given in Section 2.
Approx. site of proposed shed
Lot boundary
Text Figure 1: 2019 Aerial Image of site and surrounds (courtesy Noosa Council [NC] Interactive Maps)
Tectonic Geotechnical Pty Ltd
PO Box 899, Buderim, QLD 4556
Office: 07 5478 9016 Email: [email protected]
www.tectonicgeo.com.au
A.B.N. 83 165 727 828
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Mr Dean Frith 21182-001-Rev0
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The existing building platform has been formed by cut and fill earthworks. It is estimated that fill in this area
ranged up to 1.5 m high, with cuts up to 1 m depth. Design drawings by Best Sheds (ref. Job No.
1713555893, Sheets 1 to 8, dated 8 February 2021) detail the shed will have a plan area of 12 m by 7.5 m
and will comprise a concrete slab on ground floor with steel framing and sheet metal cladding to the walls
and roof. The shed is proposed to be located approximately 1 m off the cut face and ranges approximately
1 m to 5 m off the crest of the fill batter.
Approx. site of proposed shed Gravel access driveway
Bitumen driveway
Text Figure 2: Approximate site layout of proposed shed (courtesy of nearmap, 2021)
1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of
published geological maps, aerial photos, and a ‘soil test’ report prepared by Ground Drill (Ref. 21/213,
dated 27 April 2021).
A walk-over survey was conducted in order to make an appraisal of the general condition of the site in
regard to topography, drainage, vegetation cover, geology, erosion and slope stability and existing
structures. The walk-over survey was carried out by an experienced geotechnical engineer from Tectonic
on 14 May 2021.
The results of the desk-top study and walk-over survey are included in Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been reviewed by Mr Ashley Davey, an RPEQ with more than 25 years’ experience in
geotechnical engineering, including a number of slope stability projects.
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2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology and Subsurface Conditions
The 1:100,000 scale Gympie Special Geological Map1 (Ref. Text Figure 2 below) indicates that the site is
underlain by early to middle Triassic age meta-sedimentary geology (‘arenite, argillite, phyllite’) of the Kin
Kin Beds (pale green blue shading).
Approx. site location
Text Figure 2: Extract of Gympie Special Geology Map
Along the southern cut batter for the proposed shed pad, exposures of distinctly weathered, low strength
phyllite were observed (Ref. Text Figure 3).
Text Figure 3: Phyllite exposed in cutting along access track
The Ground Drill investigation comprised two boreholes across the proposed building platform to depths
of 0.5 m to 2 m below ground level (BGL). Borehole BH1, closest to the cut batter, encountered distinctly
weathered, low strength rock with auger refusal at 0.5 m BGL. Borehole BH2, closest to the fill batter,
1
State of Queensland, Department of Mines and Energy, Gmypie Special 1:100,000 Geological Series, Sheet 9445 and Part 9545, First
Edition 1999
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encountered 1.3 m of firm gravelly silty clay, then natural stiff silty clay to 1.8 m BGL, underlain by
distinctly weathered, low strength rock to borehole termination at 2 m BGL.
Ground Drill has classified the site as Class P in accordance with AS2870, due to the presence of
existing uncontrolled fill and poor bearing capacity; however has stated that if not for the above, the site
would have the characteristics of Class H1.
No groundwater was mentioned in the Ground Drill borehole reports, with the clay soils described as
‘moist’.
We have not sighted any documentation regarding the existing fill, and it is therefore classified as
‘uncontrolled’ in accordance with the definitions given in AS 2870-2011 Residential Slabs and Footings.
2.2 Topography
As illustrated in Text Figure 4, the property is located just down slope (north) from a gently sloping hilltop.
Inferred natural gradients measured using a hand-held clinometer above and below the proposed shed site
ranged between about 15° and 20° with more gentle slopes (10° to 12°) noted to the north of the
development area.
Previous earthworks had been undertaken for the gravel access driveway to the proposed shed and for the
building platform. Cut batters for the gravel access driveway dipped at 26°, whilst cut batters in distinctly
weathered, low strength rock dipped at 60°. The fill embankment was battered at about 35° exposing site
won clays and rock/gravel fragments.
A slump was observed within the fill batter with tension cracks also noted further west of this location. The
slump was measured to be about 1 m off the corner peg of the shed.
