Council archive · source document
Geotechnical Stability Report · OPW24/0017
31 January 2024 Project No. 24008-001-Rev0 GMW Projects Pty Ltd Attention: Greg Ward Email: [email protected] SLOPE STABILITY ASSESSMENT – PROPOSED EXTENSIONS & CARPORT 38 PLOVER STREET, PEREGIAN BEACH Dear Greg, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for the proposed extensions and carport at 38 Plover Street, Peregian Beach. This report presents the results of our slope stability assessment together with geotechnical advice for the project. In summary, subject to the implementation of the recommendations made herein, it is assessed that there would be a Low Risk of slope instability affecting the proposed development in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). 1.1 Details of the Site and Development The site is described as Lot 12 on P9313, covers an area of about 506 m2 and has a frontage to Plover Street along the western boundary. At the time of the assessment, an existing residence occupied most of the site. Residential allotments with houses surrounded the site to the north and south, with the Peregian Beach Foredune and Reserve Area to the east. A 2023 aerial image of the site and surrounds is shown in Text Figure 1 below. A more detailed description of the site is given in Section 2. Subject site Text Figure 1: Aerial image of site and surrounds from 2023 (courtesy of Nearmap imagery) 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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Architectural drawings supplied to Tectonic (Ref. Drawing Nos. 01 to 05.0 [not inclusive], Rev B, dated 23
May 2023) indicate that the proposed extensions will comprise a new carport along the south-western edge
of the residence, new internal access driveway off Plover Street, along with internal renovation of the
western part of the residence and landscaping works.
Construction will likely comprise mostly lightweight materials, with the eastern edge of the carport area
supported by a concrete blockwork wall up to about 1.5 m high. No earthworks details were available;
however, based on the provided drawings it appears that excavations less than 1 m deep and filling up to
about 1.5 m high will be required for the new carport and landscaping.
A proposed layout plan and some cross-sectional views of the proposed development are shown in Text
Figure 2 below.
Text Figure 2: Proposed layout plan (upper) and some cross-sectional views (lower)
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1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of published
geological maps, slope stability studies and a site classification ‘soil test’ report by Global Testing
(Global, Ref. Job No. 23-1410, dated 11 December 2023).
A walk-over survey was conducted to make an appraisal of the general condition of the site in regard to
topography, drainage, vegetation cover, geology, erosion, and condition of existing structures. The walk-
over survey was carried out by an experienced geotechnical engineer from Tectonic on 18 January 2024.
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 Michael Cocks, an RPEQ with over nine years of experience in
geotechnical engineering, including a number of slope stability projects on the Sunshine Coast.
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology and Subsurface Conditions
Available geological information 1 indicates that the site is underlain by the Holocene age blowout dunes,
comprising ‘quartz sand’’.
Approx. site location
Text Figure 3: Extract of Laguna Bay Surface Geology Map
The Global investigation comprised two boreholes (designated BH1 and BH2) within the proposed
external extension areas. Consistent subsurface materials were described in the borehole logs
comprising loose sand from the ground surface to depths between 0.8 m and 0.9 m below ground level
(BGL), below which both boreholes were indicated to be terminated due to collapsing. Dynamic cone
penetrometer (DCP) testing extended below the borehole termination depths, and indicated that the
sands became medium dense at depths between 1.1 m and 1.5 m BGL, extending to the DCP
termination depths of 2 m BGL.
1
Queensland Government, Department of Mines and Energy, 1:100,000 Laguna Bay Surface Geology Map, Sheet 9545, 2018
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Groundwater was not mentioned in the Global borehole reports with the soils described as ‘moist’. Global
has classified the site as Class P, as per AS2870, due to the presence of trees and poor bearing capacity;
however, has indicated that if not for these factors the site has the characteristics of Class M.
2.2 Topography
During the site walk-over, topographic features specific to the site were noted and ground slopes were
recorded using a hand-held clinometer. As illustrated in Text Figure 4, the site is located on the easterly
facing flank of an ancient sand dune which crests about 40 m to the west.
