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Appendix B - Geotechnical Assessment Report
18 November 2021 Project No. 21436-001-Rev0 Mr Tony Rossiter Email: [email protected] GEOTECHNICAL ASSESSMENT PROPOSED DAM LOT 13, #8 ROBIN PLACE, LAKE MACDONALD Dear Tony, 1.0 INTRODUCTION As requested, Tectonic has undertaken a geotechnical assessment for a proposed dam at 8 Robin Place, Lake MacDonald. This report presents the results of our slope stability assessment, together with geotechnical advice for the dam development. In summary, subject to implementation of the recommendations made herein, there would be a Low Risk of slope instability affecting the proposed dam 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 13 on SP315578, is irregular in shape, and covers a total area of about 1.53 ha. The site has frontage to Robin Place along the south-eastern boundary, and consisted of relatively densely vegetated, sloping bushland to the north and open grassland with sparse vegetation to the south. The site is located within a recently developed subdivision surrounded by similar rural residential properties. A detailed description of the site is given in Section 2. Lot boundary Robin Place Text Figure 1: Image of site and surrounds (courtesy Noosa Council [NC], 2019) 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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Mr Tony Rossiter 21436-001-Rev0
18 November 2021
Our stability assessment relates to the potential future development of a dam at the site, which would be
constructed downslope and north-west of the proposed house site. A marked up layout drawing supplied by
the client indicates the approximate position of the dam, as shown in Text Figure 2 below.
The proposed dam site is located approximately mid slope within the block. The dam footprint is across an
area where the ground surface levels range from about RL 111 m to 112 m AHD and so it is anticipated that
cut to fill earthworks may be in the order of 1 m to 2 m deep/high.
Text Figure 2: Layout plan showing location of proposed dam within the site (courtesy Client)
1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of
published geological maps, slope stability studies, and the following ‘borehole investigation’ and effluent
disposal reports prepared by Ground Dill (GD):
GD Job No. 21/504, dated 5 November 2021 (Borehole Investigation).
GD, Job No. 21/504, dated 2 November 2021 (Effluent Disposal Report).
A copy of the GD Borehole Investigation Report is attached.
A walk-over survey was conducted in order to make an appraisal of the general condition of the property in
regard to topography, drainage, vegetation cover, geology, erosion, condition of existing structures and
slope stability. The walk-over survey inspection was carried out by an experienced geotechnical engineer
from Tectonic on 27 October 2021. Results of the desk-top study and walk-over inspection are included in
Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been prepared by Mr David Pollock, an RPEQ with more than 14 years’ experience in
geotechnical engineering, including a number of slope stability projects.
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2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
The 1:100,000 scale Gympie Special Geological Map (Sheet 9445 and Part 9545) indicates that the site is
underlain by meta-sedimentary strata of the early to middle Triassic age Kin Kin Beds comprising ‘arenite,
argillite and phyllite’.
Indicative Site Location
Text Figure 3: Extract from Gympie Special geological map
As part of the subsurface investigation, GD completed three boreholes (denoted boreholes BH4 to BH6)
within the approximate footprint of the proposed dam. Borehole locations are illustrated in the site plan
prepared by GD, an extract of which is given below in Text Figure 4.
Text Figure 4: GD site plan showing borehole locations
The subsurface profile encountered in the boreholes generally consisted of a surficial topsoil layer, then stiff,
medium plasticity, sandy clay to depths of 0.5 m to 1.7 m below ground level (BGL); grading to gravelly
silty/sandy clay of very stiff consistency with depth to borehole termination at 3.0 m BGL.
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2.2 Laboratory Testing
One sample was forwarded to Morrison Geotechnic, a NATA accredited laboratory, for testing comprising
Emerson class number (ECN), particle size distribution (PSD) and Atterberg limits. Laboratory test
certificates are attached and results are summarised in Table 1 below.
Table 1: Summary of Laboratory Soil Classification & ECN Testing
Plasticity Limits PSD
Sample
ECN
Location Plasticity Liquid Linear Silt / Clay Sand Gravel
Index (%) Limit (%) Shrinkage (%) (%)* (%)** (%)***
BH5
6 19 37 9.5 42 53 5
(1.0 m-1.5 m)
*Taken as % passing 2.36 mm sieve **Taken as passing 2.36 mm sieve minus passing % passing 75 µm sieve
***Taken as sum of % retained of sieves 2.36 mm and larger
Material test results indicate that the soil tested comprises medium plasticity, sandy clay with a trace of
gravel. ECN testing indicates a low dispersion potential.
2.3 Topography
As illustrated in Text Figure 5, on a wider scale, the allotment is located on north-west facing slope that
crests at the southern boundary.