Approx. Shed location
Text Figure 4: Image showing ground surface contours (courtesy NC Interactive Maps)
Text Figures 5 to 9 overleaf illustrate the site conditions at the time of our inspection.
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Text Figure 5: Small cut pad above proposed shed location
Text Figure 6: Easterly view across access driveway and proposed shed location
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Text Figure 7: Approximate shed location (pegged out)
Slump
Corner peg
Rill erosion
Tension cracking
Text Figure 7: Looking west over slump, tension cracking and rill erosion within the fill embankment
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Text Figure 9: Looking east over slump, tension cracking and rill erosion within the fill embankment
2.3 Groundwater
No seepage or ‘springs’ were observed in the vicinity of the proposed development area. No groundwater
was encountered in the Ground Drill boreholes with the soils being described as mostly ‘moist’.
Groundwater conditions may vary seasonally, and groundwater seepage could develop, particularly
following periods of prolonged or heavy rainfall.
2.4 Surface Drainage
Broadly, natural surface drainage follows the ground surface contours to the north and north-east. A small
pond was located to the north, well downslope of the proposed shed location. A spoon drain was installed
between the cut batter and the access driveway upslope of the proposed building platform, which diverts
upslope stormwater towards the west into a grated inlet, then under the driveway, discharging onto the fill
embankment (Ref. Text Figure 10). Erosion of the fill embankment was observed at this location.
Text Figure 10: Spoon drain upslope of building platform (LHS) and discharge point onto fill embankment (RHS)
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Surface water is expected to drain well from the site considering the moderate natural gradients and low
permeability of the clay soils in the immediate subsurface, and relatively shallow depth to rockhead.
2.5 Vegetation
The immediate area of the proposed shed consisted of bare ground with exposed soils, some of which had
been covered by gravel. Medium to large size mature trees were noted on the sloping ground to the north
and south around the proposed shed, which were observed to be relatively ‘plumb’. Tree root bases were
observed within the fill embankment which had not been ‘grubbed out’ (ref. Text Figure 11). These showed
evidence of soil erosion around the root base.
Text Figure 11: Root base at crest of the fill embankment
2.6 Buildings and Other Structures
No structures were present on site at the time of our assessment. The existing driveway pavement displayed
longitudinal cracking along the edges, however this did not appear to have been caused by slope instability.
Text Figure 12: Longitudinal cracking within driveway
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The proposed shed site has generally been mapped by NC as presenting a Moderate landslide hazard area
(pale yellow brown shading). An extract of the NC mapping is given below in Text Figure 13.
Proposed shed site
Text Figure 13: Extract from NC Landslide Hazard Mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility only,
largely based on ground slope, and is superseded by this site specific assessment.
The site does not exhibit any indicators of global, natural slope instability. No landslide back scarps or
areas of natural ‘hummocky’ ground were observed at or near the proposed shed site, however minor
erosion and slumping was observed in part of the fill batter which is likely due to existing surface drainage
conditions and uncontrolled fill. In addition, the subsurface conditions appeared favourable with stiff
residual soils and weathered rock encountered at relatively shallow depths within the Ground Drill
boreholes.
3.2 Qualitative Stability Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 1 below and the
document Qualitative Terminology for Use in Assessing Risk to Property, which is also attached.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk
L Low Risk
to this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) 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 guide .
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Considering the existing site conditions (Ref. Section 2), and proposed development works, it is assessed that
there would be a Low (L) Risk of global slope instability affecting the proposed shed subject to the
implementation of recommendations given below. Regulators (NC) normally require that a Very Low or Low
Risk of landslide affecting the property must be demonstrated to enable development approval. Summarised
in Table 2 is our qualitative assessment of landslide risk for the site. A summary of qualitative terminology for
use in assessing risk to property is attached (taken from AGS 2007).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007
Assessed
Hazard Likelihood Consequence
Risk
Comments
The likelihood of a failure through the existing fill
embankment to the north of the proposed shed
location is assessed as Possible due to the firm
consistency of the fill materials and slopes of the
batters. The consequence of such a failure would be
Minor with the resultant risk being Moderate as per
1: Failure through fill Possible Moderate AGS 2007.