The natural surface profile at the site had been interrupted by historic landscaping and residential
construction. Based on a visual assessment it appeared that cut and fill earthworks in the order of about
1 m depth/height had been undertaken, most of which were supported by timber sleeper and concrete
blockwork walls. Inferred unaltered ground slopes in the western part of the site ranged up to about 10°,
reducing to <5° in the eastern part of the site.
No obvious signs of irregular features which would indicate global instability were observed across the
allotment or immediate surrounding areas.
Subject site
Text Figure 4: Ground surface contours, 1 m spacing (courtesy Noosa Council ‘Intramaps’)
Site conditions at the time of our assessment are shown in Text Figures 5 and 6 below and overleaf.
Text Figure 5: Easterly view of the site from Plover Street
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Text Figure 6: Southerly view beneath the eastern edge of the residence
2.3 Groundwater
No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over. Nor was
groundwater mentioned in the Global borehole reports. Due to the sandy nature of the subsurface
conditions, development of a standing groundwater level close to the ground surface is considered unlikely.
2.4 Surface Drainage
Surface drainage would be expected to follow the contours to the east towards the vegetated Reserve Area.
Given the sandy nature of the site and surrounds, rainfall would normally be anticipated to infiltrate the
ground surface. However, as the existing residence and proposed extensions will occupy a large part of the
block, significant run-off from hard surfaces will occur. Nevertheless, where directed into swales rather than
pipes, such run-off would be expected to infiltrate into the natural sands relatively rapidly. No evidence of
significant erosion was observed across the subject site.
2.5 Vegetation
The ground surface around the existing structures primarily comprised bare sands and grass which were
fringed by scattered shrubs and trees.
2.6 Buildings and Other Structures
The existing residence is a double storey structure mostly elevated above ground on posts. Timber sleeper
walls, less than 1 m high, and a concrete blockwork wall up to about 1.5 m high supported part of the
residence, landscaping features and previous earthworks. Access was provided via an exposed aggregate
driveway directly off Plover Street to the west. Based on an external assessment, most of the visible areas
of the existing structures appeared to be in good structural condition and did not exhibit any signs of distress
(e.g. cracking, misalignment, etc.) which could be attributed to slope instability.
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 7)
which indicates that the western and easterly edges of the site are partially mapped as having a Moderate
landslide hazard.
Subject site
Text Figure 7: Extract from NC landslide hazard overlay mapping
It should be noted that the Noosa Council landslide hazard map is a broad-scale indication of the potential
landslide susceptibility and is superseded by this site-specific assessment. No visual signs of global slope
instability were observed in the vicinity of the residence or proposed development areas. No landslide
back scarps, groundwater springs, or areas of naturally ‘hummocky’ ground were observed on the site or
in the adjacent areas.
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3.2 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 (taken from AGS 2007) is attached at the end
of this report.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and
Very High
VH implementation of treatment options essential to reduce risk to Low; may be too expensive and not practical.
Risk
Work likely to 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
Moderate Risk
M 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,
L Low Risk
ongoing maintenance required.
Very Low Risk
VL 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.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development areas. Sand dune slopes are generally close to the ‘limit of equilibrium’ for slope stability where
the surface gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands. Our
measurements indicate that the natural gradients around the site typically range up to about 10°. The limit of
equilibrium is the ‘tipping point’ to instability, where the factor of safety (FOS) of the slope is approximately
1.0. The FOS of a slope is the balance (ratio) between the forces driving instability (e.g. soil weight,
groundwater pressure) and the resisting forces (e.g. soil friction and cohesion).
Sand dune slopes generally remain stable (i.e. FOS greater than 1.0) when they are:
well vegetated (mix of ground cover, shrubs and small trees);
not subjected to concentrated stormwater flows;
not surcharged with additional loads (e.g. fill); and
not undercut by excavation or erosion.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation which can lead to the development of generally localised
instability.
