Council archive · source document
4. 220148 - Slope Stability Report (2) · 132006.2554.01
26 April 2022 Project No. 22134-001-Rev0 Seachange Builders Attention: Mr Jarrod Dean Email: [email protected] SLOPE STABILITY ASSESSMENT – PROPOSED RENOVATIONS & POOL 51 TRISTANIA DRIVE, MARCUS BEACH Dear Jarrod, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for proposed renovations to the existing residence, including a new pool, at 51 Tristania Drive, Marcus 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 5 on M11113, covers an area of about 931 m2, and has frontage to Tristania Drive along the western boundary. At the time of the assessment, the existing residence occupied most of the site. Residential allotments surrounded the site to the north, south and west, with the Marcus Beach Reserve located to the east. A 2021 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 Marcus Beach Reserve Text Figure 1: Aerial image of site and surrounds from 2021 (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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Design drawings supplied to us (Seachange Builders, Ref. 2021-73.1, Sheet Nos. 1 to 17, Revision 3,
dated 16 January 2022) indicate that the proposed development will include the following:
At the sub floor level, a wet edge pool to the rear up to 1.8 m deep, a wine cellar and a lift
connecting the three levels of the building.
New kitchen and bathroom areas on the ground floor.
Realigned bedrooms and bathrooms across the first floor.
Construction is generally expected to comprise of lightweight materials (e.g. timber framing, sheet metal
roofing) but with masonry construction around the lift and new swimming pool. The construction footprint
will generally not extend outside the current building area. No earthworks details were available at the time
of reporting; however, provided drawings indicate excavations for the new pool and underground services
may range up to about 2 m deep. Minimal filling is anticipated other than backfilling around the pool and lift
pit/wine cellar areas.
The property has existing connections to town water and sewerage. A site layout plan and some cross-
sectional views for the proposed extensions are shown in Text Figure 2 below.
Text Figure 1: Rendered view of frontage (upper) and proposed sub-floor layout plan (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 investigation report by CSI Soil Testing (CSI Ref. Job No.
14743, dated 3 March 2022).
A walk-over survey was conducted in order to make an appraisal of the general condition of the site in
regard to topography, drainage, vegetation cover, geology, erosion, and condition of existing structures.
The walk-over survey was carried out by an experienced geotechnical engineer from Tectonic on
8 April 2022. The results of the desk-top study and walk-over survey are included in Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been prepared by Mr Ashley Davey, an RPEQ with over 26 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 Quaternary age dunes, mainly parabolic
comprising ‘quartz sand’.
Approx. Site Location
Text Figure 3: Extract of Gympie Special geology map
The CSI investigation comprised one borehole (designated BH1) at the rear of the existing residence
(proposed pool area). Dynamic cone penetrometer (DCP) testing was conducted adjacent to the
borehole and at one stand-alone location (designated DC1 and DC2, respectively). The borehole
intersected very loose natural silty sand to 0.6 m below ground level (BGL), then very loose to loose
natural sand to about 2 m BGL (borehole termination depth). Medium dense sands are then indicated by
DCP testing to extend to about 3 m BGL, thereafter becoming dense to at least 3.5 m BGL. Similar
variations in soil density were indicated at the location of DC2.
Groundwater was not mentioned in the CSI borehole report, with the soils described as ‘moist’.
1
Queensland Government, Department of Mines and Energy, 1:100,000 Gympie Special Geology Map, Sheet 9445 & Part 9545, 1999
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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 a relict dune, located to the west of the primary Marcus Beach foredune. The crest of the
relict dune was located about 30 m west of the subject site, with the toe of the dune running along the
eastern boundary.
The natural surface profile at the site had been interrupted by historic landscaping and residential
construction. Based on a visual inspection it appeared that cut and fill earthworks in the order of about 2 m
depth/height had been previously undertaken, most of which were supported by retaining walls or structural
residential walls. Surrounding ground slopes measured at the time of our assessment and ranged between
about 8° and 12° to the east. A yard area retained at the front of the house sloped at about 1° to the east.
