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Reference Document - MCU24/0023
NOOSA COUNCIL
REFERENCE DOCUMENT
MCU24/0023 23 APRIL 2024
7 August 2023 Project No. 23236-001-Rev0
GJ Gardner Homes Gympie
c/- Covey Associates Pty Ltd
Email: [email protected]; Cc: [email protected]
SLOPE STABILITY ASSESSMENT FOR PROPOSED RESIDENCE
1073 POMONA KIN KIN ROAD, KIN KIN
Dear Scott
1.0 INTRODUCTION
Tectonic has undertaken a geotechnical assessment for a proposed residence at 1073 Pomona Kin Kin
Road, Kin Kin. This report includes a slope stability assessment, together with geotechnical advice for the
proposed development. In summary, subject to implementation of the recommendations made herein,
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 Site and Proposed Development
The site is described as Lot 2 on RP196994, covers an area of about 16.2 ha, with frontage to Pomona Kin
Kin Road along the eastern boundary. At the time of the assessment an existing dwelling was located in the
south-eastern part of the site. The proposed residence is to be positioned on the hill crest in the south with
access provided via a driveway from the north-east. Similar acreage allotments with scattered houses
surrounded the site in all directions. A 2022 aerial image of the site and surrounds is shown in Text Figure 1.
A detailed description of the site is given in Section 2.
Driveway alignment
Existing dwelling
Existing shed
Proposed building area
Subject Site
Text Figure 1: Aerial image of site and surrounds from 2022 (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 provided (Ref. G.J. Gardner Job No. GJGG430718 dated 4 May 2023) indicate that the
proposed residence will be sited on the hill crest in the southern part of the site. The residence will
comprise a single storey, slab on ground structure of masonry and timber framed construction and a sheet
metal roof. Based on the existing site conditions it is envisaged that only minor further levelling works will
be required to form the building pad and so fill/cut earthworks of up to about 0.5 m high/deep are
anticipated.
Text Figure 2: Extracts from design drawings, site layout plan (upper LHS), site plan (upper RHS) and elevation (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 Ground Drill
(Ref. GD Job No. 22/244, dated 24 June 2022).
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 13 July 2023.
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. David Pollock, an RPEQ with more than 17 years’ experience in
geotechnical engineering, including a number of slope stability projects.
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2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
The 1:100,000 scale Gympie Surface Geological Map 1 indicates that the site is underlain by materials
derived of the early to middle Triassic age Kin Kin Beds, which are described as “commonly cleaved and
commonly kinked mudstone…”. The Australian Stratigraphic Units Database describes the Kin Kin Beds as
“arenite, argillite and phyllite.”
Cuttings exposed an upper profile of highly deformed phyllite that was kinked and cleaved with the rockmass
essentially presenting as a clayey gravel profile. Low strength (or stronger) phyllite was exposed in the
deeper driveway cuts generally exposed in places along the table drain.
;
Text Figure 3: Weathered Phyllite exposed in driveway cuttings
The GD investigation comprised the drilling of two boreholes (designated BH1 and BH2) across the
proposed residence area. The GD boreholes encountered gravelly clay fill to depths of 0.2 m to 0.5 m below
ground level (BGL), then distinctly weathered rock upon which auger drilling refusal was encountered at
depths of 1.1 m to 1.5 m BGL.
GD classified the site Class P but, if not for the site filling, as equivalent to Class M per AS2870.
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 shown in Text Figure 4 overleaf, the site featured two distinct hill
crests in the south-western part joined by a ridgeline ‘saddle’. The flanking slopes fell away from the hill
crests in all directions with a series of narrow spurs and broader gully heads dipping into more incised gully
lines below.
1
State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, Sheet 9445, 2018
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Subject Site Boundary
Indicative Proposed House Site
Text Figure4: Image showing ground surface contours (courtesy NC ‘Intramaps’)
The proposed residence is located in a favorable position atop the crest of the hill, however the natural
slopes below were, in places, very steep (up to and exceeding 30°) particularly in the drainage or gully lines.
