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ECM_22343520_v1_Geotechnical Investigation
GEOTECHNICAL INVESTIGATION
Proposed Renovations
2 Arakoon Crescent
Sunshine Beach
Report for: Christopher Kelly
C/- Adapt Planning
22484-001-Rev0
24 February 2023
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
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023
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PROPOSED RENOVATIONS
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Document Review
Document No. Revision Prepared By Reviewed By Date Issued
001 0 Michael Cocks Ashley Davey 24 February 2023
Report Distribution
Document Revision Method of Delivery Issued To
Christopher Kelly
001 0 Electronic
[email protected]
Amanda Evans
001 0 Electronic Adapt Planning
([email protected])
Prepared By: Reviewed By:
Michael Cocks BEng MIEAust NER RPEQ 27540 Ashley Davey RPEQ 8159
Senior Geotechnical Engineer Managing Director
24 February 2023
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Executive Summary
This report details the results of a geotechnical assessment undertaken for the proposed renovations to an
existing residence at 2 Arakoon Crescent, Sunshine Beach. The investigation was carried out by Tectonic
for Christopher Kelly. Adapt Planning are the consulting town planners for the project.
Royal Haskoning Australia DHV (RHDHV) has provided a coastal engineering report, detailing a review of
the Erosion Prone Area associated with the coastal building line at the subject site (Ref. PA3303, Status
S0/P01.01, Rev R1, dated 20 February 2023). A site investigation ‘soil test’ report was completed by
Australian Soil Investigations (ASI, Ref. Report No. 220343(S), dated 22 September 2022). Tectonic
conducted three additional standalone dynamic cone penetrometer (DCP) tests down the main high dune
face and near the dune toe for an assessment of the density of the underlying sands.
This report presents the results of the geotechnical assessment and slope stability modelling to address
possible slope failure following a hypothetical future sea surge/storm event nominated by RHDHV, along with
associated geotechnical recommendations and comments.
Preliminary architectural drawings indicate that the proposed development will comprise partial demolition of
the existing residence and pool, although some of the structural elements will be retained for future utilisation
(subject to an assessment by a structural RPEQ). The proposed renovations would be superimposed over
the existing footprint, extending to two storeys height. The northern edge of the building would be excavated
into the ground for a garage level and cellar room, which will have a pool positioned directly above partially
cantilevered towards the dune slope to the east. Provided sections indicate that excavations up to about 2 m
deep will be required, which will be retained.
Although the proposed footprint does not extend significantly beyond the existing building envelope,
following demolition, it is anticipated that some form of temporary retention may be required, especially
where excavations extend close to site boundaries or existing structures/elements (to remain). Filling is
anticipated to be minor and less than 1 m high.
The ASI investigation comprised two boreholes (designated BH1 and BH2) along the eastern edge of the
existing residence. These boreholes encountered consistent subsurface conditions, summarised as
follows:
Fill – comprising loose gravelly sand and silty sand, with deleterious materials (i.e. rubbish) from
the ground surface to depths between 0.9 m and 2.1 m below ground level (BGL), then
Natural Sand – comprising loose silty sand to the borehole termination depths of 3 m BGL.
ASI continued DCP testing below the borehole termination depths which indicated that the natural sands
became medium dense below depths between 3 m and 3.6 m BGL to the DCP termination depths.
Standalone DCP testing by Tectonic along the high dune face and near the dune toe indicated the
following:
Very loose/loose sands to depths between 1.1 m and 2.3 m BGL; then
Medium dense sands to depths between 1.4 m and 4 m BGL; followed by
Dense to very dense sands with practical DCP refusal at depths between 1.6 m and 5.8 m BGL.
Our slope stability modelling considered a hypothetical storm and wave scenario nominated by coastal
engineering experts RHDHV, which included a combination of short-term erosion from a 500-year average
recurrence interval storm event and shoreline recession due to sea level rise for the 2100 climate extreme
event scenario. Section 7 of the coastal erosion assessment indicates that the existing house site and
proposed development is not likely to be impacted by beach erosion under this scenario. In accordance with
previously adopted practices along the Sunshine Beach coastal strip a 28° line was modelled from the
current dune toe which does not intersect the eastern boundary or proposed development area.
Although the extreme storm case does not appear to impact the subject site, to assess the factor of safety
(FOS) against instability for the proposed development, numerical modelling has been undertaken
considering the existing dune slope and subsurface conditions from the investigation results.
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Our analysis indicates that stability of the site hinges on the temporary and permanent retention systems.
Provided that footings for the residence and structural walls are founded at least 1 m into medium dense (or
denser) sands and the existing timber sleeper wall to the east of the residence is replaced with a new
engineer designed and certified structure founded on shear piles, the required minimum FOS for long-term
stability (i.e. 1.5) could be achieved.
Our limit equilibrium modelling indicates that relatively conventional building piles could be adopted for the
proposed development generally; however, piles socketed at least 1 m into medium dense (or denser) sand
designed with an unfactored shear force resistance of 100 kN/m (or equivalent) will be required beneath the
new retaining wall to the east of the residence. It should be noted that required shear and bending
resistance of the piles and their depth should be determined by finite element analyses based on all relevant
loads including from structures (not included in our Slope/W modelling) and all soil, fill and landscaping
loads. To facilitate a larger level garden/grassed area to the east of the residence, the new retaining wall
could be located further eastwards, however not beyond the current dune crest.
The following additional recommendations are provided for the proposed development:
The residence and any ancillary structures (including retaining walls and pools) must be supported
by footings founded at least 1 m into medium dense (or denser) natural sands. Based on the results
of the investigation founding depths are anticipated to range between about 4 m and 5 m BGL
(existing ground levels). In some areas of deeper cut, high level footings may be adopted.
Elsewhere, piles would be required. Subsurface conditions should be confirmed by deeper borehole
drilling in the western part of the site following demolition of the existing structures.
