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4895-Geotech Assessment
28 July 2021 Project No. 20362-001-Rev0
Mr Max Richardson
C/- Long View Consulting Engineers Pty Ltd
Email: [email protected]
SLOPE STABILITY ASSESSMENT
PROPOSED LOT 429
COOROY BELLI CREEK ROAD, BLACK MOUNTAIN
Dear Max,
1.0 INTRODUCTION
Tectonic has undertaken a slope stability assessment for proposed new Lot 429 on Cooroy Belli Creek
Road, 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
access and house site on Lot 429 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 proposed reconfiguration comprises a realignment of the boundaries of three existing rural allotments
described as Lot 429 on MCH482, Lot 49 on MCH1168 & Lot 158 on MCH1321 (Ref. Text Figure 1).
Existing houses
Proposed house site
Proposed house site access
Text Figure 1: Proposed lot reconfiguration plan (courtesy Max Watterson & Assoc. ref. 19-4895AB)
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 Max Richardson 20362-001-Rev0
C/- Long View Consulting Engineers Pty Ltd 28 July 2021
Existing houses with direct frontage to Cooroy Belli Creek Road will be contained within the proposed
reconfigured Lots 49 and 158. Within new Lot 429, a new access driveway is proposed to run from Cooroy
Belli Creek Road to a new house site, all located atop a ridgeline. As shown in sketch drawings supplied by
by Long View Consulting Engineers Pty Ltd (LVCE, ref. Text Figure 2), the south-eastern part of the access
driveway will follow an existing track alignment.
Proposed house site
Text Figure 2: Proposed driveway alignment plan (courtesy LVCE ref. 20.029, C01 & C02, Rev P1)
No earthworks details were available for the proposed driveway and house at the time of reporting. Given
that the first part of the driveway follows an existing track, thereafter running along a ridgeline, cut and fill for
the new access is not expected to exceed 1 m depth/height. It is anticipated that the new house may
comprise a slab on ground floor (at least in part), which would require some excavation and possible filling.
As the new house site will not have connections to town water or sewerage, on-site collection of rainwater in
tanks will be required together with on-site treatment and disposal of wastewater.
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 geotechnical engineer from Tectonic
on 17 November 2020.
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 prepared by Mr Ashley Davey, an RPEQ with more than 26 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 Map 1 (Ref. Text Figure 3 below) indicates that the site is
underlain by meta-sedimentary geology (arenite, argillite and phyllite) of the Kin Kin Beds (pale green
blue shading).
Approx. site location
Text Figure 3: Extract of Gympie Special Geology Map
Along a cutting near the proposed new access point on Cooroy Belli Creek Road and in the existing track
cutting along the proposed driveway, very low to medium strength, distinctly weathered phyllite was
exposed, confirming the mapped geology (Ref. Text Figure 4). Gravelly clay soil was observed to site
above the weathered rock to depths of less than 1 m. Such conditions are also expected at the proposed
house site, which is located in a similar geomorphological position as the observed cuttings.
Text Figure 4: Phyllite exposed along road cutting (LHS) and alongside the existing access track (RHS)
1
State of Queensland, Department of Mines and Energy, Gympie 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 5, the proposed house site and most of the proposed access driveway trace the
crest of a north-west/south-east trending ridgeline, with the south-eastern end of the proposed driveway
following an existing track just off the crest. The shape of the ground surface across these development
areas is strongly divergent, which is favourable from a slope stability viewpoint, whereas convergent gully
areas are subject to greater surface water, groundwater, and soil accumulation which is why landslides are
more prevalent in these landforms. The proposed house site and driveway alignment are well setback from
gullies. Inferred natural gradients measured using a hand-held clinometer along the proposed driveway and
at the house site typically ranged up to about 10°, but increased to 25° and steeper moving down the ridge
flanks. The existing road cutting near the proposed driveway connection was measured to dip at about 50°.
Proposed house site
Existing track
Text Figure 5: Image showing ground surface contours (courtesy Noose Council [NC] Interactive Maps)
Text Figures 6 and 7 below and on the following page illustrate the site conditions at the time of our
inspection.
Existing track
Text Figure 6: View looking north-west along existing track at south-eastern end of proposed driveway
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Text Figure 7: Westerly view along ridgeline from house site
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, either across or
adjacent to the proposed house site and driveway alignment. Groundwater conditions may vary
seasonally, and groundwater seepage could develop within permeable horizons in the soil and rock,
particularly following periods of prolonged or heavy rainfall. However, given the site position along a
ridge crest, the permanent groundwater table is expected to be quite deep, not adversely impacting on
the stability of the upper level soils.
