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slope stability report
WORKING FOR YOUR PROJECTS SUCCESS SLOPE STABILITY ASSESSMENT Address : 27 Grant Street Noosa Heads Client : Brown Rooster Pty Ltd Date : 8 February 2021 Report No : 30378(SL)
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INTRODUCTION
SCG Consulting Engineers have been commissioned to provide a Slope Stability Assessment for the above
allotment. The assessment considers the existing state of the allotment and any possible effects that the
proposed works will have on this site.
The proposed works are detailed by Dennis Hill Architect in plans 049 dated 10/12/2020 and consists of a
proposed duplex consisting of a mix of slab on ground and suspended timber frame floors.
SITE DESCRIPTION
A desk top study of the site confirms the following:-
- The site is located in an area identified as a ‘Landslip Hazard’ on the Noosa Council’s Landslip Hazard
Map. A portion of the site is identified as having a ‘Moderate Landslip Risk’.
- Level contours provided on the proposed building plans indicate an average fall within the site of 20-25%.
SCG Consulting Engineers have conducted several soil tests along Grant Street. The following soils profile are
encountered.
: LOOSE SILTY SANDS INCREASING IN DENSITY WITH DEPTH
Our walkover confirmed the following characteristics-
Site Slope : A visual assessment of the surface grades within the allotment using a hand held
inclometer suggests grades in the order of 20% exist across the proposed
construction site. The measured grades are consistent with the contours indicated
on the proposed construction.
Vegetations : Vegetation consists of grasses and scattered trees.
Slope Shape/Location : The surface profile across the allotments slope faces are generally LINEAR
DIVERGENT.
Drainage : The surface drainage is good, sub-surface drainage is good. There was no
indication of ground water seepage or rising water pressure.
Stormwater flows from upslope of the allotment will be intersected and captured by
the road reserve.
Earthworks : There is no existing earthworks apparent.
Adjacent Structures : Adjacent structures on the neighbouring properties appear to be a mix of slab on
ground and suspended timber framed floors.
Evidence of previous : None apparent.
Soil Movements
LANDSLIP MECHANISMS
Landslip is generally defined as the movement of a mass of rock or soil down a slope.
It is generally considered to occur when the strength of the soil or rock mass to support itself is exceeded causing
movement to occur within the mass.
SCG Consulting Engineers 2 SCG Ref: 30378(SL)
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This loss of strength can occur as a result of numerous circumstances. These include, but are not limited to the
following:-
- Siesmic Activity
- Erosions effects such as wind or fluid scour
- Soil Saturation from rainfall events or defective drainage measures
- Construction/Earthworks activities
- Groundwater variations/movements
- Previous Landslips
The nature of landslips vary with site specific conditions and failure mechanisms that cause the slips to occur.
Appendix B provides some diagrammatic examples of various landslips.
The speed of landslips can vary greatly from millimetres per year to a number of meters in a matter of minutes.
Once again this characteristic is governed by site specific conditions and the failure mechanism causing the slip
to occur.
The allotment is situated within a massive aeolian sand bed of Quaternary age. As a general observation dunal
horizons such as these are stable where;
- The natural slope is less than the angle of repose for sand (30°)
- Vegetation exists to protect from wind generated scour
- The drainage pattern is such that there is no risk of fluvial scour.
These horizons can become unstable when aeolian or fluvial scour erode slopes to angles greater than 30°,
particularly when coincident with vegetation removal.
LANDSLIP HAZARD ASSESSMENT
For our assessment we utilize the Guidelines For Landslip Risk Management Framework as provided by the
Australian Geomechanics Society, in their journal of March 2007. Descriptions used by this framework can be
found appended to this report.
Using the Qualitive Risk Analysis Matrix (refer appendix) we have established the following characteristics for the
allotment.
Landslip Mechanism : Translational or Rotational Flow
We consider a potential landslip mechanism will consist of either a Rotational or Translational Landslide or Creep
movement within the sloping profile. Diagramation representation of these mechanisms are appended to this
report. We are of the opinion that this movement could occur within the Uncontrolled Fill or Gravelly Clays
profiles.
Likelihood of Landslip : Possible
This is due to the evidence during our site visit which includes the surface grade within the allotment and the
earthworks for the proposed construction.
Consequences to Property : Minor
With these factors considered we classify the allotment as:-
MODERATE RISK OF LANDSLIP
Although we consider the allotment has a moderate risk of landslip, building work undertaken on the site should
follow accepted methods which assist in maintaining slope stability.
Such measures are discussed in the following section.