Excavations close to site boundaries may necessitate the installation of temporary shoring prior to
excavation. Based upon our experience with similar projects in proximity, shoring could comprise
sheet piles or contiguous piles.
Surcharge loads across the high dune crest (i.e. east of the residence) should be limited to no more
than 5 kPa, unless assessed and approved by Tectonic.
Ground vibrations during construction should be minimised and monitored. Where excessive
vibrations occur, alterations to construction techniques may be required. It is recommended that a
dilapidation survey of adjoining structures be completed prior to construction.
Revegetation of bare slopes around the development area should be undertaken as soon as
possible following construction. Similarly, improvements to the vegetation (i.e. with endemic native
species) on the natural high dune face downslope of the residence would also aid in restricting the
development of initial surficial failures.
All collected roof water should be directed by pipes to the council stormwater system along The
Esplanade (e.g. through the use of a ‘wet’ or ‘charged’ drainage system) or, where not physically
possible, surface water should be directed to infiltrate into the ground around the edges of hard
surfaces. Infiltration areas should be set back as far as possible from the crest of the dune slope.
Hydraulic design should be such that under no circumstances are uncontrolled stormwater flows
permitted over the dune face; where channelised erosion and scour features could develop.
The scour in the south-eastern corner of the site should be reinstated with a cellular confinement
system and rock fill (gabions, reno mattress or similar) designed by a civil RPEQ.
Based on the findings of our assessment it is considered that a Low Risk of slope instability could be
achieved for the proposed development, subject to the following:
Implementation of the recommendations given in this report in preparing the preliminary design;
Review of the preliminary detailed design by Tectonic to confirm compliance with the
recommendations herein; and
Installing a new engineer designed and certified retaining structure supported on piles founded at
least 1 m into medium dense (or denser) sand, designed to accommodate an unfactored shear
resistance of 100 kN/m, along the eastern edge of the level area to the east of the residence.
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Table of Contents
1.0 INTRODUCTION ........................................................................................................................................................ 1
2.0 PROPOSED DEVELOPMENT ................................................................................................................................... 1
3.0 METHOD OF INVESTIGATION ................................................................................................................................. 3
3.1 Fieldwork ...................................................................................................................................................... 3
4.0 RESULTS OF INVESTIGATION ................................................................................................................................ 3
4.1 Site Description............................................................................................................................................. 3
4.2 Topography................................................................................................................................................... 6
4.3 Geology ........................................................................................................................................................ 7
4.4 Subsurface Conditions.................................................................................................................................. 8
4.5 Groundwater ................................................................................................................................................. 8
4.6 Surface Drainage .......................................................................................................................................... 8
5.0 SLOPE STABILITY ASSESSMENT .......................................................................................................................... 9
5.1 Existing Conditions ....................................................................................................................................... 9
5.2 Stability Assessment................................................................................................................................... 10
5.2.1 Quantitative Assessment ...................................................................................................................... 11
6.0 GEOTECHNICAL RECOMMENDATIONS .............................................................................................................. 14
6.1 Earthworks .................................................................................................................................................. 14
6.1.1 Site Preparation and Trafficability ......................................................................................................... 14
6.1.2 Excavations ........................................................................................................................................... 14
6.1.3 Filling..................................................................................................................................................... 15
6.2 Building Foundations .................................................................................................................................. 15
6.2.1 Indicative Site Classification.................................................................................................................. 15
6.2.2 Footing Design ...................................................................................................................................... 16
6.3 Temporary and Permanent Retention......................................................................................................... 17
6.4 Drainage ..................................................................................................................................................... 18
7.0 CONCLUSION ......................................................................................................................................................... 19
8.0 LIMITATIONS .......................................................................................................................................................... 19
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TABLES
Table 1: Stability Risk Levels ............................................................................................................................................ 10
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007) ....................................................................... 10
Table 3: Retaining Structure Design Parameters .............................................................................................................. 18
TEXT FIGURES
Text Figure 1: Proposed layout plan (upper) and some cross-sectional views (lower) courtesy of Aboda Design
Group ................................................................................................................................................................. 2
Text Figure 2: Aerial image of the site and surrounds (courtesy of Noosa Council ‘intra-maps’) ........................................ 4
Text Figure 3: Oblique Tectonic drone image looking north ................................................................................................ 4
Text Figure 4: Drone image looking due west over dune in vicinity of site .......................................................................... 5
Text Figure 5: Westerly view of the residence from high dune crest ................................................................................... 5
Text Figure 6: Detailed survey of the site and high dune, courtesy of Minestaff Survey ..................................................... 6
Text Figure 7: Tectonic drone image of the dune face and main beach looking north ........................................................ 7
Text Figure 8: Extract of Laguna Bay 1:100,000 Scale Surface Geology Map.................................................................... 7
Text Figure 9: Scour in the southern part of the site (LHS) and bulging coir log (RHS) ...................................................... 9
Text Figure 10: Extract from Noosa Council landslide hazard overlay mapping ................................................................. 9
Text Figure 11: Geological model of the site and high dune, including extreme storm event scenario ............................. 12
Text Figure 12: Slope/W output for the proposed development ........................................................................................ 13
Text Figure 13: Footing depth required to prevent undermining........................................................................................ 17
FIGURES
Figure 1: Site Plan
APPENDICES
APPENDIX A
DCP Reports
APPENDIX B
AGS Guidelines – Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construciton
APPENDIX C
Limitations
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1.0 INTRODUCTION
This report details the results of a geotechnical assessment undertaken for the proposed renovations to an
existing residence at 2 Arakoon Crescent, Sunshine Beach. The investigation was carried out by Tectonic
for Christopher Kelly, generally in accordance with Proposal P22631-Rev0 dated 4 October 2022. Adapt
Planning are the consulting town planners for the project.
