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210533 - 108 Black Mountain Range Road, BLACK MOUNTAIN - Slope Stability Report
16 June 2022 Project No. 20332-003-Rev0 Total Concept Homes Attention: Mr Tim English Email: [email protected] SLOPE STABILITY ASSESSMENT PROPOSED GRANNY FLAT 108 BLACK MOUNTAIN RANGE ROAD, BLACK MOUNTAIN Dear Tim, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed granny flat at 108 Black Mountain Range Road, Black Mountain. This report presents the results of our slope stability assessment, together with geotechnical advice for the development. In summary, subject to implementation of the recommendations made herein, it is assessed that there would be a Low Risk of slope instability affecting the proposed building in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). 1.1 Details of Site and Development The subject site is described as Lot 1 on SP326083 and covers a total area of 43,303 m2. The site has access off Black Mountain Range Road along a small frontage to the south-west. Surrounding properties feature similar rural residential blocks to the north, east and south. Text Figure 1 below illustrates the site position. A detailed description of the site features is given in Section 2. Approx. site of proposed granny flat Existing residence Lot boundary Text Figure 1: 2019 Aerial Image of site and surrounds (courtesy Noosa Council [NC] Interactive Maps) 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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We have been supplied with architectural layout drawings and elevations (Ref. Hassell Free Designs,
Dwg. No. 01 to 06, dated 22 October 2021) for the project. These indicate that the proposed granny flat
would be located in an area approximately 32 m east/north-east of the existing residence. Ground slopes in
this area are generally indicated to be down to the south-east.
The granny flat would be about 10 m x 8.5 m in plan, consisting of a single storey, relatively lightweight clad
suspended structure, with a sheetmetal skillion roof. Drawings indicate that the structure would be
suspended on steel posts, connected to steel reinforced concrete bored piles (or similar).
At the time of our assessment, some preparatory works had been undertaken, which included an earthworks
pad across the building footprint. This was in the order of about 1.5 m depth of the upslope side; with fill on
the downslope side estimated to be about 1 m to 1.2 m height above the original ground surface.
The property does not have access to mains sewerage or a piped, potable water supply, and therefore
collection of rainwater in tanks and on-site disposal of effluent will be required.
An extract of the supplied drawings is given below in Text Figure 2.
Text Figure 2: Extract of overall layout (LHS); and section through granny flat (RHS), courtesy Hassel Free Designs
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, aerial photos, and a site classification report prepared by Ground Drill also
prepared a report to address on-site effluent disposal design and feasibility (Ref. Job No. 22/120, dated 24
March 2022).
A walk-over survey was conducted in order to make an appraisal of the general condition of the site in
regard to topography, drainage, vegetation cover, geology, erosion and slope stability and existing
structures. The walk-over survey was carried out by an experienced geotechnical engineer from Tectonic
on 2 June 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 Ashley Davey, an RPEQ with more than 27 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 and Subsurface Conditions
The 1:100,000 scale Gympie Special Geological Map 1 (Ref. Text Figure 3 below) indicates that the site is
underlain by materials derived of the early to middle Triassic age Kin Kin Beds (green shading), which are
described as “commonly cleaved and commonly kinked mudstone…” The Australian Stratigraphic Units
Database (www.asud.gov.au) describes the Kin Kin Beds as “arenite, argillite and phyllite.”
Approx. site location
Text Figure 3: Extract of Gympie Surface Geology Map
The Ground Drill investigation comprised drilling of two boreholes (designated BH1 and BH2) to the
respective downslope (fill side) and upslope (cut side) of the building pad prepared at the granny flat site.
Borehole BH1 encountered silty clay fill, in a firm condition to 1.2 m below ground level (BGL); then the
original topsoil (firm clayey silt) to 1.3 m BGL, followed by residual soils comprising high plasticity, stiff, silty
clay to the borehole termination depth of 2 m BGL. Borehole BH2 encountered distinctly weathered ‘rock’ at
surface, resulting in auger drilling refusal at 0.1 m BGL.
Three additional shallow boreholes (taken to 0.6 m BGL) were also drilled to the north of the prosed granny
flat, to provide information for the on-site effluent disposal design. These encountered topsoil to 0.1 m BGL,
then residual soils of stiff, high plasticity, silty clay to 0.5 m BGL, underlain by distinctly weathered ‘rock’ to
the termination depth at 0.6 m BGL.
All boreholes were drilled using a 4WD utility mounted drill rig, with solid flight augering equipment. In situ
strength testing comprised the dynamic cone penetrometer (DCP) method, taken from surface adjacent to
the boreholes at the site of the proposed granny flat. Pocket Penetrometer testing was also undertaken in
selected disturbed auger cuttings.