Proposed
dam location Lot boundary
Text Figure 5: Image showing ground surface contours (courtesy NC Interactive Maps)
Ground slope angles were estimated from measurements on site during the walk-over using a hand-held
clinometer. The dam site is located on a generally linear slope that dips to the north-west at angles of up to
about 6°.
Photographs illustrating the site conditions are given overleaf in Text Figures 6 and 7.
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Text Figure 6: Dam site looking north to wooded area along northern boundary
Text Figure 7: Dam site looking south-east toward Robin Place frontage
2.4 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over in proximity to the
development areas. Groundwater seepage was not logged in any of the GD boreholes undertaken at the
site.
2.5 Surface Drainage
Natural surface drainage would follow the ground surface gradients and disperse as sheet flow to the north-
west. The site is generally considered to be well drained owing to the positive gradients, being relatively
gentle, and the low permeability of the clayey surficial soils. No significant signs of erosion were noted.
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2.6 Vegetation
The southern part of the site has been cleared of trees, with grass ground cover. The northern part
comprises relatively dense bushland with a mixture of the grass ground cover, shrubs and a variety of small
to large trees. No signs of obvious tilting or bowing were noted in the larger trees, which can be an
indication of slope movement.
2.7 Buildings and Other Structures
There were no structures present on site at the time of our walk-over assessment.
3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
NC Landslide Hazard Overlay mapping (Ref. extract in Text Figure 8 below) indicates that the proposed dam
site has less than Moderate landslide hazard (no shading, presumable Very Low or Low hazard), with only
minor areas toward the south mapped as being of Moderate landslide hazard (pale brown shading).
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 hazard and is
superseded by site specific investigation and analysis works. In the vicinity of the proposed dam site
there was no signs of landslide, with the terrain and geology at these locations considered favorable from
a slope stability viewpoint.
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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 2 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached.
Table 2: 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 subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of landslide affecting the
proposed dam. Regulators (NC/Private Certifiers) normally require that a Very Low or Low Risk of landslide
affecting the property must be demonstrated to enable development approval. Summarised in Table 3 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).
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
Provided that our earthworks recommendations,
erosion protection and drainage control measures are
implemented as outlined in this report then the
1. Dam embankment failure. Unlikely Minor Low likelihood of embankment failure is considered
Unlikely. The consequence of such instability would
be Minor with the resultant risk being 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.
4.1 Dam Embankment Construction
No specific design details were available at the time of reporting however it is anticipated that embankments
for the dam construction will be in the order of 1 m to 2 m deep/high. If earthworks in excess of these
heights are proposed, Tectonic must review the design details and reassess slope stability prior to
construction. It is recommended that a suitably qualified engineer be consulted regarding the design of the
dam so that hydraulic and civil design matters are appropriately resolved.
4.1.1 Suitability of Existing Soils for Re-use as Dam Fill
Appendices 2 and 3 from the Guidelines for constructing and maintaining aquaculture containment structures
(2007) provide recommendations for the suitability of various soil types for re-use as structural fill for dam
embankment construction (in relation to small dams for aquaculture ponds).
The soil types encountered by the GD investigation in this area comprised mostly sandy clays with some
gravel content that were noted to have low dispersion potential. Such materials are expected be generally
suitable for use as structural fill for dam construction.
4.1.2 Site Preparation
After stripping and excavation to the design levels, the subgrade is expected to comprise mostly stiff sandy
clays, that may become firm (or softer) if high moisture content develops.
Following stripping, for the dam embankment footprint, ‘proof rolling’ using a fully laden water cart or
minimum 10 tonne drum roller should be carried out in order to identify any ‘soft spots’. Treatment of ‘soft
spots’ may involve excavation and replacement with select fill or tyning, appropriate moisture adjustment and
compaction. Proof rolling should be carried out in the presence of an experienced engineer.
Reactivity of the clay at subgrade level which will form the lining of the dam should be considered, as the
clay may develop cracking when dried out, which could lead to leakage. Should such leakage not be
considered to be an issue for the dam being constructed on this site, no further action would be required (in
dealing with the clay). However, if leakage is an issue, the clay should be covered with a 200 mm to
400 mm thick layer of inert/non-reactive material (e.g. gravel, sand or clayey sand), or be mixed with suitable
additives, such as gypsum at a rate of about 2.5 t/ha (this should be confirmed by agronomy assessment), to
reduce the potential shrinkage.
Use of a geosynthetic liner (e.g. HDPE) could also be considered to aid in reducing infiltration losses.
A temporary surface diversion drain should be installed upslope of the proposed dam site to shed surface
water runoff away from the construction area.