In those areas, where surface drainage was assessed to be poor, there was evidence of previous slope
instability comprising:
The backscarp of a relatively large landslide to the south-east of the house site;
Scour/slumping on the steep slope to the east;
Creep as evidenced by tilted trees to the north-west; and
A number of slumps along the driveway cutting with signs of downslope creep movements below.
Some of these features are illustrated in Text Figure 5 overleaf.
It would appear that where the ground surface dipped at ≥25°, the friction angle of the upper gravelly
rockmass was exceeded and so, under saturated conditions, where cohesion was reduced or lost, landslide
movements were prone to occur.
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Text Figure 5: Areas of slope instability noted across the site
The hill crest upon which the residence is to be sited appeared to have been somewhat levelled by cutting
down in the central part and filling to the perimeters, particularly the south-eastern part where fill appeared to
be in the order of up to 2 m high, and battered quite steeply at up to about 32° above the gully head
(containing a landslide) below.
Text Figures 6 and 7 overleaf illustrate site conditions across the proposed development area at the time of
the assessment.
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Text Figure 6: View of the proposed residence area, looking south
Text Figure 7: View of the proposed residence area, looking north
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2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed across the proposed development area
during our walk-over, nor was groundwater mentioned on the GD borehole reports, with the soils
described as ‘dry’ and ‘moist’.
2.4 Surface Drainage
Natural surface drainage follows the ground surface gradients from the proposed development area
shredding off the crest and ridgeline mostly to drainage lines to the north-east, south-east and west.
Generally, surface water is expected to drain well from the site considering the positive gradients and low
permeability of the clayey soils; however, poorer drainage conditions are anticipated on level areas.
No signs of significant erosion were observed around the proposed development area at the time of the
assessment; however, channel and rill erosion were observed along the driveway table drain.
2.5 Vegetation
During our walkover assessment, the proposed development area had grass ground cover. Relatively
dense bushland traced the drainage lines, with some clearing having been completed across the more
linear slope to the east. Tilting and leaning trees were observed on steeper ground along the gully heads
and drainage lines away from the proposed residence area.
Some cleared vegetation had been felled and left stockpiled above the south-eastern gully head which
could create an area of poor drainage. Measures to address that matter and improve drainage in other
areas of the site are given in Section 4.5 of this report.
Text Figure 8: Cleared vegetation above south-eastern gully head
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2.6 Buildings and Other Structures
At the time of the assessment the existing dwelling comprised a single storey structure located
approximately 200 m to the east of the proposed new residence.
The driveway traversed up across the northern flanking slopes and onto the ridgeline of the hill crest where
the residence is proposed. The driveway was gravel surfaced and typically either formed at grade or set into
the slope by cut and fill up to about 2.5 m deep/high. Cut batters were formed at slopes of up to about 40°
with fill batters typically flatter in the order of up to 30°. Several scour/slump features were noted (Ref. Text
Figure 5) in the driveway cuttings where surface water appeared to overtop the embankments.
Text Figure 9: Typical driveway formation comprising cut/fill across northern slope
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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 10)
which indicates that the proposed development area and immediate surrounds have a Very Low/Low
Landslide Hazard. Most of the remaining areas of the site are mapped as having Moderate to High
Landslide Hazard.
Site boundary
Proposed building location
Text Figure 10: Extract from NC Landslide Hazard Overlay Mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility only
and is superseded by this site-specific assessment. At the time of our visit, the proposed residence area
did not exhibit any indicators of global slope instability. No landslide back scarps, groundwater springs,
tension cracks, or areas of naturally ‘hummocky’ ground were observed across the hill crest however the
natural slopes below were, in places, very steep particularly in the drainage or gully lines. In these areas
where surface drainage was poor, there was evidence of previous slope instability.
