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Slope Stability Assessment Existing Earthworks - OPW21-0279
17 September 2021 Project No. 21354-001-Rev0 Anthony Kelly Email: [email protected] SLOPE STABILITY ASSESSMENT EXISTING EARTHWORKS 91 LAWNVILLE ROAD, COOROY Dear Anthony, 1.0 INTRODUCTION Tectonic has undertaken a slope stability assessment for existing earthworks at 91 Lawnville Rd, Cooroy. 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 Very Low Risk of slope instability affecting the areas where earthworks have been undertaken, 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 1 on RP223270 and covers a total area of 15.67 ha. The site is accessed via a driveway from Lawnville Road, downslope to the west. The surrounds feature similar rural acreage properties. Text Figure 1 below illustrates the site position. A detailed description of the site features is given in Section 2. Existing residence Lot boundary Text Figure 1: 2019 aerial image of site and close 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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Anthony Kelly 21354-001-Rev0
17 September 2021
There were four general areas found to form part of the earthworks already undertaken at the time of our
assessment. Broadly speaking these were as follows:
A driveway diversion running adjacent to the slope contours off the existing asphalted main driveway
to the south-west. Earthworks were limited to surface stripping, and installation of swale drains to
either side of the driveway extension.
A southern area where minor, mostly cut, earthworks had been completed to form a level to gently
sloping area of cleared ground.
Formalisation of a pre-existing historical track feature had also been undertaken connecting the
southern cleared area to the northern cleared area (see below). This was sidelong to the sloping
ground (parallel to slope contours). Earthworks had been limited to initial surficial stripping, and
cutting of a drainage ditch along the upslope (western) side of the alignment.
A northern cleared area was present to the northern side of the existing residence, being mostly in
cut due to the topographical position, with a level to gently sloping finished surface.
All features as part of the existing earthworks assessed had been left with compacted roadbase materials at
surface, and we understand that asphalting of the driveway extension and formalised access track are
proposed to finish. All cut (and minor fill) batters had been seeded via hydromulching, with some sporadic
coverage of grass already present at the time of our assessment.
An indication of the location of the site features present is given below in Text Figure 2.
Approx. northern
cleared area
Approx. formalised
existing track
Approx. southern
cleared area
Approx. driveway diversion
Text Figure 2: Recent aerial image annotated with location of site features (courtesy Near Map, 15 May 2021)
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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 and aerial photos.
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 engineering geologist from Tectonic
on 14 September 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 26 years’ experience in
geotechnical engineering, including a number of slope stability projects.
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology and Subsurface Conditions
The 1:100,000 scale Gympie Special Geological Map 1 (Ref. Text Figure 3 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 3: Extract of Gympie Special Geology Map
Where minor exposures could be observed through hydromulched cut batters, it was noted that the
materials generally comprised pale grey to pale orange brown and pale red brown, silty clay, occasionally
with gravel. This is in line with the mapped geology at the site.
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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2.2 Topography
As illustrated in Text Figure 4 below, the topography over the wider site area is largely dominated by a well
defined, rounded ridgeline, extending from further south; to around 450 m north of the subject site boundary
where it dips down to meet Six Mile Creek. The western flank of the spur dips down from the crest generally
at around 12° to 15°, shallowing significantly towards Lawnville Road, where the ground transitions to
relatively flat alluvial ground associated with Six Mile Creek. The eastern flank dips more steeply at between
15° and 25° towards the invert of a prominent, and intermittently dammed gully line.
Localised gully in
flank
Six Mile Creek
Prominent gully
invert
Text Figure 4: Image showing ground surface contours (courtesy NC Interactive Maps)
As mentioned above in Section 1.1, there were four broad areas of the development undertaken thus far,
which are described individually below.
The driveway extension to the south-west of the site area ran across the ground slope contours, ranging
from 8°close to the existing driveway, and up to about 12° to 15° at the transition to the southern cleared
area. Drains had been formed in cuts, ranging from about 0.5 m to 1 m depth to either side of the driveway.
A view of the driveway extension is shown on the following page in Text Figure 5.
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Text Figure 5: Looking downslope over driveway extension
The southern cleared area was located over previously open ground to the south of an existing shed at the
property. Cuts in the order of about 0.6 m to 1 m depth had been undertaken to the edges of the area, with
toe drainage, or the camber of the surface, feeding to the west to the swales alongside the driveway
extension, or to the north-east to the formalised access track. Batters had mostly been left in the order of
about 15° to 20°. A view of this area can be seen below in Text Figure 6.
