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21054-001-Rev0 Tristania Dr
25 February 2021 Project No. 21054-001-Rev0 Peter Williams Email: [email protected] SLOPE STABILITY ASSESSMENT – PROPOSED RESIDENCE 8 TRISTANIA DRIVE, MARCUS BEACH Dear Peter, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed residence at 8 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 residence 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 100 on M1112, covers an area of 632 m2, and has frontage to Tristania Drive, Toona Street and David Low Way along the eastern, southern, and western boundaries, respectively. At the time of the assessment a vacant allotment was located directly to the north, with existing residential houses and low-rise units further to the north and south. The Marcus Beach foredune is located on the opposing side of Tristania Drive to the east. A 2020 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 2020 (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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Peter Williams 21054-001-Rev0
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Preliminary design drawings provided to us (Ref. Zest Building Design Project No. 20042, drawing Nos
SK.01 to SK.07, Issue B, dated 2 February 2021) indicate that the proposed residence will comprise a split-
level structure incorporating at-grade and suspended concrete slab floors. Construction is to comprise
mostly preformed concrete panels, masonry walls and a concrete roof. A mostly in-ground plunge pool is
proposed off the north-western corner of the residence. Access will be provided via a new driveway off
Tristania Drive along the eastern boundary.
Based on provided plans, cut and fill earthworks for the proposed development will likely range up to about
2 m deep and 1 m high, respectively.
Text Figure 2 below shows the proposed site plan and a cross-sectional view of the development.
Text Figure 2: Proposed site plan (upper) and a cross-sectional view of the proposed development (lower)
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 CSI Soil Testing (CSI,
Ref. Job No. 13596, dated 12 August 2020).
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
12 February 2021. The results of the desk-top study and walk-over survey are included in Section 2
below.
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1.3 Qualifications of Responsible Engineer
This report has been reviewed by Mr Ashley Davey, an RPEQ with more than 25 years’ experience in
geotechnical engineering, including a number of slope stability projects
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 two boreholes (designated BH1 and BH2) across the proposed
development area. These boreholes encountered relatively consistent subsurface conditions,
summarised as follows:
Loose silty sand from the ground surface to 0.7 m below ground level (BGL); then
Loose, poorly graded sand (borehole BH1) and medium dense poorly graded sand (borehole
BH2) to the borehole termination depths (1.5 m BGL).
It should be noted that CSI continued dynamic cone penetrometer (DCP) testing beyond the borehole
termination depths, which indicated that the loose sands in borehole BH1 became medium dense below
about 3 m BGL.
Groundwater was not mentioned on the CSI borehole reports, with the soils described as ‘moist’.
CSI has classified the site as Class P, as per AS2870, due to poor bearing capacity; however, has
indicated that the soils encountered have the characteristics of a Class S site.
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
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 toe of the relict dune traces Tristania Drive along the
eastern boundary. The ground slope across the site was primarily waning (reducing) and relatively planar.
Gradients across the site were recorded using a hand-held clinometer and typically varied between about 8°
and 10° across the proposed development area, flattening to less than 5° along the eastern boundary.
Subject Site
Text Figure 4: Image showing ground surface contours (courtesy Noosa Council ‘Intramaps’)
Site conditions at the time of our assessment are shown in Text Figure 5.
Text Figure 5: Panoramic view of the site looking north-west
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
mentioned in the CSI borehole reports.
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2.4 Surface Drainage
The site has a relatively small catchment, with no natural surface features observed that would cause
concentration of surface runoff. Most of the surface water to the west of the property is anticipated to be
intercepted by the existing stormwater infrastructure along the David Low Way. Given the sandy nature of
the site and surrounds, rainfall would normally be anticipated to infiltrate the ground surface. However, as
the proposed residence 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
As illustrated in Text Figure 5 (on preceding page), the ground surface was covered by relatively dense
vegetation comprising mostly shrubs and small to medium sized trees. Some of the tress appeared to have
died and/or were slightly leaning, which may be due to shallow soil creep.