Royal Haskoning Australia DHV (RHDHV) has provided a coastal engineering report, detailing a review of
the Erosion Prone Area associated with the coastal building line at the subject site (Ref. PA3303, Status
S0/P01.01, Rev R1, dated 20 February 2023).
A site investigation ‘soil test’ report was completed by Australian Soil Investigations (ASI, Ref. Report No.
220343(S), dated 22 September 2022). Tectonic conducted three additional standalone dynamic cone
penetrometer (DCP) tests down the main high dune face and near the dune toe for an assessment of the
density of the underlying sands.
This report presents the results of the geotechnical investigation and slope stability modelling to address
possible slope failure following a hypothetical future sea surge/storm event nominated by RHDHV, along with
associated geotechnical recommendations and comments.
2.0 PROPOSED DEVELOPMENT
Preliminary architectural drawings by Aboda Design Group were supplied to Tectonic
(Ref. Drawing Nos. WD.101 to WD.601 [not inclusive], Issue DA, dated 21 September 2022). Based on the
provided drawings it is understood that the proposed development will comprise partial demolition of the
existing residence and pool, although some of the structural elements will be retained for future utilisation
(subject to the findings of the geotechnical investigation and assessment by structural engineers).
The proposed renovations would be superimposed over the existing footprint, extending to two storeys
height. The northern edge of the building would be excavated into the ground for a garage level and cellar
room, which will have a pool positioned directly above partially cantilevered towards the dune slope to the
east. Provided sections indicate that excavations up to about 2 m deep will be required, which will be
retained.
Although the footprint does not extend significantly beyond the existing building envelope, following
demolition, it is anticipated that some form of temporary retention may be required, especially where
excavations extend close to site boundaries or existing structures/elements (to remain). Filling is anticipated
to be less than 1 m high.
The proposed layout and some cross-sectional views are provided in Text Figure 1 overleaf.
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Text Figure 1: Proposed layout plan (upper) and some cross-sectional views (lower) courtesy of Aboda Design Group
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3.0 METHOD OF INVESTIGATION
3.1 Fieldwork
An experienced geotechnical engineer from Tectonic undertook a site walk-over on 5 December 2022 to
visually assess indicators of past slope instability. The walk-over included assessment of soil exposures,
surface drainage conditions, topography, evidence of erosion or past instability, and vegetation
coverage/condition.
At the time of the walkover three standalone DCP tests (denoted DCP1 to DCP3) were conducted down the
high dune face. DCP1 was conducted near the eastern property boundary, DCP2 at an intermediate point,
around half-way down the dune face, and DCP3 near the dune toe. All DCPs were completed to practical
refusal. DCP reports are attached in Appendix A, with locations shown in Figure 1.
4.0 RESULTS OF INVESTIGATION
4.1 Site Description
The site is formally described as Lot 783 on RP48111, covering an area of about 866 m2, with frontage to
The Esplanade along the western boundary. At the time of the investigation the site featured an existing
residence of mostly two storeys, with a single level garage and mostly inground pool along the western
boundary. A levelled terrace area was located along the eastern edge of the residence which was supported
by a timber sleeper wall about 0.5 m to 1 m high. Based on an external assessment, visible areas of the
existing structures appeared to be in relatively good structural condition, with no obvious signs of distress
(e.g. cracking, misalignment etc.) which would be indicative of global slope instability. Cracking <10 mm
wide was observed in parts of the rendered finish, with corrosion noted within some of the fixings.
Two to three storey houses and low-rise unit buildings were generally present in the vicinity of the site, of
varying age and construction methodology. The areas around the residence featured garden beds, small
bushes and scattered trees. A large pine tree adjacent to the south-eastern corner of the residence was
noted to be tilting and exhibited a ‘J’ shaped curve, and appeared to be undermined by an erosion scour
which extended down the high dune face below.
Patchy vegetation was observed along the upper part of the high dune face, with relatively dense vegetation
present further downslope comprising ground cover plants and a small number of trees, such as Banksia
and She-Oak.
The asphalt seal and kerb and channel along The Esplanade in the vicinity of the site was noted to be in
good condition with no obvious signs of cracking or misalignment. An aerial image of the site and surrounds
is provided on Text Figure 2 overleaf.
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Subject Site
Sunshine Beach
Text Figure 2: Aerial image of the site and surrounds (courtesy of Noosa Council ‘intra-maps’)
Site conditions at the time of our investigation are shown in Text Figures 3 to 5 below and overleaf.
Subject Site
Text Figure 3: Oblique Tectonic drone image looking north
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Subject site
Text Figure 4: Drone image looking due west over dune in vicinity of site
Text Figure 5: Westerly view of the residence from high dune crest
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4.2 Topography
During the site walk-over, topographic features specific to the site were noted, and ground slopes were
recorded using a hand-held clinometer. As illustrated in a detailed survey by Minestaff Survey (Ref. M4371-
3000-002, Rev A, dated 5 January 2023) in Text Figure 6 below, the existing residence is located on
relatively gently sloping ground above the crest of the easterly facing high sand dune slope. The eastern
property boundary extends partway down the dune slope.
Subject Site
Text Figure 6: Detailed survey of the site and high dune, courtesy of Minestaff Survey
Locally, Arakoon Crescent and The Esplanade may have been constructed by cut and fill earthworks. It is
not known if the original roadworks fill is now retained along the western boundary, or if it was removed by
excavations during residential construction. The natural surface profile at the site had been interrupted by
historic landscaping and residential construction. Based on a visual assessment it appeared that cut and fill
earthworks in the order of about 3 m deep and 1.5 m high had been previously undertaken, most of which
were supported by retaining walls. An offset of about 5 m was present between the eastern edge of the
existing residence, and the main break in slope to the high dune face.