There is potential for such slope movements to regress upslope which could put the residence at increased
risk of instability. This is particularly relevant in the south-eastern part as the building pad appears to have
been formed by filling above steep ground with a landslide backscarp in that area (Ref. Text Figure 11).
Measures to mitigate and reduce the risk of instability are given below in Sections 3.2 and 4.
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Text Figure 11: Areas at increased risk of 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 below, and
the Qualitative Terminology for Use in Assessing Risk to Property is attached.
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
M Moderate Risk 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.
VL Very Low Risk 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.
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To reduce the risk of instability to the residence, it is recommended that the building location is moved to the
north so that it is sited immediately atop the crest and adjacent to the east trending spur line (Ref. Text
Figure 12 below) and so it does not to encroach areas where unacceptable landslide risk has been identified.
The entire residence and any ancillary structures must be founded on footings taken to penetrate through
any fill and upper gravelly phyllite to socket at least 0.5 m into low strength or stronger phyllite. Filling across
the south-eastern part should be removed such that the currently over-steep batter can be reprofiled to
1V:2.5H and surface water must be diverted away from that area and directed as sheet flow to the eastern
spur.
Text Figure 12: Recommended area for building to be relocated to and areas requiring earthworks and drainage
improvements around the levelled building platform
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given above and in Section 4, it is assessed that there would a Low (L) Risk of global
slope instability affecting the proposed residence.
Regulators (NC) normally require that a Very Low or Low Risk of landslide affecting property must be
demonstrated to enable development approval. Summarised in Table 2 is our qualitative assessment of
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landslide risk for the site. A summary of qualitative terminology for use in assessing risk to property is
attached (taken from AGS 2007).
It should be noted that this risk assessment excludes geotechnical risk that may be associated with settlement
due to consolidation of any uncontrolled fill materials over time which could affect non-piled structures (e.g.
pavements, landscaping structures). This risk assessment also excludes geotechnical risk associated with
the existing structures, dams, and areas of the site outside of the proposed residence area.
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
The likelihood of a failure through fill or natural soil is
assessed as Unlikely due to the gentle to moderate
ground slopes immediate to the proposed
1: Shallow failure through fill or development area and strength of the materials.
natural soil above the foundation Unlikely Minor Low The consequence of such a failure would be Minor
depth considering the anticipated confined extent of
instability, and the depth of foundations
recommended, with the resultant risk being Low as
per AGS 2007.
The likelihood of a deep failure through low strength (or
stronger) phyllite below the foundation depth is
assessed as Rare due to the strength of these
2: Deep failure through phyllite materials, apparent lack of groundwater, and no
Rare Major Low
below the foundation depth evidence of such existing deep-seated instability across
the crest area. Although the consequence of such a
failure could be Major, the resultant risk is Low as per
AGS 2007.
The likelihood of ongoing slope instability on the
steeper, more poorly drained slopes is assessed as
Likely due to the evidence of previous instances of
3: Ongoing scours, slumps or such movement. Provided the residence is sited away
landslides on steeper slopes Likely Insignificant Low from those areas, and drainage and earthworks
and drainage lines. improvement measures are carried out as
recommended herein, the consequence of such a
failure would be Insignificant to the residential
structure. The resultant risk is Low as per AGS 2007.
The potential impacts of slope instability on the proposed development have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts so that a Low Risk
of instability would be achieved for the proposed residence.
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
To achieve a Low Risk of instability for the proposed residence, the following is recommended:
The building location must be moved to the north so that it is sited immediately atop the crest (Ref. Text
Figure 12).
All structures be founded on footings taken to penetrate through any fill and upper gravelly phyllite to
socket at least 0.5 m into low strength (or stronger) phyllite.
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Filling across the south-eastern part must be removed such that the currently over-steep batter can be
reprofiled to 1V:2.5H and surface water must be diverted away from that area and directed as sheet
flow to the eastern spur.
Further earthworks cut/fill heights shall be limited to less than 0.5 m unless assessed and approved by a
geotechnical RPEQ.