2.4 Surface Drainage
Being along a rounded ridgeline, the proposed development areas have negligible surface water catchment
and natural ground surfaces will rapidly shed stormwater away into the adjoining gullies. No significant soil
erosion was noted during our walk-over inspection. Surface water is expected to drain well from the site
considering the favourable natural gradients and low permeability of the clay soils in the immediate
subsurface, and relatively shallow depth to rockhead. Ponding of surface water may occur on any levelled
building areas, and stormwater flows may concentrate along the driveway alignment, unless drainage
measures are installed (Ref. Section 4.5).
2.5 Vegetation
The immediate area of the proposed driveway alignment and house site consisted of periodically slashed
grass. Mature trees were noted scattered around the development areas, all of which were observed to be
relatively ‘plumb’ and exhibiting no signs of instability.
2.6 Buildings and Other Structures
Although no buildings were located near the proposed driveway and house site, post and wire fences in the
vicinity were observed to be well aligned, including across adjacent steep ground.
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The proposed house site and most of the proposed driveway alignment have been mapped by NC as
presenting no landslide hazard (presumably Very Low or Low Hazard areas), however the south-eastern end
of the proposed driveway partly traverses a Moderate hazard area (pale yellow brown shading). An extract
of the NC mapping is given below in Text Figure 8.
Proposed house site
Existing track
Text Figure 8: 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 did not exhibit any indicators of global, natural slope instability. No landslide back scarps,
tension cracks in the ground surface or areas of natural ‘hummocky’ ground were observed at or near the
proposed house site and driveway alignment. Exposed geology consists of shallow residual clay soils
overlying meta-sedimentary phyllite, which is not known for the development of extensive slope instability
in the region. Topography and drainage across the proposed development areas was favourable. A 2015
aerial photograph obtained from the NC Interactive Maps website (Ref. Text Figure 9) does show possible
small landslide features on steep ground near the gully floor to the south of, but well down slope from, the
proposed house site. These likely landslides are sufficiently setback from the proposed house site and
driveway so as to pose a negligible risk to the proposed development.
Proposed house site
Possible landslide features
Text Figure 9: 2015 aerial image courtesy of NC Interactive Maps
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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 and the
Qualitative Terminology for Use in Assessing Risk to Property which is attached.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation
Very High
VH of treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to
Risk
cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
H High Risk
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 investigation, planning
M Moderate Risk and implementation of treatment options to reduce 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
L Low Risk
maintenance required.
VL Very Low Risk Acceptable. Manage by normal slope maintenance procedures.
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 Low (L) Risk of global slope instability affecting the proposed driveway and house site as
described in Section 1.1 above, on new Lot 429 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
Given the presence of stiff residual soils at surface,
and phyllite materials below at shallow depth, the
apparent lack of groundwater, and no evidence of
1: Failure through natural soils existing instability in the proposed development
or proposed fill materials at Unlikely Medium Low areas, it is considered Unlikely that a shallow
relatively shallow depth (<2 m) landslip would develop, 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
strength of these materials, no evidence of such deep
2: Deep seated failure in Barely
Major Very Low instability, and the favourable topography across the
weathered rock Credible
development area. 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 designs for the house and driveway.
4.1 Site Layout and Geotechnical Constraints
The proposed house location and driveway alignment 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, or if cut or fill earthworks are to exceed the magnitudes
described in Section 1.1.
Any proposed fill on slopes exceeding 10° (18%) shall be fully supported by an RPEQ engineer designed
and certified retaining wall.
4.2 Earthworks
Any organic rich topsoil must be stripped from the construction areas and set aside for reuse across finished
surfaces (not to be reused in controlled fills). It is recommended that the subgrades be proof rolled using a
minimum 4 tonne rubber tyred vehicle or drum roller to identify any ‘soft spots’. Any such soft spots should
be over-excavated and replaced with compacted select fill.
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.
Fill materials should be compacted at moisture contents within the range of -2 % to +2 % of optimum
moisture content for Standard Compaction. Confirmatory compaction testing must be carried out at regular
intervals and further details for control and testing of fill are given in Australian Standard AS 3798-2007
Guidelines on Earthworks for Commercial and Residential Developments.