Text Figure 6: Looking downslope over driveway extension
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A search of historical aerial photos (Ref. www.qimagery.information.qld.gov.au) indicates that the alignment
of the track formalised as part of the recent earthworks may have been present from about 1989, and
possibly earlier, although the older available aerial images (1954 onwards) are not of sufficient resolution to
provide certainty on this.
It is understood that the recent earthworks have formalised the existing alignment, including formation of a
drainage swale on the upslope side. Recent earthworks have been minimal, with the historic earthworks
being mostly in cut (up to about 1 m), with shallow fill on the downslope (eastern) side being spread out on
the natural slope, mostly outwith the track corridor. The fill material was graded to the same approximate
gradient as the natural ground (≤ 25°).
This is shown below in Text Figure 7.
Text Figure 7: Looking south along northern (LHS) and southern (RHS) portions of the formalised existing track
The cleared northern area had exploited a very gently rounded, and relatively wide, natural crest of the
ridgeline running south-north through the site. Based on available contour information and observations on
site, it was estimated that the maximum depth of cut would be in the order of about 1.5 m to the south,
pinching out to the north-east and north-west. Material removed from the south of the cleared area had
been spread out in fill batters inclined at around 20° to 25° to the periphery. A slight fall to the north-eastern
corner was noted over the surface of the cleared area.
A view of this is shown below in Text Figure 8.
Text Figure 8: Looking south over northern cleared area
Across the four main areas forming the earthworks at the site, no irregular natural surface features were
noted in the ground adjacent, and no slumping or cracking of batters having been undertaken was noted at
the time of our assessment.
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2.3 Groundwater
No seepage or ‘springs’ were observed in the vicinity of the vicinity of the earthworks. Groundwater would
also be expected to be an unlikely occurrence, given the topographical position of the site at the crest of a
prominent ridgeline. Groundwater conditions may vary seasonally, and groundwater seepage could
develop, particularly following periods of prolonged or heavy rainfall.
2.4 Surface Drainage
The natural topography at the site of the earthworks means that there would be a very small catchment for
surface water affecting the earthworks undertaken. Rainfall to surface is therefore expected to be the main
source of waters flowing to the areas identified above in Section 1.1.
Naturally the waters would tend to be spread, and be distributed as sheet flow towards the snout of the spur
to the north, and over the flanks to the east and west.
Well defined drainage channels had been formed to either side of the driveway extension to the south-west
and the cleared southern area. These fed to a culvert at the transition from the pre-existing asphalt surfaced
driveway and the driveway extension, which had been rock lined around the culvert wing-walls, and
discharged to lower ground in the direction of Six Mile Creek. This is shown below in Text Figure 9.
Pre-existing driveway
Driveway extension
Text Figure 9: Culvert at transition to pre-existing driveway from the driveway extension
A drainage ditch had also been formed on the upslope side of the formalised trackway, which also took
waters from the southern side of the northern cleared area. Drainage was fed to a culvert crossing which
discharged to the natural gully which had formed in the eastern flank of the ridgeline spur running through
the site. Rock lining (including wrapping in geofabric) had been utilised upslope of the crossing point to the
north and south; and had been used at least 10 m downslope of the discharge point.
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Text Figure 10: Rock lining at culvert collection point on upslope side (LHS); and at discharge downslope (RHS)
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. Significant channelised erosion was not noted
over the existing earthworks, or on natural ground adjacent. It was also observed that the existing cut and fill
batters had been covered with a hydromulched grass seeding.
2.5 Vegetation
Several medium to large size trees were present to the south, north and east of the earthworks areas, with
none noted to feature irregular growth or tilting (which can be, but is not necessarily always, a sign of
relatively shallow ‘creep’ type instability).
Maintained grassed lawn areas were observed around the existing residence and shed, with grazed
paddocks fringing the earthworks areas otherwise.
2.6 Buildings and Other Structures
Structures present on site at the time of our assessment included the existing one storey masonry walled
residence, a sheetmetal walled shed to the south; and concrete fence posts directly adjacent to the
earthworks areas.
