Council archive · source document
Attachment 2- soil stability report · MCU14/0017
Attached to the e-mail MCU14/0017 Detached house 26/512 David Low Way Copeland.
Report No. 13N 682
PROPOSED RESIDENCE,
LOT 26 NOOSA DUNES COURT,
CASTAWAYS BEACH
SLOPE STABILITY ASSESSMENT
FOR
MR. M. HAGGART
GTS Report Number 13N 682
Sept 2013
1
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Report No. 13N 682
TABLE OF CONTENTS
1. INTRODUCTION ..................................................................................................................... 3
1.1 Details of the Proposed Development ............................................................................... 3
1.2 Scope of Investigation ...................................................................................................... 4
1.2 Qualifications of Responsible Person................................................................................ 4
2. EXISTING CONDITIONS ..................................................................................................... 5
2.1 Local Geology................................................................................................................... 5
2.2 Topography ...................................................................................................................... 6
2.3 Vegetation ........................................................................................................................ 6
2.4 Groundwater ..................................................................................................................... 6
2.5 Drainage & Erosion........................................................................................................... 6
2.6 Existing Structures ............................................................................................................ 6
3. GEOTECHNICAL RECOMMENDATIONS FOR CONSTRUCTION ..................................... 7
3.1 Earthworks........................................................................................................................ 7
3.2 Drainage ........................................................................................................................... 8
3.3 Groundwater ..................................................................................................................... 9
3.4 Footing Design ................................................................................................................. 9
The above should also be considered should future excavation be required on adjacent sites to
facilitate construction. ................................................................................................................ 10
4. ASSESSMENT OF EXISTING SLOPE CONDITION ............................................................. 11
4.1 Slope Stability Assessment............................................................................................. 11
5. CONCLUSIONS .................................................................................................................... 12
6. IMPORTANT NOTES ABOUT THIS REPORT ...................................................................... 13
Table of Figures
Figure 1 – Aerial Photograph of Lot 26 Noosa Dunes Court, Castaway’s Beach - Sunshine Coast Council Mapping System ‘My
Maps’..................................................................................................................................................................................................................3
List of Tables
Table 1 - Summary of Borehole Logs & Interpreted Conditions from Dynamic Cone Penetrometer Testing ....................................5
Table 2 - Estimated Design Parameters for Bored Piles ............................................................................................................................9
Table 3 – Risk Level Implications ................................................................................................................................................................11
Table 4 – Qualitative Risk Assessment for Property ................................................................................................................................12
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Report No. 13N 682
Ref: 13N 682
12 September 2013
Mr. M. Haggart
c- Amelia Loftus
Loftus Studio
Dear Mark,
1. INTRODUCTION
Subsequent to your request, Geotechnical Testing Services has undertaken a slope stability
assessment for the proposed residential development at Lot 26 Noosa Dunes Estate, 512 David
Low Way, Castaways Beach.
The purpose of this report is to present the findings of this slope stability assessment and provide
geotechnical recommendations for the development.
1.1 Details of the Proposed Development
It is understood that the proposed residence will comprise a three story dwelling constructed of a
combination of lightweight materials and masonry walls. The proposed lower ground floor and pool
will be partially excavated below the natural ground surface. A paved driveway will access the
garage that is expected to be constructed partially below existing ground level.
Figure 1 – Aerial Photograph of Lot 26 Noosa Dunes Court, Castaway’s Beach - Sunshine Coast Council Mapping System ‘My
Maps’
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Report No. 13N 682
1.2 Scope of Investigation
As part of the slope stability assessment for this site, Geotechnical Testing Services undertook a
desk-top study comprising review of:
Site Classification Report (Job Number: 13/0534) prepared by Global Testing on 23 August
2013
1:100,000 Gympie Special Geological Map (Sheet 9445 & Part 9545) published by the
Department of Mines and Energy in 1999
Sunshine Coast Council Landslip Hazard Mapping
Aerial Photography of the subject area
In addition to the above, a site visit was undertaken on 2 September 2013. This site visit involved a
site walk-over to assess topography, vegetation, drainage, geology, erosion and slope stability.