The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion. At this site, the area down slope (to the east) is protected by the
relatively wide Peregian Beach foredune and Reserve Area and therefore, water erosion or significant
disturbance of the dune by natural forces (e.g. from coastal erosion), in the vicinity of the site, is considered
unlikely over the design life of the house.
Considering the existing site conditions (Ref. Section 2) and subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of global slope instability
affecting the proposed extensions and pool. 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 overleaf is our qualitative assessment of landslide risk for the development site. A summary of qualitative
terminology for use in assessing risk to property is attached (taken from AGS 2007).
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It should be noted that this assessment excludes geotechnical risk associated with non-piled structures (e.g.
driveway, landscaping etc.) due to settlement of the upper loose sands. The design of at grade structures
(including landscaping) should consider differential settlements between any piled areas (hard/rigid surfaces)
and surrounding fill and natural sands. This risk assessment also excludes geotechnical risk associated with
the existing structures (including retaining walls).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
Given the gentle to moderate gradients across the
proposed development areas, it is considered
Unlikely that a landslide would develop in these
1: Failure through very loose
materials, subject to implementation of the
to loose natural sands above Unlikely Medium Low
recommendations in Section 4. It is assessed that
the foundation depth
there could be a potential Medium impact from
such instability so the resultant risk would be Low
as per AGS 2007.
The likelihood of a deep failure through the medium
dense (or denser) natural sand below the proposed
foundation depth is assessed as Rare due to the lack
2: Deep failure through the
of precedence of such movements locally, no signs of
medium dense (or denser)
Rare Major Low any existing slope instability, apparent lack of
sands below the foundation
groundwater and the negligible risk of erosion at the
depth
toe of the slope. Although the consequence of such a
failure could be Major, the resultant risk is 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.
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. These Guidelines should be adopted in the design
and construction of works on the site.
4.1 Site Layout and Development Limitations
Should any revisions be made to the proposed extent of the development, as described in Section 1.1,
Tectonic must review these revisions prior to construction to assess if there are any impacts on slope stability.
To achieve a Low Risk of instability, the following is recommended:
The proposed development should not exert additional load on the existing structures or footing
system, unless assessed and approved by a structural RPEQ, who must also confirm the existing
footings and structural members have sufficient capacity to carry additional loads imposed by the
extensions.
All footings must found below the zone of influence of the existing structures (including retaining
walls).
Cut and fill earthworks should not exceed 1.5 m depth/height unless assessed and approved by a
geotechnical RPEQ.
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
and certified by an RPEQ. The design must include an assessment of global and internal stability.
Design and construction of footings shall be undertaken in accordance with Section 4.3.
Formal drainage should be constructed to ensure all stormwater is directed away from the existing
and proposed structures.
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4.2 Earthworks
No earthworks details were available at the time of reporting; however, the provided plans suggest that cut
and fill earthworks in the order of about 1 m deep and 1.5 m high may be required, most of which will be
supported by retaining structures. Should proposed earthworks exceed these limits, Tectonic must assess
the impacts on slope stability before construction.
4.2.1 Site Preparation and Trafficability
In construction areas, any relict construction elements (other than those approved by the structural RPEQ),
vegetation, tree stumps and large roots should be stripped and ‘grubbed out’. Any organic rich topsoil
should also be stripped and removed from site. Organic topsoils and garden soils must not be re-used as
structural fill; however, small volumes may be retained on site for later use across landscaping areas, if
required.
The natural subgrade following stripping will likely be in a loose condition. Should slab on ground
construction be proposed (e.g. new driveway, concrete pathways etc.) the ground surface should be
compacted by several passes of a vibrating plate compactor or small smooth drum roller (or similar) in
conjunction with the surface application of ample water.