The dune face sloped at around 32° to 35° just off the crest, reducing to around 30° across the majority of
the face. Beyond the eastern boundary a swale feature was apparent comprising a gently sloping bench
(<5°) for a horizontal distance of about 5 m to 8 m (increasing in width progressing southwards) and a
downslope mound estimated to be about 1.5 m high along the seaward edge. This mound had a backslope
of about 25° (west towards the bench), with the seaward facing flank dipping at about 30°. It was unclear
what created the swale feature in the central part of the dune face; however, it was likely due to previous
earthworks, presumably to make an access track. Beyond the toe of the foredune a gently undulating
incipient dune continued eastwards towards the main beach.
Excluding the swale feature, the face of the dune was generally consistent and linear, steepening to about
40° around 3 m up (vertically) from the toe. Beyond the toe of the main high dune, a trough was present of
about 10 m width; then a small foredune around 1.5 m height, with side slopes of around 15° to 20°. A
shorter trough of about 5 m width was then present to an incipient dune, around 1 m high; with side slopes of
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up to 10°. The beach extended thereafter, dropping down from about RL 2 m AHD to the shoreline (around
RL 0 m AHD).
The abovementioned geomorphological features are illustrated in Text Figure 7 below.
Subject Site
High Dune
Swale/Track
Foredune
Incipient dune
Troughs/Swales
Text Figure 7: Tectonic drone image of the dune face and main beach looking north
4.3 Geology
Available geological information 1 indicates that the site is underlain by Pleistocene age quartz dune sand
(described as “parabolic dunes”; Ref. extract of published mapping in Text Figure 8). The headlands to the
north of the site are indicated to consist of materials derived of the Tiaro Coal Measures, a late Triassic to
Jurassic age sedimentary deposit. These materials are expected to underlie the dune sands proximal to the
site at depth.
Approx. Site Location
Text Figure 8: Extract of Laguna Bay 1:100,000 Scale Surface Geology Map
1
Queensland Government, Dept. of Natural Resources, Mines and Energy, 1:100,000 Laguna Bay Surface Geology, Sheet 9545, 2018
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4.4 Subsurface Conditions
The ASI investigation comprised two boreholes (designated BH1 and BH2) along the eastern edge of the
existing residence. These boreholes encountered consistent subsurface conditions, summarised as
follows:
Fill – comprising loose gravelly sand and silty sand, with deleterious materials (i.e. rubbish) from
the ground surface to depths between 0.9 m and 2.1 m below ground level (BGL); then
Natural Sand – comprising loose silty sand to the borehole termination depths of 3 m BGL.
ASI continued DCP testing below the borehole termination depths which indicated that the natural sands
became medium dense below depths between 3 m and 3.6 m BGL to the DCP termination depths.
Standalone DCP testing by Tectonic along the high dune face and near the dune toe indicated the
following:
Very loose/loose sands to depths between 1.1 m and 2.3 m BGL; then
Medium dense sands to depths between 1.4 m and 4 m BGL; followed by
Dense to very dense sands with practical DCP refusal at depths between 1.6 m and 5.8 m BGL.
4.5 Groundwater
No signs of groundwater seepage (‘springs’) were observed at the ground surface during our walk-over. No
groundwater was mentioned on the ASI borehole reports, with the soils described as ‘moist’.
Groundwater conditions are expected to be reflective of prevailing weather conditions and seepage may
develop, particularly following periods of prolonged or heavy rainfall at various points through the dune
profile. Based upon experience at other similar sites in the area, such seepage would be expected to be
transient and of relatively minor lateral extent.
4.6 Surface Drainage
Surface drainage would be expected to follow natural surface gradients to the east towards the beach. The
site has a relatively small catchment, with some run-off expected to enter the site from upslope along The
Esplanade; however, most of which should be intercepted by the existing kerb along the frontage.
Given the sandy nature of the site, rainfall would normally be anticipated to infiltrate the ground surface
relatively quickly. However, as the proposed development will occupy a large part of the block, significant
run-off from hard surfaces will likely occur. Nevertheless, where directed into swales rather than pipes,
such run-off would be expected to infiltrate into the natural sands relatively rapidly.
A scour was observed in the south-eastern corner of the site, downslope of the existing residence. The
scour was estimated to be between 1 m and 2 m deep with side scarps dipping between 40° and 50°. A
coir log ‘silt sausage’ was observed above the head of the scour feature which was bulging with debris. A
PVC outlet was also observed upslope of the area which discharged directly onto the ground surface, with a
corresponding scour about 0.5 m deep. It appeared that concentrated stormwater runoff discharged into
this area, resulting in the observed scour features.
Anecdotal evidence by the owner suggests that stormwater previously discharged into the area including
from the road surface above, with works undertaken to attempt to divert the water away from the area.
Stormwater improvements are recommended in Section 6.4.
No other significant surface run-off features were noted across the allotment during our assessment.
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Text Figure 9: Scour in the southern part of the site (LHS) and bulging coir log (RHS)
5.0 SLOPE STABILITY ASSESSMENT
5.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 10
below) which indicates that the eastern part of the site is within a Moderate Hazard Area. Beyond the
eastern boundary the high dune face is mapped as having a Moderate to High hazard.
Subject Site
Text Figure 10: Extract from Noosa Council landslide hazard overlay mapping
It should be noted that the Noosa Council landslide hazard map is a broad-scale indication of the potential
landslide susceptibility and is superseded by this site-specific assessment. The allotment did not exhibit
any visible surface features which would indicate large scale global slope instability during our walkover:
no landslide back scarps, noteworthy groundwater seepage or areas of naturally ‘hummocky’ ground were
observed. Medium dense sands were inferred at relatively shallow depths below surface, with the high
dune standing at between 28° to 35°; and with a slightly steepened section at the toe having a gradient in
the order of 40°.
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5.2 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.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of
Very High Risk
VH treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost
more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
H High Risk required to reduce risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires investigation, planning and
Moderate Risk
M implementation of treatment options to reduce risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing
L Low Risk
maintenance required.
