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Slope Stability Assessment
26 May 2022 Project No. 22178-001-Rev0 Jane & Cameron Blake C/- Adapt Planning Attention: Liam Pinese Email: [email protected] PRELIMINARY SLOPE STABILITY ASSESSMENT PROPOSED NEW RESIDENCE, SHED, OUTBUILDINGS & HORSE ARENA 42 LUKES ROAD, COOROY MOUNTAIN Dear Liam 1.0 INTRODUCTION Tectonic has undertaken a preliminary geotechnical assessment for a proposed new residence, shed, outbuildings and a horse arena at 42 Lukes Road, Cooroy Mountain. 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 development in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). Please note Tectonic must review a suitable ‘soil test’ report, building plans and earthworks drawings for the proposed development prior to construction to confirm the geotechnical recommendations provided in this report. 1.1 Details of Site and Proposed Development The site is described as Lot 77 on MCH546, covers an area of about 43.2 ha, and has frontage to Cooroy Mountain Road and Lukes Road along the northern and eastern boundaries, respectively. At the time of the assessment a residence, shed and outbuildings were located centrally on the site. The proposed new shed, outbuildings and horse arena are to be constructed adjacent to the existing structures, with the proposed new residence in the northern part of the site. Four dams were situated on lower-lying ground along the eastern boundary and in the southern part of the site. Semi-rural residential allotments with scattered houses typically surrounded the subject site. An aerial image of the site and surrounds is shown in Text Figure 1 overleaf. A detailed description of the site is given in Section 2. 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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Page 2
Jane & Cameron Blake 22178-001-Rev0
26 May 2022
Subject Site
Approx. Proposed New Residence
Approx. Proposed Shed, Outbuildings & Horse Arena
Text Figure 1: Aerial image of site and surrounds (courtesy of Noosa Council [NC] Interactive Mapping)
Drawings supplied by Aboda Design Group (Drawing Nos. P.101 to WD.602 [not inclusive], Rev DA, dated
13 April 2022), indicate that the proposed development will comprise the following:
New shed, horse arena and upgrade to the existing outbuildings to the east of the existing residence.
New driveways, access points and a gate house will connect the existing/proposed central structures
to Lukes Road.
New residence in the northern part of the site, connecting to the existing access point via a
meandering driveway.
No designs drawings were available for the proposed new sheds, horse arena and upgrades to the existing
outbuildings; however, construction is anticipated to comprise mostly lightweight materials (e.g.
weatherboard cladding to walls, sheet metal roofing etc.) either suspended above ground on posts or
supported via slab on ground.
Preliminary drawings indicate that the proposed new residence will comprise a two to three storey structure
mostly excavated into the hillcrest. Construction will likely comprise a combination of lightweight materials
and rendered masonry (or similar). To the north of the residence a lap pool, pool room and alfresco/levelled
grassed area is proposed, with a tennis court further to the north on a lower bench. A lower floor level will be
incorporated beneath parts of northern area to include a gym, bar, games/media room and spare bedrooms.
An access driveway is proposed to meander up the ridge flanks to the proposed residence from the existing
access point along the eastern boundary.
Cut and fill earthworks for the proposed central development (horse arena, shed etc.) are anticipated to be
relatively minor (i.e. less than 1 m depth/height). Based on provided sections it is anticipated that
excavations for the proposed new residence and surrounding ancillary structures will range up to about 4 m
deep, with filling likely to be up to about 1.5 m high to form finished levels and for the grassed terrace. No
details for the proposed access driveway had been provided; however, it is envisaged that cut and fill
earthworks in the order of 1 m depth/height will be required, depending on the desired alignment.
Isolated deeper excavations may be required for underground footings and service trenches. The site does
not have connections to main water and sewer and therefore onsite effluent disposal will be required. Text
Figure 2 overleaf illustrates the proposed development layout and some cross-sectional views of the
proposed new residence.
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Text Figure 2: Proposed layout plan (upper) and some cross-sectional views of the proposed new residence (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 and slope stability studies.
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
5 May 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 reviewed by Mr Ashley Davey, an RPEQ with more than 27 years of experience in
geotechnical engineering, including a number of slope stability projects.
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2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
Available geological information 1 indicates that the site is underlain by low grade meta-sedimentary strata
forming the Triassic age Kin Kin Beds. Quaternary age alluvial deposits are mapped to the east of the site,
with Oligocene to Miocene age rhyolite (Ti) forming Cooroy Mountain mapped further to the east.