Care must be taken with the use of any vibratory equipment to ensure that there are no adverse impacts on
surrounding structures and underground services (e.g. existing Noosa Council infrastructure along the site
boundaries). The impacts from ground vibration must be closely monitored and managed during
construction. Where significant vibrations are possible, it would be prudent to conduct a dilapidation survey
of neighbouring structures prior to commencement of construction.
4.2.2 Filling
The natural sandy soils (excluding topsoil and garden soils) may be reused as structural fill, subject to
appropriate moisture adjustment. It should be noted that compaction of such relatively ‘clean’ sands as
structural fill requires the addition of substantial volumes of water (‘flooding’) combined with the use of
vibratory compaction equipment (typically a smooth drum roller or vibrating plate). Where significant
vibrations likely to damage adjacent property or infrastructure are generated, the placement of fill in thinner
layers and the use of static compaction equipment should be considered.
Should importation of fill be required, it is recommended that such fill should comply with the following:
Minimum CBR of 10%
Maximum plasticity index of 15%
Shrink-swell index ≤ 1%
Maximum particle size of 100 mm
Any fill should be compacted at moisture contents within the range of ±2% of optimum moisture content for
Standard Compaction. Select 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 small compactor 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.
Confirmatory compaction testing should be carried out at regular intervals. A minimum of two tests per
vertical lift is recommended where structural fill heights are to exceed 300 mm. 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 embankment edges must be over-filled by 0.5 m (horizontally) and then trimmed back to well compacted
material.
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4.2.3 Excavations
Excavations through the fill and natural soils encountered in the Global boreholes should be achievable
using conventional earthmoving plant (e.g. small excavators). Non-surcharged temporary cuts in the very
loose to loose sands could be formed at 1V:2H, and in medium dense sands at 1V:1.5H, for batters up to
1.5 m in height. Although unlikely, if groundwater seepage is encountered, flatter batters may be required.
Deeper cuts, if required, should be specifically assessed by Tectonic. Permanent fill batters up to 1.5 m high
(vertical height) may be battered at no steeper than 1V:3H, or otherwise retained. Any permanent soil and fill
batters will require erosion protection (e.g. revegetation or surface protection).
If the above temporary batter slopes cannot be achieved due to spatial restraints, temporary shoring will be
required. Excavations in close proximity to existing structures/infrastructure will need to be specifically
designed to prevent undermining. If undermining is a potential issue, the existing structures may need to be
underpinned or the excavation may require temporary shoring.
The advice of a suitably qualified and experienced structural RPEQ must be sought regarding any potential
undermining and design and construction of any underpins/temporary shoring (if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas to a control point(s).
4.3 Footings
A suitably qualified and experienced structural RPEQ must be consulted to ensure the existing footings and
structural elements of the residence and ancillary structures (e.g. retaining walls) have sufficient capacity to
support additional loads imposed by the proposed extensions. It is recommended that footing exposures
and/or building records searches be undertaken prior to construction. Alternatively, the proposed extensions
should be designed to be structurally separate from the existing structures.
No design loadings were available at the time of reporting. Footings for the proposed extensions, carport
and any ancillary structures (e.g. retaining walls etc.) must penetrate through any fill materials and unsuitable
natural soils (i.e. very loose to loose sands) to found at least 0.5 m into medium dense (or denser) sands
which were inferred to be present at depths between about 1.1 m and 1.5 m BGL (existing ground levels) in
the Global borehole logs. Thus, piled footings will be required unless medium dense sands are exposed at or
near the foundation level. Piles founded at least 0.5 m into medium dense (or denser) natural sand may be
designed using an allowable end bearing pressure of 300 kPa.
High-level footings may be possible in areas of future cut, where founded at least 0.3 m into natural medium
dense (or denser) sand, may be designed using an allowable bearing pressure of 100 kPa. Excavations for
footings may loosen the base so it is recommended that a vibrating plate with ample water should be used to
densify footing bases prior to the placement of reinforcing steel.
