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Slope Stability Assessment
13 February 2026 Ref.: 26020-001-Rev0
SCG Consulting Engineers
Attention: Tom Harrington
[email protected]
RE: SLOPE STABILITY ASSESSMENT
PROPOSED RESIDENCE, GYM AND STABLES, 111 LUKES ROAD, COOROY MOUNTAIN
Dear Tom,
1. Introduction
As requested, Tectonic has undertaken a slope stability assessment for a proposed residence, gym and
stables at 111 Lukes Road, Cooroy Mountain. This report presents the results of our slope stability
assessment, together with geotechnical advice for the proposed works. In summary, subject to
implementation of the recommendations made herein, there should 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). This level of risk is normally acceptable to
regulators (e.g. Noosa Council [NC]).
1.1 Details of Site and Development
The subject site is described as Lot 2 on RP214048, covering an area of about 36.78 ha with access off the
existing terminus of Lukes Road to the north-western corner. The gazetted corridor for Lukes Road
continues, undeveloped, around the western boundary of the lot. An existing residence and sheds were
located towards the south-west of the lot, with the remainder of the property comprising a mixture of
open paddocks and bushland. The surrounding allotments were of similar scale and land use. A detailed
description is given in Section 2.
Lot boundary
Existing entry point off Lukes Rd
Existing residence and sheds
Text Figure 1: Recent aerial image of site & surrounds (courtesy Near Map, 6 December 2025)
Tectonic Geotechnical Pty Ltd
A.B.N. 83 165 727 828
PO Box 899, Buderim, QLD 4556 E [email protected]
P (07) 5478 9016
W tectonicgeo.com.au 1 of 13
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Page 2
Preliminary building design drawings were supplied to Tectonic with the project brief (Ref. Skale, Job No.
2403, Dwg No. 02 to 12, Rev CD08, dated 16 July 2025). Also supplied were concept civil works drawings
by SCG (Ref. Dwg. O. 0577-CS to OW5.1, received 10 February 2026).
We understand that the proposed works would involve the demolition of the existing residence and
sheds further towards the south-west corner of the block. The new residence would be positioned
between these structures over a relatively large footprint on the crest of a topographical spur.
Preliminary earthworks information indicates that a flat building pad would be created for the residence.
Cut earthworks of up to 3 m depth would be proposed on the upslope side (to the east) with fill ranging
to a maximum of 1.9 m height on the downslope side (to the west). The building design drawings show
that the proposed residence would be formed over one storey, with some minor changes in level
between different areas of the house. A concrete ground slab is anticipated under the building, with a
mixture of masonry, clad and full height glass for the walls. A sheetmetal roof will be utilised over. A
swimming pool is indicated to the north-western corner of the building footprint, which is expected to
comprise a concrete shell being partially in-ground to the south, and completely out of ground on the
downslope/northern side.
A gym is proposed on sloping ground to the south-east/upslope of the existing residence. No structures
are present currently within the footprint of the proposed gym. The cut/fill platform for the gym would
be cut sidelong across the natural slope, resulting in about 1 m depth/height of cut/fill, which would be
retained. The gym building would have masonry and clad walls and a sheetmetal roof over one storey.
To the west of the proposed residence an existing shed would be demolished, with a larger stables
installed over the same footprint and beyond. The stables would be of a ‘barn’ style construction, with an
elevated pitched roof to the centre, and timber clad walls and sheetmetal roofing. Further west, a large
covered arena (roofing suspended on an open sided, steel portal frame) and uncovered roundyard would
be built. These elements are positioned over a gently defined, wide, rounded spur crest; and would have
an earthworks platform formed mostly in cut (up to about 1 m depth below existing levels) with fill to the
edges not likely to exceed 0.5 m height.
The property does not have access to mains water and sewerage and therefore collection of rainwater in
tanks is undertaken at the property; and a new effluent disposal system and land application area will be
added away from the house to the east to manage wastewater.
Text Figure 2 provides an extract of the architectural layout drawings.