Very Low Risk
VL Acceptable. Manage by normal slope maintenance procedures.
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the
property at risk; these are only given as a general guide.
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 global slope instability
affecting the proposed development. 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 development site. 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 Risk Comments
Given the low to moderate natural gradients through
the proposed building area, retention proposed and
1: Failure through very loose to
protection of eastern side of site using a shear pile wall
loose natural sands above the
Unlikely Medium Low it is considered Unlikely that a landslip in these
foundation depth across the
materials would affect the proposed development.
building area.
Given a potential Medium impact from such instability
the resultant risk would be Low as per AGS 2007.
The likelihood of a deep failure through the medium
dense (or denser) natural sand below the proposed
2: Deep failure through the foundation depth is assessed as Rare due to the lack of
medium dense (or denser) sands Rare Major Low precedence of such movements locally, no signs of any
below the foundation depth. existing slope instability and a lack of groundwater.
Although the consequence of such a failure would be
Major, the resultant risk is Low as per AGS 2007.
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The likelihood of a slip/failure of a temporary retention
system (sheet pile wall, or similar) as outlined in Section
6.3, is assessed as Rare, subject to detailed design by a
suitably qualified RPEQ, which accounts for the required
factors of safety against slope instability, and construction
3: Instability of temporary sheet by a specialist contractor. Although the consequence of
pile retention system, or similar, Rare Major Low such a failure could be Major, the resultant risk is Low as
as discussed in Section 6.3. per AGS2007.
Note that this assessment relates to the construction of a
new temporary retaining wall between the upper and lower
levels of the building. No assessment has been made of
any elements in loco within the existing structure.
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.
5.2.1 Quantitative Assessment
In order to estimate the factor of safety (FOS) against slope instability for the proposed development,
numerical analysis has been conducted using Slope/W. The ground surface profile has been modelled using
a combination of architectural layout drawings, survey information, NC interacting mapping and slope angles
measured in the field by Tectonic. The analysed subsurface profile was compiled based on the results of the
ASI boreholes and our subsurface investigation. Some inferences regarding subsurface conditions were
made between test locations, particularly across the western part of the site which is occupied by the existing
building. A minimum FOS value of 1.5 is the commonly adopted industry standard for long term conditions.
Considering the steep gradients to the eastern edge of the site, and the presence of loose sands; the
stability of the existing high dune slope is considered marginal. The slope is probably held in place by the
stabilising effects of negative soil pore pressures and shallow root systems. Any disturbance to the lower
part of the dune slope, such as erosion by high seas, could lead to larger scale instability.
Modelling considered a hypothetical storm and wave scenario nominated by coastal engineering experts
RHDHV, which included a combination of short-term erosion from a 500-year average recurrence interval
storm event and shoreline recession due to sea level rise for the 2100 climate extreme event scenario.
Section 7 of the coastal erosion assessment indicates that the existing house site and proposed
development is not likely to be impacted by beach erosion under this scenario. In accordance with
previously adopted practices along the Sunshine Beach coastal strip, a 28° line was modelled from the
current dune toe which does not intersect the eastern boundary or proposed development area. Text Figure
11 overleaf illustrates our geological model of the site including the extreme storm and wave scenario
nominated by RHDV and the 28° line from the high dune toe.
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Text Figure 11: Geological model of the site and high dune, including extreme storm event scenario
Although the extreme storm case does not appear to impact the subject site, and an applied 28 line from the
dune toe does not intersect the ground surface, to assess the FOS against instability for the proposed
development numerical modelling has been undertaken considering the existing dune slope.
A surcharge load of 5 kPa was applied across the site and crest of the dune to the eastern boundary to
replicate future landscaping. Building loads for the proposed residence and ancillary structures will be
transferred by a piled footing system into the natural medium dense (or denser) sands at depth and will not
surcharge the ground surface, thus have not been included in the models.
Soil shear strength was based upon effective parameters which were estimated based on our interpretation
of the subsurface investigation results, while also considering the values recommended for similar materials
in Appendix D of Australian Standard AS4678-2002 Earth Retaining Structures. Adopted soil parameters
are shown on the Slope/W output presented in Text Figure 12 overleaf. Analysis was undertaken for a
specified range of entry and exit points that could impact the proposed development.
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Text Figure 12: Slope/W output for the proposed development
Our analysis indicates that stability of the site hinges on the temporary and permanent retention systems.
Provided that footings for the residence and structural walls are founded at least 1 m into medium dense (or
denser) sands, and the existing timber sleeper wall to the east of the residence is replaced with a new
engineer designed and certified structure founded on shear piles, the required minimum FOS for long-term
stability (i.e. 1.5) could be achieved.
Our limit equilibrium modelling indicates that relatively conventional building piles could be adopted for the
proposed development generally; however, piles socketed at least 1 m into medium dense (or denser) sand
designed with an unfactored shear force resistance of 100 kN/m (or equivalent) will be required beneath the
new retaining wall to the east of the residence. It should be noted that required shear and bending
resistance of the piles and their depth should be determined by inite element analyses based on all relevant
loads including from structures (not included in our Slope/W modelling) and all soil, fill and landscaping
loads. To facilitate a larger level garden/grassed area to the east of the residence, the new retaining wall
could be located eastwards, however not beyond the current dune crest.
Subject to implementation of the recommendations made in Section 6 during building works, there would be
a Low Risk of slope instability affecting the proposed development.
It should be noted that this modelling addresses the global stability of the proposed works; however, the
stability of the retaining walls must be addressed by others as part of detailed design.
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6.0 GEOTECHNICAL RECOMMENDATIONS
6.1 Earthworks
Excavations for the lower level and new garage are anticipated to range up to about 2 m deep. Isolated
relatively deep excavations may be required for underground services and footings or other specific
elements. Filling less than 1 m high is anticipated to form final levels, with filling to the east of the residence
limited to landscaping only, which should not exceed 0.5 m high.