Indicative Site Location
Text Figure 3: Extract from Gympie Surface Geology Map
To confirm the subsurface conditions across the proposed development area a suitable soil test report must
be reviewed by Tectonic prior to construction.
1
1:100,000 Gympie Surface Geology Map, Sheet 9445, DNRM, 2018
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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 shown in Text Figure 4, the subject site is dominated by a ridge
which crests in the western part of the site. Multiple spurs emanate off the ridge crest, primarily dipping
towards the east, with drainage lines/gullies on either side of the spur crests. The main residence is
proposed to be cut into the crest of the ridge, with the proposed access driveway meandering up the eastern
flank. Generally, the existing residence, sheds and outbuildings were located near the crest of a south-
easterly dipping spur, emanating off the main ridgeline. Four dams were located on lower-lying areas along
the eastern boundary and to south of the property.
Cut and fill earthworks had been undertaken in the central part of the site presumably during construction of
the existing structures. Based on a visual appraisal it appeared that cut and fill earthworks in the order of
about 1 m depth/height had been undertaken. Unretained fill batters typically were battered at less than 20°.
Natural slopes across the central part of the site (i.e. proposed new shed, outbuildings and horse arena),
typically were less than 10°, reducing to <5° along the eastern boundary. Gradients across the proposed
new residence area generally varied from relatively level atop the ridge crest, up to about 10° immediately
down the flanks. Steeper gradients were recorded further down the ridge flanks and within drainage lines
generally ranging up to about 25°.
Approx. Proposed New Residence
Approx. Proposed Sheds, Outbuildings and Horse Arena
Text Figure 4: Image showing ground surface contours (courtesy SCC MyMaps)
Text Figures 5 to 7 below and overleaf illustrates site conditions at the time of the assessment.
Text Figure 5: North-easterly view of the central part of the site and proposed development area
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Proposed Residence
Text Figure 6: View of the northern part of the site, looking northwards towards the proposed main residence
Text Figure 7: Panoramic view of the new residence area, looking north
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed across the proposed development areas
during our walk-over.
2.4 Surface Drainage
Natural surface drainage follows the ground surface contours from the proposed development areas,
primarily towards the east in the central development area and in all directions off the ridge crest at the
proposed new residence location. Generally, surface water would following the natural drainage lines and
positive gradients towards the existing dams and lower lying ground to the east/south. Surface water is
expected to drain well from the site considering the positive gradients and low permeability of the expected
clayey soils; however, poorer drainage conditions would be anticipated on level areas. No significant signs
of erosion were observed across the proposed development areas during the walkover assessment.
2.5 Vegetation
Vegetation across the site had primarily been cleared, with maintained grass around the existing residence
and mostly long grass/weeds elsewhere.
2.6 Buildings and Other Structures
At the time of the assessment the existing residence comprised a double storey structure of masonry
construction. Most of the existing sheds and outbuildings were of lightweight construction (e.g. sheet metal
roofing, timber flooring etc.). Based on an external visual assessment the existing structures appeared to be
in relatively good structural condition, with no significant signs of distress (e.g. cracking, misalignment etc.)
which could be attributed to slope instability. The existing sheds and outbuildings displayed age related
dilapidation (e.g. rotten panels etc.).
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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 8)
which indicates that the proposed central development area has a Very Low/Low Landslide Hazard whilst
the proposed new residence has a Very Low/Low to Moderate Landslide Hazard. A small area in the
southern part of the site is mapped as having a High Landslide Hazard which is well away from the proposed
development areas.
Proposed New Residence
Subject Site
Proposed Sheds, Outbuildings and Horse Arena
Text Figure 8: Extract of NC landslide hazard overlay mapping
It should be noted that the map is a broad-scale indication of the potential landslide susceptibility only and
is superseded by this site-specific investigation and analysis works. At the time of our visit, the proposed
development areas 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.
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 overleaf, and
the Qualitative Terminology for Use in Assessing Risk to Property is attached.
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Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research,
Very High
VH planning and implementation of treatment options essential to reduce risk to Low; may
Risk
be too expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation
H High Risk of 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
L Low Risk
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.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given in Section 4, it is assessed that there would a Low (L) Risk of global slope instability.