Subject Site
Text Figure 4: Ground surface contours, 1 m spacing (courtesy NC interactive mapping)
Site conditions at the time of our assessment are shown in Text Figures 5 and 6 below and overleaf.
Text Figure 5: View of the rear yard, looking west
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Text Figure 6: Property frontage, undergoing demolition at time of assessment
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
mentioned in the CSI borehole report.
2.4 Surface Drainage
Surface drainage would be expected to follow the contours to the east. However, run-off would be minimal
due to the sandy nature of the soils at the site, and is currently interrupted by the existing residence,
terracing and landscaping features.
It is acknowledged that a surface water catchment for the site exists to the west; however, most of the
stormwater to the west of the property is anticipated to be intercepted by the existing stormwater
infrastructure along Tristania Drive. 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
renovations 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.
Two field inlets pits for stormwater discharge were observed along the rear (eastern) boundary.
2.5 Vegetation
The ground surface around the existing residence primarily comprised areas of short grass which were
fringed by scattered shrubs and juvenile trees. The beach reserve to the east was densely vegetated.
2.6 Buildings and Other Structures
The existing residence is a three level, rendered masonry structure. Most of the ground surface surrounding
the residence was levelled by previous earthworks, supported by grouted rock or blockwork walls. Based on
an external assessment, most of the visible areas of the existing structures and retaining walls appeared to
be in good condition and did not exhibit any signs of significant distress (e.g. cracking, misalignment, etc.)
which could be attributed to slope instability. Minor hairline cracking was observed in the building render.
The existing pool was empty of water and appeared free from cracking.
Some cracking was observed along the grouted rock retaining wall along the eastern edge of the existing
pool, however we understand that these structures are to be demolished.
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Text Figure 7: Existing empty pool (LHS) and northern side of house (RHS)
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 8),
which indicates that the eastern part of the site has a Moderate (pale brown shading) landslide hazard.
Subject Site
Text Figure 8: Extract from NC landslide hazard overlay mapping
It should be noted that the Noosa Council landslide hazard map is a broad-scale indication of potential
landslide susceptibility and is superseded by this site-specific assessment. No visual signs of global slope
instability were observed across the site or adjacent areas. No landslide back scarps, groundwater
springs, or areas of naturally ‘hummocky’ ground were observed. Review of historical aerial photos from
NearMap and QImagery, at least one every decade back to the 1950’s, indicates that the foredune area
adjacent to the site has remained relatively intact over that period, and does not appear to have been
subjected to substantial erosion over a period of at least 70 years to date.
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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 is also attached in Appendix B.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to
L Low Risk
this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. 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 ground surface gradients around the proposed development area range up to
about 12°. Mapping indicates that the foredune within the reserve area to the east of the site has slopes in
the order of 30° over short distances (up to about 10 m), however the reserve is densely vegetated and about
100 m wide.
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, and 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, such as due to extreme weather events. At this site, the toe of
the dune (along the eastern property boundary) is setback about 100 m from the beach and protected by a
densely vegetated reserve including a well developed foredune. Therefore, water erosion or significant
disturbance of the dune by natural forces, in the vicinity of the site, is considered unlikely.
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 development. Regulators (NSC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in Table
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2 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).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence Risk Comments
Given the low to moderate gradients around the
proposed development area it is considered Unlikely
1: Failure through very loose to that a landslip would develop in these materials,
loose natural sands above the Unlikely Medium Low subject to implementation of the recommendations in
foundation depth. Section 4. It is assessed that 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 of
2: Deep failure through the
precedence of such movements locally, no signs of any
medium dense (or denser) sands Rare Major Low
existing slope instability and apparent lack of
below the foundation depth.
groundwater. 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.
4.1 Site Layout and Development Limitations
Should any revisions be made to the proposed extent of the renovations as described in Section 1.1, Tectonic
must review these prior to construction to assess if there are any impacts on slope stability.