Proposed residence
Proposed gym
Proposed arena & stables
Text Figure 2: Extract of layout drawings (courtesy Skale)
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1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of
published geological maps, NC overlays, slope stability studies and a ‘soil test’ report by Australian Soil
Investigations (ASI, Ref. Report No. 250385 [S], dated 23 January 2026). An effluent disposal design
report and site evaluation were also provided (ASI, Ref. Report No. 250385 [A] dated 19 January 2026). A
walk-over survey was conducted to make an appraisal of the general condition of the site regarding
topography, drainage, vegetation cover, geology, erosion, condition of existing structures and slope
stability. The walk-over was carried out by an experienced engineering geologist from Tectonic on
9 February 2026. 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 Ashley Davey, an RPEQ with more than 30 years’ experience in
geotechnical engineering, including numerous slope stability projects.
2. Description of Existing Conditions
2.1 Geology
A published geological map for the region1 indicates that the site is underlain by the middle Triassic age
Kin Kin Beds (dark green/blue shading); a metasedimentary unit described as “strongly cleaved and
commonly kinked mudstone with thin mudstone laminae, minor volcanilithic sandstone.” An extract of the
relevant geological mapping is shown below in Text Figure 3.
Approx. site location
Text Figure 3: Extract of Nambour Surface Geology Map
At the time of our site visit the surface across the site was relatively well maintained with established
vegetation, and lawns around the existing residence; and slashed paddocks in the surrounds. Along the
1
1:100,000 Scale Gympie Surface Geology Map, Sheet 9445, Dept. Natural Resources, Mines & Energy, 2018
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unsealed driveway west of the residence pale orange brown, silty clay soils were exposed in some small
erosion rills. This type of soil is analogous with the weathered materials of the Kin Kin Beds.
The ASI ‘soil test’ comprised the drilling of four boreholes, denoted BH1 to BH4 around the perimeter of
the proposed residence. Boreholes were drilled using a 4WD utility mounted, solid flight auger drill rig.
The boreholes featured similar subsurface profiles, with up to 300 mm of topsoil logged at surface, then
stiff, high plasticity clay to 1 m below ground level (BGL), followed by very stiff, silty clay to the borehole
target depths of 3 m BGL (boreholes BH2 to BH4). Deeper probing was completed in borehole BH1 to
5.8 m depth BGL, where “dense gravels” were noted on the log.
Groundwater seepage was not mentioned in the ASI borehole reports, with the soils described as “moist.”
ASI classified the site as Class M in accordance with AS2870, with a characteristic surface movement due
to normal seasonal moisture variation (ys) only therefore of up to 40 mm.
2.2 Topography
Ground surface contours across the property and close surrounds are illustrated in Text Figure 4 below.
Lot boundary
Central drainage line
Approx. stables Approx. proposed gym
& arena
Approx. proposed residence
Text Figure 4: Topography of site and surrounds (courtesy Qld Globe)
The site consists of an elongated shape, centred on a relatively wide valley. The valley has a head in the
eastern part of the lot, and drains to the east/north-east. The ground slopes down from much higher
ground off site to the north in to the central valley, however the proposed buildings would be located
along the crest of a very gently defined, rounded spur crest. The spur trends to the east.
Around the site of the proposed residence the crest area of the spur was >20 m wide, dipping to the east
at about 4°. The southern flank of the spur was inclined at about 5°, and the northern flank ranged up to
9°. The spur crest was wider around the proposed stables and arena, however it was noted that the
southern edge of the building area dipped down at about 8°. Behind the existing residence, the upper
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slopes of the spur (where the gym is proposed) were at around 6°, with landscaping and the original
earthworks cut adjacent to the house steepened to about 10°.
Downslope of the existing residence a garden terrace had been formed by fill earthworks, which also
contained an in-ground swimming pool. The fill was estimated to be about 1.2 m height above natural
ground, and had been left battered at about 16° to 17°.
Ground slope angles were measured at the time of our site visit using a hand-held clinometer.