2.6 Buildings and Other Structures
There were no existing structures on the site at the time of the investigation. Surrounding properties to the
south comprised single to double storey structures having slab on ground floors, with observable external
walls noted to be in good condition and showing no obvious signs of distress (e.g. cracking, etc.). The
pavement along Tristania Drive was noted to be in relatively good condition. Minor cracking was evident in
the asphalt seal, which was likely due to consolidation of the surficial materials rather than slope instability.
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 6),
which indicates that most of the site and development area mapped as having a Moderate (pale yellow
shading) landslide hazard. Areas of Very Low/Low (no shading) landslide hazard are mapped in the north-
eastern and south-western corner of the site.
Subject site
Text Figure 6: Extract from NC landslide hazard overlay mapping
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It should be noted that the NSC natural hazard overlay mapping is a broad-scale indication of the potential
landslide susceptibility only, and is superseded by this site specific assessment. It is inferred that the
landslide hazard rating is a result of natural ground slopes exceeding 15%.
At the time of our visit, no signs of global slope instability were observed across the site or surrounding
properties. No landslide back scarps, groundwater springs, or areas of naturally ‘hummocky’ ground were
observed. Tilting trees were noted on the subject site which can be an indicator of shallow soil creep.
However, the allotment and areas immediately around did not exhibit any indicators of larger scale global
slope instability.
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.
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
L Low Risk
to 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.
Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°. The
overall slope across the proposed development area is generally in the order of 10˚ to 12˚. 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).
Steep sand dune slopes generally remain marginally stable (i.e. FOS just 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
when the ‘toe’ of the dune is not disturbed.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Areas outside of structures and concrete ground slabs are generally
covered by vegetation being either grass, shrubs or trees. Discharge of stormwater onto dune slopes may
cause channel erosion, undermining and disturbance of vegetation, resulting in development of generally
localised instability. Stormwater runoff must be collected (or re-directed) and discharged from the site as
discussed in Section 4.5 below.
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. Sand dune instability due to undercutting of the toe generally
occurs as a translational-type slide where a ‘slab’ of sand on the dune face slides down slope. The depth of
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such sliding may range up to a few metres during one event, but on-going erosion can cause retrogressive
failure of dune slopes leading to multiple sliding events, if left unchecked. The risk such an occurrence at this
site is minimal, considering the location of the site being away from features such as high flow natural
drainage lines that could cause toe erosion.
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 residential 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 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 natural gradients through
the proposed building area and setback from the
beach (affording protection to the toe of the dune), it
1: Failure through very loose to
is considered Unlikely that a landslip would develop
loose natural sands above the Unlikely Medium Low
in these materials, subject to implementation of the
foundation depth
recommendations in Section 4. Given a potential
Medium impact from such instability 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
2: Deep failure through the
foundation depth is assessed as Rare due to the lack
medium dense (or denser)
Rare Major Low of precedence of such movements locally, no signs of
sands below the foundation
any existing slope instability and a lack of groundwater.
depth
Although the consequence of such a failure would 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 Building Location & Limitations
We understand that the proposed building envelope is to be as shown in Text Figure 2. This position is
acceptable from a geotechnical viewpoint; however, Tectonic must re-assess slope stability if the proposed
development area is altered.
4.2 Earthworks
It is anticipated that cut and fill earthworks for the proposed development will range up to about
2 m deep and 1 m high, respectively. Should proposed earthworks exceed these quantities, Tectonic must be
notified and a re-assessment of slope stability may be required.
Preliminary plans indicate that the majority of earthworks will be supported by engineer designed retaining
walls. Any fill, and cuts exceeding 1 m, must be supported by engineer designed and certified retaining
walls.
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Following clearing of vegetation, tree stumps and large roots must be ‘grubbed out’, with any organic rich
topsoil stripped from construction areas. Organic topsoils and garden soils must not be re-used as structural
fill, however they may be retained on site for later use across landscaping areas, if required. Sandy soils
where free of deleterious materials (e.g. vegetation, etc.) 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 vibrating plate). Care must be taken with the use of vibratory equipment to ensure that there are no
adverse impacts on surrounding structures. 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.