Minor
embankments (Unlikely) (Low)
Failure of the fill embankment following remediation
works as specified in Section 4 would be Unlikely.
The consequence of such a failure would be
Medium, with the resultant risk would reduce to
Low as per AGS 2007.
Given the presence of stiff residual soils and
weathered rock materials at shallow depth, apparent
lack of groundwater, and no evidence of instability in
natural strata the vicinity of the proposed shed, it is
2: Failure through natural soils
Unlikely Medium Low considered Unlikely that a landslip would develop,
above the foundation depths
subject to implementation of the recommendations
in Section 4. With a potential Medium impact from
such shallow instability the resultant risk would be
Low as per AGS 2007.
The likelihood of a deep failure through the phyllite
rock is assessed as Barely Credible due to the
3: Deep seated failure in Barely strength of these materials and no evidence of existing
Major Very Low
weathered rock Credible instability in these materials. Although the
consequence of such a failure could be Major, the
resultant risk is Very Low as per AGS 2007.
The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts.
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4.0 RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached. This document should be referred to by those
preparing the design for the shed at the site.
4.1 Site Layout and Geotechnical Constraints
The proposed shed location described in Section 1.1 is considered acceptable from a slope stability
viewpoint. Tectonic must be consulted to assess the impacts on slope stability if development is proposed
elsewhere on the site.
4.2 Key Mitigation Measures
Recent instability has occurred in the fill embankment to the north. The following mitigation measures are
recommended from a geotechnical viewpoint to achieve a Low Risk of slope instability the shed from the
existing earthworks. It should be noted that this risk assessment excludes geotechnical risk that may be
associated with settlement due to consolidation of the uncontrolled fill material over time which could affect
non-piled structures (e.g. driveways, landscaping structures).
1. The shed must be setback at least 1 m from the crest of the fill embankment.
2. The fill embankment will need to be trimmed back to well compacted materials (to the lines of
tension cracking) and flattened to a maximum grade of 1V:2.5H, or otherwise supported by an
engineer designed and certified retaining wall.
3. All batters should be protected from erosion by surface protection, the type of which will be
dependent upon a number of factors including batter slope, material type, aspect, aesthetic finish
required, etc. As a minimum, all soil batters should be topsoiled and seeded (or hydromulched) to
establish a permanent grass (or similar vegetation) cover.
4. As the existing fill platform is deemed to be uncontrolled, it is possible that unsuitable fill materials
may be present within the fill. Therefore, to minimise future settlement due to consolidation of this fill
it is recommended that the platform subgrade should be proof rolled using a fully laden water cart (or
similar heavy onsite equipment) to identify any ‘soft spots’. Any such soft spots should be excavated
and replaced with select fill material. Tree stumps and large roots must be ‘grubbed out’.
5. Ongoing consolidation settlement of the fill is expected to occur over the long term as the particles
realign to find an equilibrium position. Given the uncontrolled nature of the fill, settlement might be
expected to be in the order of 1%H up to 2%H, where H is the height of fill. Differential settlements
across the building pad from the cut to fill areas should be expected and accounted for in the design
of adjacent surface or in ground elements (e.g. driveway slab and underground services).
6. Diversion drainage must be installed above the crest of any cut/fill embankments to divert
stormwater away. All stormwater must be directed by pipes or lined channels to discharge at least
10 m downslope from any structure or embankment and not upslope of any neighbouring
structures/embankments.
Subject to the above rehabilitation earthworks, and relatively conventional hillside construction practices
such as ensuring all footings for the proposed shed and ancillary structures (e.g. retaining walls) penetrate
through any fill and natural soils to found into underlying weathered rock, limiting future fill earthworks and
formalising onsite drainage, it is assessed that a Low Risk of instability affecting the proposed shed could be
achieved.
Specific recommendations relating to the above key measures and earthworks, footing design, retaining
walls and drainage are outlined in the sections below.
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4.3 Earthworks
No further cutting (with the exception for remediation works) or filling should be carried out on site for the shed
area unless reviewed and approved by Tectonic. If further earthworks are planned Tectonic must review
potential impacts on slope stability.