Groundwater seepage was not encountered during the drilling of the investigation boreholes, with all soils
being described as “moist” and weathered ‘rock’ materials as “dry”.
1
State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, Sheet 9445, 2018
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The site was classified by Ground Drill as Class P in accordance with AS2870-2011 due to the presence
of fill, poor bearing capacity, and trees. If not for this factor the site would have been classified Class H1
with characteristic surface movements (ys) of up to 55 mm reported by Ground Drill.
On the upslope side of the granny flat building pad, an earthworks cut which ranged to about 1.5 m depth
was present. The upper 1 m BGL (approximately) was observed to featured stiff, silty clay, with varying
gravel content; underlain at the toe by weathered phyllite.
A view of the exposed weathered phyllite in the cut is shown below in Text Figure 4.
Text Figure 4: Looking south along cut (LHS) and detail view of weathered phyllite (RHS)
2.2 Topography
As illustrated in Text Figure 5 on the following page, the wider area around the site consists of a series of
approximately parallel ridgelines and gullies, aligned east-west. The ridgelines within the subject allotment
lead east from Black Mountain Range Road, which traces a crest forming a drainage divide between
eastward and westward draining creek lines.
Just off Black Mountain Range Road, to the south-western frontage of the allotment, a rounded spur
extended through the site of the existing residence and along the northern lot boundary. The granny flat
would be proposed just off the crest of this spur, on ground dipping down to the south-east. Natural slopes
above the granny flat were in the order of about 8°, split by some low-relief terracing works (around 0.5 m
high per bench) inferred to be as a result of previous orcharding. Below the earthworks pad, ground slopes
were around 15°, and developed in to a relatively wide, gully head feature which drained eastward through
an intermittently dammed drainage line.
The cut batter backing the earthworks pad at the granny flat was around 1.5 m depth, and inclined at about
70°. The fill on the downslope side was battered at about 25°.
Ground slopes were measured at the time of the site visit by Tectonic using a hand held clinometer.
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Intermittently dammed gully
Approx. granny flat site
Text Figure 5: Image showing ground surface contours (courtesy NC Interactive Maps)
Text Figures 6 and 7 below illustrate the site conditions at the time of our inspection.
Granny flat earthworks pad
Intermittently dammed gully
downslope
Historic orcharding terraces
Text Figure 6: View looking north-east across site of proposed granny flat
Existing residence
Text Figure 7: View looking east over granny flat pad to existing residence above
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, and neither was any
seepage mentioned on the Ground Drill borehole reports. Groundwater conditions may vary seasonally,
and could rise to shallower depths, particularly following periods of prolonged or heavy rainfall.
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2.4 Surface Drainage
Broadly, the natural surface drainage follows the ground surface contours, which over the proposed
development area tended to the gully to the south-east/east. An upslope catchment does exist at the granny
flat site, extending to the crest of the spur running through the lot (the highest topographical point locally
being around the location of the existing residence).
A gravel trackway running from the residence to the site of the proposed granny flat was noted to be locally
eroded, appearing to direct water to the earthworks pad. Some minor slumping at the crest of the fill
embankment was noted, with some rill erosion evident.
Surface water is expected to drain well from the site considering the moderate to steep natural gradients
and relatively low permeability of the soils in the immediate subsurface, and relatively shallow depth to
apparent ‘rockhead.’
2.5 Vegetation
In the vicinity of the proposed granny flat, the vegetation mostly consisted of maintained grass. Sparse
shrubs and large size mature trees were noted on the sloping ground to the west of the proposed granny
flat, which were observed to be relatively ‘plumb’. Tilting or distorted growth can sometimes be an indicator
of slope instability.
2.6 Buildings and Other Structures
The existing residence comprised a one storey, low set, masonry walled structure, constructed over a cut
and fill earthworks pad. The residence was mostly on cut to the south, which ranged up to around 1.5 m
depth. Fill on the northern side of the residence was supported by a grouted rock wall and ranged up to 2 m
height. The grouted rock facing supporting the fill did not show any signs of global instability having affected
the structure; with the residence generally being in good structural condition, other than some minor cracking
and separation of the concrete apron to the slab. It was noted that some of the cracking coincided with the
daylighting point for underslab pipework (assumed to be for wastewater and water).
Text Figure 8: Looking west along grouted rock wall (LHS); & cracking in apron surrounding existing residence
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The crest area of the spur running south-west to north-east through the site, which incorporates the area of
the existing residence, has been mapped by NC as presenting a Low/Very Low landslide susceptibility
(clear/no shading). However the initial flanks of the crest, where the granny flat is proposed are mapped as
a Moderate hazard area (pale yellow brown shading). Towards the inverts of gullies away from construction
areas, High risk areas have been assigned (pale orange shading). An extract of the NC mapping is given
below in Text Figure 9.