Given the sandy soil profile and depths of footings required, steel screw piles are an appropriate footing type
for the proposed development. Due to the presence of loose sands, together with the close proximity to
surrounding infrastructure, open bored piers or driven piles are not considered to be appropriate for this site.
Steel reinforced concrete bored piles could be feasible; however, the use of sacrificial liners (e.g.
‘formatube’) would be required to prevent the caving of upper loose sands.
The advice of a specialist piling contractor must be sought regarding feasibility, design and installation of the
piling system for this site, including an assessment of potential vibration effects on nearby structures, and
monitoring requirements.
If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling may
not encounter sufficient drilling resistance (from torque measurement) in sands at the suggested founding
depth and piles could extend to much greater depth than necessary.
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It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
deflections under tension and compression cycles) should also be undertaken prior to construction.
Kentledge for static testing, or simulated cyclic loads, should prove the piles to a minimum of two times the
expected working load within tolerable deflections.
All footings should found such that they are not adversely affected by (or affect) 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 8.
Where footings are located within a horizontal distance of 1.5H (H being the batter height) of any cut or fill
batter crest, these footings must be founded at a depth level with the toe of the batter.
Text Figure 8: Footing depth required to prevent undermining
Site drainage should be designed to readily remove surface water and to prevent ponding of water on
subgrade areas during construction and also adjacent to foundations and perimeter slabs once construction
is completed.
If bored piles are adopted, it is recommended that footings be inspected by an RPEQ prior to placement of
steel reinforcement and pouring of concrete to confirm allowable bearing pressures and cleanliness of
excavations.
If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic and Global should be notified immediately, and an inspection carried out to determine
if changes to footing design are required.
4.4 Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.3 above. Retaining
structures greater than 1 m high, or those required to support surcharge loads or sloping backfill, will require
engineer design and certification of construction. Retaining wall design should be carried out in accordance
with AS4678-2002 Earth retaining structures.
The parameters outlined in Table 3 overleaf may be adopted for retaining wall design. The values in Table 3
assume that the walls are fully drained, and no allowance is included for hydrostatic pressures, construction
loading, up-slope footing loadings, the effects of sloping ground and backfill compaction stresses. Any
temporary shoring required to support excavations during construction may also be designed using the
parameters in Table 3.
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Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle Ø Ko
Ka
(kN/m3) (°) (Non-yielding Kp
(Cantilever wall)
wall)
Very Loose/ Loose
17 28 0.36 0.53 2.77
Sands
Medium Dense (or
18 33 0.29 0.46 3.39
Denser) Sands
Future Fill * * * * *
Note: * depends on fill material type and level of compaction
As a minimum, retaining structures must be founded as per the recommendations in Section 4.3 above with
the footings also designed to account for lateral loads imposed by the retaining wall. Detailed design must
include an assessment of internal and global stability, together with required bearing capacity and founding
depth, by the retaining wall design engineer.
4.5 Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment (e.g.
grassed or landscaped swales or diversion mounds). Adequate site drainage must be installed to ensure
that stormwater runoff is directed away from buildings, walls and footings. Although unlikely, if groundwater
seepage is encountered during excavations, Tectonic must be notified to assess the impact on stability.
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. It is assumed that the pool floor will have a hydrostatic
pressure relief valve, which must daylight at the ground surface away from the proposed and existing
footings.
All excess stormwater collected around the residence (including overflow from rainwater tanks, pool etc.)
must be directed by pipes or lined channels to discharge at least 5 m downslope of any structure or
embankment and towards the vegetated Reserve Area to the east.
Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.
5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed extensions and carport from slope instability
subject to implementation of the recommendations given in this report.
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6.0 LIMITATIONS
This assessment relates only to the proposed extensions and carport. Existing structures, slopes, and
driveways may be subject to risk of slope instability, but assessment of those matters was beyond the scope
of our commission.
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
Michael Cocks BEng NER RPEQ 27540
Senior 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
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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.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 16
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 18
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)