Site conditions at the time of our walk-over are shown in Text Figures 5 to 7 below, and on the following
page.
Text Figure 5: Looking east across site of proposed residence
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Text Figure 6: Looking west over existing shed (site of proposed stables)
Text Figure 7: Ground on southern edge of proposed stables (LHS) & proposed arena location, looking
west (RHS)
2.3 Groundwater
No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over around
the proposed building sites, and groundwater strikes were apparently not encountered in the ASI ‘soil
test’ boreholes.
2.4 Surface Drainage
The site has a theoretical catchment extending upslope for about 150 m to the east of the buildings (the
gym) to a localised knob in the rounded spur.
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The spur becomes narrow beyond this, with a number of drainage lines shedding to the north and south. Therefore, the catchment is limited to the site, and would be reduced further by the divergent shape of the spur crest and shallow gradients. Thus waters would be spread slightly, with little or no hydraulic acceleration towards the site. While the spur of the rounded spur on which construction is proposed was relatively shallow in gradient, and wide, it is acknowledged that within a short distance there would be positive gradients away from the future develop areas. No signs of significant erosion or exposed soil was observed in the vicinity of the proposed buildings, or existing slopes and earthworks batters in existing garden areas. 2.5 Vegetation Around the south-western corner of the block there were a mixture of grazed and slashed, maintained paddocks. To the southern edge of the existing shed (where the stables are proposed) were small orchard trees, and slightly larger mature trees were located along the southern edge of the earthworks footprint for the proposed residence. Around the existing residence, a mixture of overgrown gardens and garden beds were observed, with some medium size trees closer to the house. On the steepest slopes observed during our walk-over near the development area (up to 10° to the north-east, where effluent disposal is proposed) there were tall, mature bunya and gum trees. The trees were not notably leaning or otherwise distorted in their growth, such that signs of slope instability could be inferred. 2.6 Buildings and Other Structures The existing residence was accessed via. an unsealed driveway off Lukes Road, which ran past the existing shed, and along the southern edge of the building footprint for the proposed residence. The existing residence consisted of a single main level, being on ground to the east, and with an undercroft installed to the west. The house had rendered masonry block walls, with timber suspended verandahs to the northern/north-western edge. A smaller shade house was located to the north-east of the residence, with a concrete floor slab, timber framing and walls and a sheetmetal roof. Beyond this, were in-ground concrete water tanks. As mentioned above, an in-ground pool was noted to the north of the house, in an area of fill. The pool had a concrete shell and tiled coping, and did not appear to be cracked or leaking. The existing structures on site appeared to be in relatively good condition from a slope stability perspective, with no leaning, bulging, cracking or other structural distress observed that could be attributed to slope instability. 3. Assessment of Land Stability 3.1 Existing Conditions NC landslide susceptibility mapping (Ref. extract in Text Figure 8, overleaf) indicates that the sites of the proposed buildings mostly have a Low/Very Low susceptibility to landslide (no shading) with some isolated areas of Moderate susceptibility to the north, on the lower flanks and invert of the central drainage line. 13 February 2026 26020-001-Rev0 7 of 13
Page 8
Lot boundary
Approx. proposed building locations
Text Figure 8: 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
and is superseded by site specific investigation and this assessment. Near the proposed residential
buildings there were no signs of global, natural 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 which is also attached.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
VH Very High Unacceptable without treatment. Extensive detailed investigation and research, planning and
Risk implementation of treatment options essential to reduce risk to Low; may be too expensive and
not practical. Work likely to cost more than value of property.
H High Risk Unacceptable without treatment. Detailed investigation, planning and implementation of
treatment options required to reduce risk to Low. Work would cost a substantial sum in relation to
the value of the property.
M Moderate May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Risk investigation, planning and implementation of treatment options to reduce risk to Low. Treatment
options to reduce to Low risk should be implemented as soon as practicable.
L Low Risk Usually acceptable to regulators. Where treatment has been required to reduce the risk to this
level, ongoing maintenance required.
VL Very Low 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.