All vegetation, tree stumps and large roots must be ‘grubbed out’ scross proposed building areas. Any
organic rich topsoil (although not anticipated) should then be stripped and set aside. Organic topsoils and
garden soils must not be re-used as fill; however, they may be retained on site for later use across
landscaping areas, if required. Ground surfaces should be graded to shed surface water runoff away from
construction areas.
Permanent batter slopes should be constructed as follows:
▪ Fill embankments to a maximum grade of 1V:2.5H
▪ Cut batters in natural soil consisting of stiff (or stiffer) clay to a maximum grade of 1V:2H
▪ Cut batters in weathered rock to a maximum grade of 1V:1H
Steeper batters should be re-profiled. Where spatial restrictions exist, batters should be retained by
engineer designed retaining walls (ref. Section 4.4 below).
Although trafficability should generally be good in ‘dry’ conditions, the soils could be subject to strength loss
from repetitive vehicle loading (particularly from rubber tyred plant) or if they become wet. Due to the
potential for poor trafficability during and after wet periods, placement of a minimum 150 mm thick granular
working platform may be required, particularly along access ways.
4.4 Retaining Structures
Retaining structures must be founded as described in Section 4.5 below. Retaining structures greater than
1 m high will require engineer design and certification of construction. Retaining structures should be
engineer designed in accordance with AS4678-2002 Earth-retaining structures.
We suggest the following parameters may be adopted for retaining wall design.
Table 3: Retaining Wall Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle
Ka Ko
(kN/m3) (Degrees) Kp
(Cantilever wall) (Non-yielding wall)
Firm to Stiff Clay 19 25 0.40 0.57 2.46
Weathered Rock 21 36 0.26 0.41 3.85
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.
4.5 Footing Design
Footings for the proposed shed and any ancillary structures (e.g. tanks or retaining walls) should penetrate
through any fill, and residual soils, to found a minimum of 0.3 m into weathered rock. Footings so founded
may be designed using an allowable bearing pressure of 400 kPa. Footings located in cut areas may be
founded directly on weathered rock, and may therefore comprise conventional slab edge/internal beams,
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with mass concrete placed to achieve the required depth as necessary. Elsewhere, given the depth to
weathered rock, it will be necessary to support structures on piles. Concrete bored piles are recommended
due to the presence of rock particles within the fill.
Footing design should also account for potential soil reactivity, advised by Ground Drill to be equivalent to
Class H1. Ongoing consolidation settlement of the fill is expected to occur over the long term as the particles
realign to find an equilibrium position. Given the uncontrolled nature of the fill and, settlement might be
expected to be in the order of 1%H up to 2%H, where H is the height of fill. Differential settlements across
the building pad from the cut to fill areas should be expected and accounted for in the design of adjacent
surface or in ground elements (e.g. driveway slab and underground services).
All footings should found such that they are not adversely affected by any adjacent excavations, trenches, or
retaining walls that are not designed to support building loads. Footings should found at least below a plane
extending 1 m horizontally from the base of trenches/excavations/retaining walls, then rising up at 1V:1H, as
illustrated in Text Figure 14.
Text Figure 14: Footing depth required to minimise the risk of undermining and surcharge loading
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. A suitably qualified and experienced engineer should inspect the footing excavations to
confirm appropriate foundation materials and cleanliness prior to pouring concrete.
If any soil conditions encountered during construction are found to differ from those noted above, Tectonic
and Ground Drill must be notified immediately and an inspection carried out to determine if changes to
footing design are required.
4.6 Drainage
Adequate site drainage should be installed to ensure that stormwater runoff is directed away from the
building walls and footings (e.g. swales in landscaped areas or grated drains across pavements up slope of
structures). Diversion drainage should be installed above the crests of all cut/fill batters, and toes of all cut
batters. Future landscaping must be carried out to maintain good drainage.
Diversion drainage must be installed above the crest of any cut/fill embankments to divert stormwater away.
All stormwater must be directed by pipes or lined channels to discharge at least 5 m downslope from any
structure or embankment and not upslope of any neighbouring structures/embankments. Surface dissipation,
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such as a rock weir or similar, must be installed at the stormwater outlet(s) to minimse the risk of soil erosion
at the discharge point.
Subsurface disposal of stormwater must not be undertaken.
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
impacting the proposed shed. This advice is subject to implementation of the recommendations given in this
report.