Formalised drainage should be constructed to ensure all stormwater is directed away from the proposed
structures.
Should any revisions be made to the proposed extent of the development, Tectonic must review these prior to
construction to assess if there are any impacts on slope stability.
4.2 Earthworks
It is anticipated that only minor earthworks (< 0.5 m deep/high) will be required to form final levels for the
residence. Deeper excavations may be required for underground footings and service trenches. Filling
should not exceed 0.5 m high, unless assessed and approved by Tectonic prior to construction.
4.2.1 Site Preparation and Trafficability
Any organic rich topsoil, uncontrolled fill, firm/loose (or weaker) soils and overly wet materials must be
stripped and removed from any pavement or at grade construction areas. Topsoil materials must not be
used in structural fill; however, may be placed across landscaping areas. It is recommended that any at
grade building/pavement subgrades be proof rolled using a minimum 4 tonne pad foot roller to identify any
‘soft spots’. Any such soft spots should be excavated and replaced with select fill material, constructed as
recommended in Section 4.2.3.
The subgrade materials exposed after site stripping and excavations are expected to comprise stiff (or stiffer)
gravelly clays. The clayey soils could be subject to strength loss from repetitive vehicle loading (particularly
from rubber tyred plant) or if they become wet. Due to the potential for poor trafficability during and after wet
periods, placement of a minimum 150 mm thick granular working platform may be required, particularly along
access ways and any laydown areas.
The site should be graded to shed surface water runoff away from construction areas. It should be noted
that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion and
sediment control measures must be installed prior to commencement of earthworks at this site.
4.2.2 Excavations
Excavations within the soils to the GD investigation depths, should be achievable using small to medium
sized conventional earthmoving plant (e.g. 5 to 14 tonne excavators) with toothed buckets, or ripper
attachments. Although not anticipated, should excavations extend into any low strength (or stronger)
weathered rock, the use of larger earthmoving plant with ripper/hydraulic breaking attachments may be
required.
Temporary, non-surcharged batter slopes may be formed at 1V:1H in the stiff (or stiffer) soils, provided
groundwater seepage is not encountered. Flatter batters may be required if groundwater seepage is
encountered, or if earthworks are undertaken during a period of heavy rainfall. Permanent non-surcharged
batters in stiff (or stiffer) natural soils and controlled fill must be formed no steeper than 1V:2.5H. Where such
battering is not possible, temporary and/or permanent retention will be required.
Existing over-steep cut and fill batters shall be trimmed back to permissible grades as outlined above or
retained.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
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4.2.3 Filling
Site soils where free of deleterious materials (e.g. vegetation, oversize etc.), are expected to be suitable for
re-use as structural fill, subject to appropriate moisture adjustment. It should be noted that placement of
highly reactive clays could be difficult and impacts on site classification and pavement design 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
Controlled fill materials should be compacted at moisture contents within the range of -2% to +2% of OMC for
Standard Compaction. Confirmatory testing must be carried out at regular intervals and further details for
control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for
Commercial and Residential Developments. 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.
Fill must be ‘benched’ into the natural soil, over-filled by 0.5 m (horizontally) and then trimmed back to the
well compacted material.
4.3 Footings
All footings for the proposed residence and ancillary structures must penetrate through any fill (existing and
future) and natural soils to found at least 0.5 m into low strength (or stronger) phyllite. Based on the GD
boreholes, founding depths of about 1 m to 1.5 m BGL are anticipated. It is therefore recommended that the
building should be supported on bored piles or ‘bucket piers’.
Footings so founded may be designed using an allowable bearing pressure of 250 kPa.
Footing design should also account for potential soil reactivity, advised by GD to be equivalent to Class M. It
should be noted that the placement of highly reactive clays, such as the onsite materials may result in a
more adverse site classification.
All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
retaining walls, or in ground structures that are not designed to support building loads. 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. 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 in Text Figure 13 overleaf.