The advice give herein should be treated as preliminary to aid in project planning, and Tectonic must review
detailed design drawings for the residence prior to construction.
6.1.1 Site Preparation and Trafficability
Following clearing of existing vegetation, any tree stumps and large roots should be ‘grubbed out’. Any
organic rich topsoil or uncontrolled fill should then be stripped away from at grade or building subgrades and
set aside. Similarly, all footings and remaining structural elements from the existing residence should be
removed and ‘grubbed out.’ Based on the ASI boreholes and site observations, stripping depths to remove
existing unsuitable upper soils should be relatively shallow however, deeper areas of unsuitable materials
may be present across the site, particularly related to the existing structures (e.g. retaining wall backfill,
service trenches, footing excavations). Topsoil materials must not be used in structural fill; however, may be
placed across landscaping areas.
It should be noted that compaction of such relatively ‘clean’ sands as structural fill requires the addition of
substantial volumes of water (‘flooding’) combined with the use of vibratory compaction equipment (typically
a vibrating plate). Care must be taken with the use of vibratory equipment (e.g. compactors or sheet piling)
to ensure that there are no adverse impacts on surrounding structures. The impacts from ground vibration
must be closely monitored and managed during construction. It would be prudent to conduct a dilapidation
survey of neighbouring structures prior to commencement of construction; and furthermore it is
recommended that a suitably qualified and experienced civil/structural RPEQ is engaged to provide a
vibration management plan for the proposed construction works. Our preliminary recommendation would be
that allowance is made for equipment and works such that the peak particle velocity does not exceed
10 mm/s, where measured at the property boundary.
The exposed natural subgrade following excavations will likely be in a loose condition and must be
compacted by several passes of a vibrating plate or minimum 4 tonne vibrating smooth drum roller, in
conjunction with surface application of ample water, to achieve the required compaction.
Due to the potential for development of poor trafficability during construction, placement of a minimum
150 mm thick granular working platform may be required across laydown areas. Specific platform design
would be required prior to mobilisation and trafficking of any heavy mobile plant on the site (e.g. mobile
cranes, concrete pumps or piling rigs).
Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).
6.1.2 Excavations
Excavations through the soils should be achievable using conventional earthmoving plant (e.g. small or
larger excavators with toothed buckets). Temporary unsurchaged cuts in the very loose to loose sand could
be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to 3 m in height. Flatter
batters or temporary retention may be required if groundwater seepage is encountered or if sloughing of dry
sand occurs at the batter surface. Deeper cuts, if required, will require temporary shoring, unless assessed
and approved by Tectonic during construction.
Permanent cuts or fills greater than 1 m must be retained. Permanent cut and fill batters up to 1 m high shall
not exceed 1V:2.5H. Any permanent soil and fill batters will require erosion protection (e.g. revegetation or
surface protection). Similarly, improvements to the vegetation (i.e. with endemic native species) on the
natural high dune face downslope of the residence would aid in restricting the development initial surficial
failures.
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It is likely that the above temporary batter slopes will not be able to be achieved, at least along the western
boundary, due to spatial restraints, and so temporary shoring may be required (e.g. sheet piles) as per the
recommendations in Section 6.3 below. Excavations in close proximity to existing structures and
infrastructure (e.g. underground services), including on neighbouring properties, will need to be specifically
designed to prevent undermining. It is recommended that footing exposures or confirmation of existing
foundations via certified building plans be undertaken to assess the possibility of undermining. If
undermining is a potential issue the existing structures may need to be underpinned or excavations may
require temporary shoring.
The advice of a suitably qualified and experienced structural RPEQ must be sought regarding any potential
undermining and design and construction of any underpins/temporary shoring (if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
6.1.3 Filling
The natural sandy soils where free of deleterious materials (e.g. vegetation, organics etc.) may be reused as
structural fill (retained under the building), subject to appropriate moisture adjustment. It should be noted that
compaction of such relatively ‘clean’ sands as structural fill requires the addition of substantial volumes of
water (‘flooding’) combined with the use of vibratory compaction equipment (typically a smooth drum roller or
heavy vibrating plate). Should importation of fill be required, it is recommended that such fill should comply
with the following:
Minimum CBR of 10 %
Maximum plasticity index of 15 %
Shrink-swell index ≤ 1 %
Maximum particle size of 100 mm
The primary recommendation would be to re-use site won material, or use a similar granular soil where
import is required. Any fill should be compacted at moisture contents within the range of ±2 % of optimum
moisture content for Standard Compaction. Fill material should be placed in 250 mm thick (loose thickness)
layers and be compacted by repeated rolling using a minimum 4 tonne smooth drum roller (or equivalent,
with vibration on if safe to do so) to achieve a dry density ratio of at least 95 % of the maximum Dry Density
Ratio (DDR), based upon the Standard Compaction method, or a Dry Density Index of at least 70 %.
Confirmatory compaction testing must be carried out at regular intervals. A minimum of two tests per vertical
lift is recommended where structural fill heights are to exceed 300 mm. Further details for control and testing
of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and
Residential Developments.
6.2 Building Foundations
6.2.1 Indicative Site Classification
Site classifications for footing design are determined in accordance with AS 2870-2011 Residential Slabs and
Footings, and depend on the depth of cut / fill undertaken at a site, variations in the natural soil/rock depths,
and the reactivity of the in situ soil / rock and any imported fill materials.
Based on the results of the investigation, the site is classified as Class P in accordance with AS 2870-2011
Residential Slabs and Footings, due to the presence of poor bearing capacity soils at shallow depths, and
trees. If not for these factors, the site would have the characteristics of Class S, with the estimated
characteristic surface movements (‘ys’) ranging up to about 20 mm. However, due to the potential variability
in subgrade density a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footing and plumbing design.