The exposed natural subgrade following excavations will likely be in a loose condition and must be
compacted by several passes of a vibrating plate or minimum 4 tonne vibrating smooth drum roller, in
conjunction with surface application of ample water, to achieve the required compaction.
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. Fill material should be placed in 250 mm thick (loose thickness) layers and be
compacted by repeated rolling using a minimum 4 tonne smooth drum roller (or equivalent, with vibration on
if safe to do so) to achieve a dry density ratio of at least 95% of the maximum Dry Density Ratio (DDR),
based upon the Standard Compaction method, or a Dry Density Index of at least 75%.
Confirmatory compaction testing must 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.
Excavations should be achievable using conventional earthmoving plant (e.g. small to medium sized
excavators with toothed buckets).
Temporary cuts in the natural sands could be formed at 1V:1.H, for unsurcharged batters up to 2 m in height,
provided groundwater seepage is not encountered. Deeper cuts, if required, should be specifically assessed
by Tectonic. All fills, and permanent cuts exceeding 1 m, must be retained. Permanent cut batters up to 1 m
height shall not exceed 1V:2.5H. Any permanent soil and fill batters will require erosion protection (e.g.
revegetation or surface protection).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Any excavations required adjacent to existing structures will need to be specifically designed to ensure that
the structures are not adversely affected ('undermined').
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), prior
to off-site release.
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4.3 Footing Design
No design loadings were available at the time of reporting. Footings for the proposed residence and any
ancillary structures (e.g. retaining walls, pool, etc.) must penetrate through any fill materials and unsuitable
natural soils (i.e. very loose to loose sands) to found into medium dense (or denser) sands which were
mentioned in the CSI borehole and DCP reports at depths between 1 m and 3 m BGL. As a minimum it is
recommended that footings found at least 2.5 m BGL (existing ground levels), or deeper in areas where
loose sands extend below this depth (e.g. CSI borehole BH1).
Given the sandy soil profile and depths of footings required, steel screw piles are considered to be a suitable
foundation type, generally. However, in areas of deeper cuts exposing medium dense (or denser) sands at
the subgrade level, conventional slab on ground or high-level strip or pad footings may 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.
Screw piles socketed at least 0.5 m into medium dense (or denser) natural sands and at least 2.5 m BGL
(existing ground levels) may be designed using an allowable end bearing pressure of 300 kPa. In areas of
deeper excavations exposing medium dense (or denser) natural sands at the subgrade level, provided the
natural subgrade is compacted as outlined in Section 4.2, slab edge beams, internal beams and any isolated
strip or pad footings must found at least 0.5 m into medium dense (or denser) natural sands and may be
designed using an allowable bearing pressure of 100 kPa.
Footing design should also account for potential soil reactivity, advised by CSI to be equivalent to Class S. It
should be noted that placement of reactive clay may result in a more severe equivalent site classification.
All footings should found such that they are not adversely affected by any adjacent excavations, batter
slopes, trenches, retaining walls, or in ground structures 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/batter
slopes/excavations/retaining walls, then rising up at 1V:1H, as illustrated in Text Figure 7 below.
Text Figure 7: Footing depth required to prevent undermining or surcharge loading
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. It is recommended that footings 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 of excavations. It will be important to ensure close supervision of screw pile
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installation, as piling will not encounter drilling refusal at the recommended founding depth, and piles could
extend to much greater depth than necessary.
If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic 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.
We suggest the parameters outlined in Table 3 below 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.
Table 1: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Loose Sands 1.8 30 0.33 0.5 3.0
Medium Dense (or
1.9 34 0.28 0.44 3.5
Denser) Sands
Future Controlled 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. It is assumed that the pool floor will include a
hydrostatic pressure relief valve.
All excess stormwater collected around the residence (including overflow from rainwater tanks and pool)
should be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Tristania Drive.
Subsurface disposal of stormwater must not be undertaken.
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5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment, there would be a Low Risk of slope instability affecting the
proposed residence 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
Michael Cocks BEng (Civil/Geotechnical) MIEAust Ashley Davey BEng (Geological) RPEQ 8159
Geotechnical Engineer Principal Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 13
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 14
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
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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)