This is provided a suitable soil test report, building plans and earthworks drawings are reviewed by Tectonic
prior to construction. 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 below is our
qualitative assessment of 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 the
existing structures, dams and areas of the site outside of the proposed development areas.
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
The likelihood of a failure through future fill or natural
residual soil materials impacting the proposed
development is assessed as Unlikely due to the low
to moderate ground slopes across the proposed
1: Shallow failure through future
development area, favourable topography and
fill or natural soils above the Unlikely Minor Low
apparent lack of groundwater. The consequence of
foundation depth
such a failure would be Minor 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 very stiff residual
soils or weathered phyllite below the foundation depth is
2: Deep failure through very stiff
assessed as Rare due to the strength of these materials
(or stiffer) residual soil or
Rare Major Low and no signs of previous deep-seated instability across
weathered phyllite below the
the proposed development areas. Although the
foundation depth.
consequence of such a failure could be Major, 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 secondary dwelling.
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3.2.1 Effluent Disposal
No effluent disposal information for the proposed new residence had been provided at the time of reporting,
however the effluent disposal area should be located so that it is setback at least 15 m from the proposed
dwelling and run-off does not flow towards the development area, driveway or cuttings/embankments (i.e.
wastewater application areas should either be to the side(s) or down slope). The existing effluent
disposal/septic system should be inspected by a suitably qualified person to confirm it is performing
adequately and updated accordingly (if required).
It is recommended that diversion drainage be installed upslope of the irrigation area to avoid surface water
infiltrating into shallow trenches the land application area be revegetated as soon as possible. Vegetation
should include native species which are used for stabilisation of steep slopes and suitable for wet soils with
high nutrient loads (e.g. Lomandra).
The on-site effluent disposal system must be located a suitable distance away from any watercourse,
dwellings and recreation areas. Setback distances are subject to the level of treatment proposed and shall
be in accordance with Local and State Government requirements.
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
The planned extents of the proposed development shown in Text Figure 2 are acceptable from a slope
stability viewpoint. 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.
To achieve a Low Risk of instability, the following is recommended:
Tectonic must review a suitable soil test report, development plans and earthworks drawings
(including the proposed driveway alignment), with this report revised prior to construction.
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be RPEQ designed
and certified, which must include an assessment of global and internal stability.
Fill heights shall be limited to no more than 1 m on slopes exceeding 10°, unless assessed and
approved by a geotechnical RPEQ, and fully supported by an engineer designed and certified
retaining wall.
Design and construction of footings shall be undertaken in accordance with Section 4.3.
Formalised drainage should be constructed to ensure all stormwater is directed away from
structures and footings.
4.2 Earthworks
Cut and fill earthworks for the proposed central development (horse arena, shed etc.) are anticipated to be
relatively minor (i.e. less than 1 m depth/height). Based on provided sections it is anticipated that
excavations for the proposed new residence and surrounding ancillary structures will range up to about 4 m
deep, with filling likely to be up to about 1.5 m high to form finished levels and for the grassed terrace. No
details for the proposed access driveway has been provided; however, it is envisaged that cut and fill
earthworks in the order of 1 m depth/height will be required, depending on the desired alignment.
4.2.1 Site Preparation and Trafficability
Any organic rich topsoil, 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’ in the presence
of a suitably qualified and experienced civil/geotechnical engineer. Any such soft spots should be excavated
and replaced with select fill material, constructed as recommended in Section 4.2.3.
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The subgrade materials exposed after site stripping is expected to comprise stiff (or stiffer) clays, possibly
with some firm zones. 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 existing soils, should be achievable using small to medium sized conventional
earthmoving plant (e.g. 5 to 14 tonne excavators) with toothed buckets. Should excavations extend into any
very low strength (or stronger) weathered rock, the use of larger earthmoving plant with ripper/hydraulic
breaking attachments may be required. Should any high strength (or stronger) rock require excavation, the
use of other specialised excavation techniques (e.g. pre-splitting the rock with expansive grout, etc.) may be
required.
Temporary non-surcharged batter slopes may be formed at 1V:1H in stiff/medium dense (or stronger)
materials, 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
batters in stiff/medium dense (or stronger) natural soils and weathered phyllite up to 1 m high must be formed
no steeper than 1V:2.5H. Steeper temporary and permanent batters may be permissible in low strength (or
stronger) weathered rock, subject to an assessment by a geotechnical RPEQ.