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Text Figure 13: Zone of influence diagrams – footings to be founded below these lines
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. Should bored piles be required, it is essential that the piling contractor have a ‘mud bucket’
or similar to enable clean out of the pile bases.
A structural/geotechnical RPEQ should inspect the footing excavations to confirm appropriate foundation
materials and cleanliness prior to pouring concrete. If any soil conditions encountered during construction
are found to differ from those noted above, Tectonic and Ground Drill must be notified immediately, and an
inspection carried out to determine if changes to footing design are required.
4.4 Retaining Walls
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.
Design for retaining structures could be carried out using a pressure distribution applicable to the wall type
and based on the parameters listed in Table 3. These values do not include allowance for hydrostatic
pressures nor transient (e.g. construction) or adjacent footing loadings.
Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(kN/m3) Kp
(Cantilever wall) (Non-yielding wall)
Stiff (or stiffer) Clays 18 27 0.38 0.55 2.66
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.
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4.5 Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area.
Permanent site drainage must be installed to ensure that stormwater runoff and residence overflow is
directed away from structures and footings (e.g. positive gradients and/or channel drains), and also not
allowed to flow over adjacent fill areas. Uncontrolled flow of stormwater over fill batters (e.g. from residence
overflow during high intensity storms) has the potential to saturate and weaken the fill, potentially leading to
erosion and slumping failure. Any erosion affected areas should be reinstated as soon as possible.
Specific drainage recommendations are as follows:
Capture any stormwater from level areas and direct it into controlled drainage to prevent saturation
of foundation materials around the structures.
Create diversion drains (i.e. contour berms) above the gully heads as shown in Text Figure 12.
Remove any stockpiles of vegetation or debris which might create poor drainage on the slopes
below the residence and also re-profile uneven ground around removed tree root balls to promote
run off of surface water and prevent ponding.
Apply surface and erosion protection to any exposed slopes including as a minimum revegetation
with surface protection matting and scour protection in areas where required.
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 proposed development should be directed by pipes or surface
channels (e.g. rock lined ‘dry creek’ beds) to discharge at least 10 m downslope of any structure,
embankment or effluent disposal area. Stormwater outlets should be positioned on natural spurs. A surface
dissipation device such as rock weir or rock lined contour drain must be installed at the stormwater outlet(s).
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).
4.6 Driveway
Ongoing instability in the form of batter scours or slumps is likely to occur on both the cut and fill
embankments along the upper, north-western part of the driveway, thus requiring maintenance and repairs
on an ongoing basis.
To minimise maintenance requirements, the following mitigation and drainage control measures are
recommended for the driveway:
Any over-steep batters should be reprofiled to batter slopes of 1V:2.5H (or 1V:1H in low strength or
stronger rock). Where space and slopes prevent re-profiling retention using engineer designed
retaining structures could be considered.
Revegetate any bare soil exposed. The use of native species appropriate to steep slopes and this
region are recommended. Hydromulch may be used on soils exposed in batters.
Create diversion drains above the crest of the cut and fill embankments and direct flows to chute
drains at preferable locations to outlet below in a controlled fashion.
Install water bars at regular intervals to direct surface water into a swale drain along the up slope
side of the driveway.
Install appropriately sized culverts at regular intervals to transfer stormwater flows from the swale
drain to the down slope side of the driveway.
Protect the swale and culvert inlets/outlets from erosion by placement of rock fill, or similar.
Regularly inspect and clean out culvert inlets to prevent blockage by leaf litter, soil, vegetation, etc.
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5.0 CONCLUSION
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
affecting the proposed residence subject to implementation of the recommendations given in this report. It
will be necessary to re-site the building envelope to the north and implement other measures to achieve a
low risk.
6.0 LIMITATIONS
This assessment relates only to the proposed residence. Existing structures, slopes, dams, 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
David Pollock BE(Civil) RPEQ 14121
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 19
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 20
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
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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 22
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)