4.1.3 Filling
Fill for the dam embankment should be compacted at moisture contents within the range of +1 % to +2 % of
optimum moisture content for Standard Compaction. Fill should be placed in maximum 150 mm to 200 mm
thick (loose thickness) layers and be compacted by at least four to six passes with an pad foot compactor (or
similar) to achieve a dry density ratio of at least 95 % of the maximum Dry Density Ratio (Standard
Compaction method).
Details for inspection and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on
Earthworks for Commercial and Residential Developments. Confirmatory compaction testing must be
carried out at regular intervals. Filling should be carried out under Level 2 geotechnical testing. It is
recommended that the frequency of fill density testing carried out should be in accordance with Type 3 per
Table 8.1 of AS3798.
The above requirements should also be applied when constructing the key-in trench beneath the dam.
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4.1.4 Batter Slopes
Given the space available for construction, it is expected that a 1V:3H downstream face; 1V:2.5H upstream
face; and 3 m wide crest can be adopted for the design of the dam. Batters for the sides of the cut-off trench
beneath the dam should be formed no steeper than 1V:1H provided that the depth does not exceed 1.5 m.
Assuming typical soil parameters for reworked natural sandy clay compacted as detailed in Section 4.1.3,
the FOS against slope failure should be at least 1.5, with a resultant Low Risk of slope instability affecting
the dam. Stability analysis should be carried out if a fill embankment higher than 2 m is proposed.
4.1.5 Spillway Construction
The width of the spillway inlet and outlet should be determined to ensure that the maximum likely flood flows
can be carried at safe velocities of less than 2.5 m/s.
It may be necessary for the surface of the spillway be protected with rock embedded in concrete or
shotcrete. Rock (e.g. rip-rap) embedded in concrete may be a preferred option to assist in dispersing strong
flows. A key-in trench should be installed a minimum of 300 mm deep at the upstream edge of the inlet, and
the downstream edge of the outlet, in order to prevent undermining or lifting of the concrete lining of the
spillway.
4.2 Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area, to prevent saturation of working subgrades.
5.0 REFERENCES
Design and Construction of Small Earth Dams, Nelson, K.D., EA Books, 2010 (3rd Ed.)
Guidelines for the Construction of Earth-Fill Dams, Water Resources Policy #2008/1, Water Resources
Division, Dept of Primary Industries and Water, Tasmania, 2008
Queensland Dam Safety Management Guidelines, The State of Queensland (Dept of Natural Resources and
Mines), 2002
Guidelines for constructing and maintaining aquaculture containment structures, The State of Queensland
(Dept of Primary Industries and Fisheries), 2007
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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
David Pollock BE(Civil) RPEQ 14121
Senior Geotechnical Engineer
Attachments: Ground Drill Borehole Reports
Laboratory Test Certificate
Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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Ground Drill Pty Ltd
PH – 0407035220
Web- www.grounddrill.com.au
Email [email protected]
QBCC Lic – 15034580
Cedar Pocket 4570
Borehole Investigation
Job No: 21/504
Date: 05/11/21
Client: T. Rossiter
Site Address: Lot 13, 8 Robin Place, Lake MacDonald
Client Scope – 3 x 3m Boreholes
Page 12
Borelogs
BH 4 Soil Description DCP BH 5 Soil Description DCP
0 SANDY CLAYEY SILT (ML) – 0 SANDY CLAYEY SILT (ML) – Grey/brown,
Grey/brown, low plasticity fines, fine to low plasticity fines, fine to medium sand,
medium sand, moist, firm. moist, firm.
100 3 100 SANDY CLAY (CI) – Medium plasticity, pale 2
brown/orange, fine to medium sand, moist,
firm-stiff.
200 SANDY CLAY (CI) – Medium plasticity, 3 200 2
pale brown/orange, fine to medium sand,
moist, firm-stiff.
300 2 300 3
400 2 400 3
500 PP-75kpa 2 500 GRAVELLY SANDY SILTY CLAY (CH) – 2
High plasticity, orange, fine to medium
sand, fine to medium gravel, moist, stiff.
600 SANDY CLAY (CI) – Medium plasticity, 3 600 5
pale brown/orange, fine to medium sand,
moist, stiff.
700 3 700 2
800 2 800 3
900 3 900 4
1000 PP-175kpa 4 1000 4
1100 6 1100 6
1200 10 1200 5
1300 9 1300 5
1400 4 1400 PP-180kpa 5
1500 4 1500 5
1600 1600 GRAVELLY SANDY CLAY (CL) – Low
plasticity, orange, fine to medium sand, fine
to medium gravel, moist, very stiff.