This walk-over was undertaken by an experienced engineering geologist from our Noosaville
office.
Information relevant to this site and presented in published sources has been correlated with
observations from the site walk-over to provide a description of the existing conditions at the site
and have been included in the following section.
1.2 Qualifications of Responsible Person
This report has been prepared by Scott Prince, a Senior Engineering Geologist with over 9 years
geotechnical engineering experience, with significant demonstrable exposure to slope stability
assessment including design, remediation and investigation in mining and civil environments.
Review of this report has been undertaken by Mr. Ashley Griffiths an RPEQ with more than 14
years geotechnical experience including slope stability assessment in mining and civil construction.
Ashley has worked professionally with Scott for more than 8 years and can attest to his
competency in slope stability works.
Geotechnical Testing Services are a multi-disciplinary company specialising in geotechnical
consultancy and testing. They have been providing geotechnical engineering consulting services to
the civil and mining industries for more than 10 years with demonstrated experience with Sunshine
Coast Council on the Noosa and Buderim Landfill sites.
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Report No. 13N 682
2. EXISTING CONDITIONS
2.1 Local Geology
Regional geological maps indicate that the site is underlain by Quaternary age Holocene parabolic
dunes comprised of predominantly quartz sands.
These general conditions were confirmed by the site classification boreholes and during the site
walkover.
Three (3) boreholes were undertaken at the site to assess subsurface conditions. Auger refusal on
rock was not encountered in any of the boreholes which were terminated at 3.0m depth below
existing surface level.
A summary of the site investigation undertaken by Global Testing on 22 August 2013 is provided
below.
Table 1 - Summary of Borehole Logs & Interpreted Conditions from Dynamic Cone Penetrometer Testing
Material Assumed
Description Geological Origin Borehole No. 1 Borehole No. 2 Borehole No. 3
Silty SAND -
Aeolian 0 – 0.6m 0 – 1.9m 0 – 1.9m
Loose
Silty SAND –
Aeolian 0.6 – 3.5m 1.9 – 3.3m 1.9 – 3.6m
Medium Dense
Silty SAND -
Aeolian 3.5 – 3.8m 3.3 – 4.4m 3.6 – 4.1m
Dense
Silty SAND – Very
Aeolian 3.8 – 4.4m - 4.1 – 4.4m
Dense
Sub-surface conditions interpreted from boreholes at the site indicate a consistent soil profile of
Aeolian sands from surface to approximately 3.0m depth where boreholes were terminated.
Below 3.0m depth, additional data on the in-situ density of soils was collected/interpreted using a
Dynamic Cone Penetrometer (DCP). DCP testing was undertaken adjacent to each borehole
location to maximum depths of 4.3m.
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Report No. 13N 682
While the general profile has been interpreted as being relatively uniform, some variation may be
present in areas not covered by this investigation.
2.2 Topography
The site is located on the eastern side of Noosa Dunes Court and slopes relatively consistently
towards the east at gradients of approximately 15%. Minor undulations at the surface may be
attributed to the removal of trees/vegetation.
2.3 Vegetation
The site is covered with patchy/clumpy low vegetation comprising grasses and shrubs. Small to
medium sized trees (2m – 5m height) are located across the site, and are all generally inclined
towards the west. This inclination is likely the effect of prolonged exposure to predominant easterly
sea breezes.
2.4 Groundwater
Ground conditions at the site were observed to be dry to moist, with no surface water or springs
noted.
2.5 Drainage & Erosion
The drainage of the natural slope is towards the east. Due to the relatively free draining nature of
the near surface soils, it is expected that only minor surface run-off will traverse the site from the
adjoining sites to the north and south, and only during extended periods of very high rainfall.
2.6 Existing Structures
A low rock retaining wall and garden were observed along the north western street frontage of the
site. No other existing structures were observed across the site.