Fill should have a maximum particle size of 100 mm for an uncompacted layer thickness of 200 mm; and
shall be compacted by repeated rolling with a roller to achieve a dry density ratio of at least 95% of the
Standard Maximum Dry Density for cohesive soils, or 70% Dry Density Index for any imported cohesionless
soils. Any imported fill should comprise a good quality material meeting the following specifications:
Soaked CBR ≥ 10 %
Shrink-swell index (Iss) ≤1 %
Maximum particle size ≤ 100 mm
It is anticipated that site earthworks could generally be completed by medium size excavators (10 tonne to
20 tonne) with toothed bucket attachments. However, strong phyllite could be expected within 1 m of the
ground surface. Such materials may require the use of ripping attachments, or hydraulic breaking.
Temporary batters in natural soils of stiffer (or stiffer) consistency, and future controlled fill may be formed at
1V:1H; with permanent batters formed no steeper than 1V:2H up to a maximum of 2 m in height. Near
vertical temporary batters up to 2 m high would be permissible in very low strength (or stronger) phyllite, with
permanent batters to be no steeper than 1V:1H. Flatter batters may be required if groundwater seepage is
encountered. Permanent batters will require erosion protection such as covering with topsoil, mulch, erosion
matting (e.g. Jute mat) and vegetation. Where spatial constraints prevent adoption of the above
recommended batter slopes, cut or fill batters would need to be supported by retaining walls.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Any excavations required adjacent to existing structures, will need to be specifically designed to ensure that
the structures are not adversely affected (e.g. 'undermined').
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Ground surfaces should be graded to shed surface water runoff away from construction areas. It should be
noted that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion
and sediment control measures must be installed prior to commencement of earthworks at this site.
4.3 Footing Design
Footings for the proposed residence and any ancillary structures (e.g. retaining walls) must penetrate
through any future fill and residual soils to ‘socket’ at least 0.3 m into very low strength (or stronger) phyllite.
Normally dimensioned, conventional high-level footings would likely be appropriate in cut areas, extended
locally via the use of steel reinforced concrete bored piles (or ‘bucket piers’) in fill areas or where soil depths
are greater.
Footings could be designed with an allowable bearing pressure of 400 kPa very low strength (or stronger)
phyllite.
Footing design should also account for the characteristic surface movement (ys), to be advised by others
following completion of a ‘soil test’ for the residence in accordance with QBCC requirements.
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 10.
Text Figure 10: 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. Where piles are required, it is essential that the piling contractor have a ‘mud bucket’ or
similar to enable clean out of the pile bases. 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
must be notified immediately and an inspection carried out to determine if changes to footing design are
required.
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4.4 Retaining Structures
Any retaining walls must be founded as described in Section 4.3 above. Any proposed fill on slopes
exceeding 10° (18%) shall be fully supported by an RPEQ engineer designed and certified retaining wall.
Engineer design and certification of retaining walls is required where such walls:
Exceed 1 m height.
Are subjected to surcharge loads (e.g. from backslope, adjacent structures, etc.).
Are located within close proximity to existing structures.
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)
Stiff to Very Stiff
18 26 0.39 0.56 2.56
Natural Clays
Weathered Phyllite
23 35 0.27 0.43 3.69
Rock
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 Drainage
During construction temporary swales and diversion drainage may be required to divert seepage and/or
runoff away from the building and driveway areas, to prevent saturation of subgrades.
Surface diversion drainage should be constructed above the crest of any cut or fill embankment (e.g.
grassed or landscaped swales, diversion mounds or graded surfaces). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building and retaining wall footings (e.g.
positive gradients and/or channel drains), and also not allowed to flow over fill embankments.
Uncontrolled flow of stormwater over fill batters (e.g. from gutter overflow during high intensity storms) has
the potential to saturate and weaken the fill, potentially leading to slumping failure of the fill. Future building
and landscaping must be carried out to maintain good drainage.
Along the driveway, the finished surface should grade into a ‘table drain’ cut into the up slope side, with
collected waters directed to discharge at regular intervals via culverts under the alignment. If an unsealed
gravel pavement is adopted, it would be prudent to install water bars at regular intervals on steeper sections
to intercept long stormwater flow paths and prevent erosion of the driveway surface.
Subsurface drainage must be installed behind the retaining wall (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.
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All collected waters (including overflow from rainwater tanks, driveway culvert discharge, etc.) must be
directed by pipes or turf/rock-lined channels and not onto any cut or fill batter to discharge at least 5 m down
slope of any structures and earthworks, with the outlet zone protected by stacked rock or grouted stone
pitching.
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).
It is anticipated that land application of treated wastewater from the proposed residence will likely take place
further west of the proposed house site on the ridgeline. It is recommended that surface irrigation of
wastewater should be adopted and that irrigation should only occur on slopes of 20% or less.
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 development. 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
Ashley Davey BEng (Geological) RPEQ 8159
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 13
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 14
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 16
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)