SCG Consulting Engineers 3 SCG Ref: 30378(SL)
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HAZARD MITAGATION – CONSTRUCTION RECOMMENDATIONS We consider the primary risks for landslip occurring within this allotment is over excavation or, stormwater scour and vegetation removal. The construction of any cut/fill platforms within the site can lead to an increased risk of landslip. Any over excavated (steep) cuts may lead to landslip. Similarly, steep fill batters may be susceptible to slip or erosion. Any exposed fill batters are susceptible to weakening from saturating rains and stormwater scour. This risk is compounded, where poor compaction is achieved when placing the fill. Any fill batter profiles should be limited to a maximum grade of 1 to 3 (vertical to horizontal). Where possible the fill platform should be benched into the natural profile after the removal of all organic material including topsoils. The fill material should be placed and compacted to achieve a minimum relative field density of 95%. Surface stormwater runoff should be diverted around fill batters. Vegetating the fill batter can improve the batters ability to resist slip and or scour. Where these recommendations are unachievable, benching, shoring or retaining measures should be implemented. Any permanent exposed cut batters should be limited to a maximum slope of 1 to 2 (vertical to horizontal). The face should be protected from stormwater scour with cut off drains above the cut. Catch drains should also be provided along the base of the cut. The drains must dissipate all stormwater from above and below the cut. Vegetating the cut batter can improve the batters ability to resist slip and scour. Steeper batters should be retained with engineer designed retaining walls. All excavations must be adequately battered or retained to prevent collapse. Temporary excavations for retaining walls can have exposed cut batters of 1 to 1.7 (vertical to horizontal) taken from the base of the excavation. These earthworks must incorporate shallow surface spoon drains at the base and along the high side of the cut. Drains must have an efficient outflow down-slope and clear of the construction area. Any exposed soil face should also be protected against the potential for stormwater erosion. Saturation of the exposed cut batter from prolonged rainfall will potentially de-stabilise the cut batter. Programming of the works must consider this possibility. Temporary back propping of the slope may be required. Should these recommendations be un- achievable the builder should seek guidance from this office on temporary shoring, benching or other suitable techniques. The allotment will support a typical form of residential construction. Structural foundations should be designed by a competent person in accordance with AS 2870, accepted engineering theory and the geotechnical conditions. All earthworks should be constructed in accordance with AS3798-1996 ‘Guidelines on earthworks for commercial and residential developments’ and sound engineering theory. The builder must ensure the stability of neighbouring structures is maintained through not undermining their foundations. This is achieved via restricting any earthworks to above a line radiating downwards at an angle of 1 in 1.2 (vertical to horizontal) from the base of the foundation/structure to be protected. Retaining walls for landscaping purposes would need to be constructed with consideration of maintaining stability of the surrounding ground during construction. Such measures would include limiting the slope of any excavations to a slope of 1 vertical to 1.7 horizontal and protecting the works against stormwater scour. The retaining wall designs must allow for the lateral soil pressures expected including additional surcharging from sloping backfills. Although we consider the allotment stable in its current form there will always be a risk of landslip under severe stormwater conditions. Measures that will reduce this risk include the following but are not limited to. The finished ground surface should be leveled and graded to ensure no concentration of stormwater runoff leading to ground scour can occur. Any collected stormwater from areas other than the roof should be disposed of downslope from the proposed dwelling and should also ensure no ground scour can occur. SCG Consulting Engineers 4 SCG Ref: 30378(SL)
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All stormwater runoff from hard landscaped areas should be directed around the perimeter of the house and
contained within the site via diversion bunding, scour proof drains and field inlets. Where possible stormwater
runoff from roofed areas should be disposed of via piping to the road reserve via approved kerb adaptors.
Otherwise, piped roof stormwater runoff should be connected to the inter-allotment drainage along the rear of the
site.
It should be noted that concentrating stormwater infiltration into soil profiles can lead to instabilities in moderately
sloping areas.
No stormwater runoff should be allowed to flow over any exposed excavated embankment faces or filled batters.
All existing cross allotment flows from neighbouring properties must also be considered, managed and
maintained. Clear unhindered drainage paths must be provided and maintained during and beyond construction.
All constructed retaining walls must have effective drainage measures upslope and beyond the walls to relieve
any sub-surface water pressure/build-up. These drains must be maintained and flushed out regularly. To achieve
this we recommend bringing the upstream end of the drain to surface and capping off to allow future flush out.
We have attached guidelines for hillside construction extracted from Australian Geomechanics vol. 35 March
2000 – “Landslide Risk Management”. These references provide commentary on good design and building
practice.
We trust this meets with your requirements, however should you have any queries on the above matter please
contact the undersigned.
Yours faithfully
Sean Richardson RPEQ 5835
For and on behalf of
SCG CONSULTING ENGINEERS
Encs Appendix A - Some Guidelines for Hillside Construction
Appendix B - Landslip Mechanisms
Appendix C - Landslip Risk Assessment Matrix
SCG Consulting Engineers 5 SCG Ref: 30378(SL)
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APPENDICES A. SOME GUIDELINES FOR HILLSIDE CONSTRUCTION B. LANDSLIP MECHANISMS C. LANDSLIP RISK ASSESSMENT MATRIX
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX A - 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
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APPENDIX B
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
Australian Geomechanics Vol 42 No 1 March 2007
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
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
-2
20 years The event will probably occur under adverse conditions over the
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
-4 The event might occur under very adverse circumstances over the
10 10,000 years
design life.
UNLIKELY D
5x10-5 20,000 years
10-5 100,000 years
The event is conceivable but only under exceptional circumstances
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
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD 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)
B - LIKELY 10-2 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
E - RARE 10-5 M L L VL VL
F - BARELY CREDIBLE 10-6 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.
VL VERY LOW RISK Acceptable. Manage by normal slope maintenance procedures.
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only given as a
general guide.
Australian Geomechanics Vol 42 No 1 March 2007