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6.2.2 Footing Design
Should any of the existing structural elements remain, the residence and ancillary structures must not exert
additional load on these elements, unless assessed and approved by a structural RPEQ, who must also
confirm the existing footings and structural members have sufficient capacity to carry additional loads
imposed by the development.
No design loadings were available at the time of reporting. Footings for the proposed residence and any
ancillary structures (e.g. retaining walls, pool etc.) must penetrate through any fill materials and unsuitable
natural soils (i.e. very loose to loose sands) to found at least 1 m into medium dense (or denser) natural
sands. High-level footings may be possible in areas of deeper excavation if medium dense (or denser) sands
are exposed at or near the subgrade. This would be subject to an assessment by a suitably qualified and
experienced geotechnical RPEQ. High-level footings, so founded, could be designed using a maximum
allowable bearing pressure of 100 kPa.
Elsewhere, based on the ASI boreholes, pile founding depths between about 4 m and 5 m BGL (existing
ground levels) are anticipated. Deeper pile depths may be necessary subject to assessment of building loads
at the detailed design stage. To confirm founding depths, deeper borehole drilling should be undertaken
following partial or complete demolition of the existing structures to assess subsurface conditions in the
eastern part of the site.
Excluding the eastern retaining wall, steel screw piles are considered to be an appropriate footing option for
the proposed development. For the eastern boundary retaining wall to achieve the required unfactored shear
resistance of 100 kN/m it is envisaged that steel reinforced concrete bored piles or grout-injected CFA piles
(or similar) will be required. Steel reinforced concrete bored piles would require the use of sacrificial liners
(e.g. Formatube) to prevent caving of the upper sands. Construction water loss would also need to be
carefully monitored and controlled should CFA piles be adopted. The capacity of the piles to achieve this
shear capacity or an equivalent amount should be checked as part of the structural design (lateral capacity).
If steel screw piles are adopted for the main residence and internal walls, it will be important to ensure close
supervision of installation, as piling may not encounter sufficient drilling resistance (from torque
measurement) in sands at the suggested founding depth, and piles could extend to much greater depth than
necessary.
It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
deflections under tension and compression cycles) should also be undertaken prior to construction.
Kentledge for static testing, or simulated cyclic loads, should prove the piles to a minimum of two times the
expected working load within tolerable deflections.
Ultimately, the advice of a specialist piling contractor should be sought regarding pile design and
construction. However, where steel screw piles are adopted site specific load testing and the results of this
geotechnical investigation should be considered by the structural engineers in validating any generic torque
to capacity ratios provided as part of the proposed design. An assessment of potential vibration effects on
nearby structures and monitoring requirements should also be included.
Footing design should also account for potential soil reactivity to be equivalent to Class M. It should be noted
that the placement of reactive clay may result in a more significant equivalent site classification, such as Class
H1 (or higher).
All footings should found such that they are not adversely affected by any adjacent excavations, batter slopes,
trenches, or retaining walls that are not designed to support building loads. Footings adjacent to any fill batter
crests should be deepened to found below the toe of the batter. Footings should found at least below a plane
extending 1 m horizontally from the base of trenches/underground services/excavations/retaining walls, then
rising up at 1V:1H, as illustrated in Text Figure 13 overleaf.
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Text Figure 13: Footing depth required to prevent undermining
Site drainage should be designed to readily remove surface water and to prevent ponding of water on
subgrade areas during construction and also adjacent to foundations and perimeter slabs once construction
is completed.
It is recommended that footings be inspected by a suitably qualified and experienced RPEQ prior to
placement of steel reinforcement and pouring of concrete to confirm allowable bearing pressures and
cleanliness of excavations.
If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic and ASI should be notified immediately and an inspection carried out to determine if
changes to footing design are required.
6.3 Temporary and Permanent Retention
Excavations are anticipated to range up to about 2 m deep. Due to the close proximity to the property
boundaries, existing structures that are to remain, and surrounding infrastructure, some form of temporary
shoring may be required.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, however Unitywater and Noosa Council requirements for the existing infrastructure may
restrict the use of sheet piles. Shoring of deeper cuts may require lateral restraint, and this is conventionally
achieved by helical steel anchors (similar to screw piles). However, the proximity to neighbouring properties
would require permissions to be obtained for subterranean access. Therefore, where lateral support by
cantilever only is not feasible, it is suggested that internal struts/propping be considered to provide lateral
restraint for the shoring system.
Anchored or propped sheet piles may require pre-drilling to prevent adverse vibrations during sheet pile
installation.
Other shoring methods that could be considered include bored contiguous or secant piles. Non-contiguous
or soldier pile walls are not feasible for the proposed development due to the likelihood that loose sands
would ‘flow’ through gaps between the piles.
An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the
permanent walls, whereas sheet piles would be temporary to provide retention during construction only,
therefore requiring a permanent structural wall to be constructed. Bored pile walls generally require greater
width (horizontally) for installation as compared to sheet piles and are often more expensive.
The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation of
the proposed retention system. It will be necessary to conduct numerical analysis to optimise the retention
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system. Detailed design will also need to account for relevant factors of safety to maintain the temporary
and long-term stability of the retention system.
Retaining structures greater than 1 m high, or those required to support surcharge loads or sloping backfill,
will require engineer design and certification of construction. Retaining wall design should be carried out in
accordance with AS4678-2002 Earth retaining structures.
Design for temporary and permanent retaining structures could be carried out using a trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type and based on the parameters
listed in Table 3. These values do not include allowance for hydrostatic pressures, backslopes (assumed to
be horizontal), nor transient (e.g. construction) or adjacent footing loadings.
Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Very Loose/ Loose
1.8 27 0.38 0.55 2.66
Sands
Medium Dense Sands 1.9 34 0.28 0.44 3.54
Dense (or Denser)
1.9 36 0.26 0.41 3.85
Sands
Future Controlled Fill* * * * * *
Note: * Depends on fill material type and level of compaction.
As a minimum, retaining structures must be founded as per the recommendations in Section 6.2.2 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.