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3. GEOTECHNICAL RECOMMENDATIONS FOR CONSTRUCTION
In addition to the implementation of good hillside building practices outlined in “Some Guidelines
for Hillside Construction” (which has been provided as an attachment to this report), GTS also
provide the following recommendations and geotechnical constraints for the construction of the
proposed residence.
3.1 Earthworks
Drawings provided by Loftus Studio indicate that the proposed residence will comprise a three
story dwelling, constructed using a combination of lightweight materials and masonry walls,
predominantly on or below ground level.
Based on the drawings provided (Loftus Studio Project No. 1301, WD2.1) some excavation and
filling will be required to facilitate construction of the lower ground and ground floors.
These excavations are expected to be no greater than 1.5m to 2.0m depth with the deepest
excavation adjacent to the proposed stair landing.
Borehole information for the site suggests that excavations should be expected to encounter loose
to medium dense silty sand to a minimum of 3.0m depth. Based on local geological mapping, it is
reasonable to assume that this material extends to depths greater than the extent of borehole
investigations.
While these materials can be easily excavated, excavations in this material are expected to exhibit
extremely poor stability even in the short term. Subsequently, it is recommended that all
excavations/batters be constructed to the following maximum grades:
Where temporary batters are required,
- 1V:1H - For all excavations in silty sands.
Where permanent batters are required the following maximum grades should be adopted:
- 1V:2H - For all excavations in silty sands.
Grade recommendations may be increased should ground conditions during construction be more
favourable than those expected based on the results of the site investigation.
All permanent slopes should be protected against erosion in the long term by the planting of
vegetation or the installation of appropriate erosion protection.
No personnel should be permitted to enter excavations greater than 1.5m vertical height unless the
excavations are battered or shored appropriately. Personnel should also exercise caution when
working in close proximity (<1m) of cut slopes due to the risk of slope instability especially during
or after periods of wet weather.
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It is further recommended that cut slopes be inspected at intervals during construction by a suitably
experienced geotechnical professional.
It is expected that spoil excavated on site will be re-used to construct the fill platforms proposed
under the garage and pool.
Based on the borehole logs, no material of maximum particle size greater than 100mm is
expected. If material greater than 100mm diameter is encountered this should be removed prior to
the placement of fill. All fill should be placed in uncompacted layers no greater than 200mm
thickness. Layers should be compacted with either a small roller or compactor to a minimum
density index of 70% (AS1289 5.6.1) for the relatively cohesionless soils expected at the site.
Compaction testing of the backfilled material should be undertaken at regular intervals in
accordance with Australian Standard AS3798-2007 “Guidelines on Earthworks for Commercial and
Residential Developments”.
All filling should be benched into the natural soil with layers over-filled horizontally by 1m and
trimmed back to the required profile ensuring finished surfaces are well-compacted.
Any fill placed greater than 1m height should be retained by an engineer designed and approved
retaining structure founded in dense silty sand at depths greater than 3.6m below surface.
3.2 Drainage
While the sandy nature of the soils encountered during the borehole investigation should be
relatively free draining it will still be necessary to ensure adequate drainage to direct surface water
away from footings of proposed structures. This could be achieved by the construction of shallow
spoon drains at the crest of cut slopes and batters which are directed away from construction
areas and towards natural drainage (gullies).
Some sediment and erosion management in the form of sand bags, sediment socks or temporary
retention/settlement pits may be required to minimise surface erosion and the amount of
suspended sediment being directed into the natural drainage.
Where retaining structures are to be constructed, it is recommended that subsurface drainage be
installed behind these structures to mitigate the development of hydrostatic pressure in the
retained soils. Adequate subsurface drainage could comprise the installation of slotted PVC
agricultural pipe wrapped in a suitable filter „sock‟ of geotextile or similar and placed in a free
draining gravel backfill.
Should surface seepage be encountered during or following construction it is recommended that
this flow is also directed away from all structures towards stormwater pits or street-side drainage.