Lot boundary
Approx. granny flat site
Text Figure 9: 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,
largely based on ground slope, and is superseded by this site-specific assessment.
The site does not exhibit any indicators of global, natural slope instability. No landslide back scarps or
areas of natural ‘hummocky’ ground were observed, the existing structures on site were in good condition,
with only minor erosion features observed over the existing access track alignment, or other earthworks
features (e.g. horticultural terraces, granny flat earthworks pad). In addition, the subsurface conditions
were favourable with stiff to very stiff soils present from near surface, and weathered phyllite rock logged
at relatively shallow depth in both the boreholes and exposed in earthworks cuttings.
3.2 Qualitative 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 on the
following page and the Qualitative Terminology for Use in Assessing Risk to Property is also attached.
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Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk
L Low Risk
to this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and
may depend on the nature of the property at risk; these are only given as a general guide.
Considering the existing site conditions (Ref. Section 2), and proposed granny flat, it is assessed that there
would be a Low (L) Risk of global slope instability affecting the proposed development subject to the
implementation of recommendations given below. 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 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 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 and areas of the site outside of the proposed development area.
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007
Assessed
Hazard Likelihood Consequence
Risk
Comments
Given the relatively shallow depth of fill (1.2 m),
presence of stiff (or stiffer) residual soils below, the
rounded nature of the spur crest where construction
1: Proposed granny flat - is proposed, apparent lack of groundwater, and no
Failure through earthworks fill, or evidence of existing instability in the vicinity of the
Unlikely Medium Low
natural soils above the proposed granny flat it is considered Unlikely that a
foundation depth landslip would develop, subject to implementation of
the recommendations in Section 4. With a potential
Medium impact from such shallow instability the
resultant risk would be Low as per AGS 2007.
The likelihood of a deep failure through extremely
weathered materials or weathered phyllite below the
2: Proposed granny flat - foundation depth is assessed as Rare due to the
Deep seated failure in extremely strength of these materials (based on drilling refusal),
weathered materials or Rare Major Low no evidence of existing instability, and relatively
weathered phyllite below the consistent nature of the sloping ground across the
foundation depth development area. Although the consequence of such
a failure could be Major, the resultant risk is Low as
per AGS 2007.
The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts.
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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. This document should be referred to by those
preparing the concept and designs for future residential development at the site.
4.1 Site Layout and Geotechnical Constraints
The general building location denoted in Text Figure 2, and as visited by Tectonic during our walk-over is
considered reasonable from a slope stability viewpoint, although this should not extend further downslope to
the east on to the slightly steeper slopes (measured at up to 15°).
Tectonic must be consulted to assess the impacts on slope stability if the granny flat is moved elsewhere, or
if the extent of earthworks is altered (other than as recommended in this report).
To achieve a Low Risk of instability, the following is recommended:
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
and certified, which must include an assessment of global and internal stability.
Unretained fill heights shall be limited to no more than 1 m, such as for landscaping. Any proposed
fill exceeding 1 m must be fully supported by an engineer designed and certified retaining structure,
unless assessed and approved by a suitably qualified and experienced geotechnical RPEQ.
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 the
proposed and existing structures.
4.2 Earthworks
In construction areas, all vegetation, tree stumps and large roots must be ‘grubbed out’, with any organic rich
topsoil removed. Any slumped materials on the batter face and crest of the existing fill batter should be
selectively removed, and recompacted as per the recommendations below. It is recommended that finished
subgrades be proof rolled using a minimum 4 tonne rubber tyred vehicle or drum roller to identify any ‘soft
spots’. Any such soft spots should be over excavated and replaced with compacted select fill.
Although trafficability should generally be good in ‘dry’ conditions, the 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.
Fill materials should be compacted at moisture contents within the range of -2 % to +2 % of optimum
moisture content for Standard Compaction. Confirmatory compaction 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.
Fill (site won and imported) 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 roller 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. Topsoil must not be re-used as structural fill, but may be used for landscaping.
Any imported fill should comprise a good quality material meeting the following specifications:
Soaked CBR ≥ 10 %
Shrink-swell index (Iss) ≤1 %
Maximum particle size ≤ 100 mm
Following stripping of topsoil and any organic laden surface soils, fill must be ‘benched’ into the natural soil,
with batters over filled by 0.5 m (horizontally) and then trimmed back to the well compacted material.