Any excavation required adjacent to existing structures will need to be specifically designed to ensure that
the structures are not undermined.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
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.
We recommend that any future fill be placed as a minimum under Level 2 testing, with density testing
conducted at a Type 2 frequency. Fill must be ‘benched’ into the natural soil, over-filled by 0.5 m
(horizontally) and then trimmed back to the well compacted material.
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4.3 Footings
All footings for the proposed structures (including retaining walls) must penetrate through any fill and
unsuitable natural soils to found as follows:
On slopes of 10° (18%), or less – at least 0.3 m into stiff/medium dense (or stronger) natural
materials.
On slopes exceeding 10° - at least 0.3 m into extremely weathered (XW) materials/very low strength
(or stronger) weathered rock.
High-level footings on slopes greater than 10° would be possible in areas of cut where the target foundation
stratum (i.e. XW materials/very low strength rock) is exposed at the subgrade level.
High level footings may be designed using allowable bearing pressures of:
100 kPa in the stiff/medium dense or stronger natural soils.
400 kPa in the XW materials or very low strength (or stronger) weathered rock.
Piled foundations may be designed using allowable end bearing pressures of:
150 kPa in the stiff/medium dense (or stronger) natural soils.
600 kPa in XW materials or very low strength (or stronger) weathered rock.
Note: A suitable soil test must be reviewed by Tectonic to assess the subsurface conditions across the
proposed development areas and to confirm the geotechnical recommendations prior to construction.
Inspections must be undertaken by a suitably qualified and experienced geotechnical/structural RPEQ during
construction to confirm the foundation stratum and bearing capacity.
Footing design should also account for potential soil reactivity, to be advised by others. 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 or affect any adjacent excavations,
trenches, footings or retaining walls that are not designed to support building loads. Footings should found
at least below a plane extending 1 m horizontally from the base of trenches/excavations/retaining walls (i.e.
the boulder wall downslope of the proposed pool), then rising up at 1V:1H, as illustrated in Text Figure 9.
Text Figure 9: 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. It is essential that the piling contractor have a ‘mud bucket’ or similar to enable clean out of
the pile bases.
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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.
Preliminary 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. Please note these values must be confirmed
following review of a suitable soil test. 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
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Stiff (or stiffer) Clays 1.8 25 0.41 0.58 2.46
Weathered Rock 2.0 35 0.27 0.43 3.69
Future Fill* * * * * *
Note: * Depends on fill material type and level of compaction.
As a minimum, retaining structures must be founded as per the recommendations in Section 4.3 above with
the footings also designed to account for lateral loads imposed by the retaining wall. Detailed design must
include an assessment of internal and global stability, together with required bearing capacity and founding
depth, by the retaining wall design engineer.
4.5 Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction areas.
Permanent site drainage must be installed to ensure that stormwater runoff and pool 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 pool overflow during
high intensity storms) has the potential to saturate and weaken the fill, potentially leading to erosion and
slumping failure. Specific drainage recommendations are as follows:
To prevent the migration of stormwater into the proposed effluent disposal area, installation of
diversion drainage (e.g. landscaped swale) must be completed along the upslope side of the effluent
proposed disposal area(s).
Diversion drainage (e.g. landscaped swale) should be constructed upslope of the central
development areas.
Formalised drainage systems (e.g. concrete culverts etc.) will be required across the driveway
alignments if natural drainage lines are crossed. Concrete or rock lined channel drains should be
constructed along the cut side of the proposed driveways.
If gravel driveways are proposed, installation of ‘water bars’ across the driveway at intervals of not
more than 10 m is recommended, with water directed into the channels drains.
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, which must daylight at the ground surface.
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All excess stormwater collected around the proposed development and existing structures, including
discharge from the pressure relief valve, should be directed by pipes or surface channels (e.g. rock lined ‘dry
creek’ beds) to discharge at least 5 m downslope of any structure, embankment or effluent disposal area. 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).
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
affecting the proposed development subject to implementation of the recommendations given in this report
and review of a suitable soil test report, building plans and earthworks drawings prior to construction.
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 NER RPEQ 27540 Ashley Davey BEng (Geological) RPEQ 8159
Geotechnical Engineer Managing Director
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 15
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 16
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 18
LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)