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3.3 Groundwater
Groundwater is not expected to be of concern at the site due to the relatively free draining nature
of the in-situ sandy soils.
3.4 Footing Design
It is recommended that the proposed residence and all associated structures including the pool
and garage be founded in dense silty sand or better, expected at depths greater than 3.6m below
natural ground level.
In accordance with good hillside building practice, and to provide lateral resistance to potential
downslope soil movement, it is recommended that steel reinforced concrete bored piles be
adopted for the foundation of the residence at this site.
Based on the results of the site investigation undertaken by Geotechnical Testing Services it is
recommended that the following estimated parameters be adopted for design of the proposed
footings:
Table 2 - Estimated Design Parameters for Bored Piles
Ultimate Unit Stresses (kPa)
Founding Material Description End Bearing Shaft Adhesion
Silty Sand - Dense 300 -
In order to adequately assess the pile capacity, a geotechnical strength reduction factor (Φg)
should be applied to the ultimate unit stresses estimated above in accordance with AS2159-2009 –
“Piling – Design and Installation). The geotechnical strength reduction factor (Φg) will depend on
various assessed factors associated with this site including the geological complexity of the site,
extent, amount and quality of geotechnical data, design experience in site geology and method of
assessment for assigned geotechnical parameters.
Based on general assumptions about the use of bored piles at the site, it is assessed that a
geotechnical strength reduction (Φg) factor of between 0.40 and 0.47 be adopted for preliminary
pile design purposes at the site. It is however the responsibility of the designer to make an
appropriate assessment of the geotechnical strength reduction factor (Φg) based on their risk
circumstances.
This assessment is based on the assumed absence of any pile load testing of constructed piles at
the site. Should pile load testing be undertaken a higher geotechnical strength reduction factor
may be adopted in accordance with AS2159-2009 – “Piling – Design and Installation.
Shaft adhesion should not be considered in the material encountered at this site.
As a general rule, a length to diameter ratio (L/D) of 4 or greater is recommended for the socket
length into the recommended founding material (dense silty sand).
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Upon excavation of piles, should founding conditions differ from those described in the
geotechnical investigation, GTS should be notified immediately and a site visit undertaken to
assess the likely impact of these conditions on footing design and whether changes to the design
are necessary.
Following the drilling of bored piles, the base and sides of all excavations should be adequately
cleaned of loose material and inspected by a suitably qualified professional.
To minimise the risk of detrimental interaction between foundation loads and existing or proposed
structures or services, no foundation should be constructed in the zone defined as:
1m horizontally from any existing or proposed structure or service and an imaginary line at
45° from the base of this structure or service to ground level.
The above should also be considered should future excavation be required on adjacent sites to
facilitate construction.
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Report No. 13N 682
4. ASSESSMENT OF EXISTING SLOPE CONDITION
This site has not been classified under the Sunshine Coast Council (SCC) Landslip Hazard
mapping.
While this mapping would generally provide an indication of the potential for slope instability in the
area it should be understood that this site specific investigation will provide more relevant data for
the purposes of this assessment.
4.1 Slope Stability Assessment
To adequately assess the potential risk of slope instability at the site, it is necessary to quantify
levels of risk and determine their potential implications for the proposed development. The AGS
2007 “Practice Note Guidelines for Risk Management” define these levels according to the table
below.
Table 3 – Risk Level Implications
RISK LEVEL EXAMPLE IMPLICATIONS (1)
Unacceptable without treatment. Extensive detailed investigations and research,
planning and implementation of treatment options essential to reduce risk to Low;
VH VERY HIGH RISK may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and
H HIGH RISK implementation of treatment options required to reduce risk to Low. 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 implementation of treatment options top
M MODERATE RISK reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulator's. Where treatment has been required to reduce
L LOW RISK the risk to this level, ongoing maintenance is required.
VL VERY LOW RISK Acceptable. Manage by normal slope maintenance procedures.
(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.