6.0 LIMITATIONS
Your attention is drawn to the document Limitations, which is attached to this letter report.
Please contact the undersigned should you wish to discuss any of the above matters.
Yours faithfully
TECTONIC GEOTECHNICAL PTY LTD
Carissa Bruggemann BAppSc MEngSc Ashley Davey RPEQ 8159
Geotechnical Engineer Principal Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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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
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years 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
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
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
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
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.
(3) 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 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
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 16
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 CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk 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
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. 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
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
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 guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
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Foundation Maintenance (i) and Footing Performance: <,., A Homeowner's Guide aa Buildings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause of movement in buildings can usually be related to one or more problems in the foundation soil. It 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 building movement. This Building Technology File is designed to identify causes of soil-related building movement, and to suggest methods of prevention of resultant cracking in buildings. Soil Types The types of soils usually present under the topsoil in land zoned for residential buildings can be split into two approximate groups — granular and clay. Quite often, foundation soil is a mixture of both types. The general problems associated with soils having granular content are usually caused by erosion. Clay soils are subject to saturation and swell/shrink problems. Classifications for a given area can generally be obtained by application to the local authority, but these are sometimes unreliable and if there is doubt, a geotechnical report should be commissioned. As most buildings suffering movement problems are founded on clay soils, there is an emphasis on classification of soils according to the amount of swell and shrinkage they experience with variations of water content. The table below is Table 2.1 from AS 2870, the Residential Slab and Footing Code. [Seuses of Movement Settlement due to construction There are two types of settlement that occur as a result of construction: * Immediate settlement occurs when a building is first placed on its foundation soil, as a result of compaction of the soil under the weight of the structure. The cohesive quality of clay soil mitigates against this, but granular (particularly sandy) soil is susceptible. * Consolidation settlement is a feature of clay soil and may take place because of the expulsion of moisture from the soil or because of the soil’s lack of resistance to local compressive or shear stresses. This will usually take place during the first few months after construction, but has been known to take many years in exceptional cases. These problems are the province of the builder and should be taken into consideration as part of the preparation of the site for construc- tion. Building ‘Technology File 19 (BTF 19) deals with these problems. Erosion All soils are prone to erosion, but sandy soil is particularly susceptible to being washed away. Even clay with a sand component of say 10% or more can suffer from erosion. Saturation This is particularly a problem in clay soils, Saturation creates a bog- like suspension of the soil that causes it to lose virtually all of its bearing capacity. To a lesser degree, sand is affected by saturation because saturated sand may undergo a reduction in volume — particularly imported sand fill for bedding and blinding layers. However, this usually occurs as immediate settlement and should normally be the province of the builder. Seasonal swelling and shrinkage of soil All clays react to the presence of water by slowly absorbing it, making the soil increase in volume (sce table below). ‘The degree of increase varies considerably between different clays, as does the degree of decrease during the subsequent drying out caused by fair weather periods. Because of the low absorption and expulsion rate, this phenomenon will not usually be noticeable unless there are prolonged rainy or dry periods, usually of weeks or months, depending on the land and soil characteristics. ‘The swelling of soil creates an upward force on the footings of the building, and shrinkage creates subsidence that takes away the support needed by the footing to retain equilibrium. Shear failure This phenomenon occurs when the foundation soil does not have sufficient strength to support the weight of the footing. There are two major post-construction causes: * Significant load increase. ¢ Reduction of lateral support of the soil under the footing due to erosion or excavation. * In clay soil, shear failure can be caused by saturation of the soil adjacent to or under the footing. GENERAL DEFINITIONS OF SITE CLASSES Class Foundation A Most sand and rock sites with little or no ground movement from moisture changes SI Slightly reactive clay sites with only slight ground movement from moisture changes M Moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes H Highly reactive clay sites, which can experience high ground movement from moisture changes [ Extremely reactive sites, which can experience extreme ground movement from moisture changes AtoP Filled sites P Sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise
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Tree root growth Trees and shrubs that are allowed to grow in the vicinity of footings can cause foundation soil movement in two ways: * Roots that grow under footings may increase in cross-sectional size, exerting upward pressure on footings. * Roots in the vicinity of footings will absorb much of the moisture in the foundation soil, causing shrinkage or subsidence. _Unevenness of Movement The types of ground movement 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 construction. ° Differing moisture content of foundation soil prior to construction. Movement due to non-construction causes is usually more uneven still. Erosion can undermine a footing that traverses the flow or can create the conditions for shear failure by eroding soil adjacent to a footing that runs in the same direction as the flow. Saturation of clay foundation soil may occur where subfloor walls create a dam that makes water pond. It can also occur wherever there is a source of 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 of the building firsc, then gradually spreads to the interion The swelling process will usually begin ac 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 Soil 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 gap or cantilevering until the bricks or the mortar bedding fail. Older masonry has little resistance. Evidence of failure varies according to circumstances and symptoms may include: * Step cracking in the mortar beds in the body of the wall or above/below openings such as doors or windows. * Vertical cracking in the bricks (usually bur not necessarily in line with the vertical beds or perpends). Isolated piers affected by erosion or saturation of foundations will eventually lose contact with the bearers they support and may tilt or fall over. The floors that have lost this support will become bouncy, sometimes rattling ornaments etc. Seasonal swelling/shrinkage in clay Swelling foundation soil due to rainy periods first lifts the most er | extremities of the footing system, then the remainder of the perimeter footings while gradually permeating inside the building footprint ro 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 thar 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 oor can be bouncy. Externally there may be visible dishing of the hip or ridge lines. As the moisture absorption process completes irs journcy to the innermost areas of the building, the internal footings will rise. If the spread of moisture is roughly even, it may be thar the symptoms 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 stri footings or piers under walls, creating noticeable doming of flooring. a) oat Wall cracking due to uneven footing settlement 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 of lowering the external footings, The doming is accentuated and cracking reduces or disappears where it occurred because of dishing, but other cracks open up. The roof lines may become convex. Doming and dishing are 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 is toward dishing. Movement caused by tree roots In general, growing roots will exert an upward pressure on footings, whereas soil subject wo drying because of tree or shrub roots will tend to remove support from ander 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 co another, The net result of all these forces is usually rorational. This resultant force often complicates the diagnosis because the visible symptoms 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. It will attempt to 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 iintil the original cause has been remedied, or until the subsidence has completely neutralised the affected portion of footing and the structure has stabilised on other 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 not be exactly reversed, and it is also usual chat brickwork will sectle in its new position and will resise-the forces trying co return it to 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 that the cracking will become wider until the sections of brickwork become virtually independent. With repeated cycles, once the cracking is established, if there is no other complication, it is normal for the incidence of cracking to stabilise, as the building has the articulation it needs to cope with the problem. This is by no means always the case, however, and monitoring of cracks in walls and floors should always be treated seriously. Upheaval caused by growth of tree roots under footings is not a simple vertical shear stress, There is a tendency for the root 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 brick- work 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 that 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 any case, externally visible cracking is important as a guide to stresses on the structure generally, and it 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 swell/shrink 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 cause a footing to fall away, this can double the span which a wall must bridge. This additional stress can create cracking in wall linings, particularly where there is a 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 or all of the above symptoms for a considerable period. The same warning period should apply in the case of upheaval. It should be noted, however, that where framed buildings are supported by strip footings there is only one 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 of roof, the building can be expected to behave as a framed structure, except that the external masonry will behave in a similar way to 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 a 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 etc. * 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 that 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 is a cosmetic nuisance only and can be kept in repair or even ignored. The table below is a reproduction of Table C1 of AS 2870. AS 2870 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. Where building movement 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 gully 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 result 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 run 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 trench 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 protonged rain. If necessary, a grated drain system connected to the stormwater collection system is usually an easy solution. It 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 It is essential to 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 most 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 CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS Description of typical damage and required repair Approximate crack width Damage limit (see Note 3) category Hairline cracks <0.1 mm 0 Fine cracks which do not need repair <l mm 1 Cracks noticeable but easily filled. Doors and windows stick slightly <5 mm 2 Cracks can be repaired and possibly a small amount of wall will need 5-15 mm (or a number of cracks 3 to be replaced. Doors and windows stick. Service pipes can fracture. 