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Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there should be a Low Risk of global slope instability
affecting the proposed shed. Regulators (NC) normally require that a Very Low or Low Risk of landslide
affecting the property must be demonstrated to enable development approval.
Summarised in Table 2 is our qualitative assessment of landslide risk for the extensions. 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 Comments
Risk
The likelihood of a landslide within the fill or
natural soil materials affecting the proposed
structures is considered Unlikely, given that no
evidence of such slope movement was noted
Landslide extending
on the site or in the immediate vicinity,
through proposed fill
moderate ground slopes (even on the
and/or natural residual Unlikely Medium Low
spur/drainage line flanks, being ≤10°) apparent
soils to cause damage to
lack of groundwater seepage, and that there
the proposed structures
was an opportunity to create favourable
surface water drainage. The consequence of
such instability would be Medium, with the
resultant risk being Low as per AGS 2007.
Given the strength of the deeper residual soils,
limited extent of deeper earthworks filling, and
absence of indicators of slope instability, it is
Deeper landslide (>2 m) considered Rare that a deeper landslide would
through very stiff (or occur extending below the proposed
Rare Major Low
stiffer) residual soils below foundation depths, subject to implementation
the foundation depth of the recommendations in Section 4. Even for
a Major consequence from such instability
there would be a resultant Low risk as per AGS
2007.
4. 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 Extent of Development
The location and nature of the proposed residence, stables and arena (including earthworks) as
described in Section 1.1 shall not be altered unless assessed and approved by Tectonic.
4.2 Earthworks
Future earthworks in building areas are expected to be most significant for the proposed residence,
extending to about 3 m maximum depth of cut and up to about 1.9 m height of fill. Deeper isolated
excavations may be required for footings and underground services.
Any loose or soft soils, saturated silts, organic rich topsoil and severely root affected soils, must be
removed from the proposed construction areas. Following site stripping subgrades should be proof
rolled using a minimum 4 tonne roller in the presence of a civil/geotechnical RPEQ to identify any ‘soft
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spots’. Materials in any such soft spots should be excavated and replaced with select fill material, constructed as recommended below. Any fill materials utilised should be free of organic or otherwise unsuitable materials and compacted at moisture contents within the range of -2 % to +2 % of optimum moisture content for Standard Compaction. 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. All fill must be ‘benched’ into the sloping ground. Fill should be overbuilt and then trimmed back to the well compacted material. 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. Given the extent of earthworks proposed, it is suggested that Level 1 geotechnical inspection and testing should be conducted at a Type 3 frequency. Excavation through the natural residual soils should be achievable using conventional earthmoving plant (e.g. excavators with toothed buckets/soil augers) to the maximum depth of the borehole drilling. Non-surcharged, temporary batter slopes should be formed no steeper than 1V:1H in stiff (or stiffer) natural soils. Non-surcharged, permanent batter slopes should be constructed at a maximum grade of 1V:2H in stiff (or stiffer) natural soils or controlled fill up to 2 m high. Permanent batter slopes above this height are not anticipated within the scope of the project works. Any permanent soil or fill batters would nonetheless require erosion protection (e.g. revegetation or surface protection). 4.3 Retaining Structures Retaining structures must be founded as described in Section 4.4 below. Engineer design and certification of retaining walls is required where such walls: ▪ Exceed 1 m height. ▪ Are subjected to surcharge loads (e.g. from backslope, adjacent structures, etc.). ▪ Are located within close proximity to other structures (e.g. the existing shed, proposed dwelling). We suggest the parameters as shown in Table 3 over page may be adopted for retaining wall design. 13 February 2026 26020-001-Rev0 10 of 13
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Table 3: Retaining Wall Design Parameters
Lateral earth pressure coefficients
Unit weight K0
Retained material Friction angle (°) Ka
(kN/m3) (Non-yielding Kp
(Cantilever wall)
wall)
Natural Stiff to
19 27 0.38 0.55 2.66
Very Stiff Residual
Soils
Future/Existing Fill * * * * *
* Depends on type of fill material used and compaction achieved
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Retaining structures should be engineer designed in accordance with AS4678-2002 Earth-retaining
structures. Retaining wall footings are to be founded as required in Section 4.4 (at a minimum). Internal
stability of any proposed retaining walls, including confirmation of sufficient resistance to sliding, over-
turning and bearing capacity failure, as well as global stability (if required), shall be assessed by the wall
designers.