6.4 Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment or
retaining walls (e.g. grassed or landscaped swales or diversion mounds). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building walls, footings, batters and retaining
walls.
Although unlikely, if groundwater seepage is encountered during excavations, Tectonic must be notified to
assess the impact on stability.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure. It is assumed that the pool floor will include a
hydrostatic pressure relief valve. The outlet pipe from this valve must be taken to ‘daylight’ at the ground
surface.
All collected roof water should be directed by pipes to the council stormwater system along The Esplanade
(e.g. through the use of a ‘wet’ or ‘charged’ drainage system) or, where not physically possible, surface
water should be directed to infiltrate into the ground around the edges of hard surfaces, with infiltration
areas set back as far as possible from the crest of the dune slope. Hydraulic design should be such that
under no circumstances are uncontrolled stormwater flows permitted over the dune face; where channelised
erosion and scour features could develop.
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To reduce the likelihood of further scouring and undermining of the site, we recommend the following
regarding the scour in the south-eastern part of the site (Ref. Section 4.6):
If possible, diversion drainage should be upgraded/constructed upslope of the area to ensure
stormwater flows, including from the road, are diverted away from the high dune face and ideally into
the existing stormwater infrastructure along The Esplanade. Diversion drainage should not
discharge directly onto the surrounding dune slope which may create new scour issues elsewhere.
The scour should be reinstated with a cellular confinement system (e.g. ‘MiraCell’, gabions, reno
mattress, or similar) and rock fill appropriate to the chosen system. It will be important to undertaken
hydraulic analyses to confirm the estimated stormwater flow rates and suitability of the chosen
confinement system and rock fill type.
Design of the stormwater system should be undertaken by a suitably qualified and experienced civil RPEQ.
7.0 CONCLUSION
Based on the findings of our assessment it is concluded that a Low Risk of slope instability could be
achieved for the proposed development, subject to the following:
Implementation of the recommendations given in this report to prepare the preliminary design;
Review of the preliminary detailed design by Tectonic to confirm compliance with the
recommendations herein; and
Installing a new engineer designed and certified retaining structure supported on piles founded at
least 1 m into medium dense (or denser) sand, designed to accommodate an unfactored shear
resistance of 100 kN/m, along the eastern edge of the level area to the east of the residence.
8.0 LIMITATIONS
This assessment relates only to the proposed building development at the site. Slopes outside the
proposed development area may be subject to risk of slope instability, but assessment of those matters
was beyond the scope of our commission.
Your attention is drawn to the document Limitations, which is included in Appendix C of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of this
report should be, and to present you with recommendations on how to minimise the risks associated with the
services provided by Tectonic for this project.
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Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Office: 07 5478 9016
Email: [email protected]
PROJECT:
Subject site GEOTECHNICAL INVESTIGATION
2 ARAKOON CRESCENT
SUNSHINE BEACH
BH2
TITLE:
DCP1 SITE PLAN
DCP2
DCP3 CLIENT:
BH1 CHRISTOPHER KELLY
A’
DRAWN: DATE:
MCC 23/02/2023
CHECKED: DATE:
ACD 23/02/2023
SCALE:
AS SHOWN (APPROX.)
LEGEND:
APPROX. STAND-ALONE DCP
LOCATION
APPROX. ASI BOREHOLE
LOCATIONS
APPROX. SLOPE/W SECTION
Notes
FIGURE:
1. Based plan courtesy of Noosa Council Intra-Maps, dated 2019 PROJECT NO.: REV:
22484 1 0
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APPENDIX A
DCP Reports
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CLIENT : Christopher Kelly SHEET : 1 OF 1
CONTRACTOR :
PROJECT : Proposed Renovations
LOCATION : 2 Arakoon Crescent, Sunshine Beach
PROJECT No. : 22484
TEST: DCP1 TEST: DCP2 TEST: DCP3
LOGGED: LM 5/12/2022 CHECKED: MCC LOGGED: LM 5/12/2022 CHECKED: MCC LOGGED: LM 5/12/2022 CHECKED: MCC
EASTING: 511274.0 m NORTHING: 7079780.0 m EASTING: 511287.0 m NORTHING: 7079794.0 m EASTING: 511314.0 m NORTHING: 7079791.0 m
MGA2020 Zone 56 MGA2020 Zone 56 MGA2020 Zone 56
Depth (m)
Depth (m)
Depth (m)
Blows per 100 mm (AS 1289.6.3.2-1997) Blows per 100 mm (AS 1289.6.3.2-1997) Blows per 100 mm (AS 1289.6.3.2-1997)
0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25
HW to 0.7 m HW to 0.8 m HW to 0.2 m
0.5 0.5 0.5
1.0 1.0 1.0
1.5 1.5 1.5
>20
2.0 2.0 2.0
TECTONIC 3.00.2 LIB.GLB Log IS DCP 3 PER PAGE 22484.GPJ <<DrawingFile>> 23/2/2023 11:01 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
2.5 2.5 2.5
3.0 3.0 3.0
>20
3.5 3.5 3.5
4.0 4.0 4.0
4.5 4.5 4.5
>14
5.0 5.0 5.0
Comments Comments Comments
HW - Hammer Weight HW - Hammer Weight HW - Hammer Weight
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APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in
Assessing Risk to Property
Some Guidelines for Hillside Construction
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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
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023
Page 31
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
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023
Page 32
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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Document No.
Set ID:68029 RL2 August 2010
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Version: 1, Version Date: 16/03/2023
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Document Set No.
Form ID: 22343520
68029 RL2 August 2010
Version: 1, Version Date: 16/03/2023
Page 34
PROPOSED RENOVATIONS
2 ARAKOON CRESCENT, SUNSHINE BEACH
APPENDIX C
Limitations
24 February 2023
Report No. 22484-001-Rev0
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023
Page 35
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)
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023