3 mm or more in one group) Weathertightness often impaired Extensive repair work involving breaking-out and replacing sections of walls, 15-25 mm but also depend 4 especially over doors and windows. Window and door frames distort. Walls lean on number of cracks or bulge noticeably, some loss of bearing in beams. Service pipes disrupted
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Clump of trees; height selected for distance from house — |. Drained pathway Carport Path—+ Garden bed covered with [2 ay) mulch pment gp inca arrears BUTT ete areenri ; a areata should extend outwards a minimum of 900 mm (more in highly reactive soil) and should have a minimum fall away from the building of 1:60. The finished paving should be no less than 100 mm below brick vent bases. It is prudent to relocate drainage pipes away from this paving, if possible, o-ayoid complications from future leakage, If this is not pesaics. earthenware pipes should be replaced by PVC and ackfilling should be of the same soil type as the surrounding soil and compacted to the same density, Except in areas where freezing of water is an issue, it is wise to remove taps in the building area and relocate them well away from the building — preferably not uphill from it (see BTF 19). It may be desirable to install a grated drain at the outside edge of the paving on the uphill side of the building. IF subsoil drainage is needed this 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 thete is little clearance berween the floor and the ground. Condensation adds ro the moisture already present in the subfloor and significantly slows the process of drying our. Installation of an adequate subfloor ventilation system, either natural or mechanical, is desirable. Warning: Although this Building Technology File deals with cracking in buildings, it should be said thar subfloor moisture can result in the development of other problems, notably: © Water that is transmitted into masonry, metal or timber building elements causes damage and/or decay to those elements. * High subfloor humidity and moisture content create an ideal environment for various pests, including termites and spiders. ¢ Where high moisture levels are transmitted to 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 to inhabitants, particularly those who are abnormally susceptible to respiratory ailments. The garden ‘The ideal vegetation layout is to haye lawn or plants that require only light watering immediately adjacent to the drainage or paving edge, then more demanding plants, shrubs and trees spread out in that order. Overwatering due to misuse of automatic watering systems is a common cause of saturation and water migration under footings. If it is necessary to use these systems, it is important to remove garden beds to a completely safe distance from buildings. Existing trees Where a tree is causing a problem of soil drying or there is-the existence or threat of upheaval of footings, if the offending roots are subsidiary and their removal will nor significantly damage the tree, they should be severed and a concrete or metal barrier placed vertically in the soil to prevent furure root growth in the direction of the building. IF it is not possible co remove the relevant roots without damage to the tree, an application to remove the tree should be made to the local authority. A pradent plan is to transplant likely offenders before they become a problem. Information on trees, plants and shrubs State departments overseeing agriculture can give information regarding root patterns, volume of water needed and safe distance from buildings of most species. Botanic gatdens are also sources of information. For information on plant roots and drains, see Building Technology File 17. Excavation Excavation around footings must be properly engineered. Soil supporting footings can only be safely excavated at an angle that allows the soil under the footing to remain stable, This angle is called the angle of repose (or friction) and varies significantly between soil types.and conditions. Removal of soil within the angle of repose will cause subsidence. peomesiauce Where erosion has occurred that has washed away soil adjacent to footings, soil of the same classification should be introduced 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 specialist consultant. Where isolated footings rise and fall because of swell/shrink effect, the homeowner may be tempted to alleviate floor bounce by filling the gap that has appeared between the bearer and the pier with blocking. The danger here is thar when the next swell segment of the cycle occurs, the extra blocking will push the floor up into an accentuated dome and may also cause local shear failure in the soil. IF it is necessary to use blocking, it should be by a pair of fine wedges and monitoring should be cartied out fortnightly. This BTF was prepared by John Lewer FAIB, MIAMA, Partner, Construction Diagnosis. The information in this and other issues in the series was derived from various sources and was believed to be correct when published. The information is advisory. It is provided in good faith and not claimed to be an exhaustive treatment of the relevant subject. Further professional advice needs to be obtained before taking any action based on the information provided. Distributed by CSIRO PUBLISHING PO Box 1139, Collingwood 3066, Australia Freecall 1800 645 051 Tel (03) 9662 7666 Fax (03) 9662 7555 www.publish.csiro.au Email: [email protected] © CSIRO 2003. Unauthorised copying of this Building Technology file is prohibited
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LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)