4.4 Footing Design
It is recommended that all footings are taken to found a minimum of 1 m below ground level and a
minimum of 0.3 m into stiff (or stiffer) natural residual soils (whichever is greatest). Footings so founded
could be designed using a maximum allowable bearing pressure of 100 kPa (high level footings) or
150 kPa maximum allowable end bearing pressure for piles. Soils greater than 1 m depth below the
existing surface were noted by ASI to be very stiff, and therefore allowable bearing pressures of up to
200 kPa could be tolerated where this material is verified in the foundation subgrade. It is expected that
high-level footings could be utilised following site stripping in areas of earthworks cut. A transition
through mass poured concrete pads (‘bucket piers’) and piles would be required in deeper areas of
proposed filling.
Footing design should also account for the characteristic surface movement (ys), including allowance for
the proposed earthworks.
All footings should found such that they are not adversely affected by (or affect) any adjacent
excavations, trenches, or retaining walls that are not designed to support building loads. Footings
should found at least below a plane extending 1 m horizontally from the base of trenches/excavations/
retaining walls, then rising up at 1V:1H, as illustrated in Text Figure 9 on the following page.
Additionally, footings should not be founded closer than 1.5H from the crest of the batter slopes
(H being the maximum height of fill).
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Text Figure 9: Footing depth required to minimise risk of undermining A suitably qualified and experienced RPEQ engineer must inspect the footing excavations prior to placement of steel and concrete to ensure achievement of the required depth, founding stratum and cleanliness. It will be essential that the foundation contractor have available a ‘mud bucket’ to clean out the pile bases if bored piles are adopted for deepened footings, such that the recommended founding stratum is exposed over the full base area and not obscured by ‘fall in’ or other drilling spoil. If any ground conditions encountered during construction are found to differ from those noted in Section 2.1 above, Tectonic and ASI should be notified immediately and an inspection carried out to determine if changes to footing design are required. 4.5 Drainage Adequate site drainage shall be installed to ensure that stormwater is directed away from buildings, retaining walls and earthworks batters. All collected stormwater (including overflow from rainwater tanks) should be taken to discharge a minimum of 10 m downslope of any structure and earthworks (e.g. cut/fill batters), and not upslope of any effluent disposal areas. This should be via pipes or lined channels, in order that stormwater is not allowed to flow down slope in an uncontrolled fashion. Surface dissipation, such as a rock weir or similar, must be installed at the stormwater outlet(s) to prevent soil erosion. 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 assess hydrostatic pressure to be accounted for in the design of the retaining structure. This should be collected and discharged as discussed above. Subsurface disposal of stormwater must not be undertaken, including (but not limited to) infiltration pits. 5. Limitations Your attention is drawn to the document Limitations, which is attached to this letter report. This assessment relates only to the proposed development as described in Section 1.1. Please contact the undersigned (given overleaf) should you wish to discuss any of the above matters. 13 February 2026 26020-001-Rev0 12 of 13
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Yours faithfully, Tectonic Geotechnical Pty Ltd Robert Gibb BSc (Hons) Ashley Davey BEng (Geological) RPEQ 8159 Senior Engineering Geologist Director/Principal Geotechnical Engineer Attachments: A. Qualitative Terminology for Use in Assessing Risk to Property B. Some Guidelines for Hillside Construction C. Limitations 13 February 2026 26020-001-Rev0 13 of 13
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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
68039 Landslide Risk 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
68039 Landslide Risk 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
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 114 Australian Geomechanics Vol 42 No 1 March 2007
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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. Limitations August 2024 | Rev2