No earthworks details were available at the time of reporting; however, provided drawings indicate
excavations in the order of about 2 m depth may be required for the new pool and underground services.
Minimal filling other than backfilling around the pool and lift/wine cellar walls is anticipated. Should proposed
cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability before construction.
4.2 Earthworks
4.2.1 Site Preparation and Trafficability
In construction areas, any relict construction elements, 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. Where slab on ground
construction is proposed the ground surface should be compacted by several passes of a vibrating plate
compactor or small smooth drum roller (or similar), in conjunction with surface application of ample water.
Care must be taken with the use of vibratory equipment to ensure that there are no adverse impacts on
surrounding structures and underground services. The impacts from ground vibration must be closely
monitored and managed during construction. It would be prudent to conduct a dilapidation survey of
neighbouring structures prior to commencement of construction.
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4.2.2 Filling
The existing sandy soils (excluding topsoil) 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).
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.
4.2.3 Excavations
Excavations through the natural soils encountered in the 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, for batters up to 2 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 m high (vertical height) may be battered at no steeper than 1V:3H. All
remaining permanent cut/fill batters must be supported by engineer designed retaining walls. 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 (e.g. sheet piles). 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, and to a control point(s).
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4.3 Footings
No design loadings were available at the time of reporting. New footings for the proposed renovations (e.g.
lift base, pool, etc.) must penetrate through any fill materials and unsuitable natural soils (i.e. very loose to
loose sands) to found at least 1 m into medium dense (or denser) sands which were mentioned at about 2 m
BGL (existing ground levels) in the CSI borehole/DCP tests. A minimum founding depth of 3 m BGL
(existing ground levels) is recommended.
Given the sandy soil profile and depth of footings required, steel screw piles could be adopted. 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 suitable 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. Due to the extent of the residence, access is restricted, so specialised tight access
pilling equipment will likely be required.
Please note that a suitably qualified and experienced structural RPEQ must be consulted to ensure that
existing footings have sufficient capacity to support any additional loads imposed by the proposed
renovations.
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.
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 be founded such that they are not adversely affected by or affect any adjacent
excavations, trenches, retaining walls, or in ground structures that are not designed to support building
loads. Footings should be founded at least below a plane extending 1 m horizontally from the base of
trenches/excavations/retaining walls/swimming pool, then rising up at 1V:1H, as illustrated in
Text Figure 9.
Text Figure 9: Footing depth required to prevent undermining
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If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic and CSI should be notified immediately, and an inspection carried out to determine if
changes to footing design are required.
Should bored piles be adopted, it is essential that the piling contractor have a ‘mud bucket’ or similar to
enable clean out of the pile bases. It is recommended that bored pile sockets be inspected by a suitably
qualified and experienced engineer prior to placement of steel reinforcement and pouring of concrete to
confirm allowable bearing pressures and cleanliness.
4.4 Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.3 above with the piled
footings also designed to resist lateral loads imposed by the retaining wall. 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. Detailed design must include an assessment of global stability.
The parameters outlined in Table 3 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.
Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle Ø
Ka Ko
(kN/m3) (°) Kp
(Cantilever wall) (Non-yielding wall)
Very Loose/ Loose
17 30 0.33 0.50 3.00
Sands
Medium Dense (or
18 34 0.28 0.44 3.54
Denser) Sands
Future Fill * * * * *
Note: * depends on fill material type and level of compaction
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 building 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.
All excess stormwater collected around the residence (including overflow from rainwater tanks, pool etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure.
Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.
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5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed renovations and pool from slope instability
subject to implementation of the recommendations given in this report.
6.0 LIMITATIONS
Your attention is drawn to the document Limitations, which is attached to this letter report.
Please contact the undersigned should you wish to discuss any of the above matters.
Yours faithfully
TECTONIC GEOTECHNICAL PTY LTD
Ashley Davey BE (Geological) RPEQ 8159
Principal Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 14
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 15
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
Page 16
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 17
LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)
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