By considering the existing site conditions outlined in Section 2 and assuming implementation of
the geotechnical recommendations for construction detailed in Section 3 we can provide the
following qualitative assessment of risk of potential landslip for the proposed development.
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Report No. 13N 682
Table 4 – Qualitative Risk Assessment for Property
POTENTIAL Assessed
HAZARD Likelihood Consequence Risk Comment
The likelihood of this outcome is assessed as
Unlikely due to the predominant subsurface
conditions and the relative absence of
groundwater likely due to the free draining
Shallow failure in nature of the subsurface soils. A consequence of
surface soils above the Unlikely Insignificant Very Low Insignificant has been assigned as the impact of
foundation zone this on the recommended steel reinforced
concrete bored piles will be negligible due to the
founding into dense silty sand at depth. In
accordance with AGS 2007 the assessed risk is
therefore Very Low.
The likelihood of failure through the dense sand
is assessed as Rare due to the estimated
strength parameters of the material assessed by
in-situ testing and the free draining nature fot he
Deep failure through
material. A consequence of Major has been
weathered rock below
the base of piled
Rare Major Low assigned as the impact of this type of failure on
the proposed residence and potentially
foundations
surrounding residences could require significant
stabilization and repair works at a cost close to
the value of the property. In accordance with
AGS 2007 the assessed risk is therefore Low.
An explanation of the terminology adopted from “Practice Note Guidelines for Landslide Risk
Management” AGS 2007 and utilized in the tables above has been provided as an attachment to
this report.
5. CONCLUSIONS
As a general rule, regulators such as Sunshine Coast Council require that developments are
assessed as having a Very Low Risk to Low Risk of landslip affecting the property for
development approval. Based on the findings outlined previously, we assess that there is a Low
Risk of slope instability affecting the proposed residence provided that the geotechnical
recommendations included in this report are implemented.
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Report No. 13N 682
6. IMPORTANT NOTES ABOUT THIS REPORT
This assessment has been carried out in goodwill and under the instructions of Mr. Tony Walton on
behalf of Mr. Mark Haggart. The assessment has been undertaken with the care and skill of
competent personnel as defined within Geotechnical Testing Services quality system. It is not a
comprehensive investigation but a guide to the slope conditions at the site as defined in the
proposal.
Where assessment has been undertaken based on data supplied by the Client or other published
sources it is assumed that this information is correct. No responsibility is accepted by Geotechnical
Testing Services for inaccurate, false, incomplete or misleading data supplied by the Client or
others.
This assessment by Geotechnical Testing Services has been prepared based on information
available at the time of reporting and should not be considered an assessment of all potential
conditions at the site. Variations in conditions may be encountered across the site and between
known investigation locations which have not been reported in this assessment.
Strength characteristics of soils often exhibit a large variation between wet and dry conditions. Soil
characteristics of a soil profile are given on the soil conditions at the time of the investigation or
reporting by others. Further investigation and assessment may be required to account for seasonal
changes in strength characteristics of the soil profile.
This document has been prepared for Mr. Mark Haggart, and hence no responsibility or liability is
being accepted to any third party, where any part of the report is used in either isolation or without
consideration of the whole document.
This document is not appropriate where there has been a significant change in the project or either
for the specific needs of the reader.
Please, don‟t hesitate to contact the undersigned if you require any further information or
assistance.
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GTS Report No. 13N 682 is OTEGHNIGAL STING SERVICES P Yours Sincerely, Geotechnical Testing Services ‘Pty Ltd Az_= Scott Prince (BSc (Applied Geology)) Senior Engineering Geologist Checked: Mr Ashley Griffiths For Creative Mined Enterprises RPEQ No. 11160 4A
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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
91 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)
-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.
92 Australian Geomechanics Vol 42 No 1 March 2007
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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
Page 18
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 114 Australian Geomechanics Vol 42 No 1 March 2007
Used in these records
Attachment 2- soil stability report.pdfRelated documentApplication case desk ↗
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