None of the structures present on site exhibited any signs of structural distress, or otherwise, which could
easily be attributed to global slope instability.
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
In the area of the earthworks undertaken at the site, the NC Landslide Hazard Overlay has assigned a risk
rating of Moderate to High. An extract of the NC mapping is given below in Text Figure 11.
Lot boundary
Text Figure 11: Extract from NC Landslide Hazard Overlay 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 (e.g. a higher risk has been allotted east of the site area, coincident with
the natural gully feature, with slightly steeper sideslopes to the adjacent ground), and is superseded by
site specific assessment works.
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 earthworks, with shallow batter slopes
and drainage already in place, as well as erosion control measures (revegetation). In addition, the
subsurface conditions appeared favourable with residual soils of the Kin Kin beds exposed consistently in
cut slopes observed around the site area.
There was also a significant offset of at least 10 m to any existing structures on the site (e.g. shed,
residence etc.). Downslope of the earthworks, no structures were present, and the subject site extended
well to the east and west. To the north, an offset of over 20 m was present between the toe of the fill
pushed out as part of the northern cleared area, and the nearest property boundary.
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.
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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.
Considering the existing site conditions (Ref. Section 2), and proposed development works, it is assessed that
there would be a Very Low (VL) Risk of global slope instability affecting the existing earthworks 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 cut
batter or fill embankment faces created as part of
the recent earthworks is considered Rare, given the
minimal depth/height of the works, shallow finished
1: Failure through cut or fill inclinations, revegetation of permanent slopes, and
Rare Insignificant Very Low
embankments drainage installed to date. The consequence of such
a failure would be Insignificant given the offset to
existing structures upslope and downslope and very
limited scope of remediation, with the resultant risk
being 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.
4.1 Site Layout and Geotechnical Constraints
The earthworks undertaken at the site have been minimal in scope, and largely exploit existing features,
aided by the topographical position close to the crest of a well rounded ridgeline. We understand that the
driveway extension, southern cleared area, and formalised trackway may be asphalted at surface in the
future, which would be acceptable from a slope stability perspective.
The below recommendations have been made as a minimum from a slope stability perspective, and it is
noted that the works extant are in general accordance.
4.2 Earthworks
No further cutting (with the exception for remediation works) or filling is anticipated within the scope of the
earthworks. If further significant earthworks are planned Tectonic must review potential impacts on slope
stability.
Ground surfaces should be graded to shed surface water runoff away from construction areas.
Permanent batter slopes in natural soil consisting of stiff (or stiffer) clays should be constructed at a
maximum grade of 1V:2H.
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. The existing roadbase materials present
should be sufficient from a trafficability perspective.
4.3 Drainage and Maintenance
Apart from the general advice provided above, implementation of the following specific risk minimisation
measures is recommended to maintain a Very Low Risk to property with regards the earthworks undertaken
at this site.
Ensure that the finished driveway surface has a cross-fall ‘into’ the hillside such that stormwater is
not allowed to flow in an uncontrolled manner onto the down slope.
Form a v-notch swale above the crest of the driveway cutting to divert stormwater away from the cut
batters to a point(s) where the collected stormwater may be transferred down the cut batter and into
the roadside table drain in a controlled manner (e.g. using a prefabricated or rock-lined chute[s]).
Install drainage to transfer stormwater flows from the driveway into the natural gully to the east, or in
the direction of Six Mile Creek to the west.
Protect approaches to, and any culvert inlets/outlets, from erosion by placement of geofabric and
rock fill, or similar.
Regularly inspect and clean out culvert inlets to prevent blockage by leaf litter, soil, vegetation, etc.
Revegetate any bare soil exposed by the works. Hydromulch has already been placed on soils
exposed in batters.
It is noted that measures in place at the site at the time of our inspection largely cover the above points,
however, attention to cambering the finished road surface approaching sealing should be considered. The
cleared southern and northern areas should also include a cross-fall (or sections of grading), at no less than
1V:100H to direct surface water to existing fringe drainage and minimise ponding of water at surface.
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5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Very Low Risk of slope instability
impacting the site as a result of the earthworks undertaken. 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
GOLDER ASSOCIATES PTY LTD
Robert Gibb BSc (Hons) Ashley Davey BEng (Geological) RPEQ 8159
Engineering Geologist 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 14
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
Page 15
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. 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. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 17
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)