1700 GRAVELLY SANDY CLAY (CI-CH) – 1700
Medium to high plasticity, orange, fine to
medium sand, fine to medium gravel,
moist, very stiff.
1800 1800
1900 1900
2000 2000
2100 2100
2200 2200 GRAVELLY SILTY CLAY (CH) – High
plasticity, red mottled white, fine to medium
gravel, moist, very stiff.
2300 GRAVELLY SANDY CLAY (CL) – Low 2300
plasticity, pale brown, fine to medium
sand, fine to medium gravel, very stiff.
2400 2400
Page 13
2500 2500 2600 2600 2700 2700 2800 2800 2900 2900 3000 Hole Terminated. 3000 Hole Terminated. [OCR of page images] 2500 2500 2600 2600 2700 2700 2800 2800 2900 2900 3000 | Hole Terminated. 3000 | Hole Terminated.
Page 14
BH 6 Soil Description DCP Soil Description DCP
0 SANDY CLAYEY SILT (ML) – 0
Grey/brown, low plasticity fines, fine to
medium sand, moist, firm.
100 SANDY CLAY (CI) – Medium plasticity, 2 100
pale brown/orange, fine to medium sand,
moist, firm-stiff.
200 2 200
300 3 300
400 4 400
500 GRAVELLY SANDY SILTY CLAY (CH) – 4 500
High plasticity, orange, fine to medium
sand, fine to medium gravel, moist, stiff.
600 4 600
700 5 700
800 5 800
900 4 900
1000 5 1000
1100 6 1100
1200 4 1200
1300 5 1300
1400 8 1400
1500 5 1500
1600 GRAVELLY SANDY CLAY (CL) – Low 1600
plasticity, orange, fine to medium sand,
fine to medium gravel, moist, very stiff.
1700 1700
1800 1800
1900 1900
2000 PP-225kpa 2000
2100 2100
2200 GRAVELLY SILTY CLAY (CH) – High 2200
plasticity, red mottled white, fine to
medium gravel, moist, very stiff.
2300 2300
2400 2400
2500 2500
2600 2600
2700 2700
2800 2800
2900 2900
3000 Hole Terminated. 3000
Page 15
Site Photos [OCR of page images] Site Photos Bio co uemirlrieret ace)
Page 16
Material Test Report
Report Number: ML21/242-1
Issue Number: 1
Morrison Geotechnic Pty Ltd
Date Issued: 10/11/2021 ABN: 51 009 878 899
Client: TECTONIC GEOTECHNICAL PTY LTD Maroochydore Laboratory
PO BOX 899, BUDERIM QLD Unit 4 / 81 Wises Road Maroochydore QLD 4558
Contact: David Pollock Phone: (07) 5443 9522
Project Number: ML21/242 Email: [email protected]
Project Name: Geotechnical Investigation - Robin Place Accredited for compliance with ISO/IEC 17025 - Testing
Project Location: Robin Place, Lake MacDonald
Client Reference: 21436
Work Request: 4698
Sample Number: M21-4698A Approved Signatory: Ryan Osborne
Date Sampled: 01/11/2021 Senior Technician
Dates Tested: 01/11/2021 - 09/11/2021 NATA Accredited Laboratory Number: 1169
Sampling Method: Sampled by Client
The results apply to the sample as received
Site Selection: Selected by Client
Sample Location: BH5, Depth: 1.0-1.5
Material: Refer to logs
Material Source: Insitu
Atterberg Limit (AS1289 3.1.2 & 3.2.1 & 3.3.1) Min Max
Sample History Oven Dried
Preparation Method Dry Sieve
Liquid Limit (%) 37
Plastic Limit (%) 18
Plasticity Index (%) 19
Linear Shrinkage (AS1289 3.4.1) Min Max
Moisture Condition Determined By AS 1289.3.1.2
Linear Shrinkage (%) 9.5
Cracking Crumbling Curling None
Particle Size Distribution (AS1289 3.6.1)
Sieve Passed % Passing Retained % Retained
Limits Limits
13.2 mm 99 1
9.5 mm 97 1
6.7 mm 96 1
4.75 mm 96 0
2.36 mm 95 1
1.18 mm 91 4
0.6 mm 84 7
0.425 mm 78 7
0.3 mm 67 11
0.15 mm 50 17
0.075 mm 42 8
Emerson Class Number of a Soil (AS 1289 3.8.1) Min Max
Emerson Class 6
Soil Description Gravelly Sand
Nature of Water Distilled
Temperature of Water (oC) 25
This document shall not be reproduced except in full without approval of the laboratory.
Report Number: ML21/242-1 Results relate only to the items tested/sampled. Page 1 of 1
Page 17
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 18
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 19
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
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LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)