It is anticipated that excavations (e.g. footings, underground services) could generally be completed by
medium size excavators (8 tonne to 10 tonne) with toothed bucket attachments. However, the ground profile
indicated by the Ground Drill boreholes suggests that weathered rock could be expected from the current cut
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level on the upslope side of the pad. Such materials may require the use of ripping attachments, or selected
hydraulic breaking.
Temporary batters in natural soils of stiffer (or stiffer) consistency, and future controlled fill may be formed at
1V:1H; with permanent batters formed no steeper than 1V:2H. Flatter batters may be required if
groundwater seepage is encountered. Permanent batters will require erosion protection such as covering
with erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
batter slopes, cut or fill batters would need to be supported by retaining walls. All existing batters must be
trimmed back to permissible grades as outlined above, or otherwise retained.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Any excavations required adjacent to existing structures, will need to be specifically designed to ensure that
the structures are not undermined.
Ground surfaces 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,
especially given the presence of natural gullies and dams within the site boundary downslope of the
proposed construction areas.
4.3 Footing Design
Footings for the proposed building and any ancillary structures (e.g. retaining walls) must penetrate through
any future fill to ‘socket’ at least 1 m into natural residual soil comprising stiff (or stiffer) clay or a minimum of
0.3 m in to very low strength (or stronger) weathered phyllite. Based on the Ground Drill boreholes,
depending on the construction methodology adopted, normally dimensioned, conventional high-level footings
would be appropriate in areas of cut exposing very low strength (or stronger) phyllite, or extended on the
downslope side via the use of steel reinforced concrete bored piles (or ‘bucket piles’).
Footings could be designed with an allowable bearing capacity of 100 kPa when founded in stiff (or stiffer)
natural residual clays or 400 kPa in very low strength (or stronger) weathered phyllite.
Footing design should also account for the characteristic surface movement (ys), indicated by the site
classification report by Ground Drill to be equivalent to Class H1. The re-use of the site-won clay materials
for filling works could result in greater estimates of characteristic ground surface movement and should be
considered by the design engineer if further earthworks are planned.
All footings should found such that they are not adversely affected by 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 10 overleaf.
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Text Figure 10: Footing depth required to minimise the risk of undermining and surcharge loading
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. Should bored piles be adopted, it is recommended that the piling contractor have a ‘mud
bucket’ or similar device to enable clean out of the pile bases. ‘Spinning’ of the augers is not acceptable
and will not sufficiently remove drill spoil from 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 Structures
As a minimum any retaining walls must be founded as described in Section 4.3 above. 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 existing structures.
Retaining structures should be engineer designed in accordance with AS4678-2002 Earth-retaining
structures. We suggest the parameters given in Table 3 overleaf may be adopted for retaining wall design.
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Table 3: Retaining Wall Design Parameters
Unit Lateral earth pressure coefficients
Friction
weight
Retained material angle
(kN/m3 Ka Ko
(Degrees) Kp
) (Cantilever wall) (Non-yielding wall)
Fill Embankment
* * * * *
Materials
Stiff (or stiffer)
18 26 0.39 0.56 2.56
Natural Clays
Very Low Strength
(or stronger) 21 35 0.27 0.43 3.69
Weathered Phyllite
*depends on type of material and compaction achieved
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
Adequate site drainage should be installed to ensure that stormwater runoff is directed away from the
building walls and footings (e.g. swales in landscaped areas or grated drains across pavements up slope of
structures including retaining walls for the driveway). Diversion drainage should be installed above the crest
of all cut/fill batters, and toe of all cut batters. Uncontrolled flow of stormwater over embankment batters (e.g.
from gutter overflow during high intensity storms) has the potential to saturate and weaken the embankment
materials, potentially leading to slumping failure. Future building and landscaping must be carried out to
maintain good drainage.
Specific drainage recommendations are as follows:
Install diversion drainage (e.g. turfed, rock lined or concrete spoon drain) along the upslope side of
the proposed granny flat.
Install diversion drainage along the upslope side of any effluent disposal areas to prevent stormwater
migrating into shallow subsurface trenches.
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 on site must be directed by pipes to the legal point of discharge, at least 5 m
down slope from any proposed/existing structures, embankments or effluent disposal areas. Surface
dissipation, such as a rock weir or similar, must be installed at the stormwater outlet(s) to minimse the risk of
soil erosion at the discharge point.
Subsurface disposal of stormwater must not be undertaken.
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
impacting the proposed granny flat. This advice is subject to implementation of the recommendations given
in this report.
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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
Robert Gibb BSc (Hons) Ashley Davey BEng (Geological) RPEQ 8159
Engineering Geologist 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
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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)