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RFI - 21327-001-Rev0 Plover St
GEOTECHNICAL INVESTIGATION
Proposed Residence
34 Plover Street, Peregian Beach
Report for: Nick & Katie Jorss
21327-001-Rev0
10 September 2021
Tectonic Geotechnical Pty Ltd
PO Box 899, Buderim QLD 4556
Office: 07 5478 9016
Email: [email protected]
www.tectonicgeo.com.au
A.B.N. 83 165 727 828
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Page 2
PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
Document Review
Document No. Revision Prepared By Reviewed By Date Issued
001 0 Michael Cocks Ashley Davey 10 September 2021
Report Distribution
Document Revision Method of Delivery Issued To
Nick & Katie Jorss
001 0 Electronic
([email protected])
Mat Bruton
001 0 Electronic Alta Projects
([email protected])
Michael Cornish
001 0 Electronic Sparks Architects
([email protected])
Prepared By: Reviewed By:
Michael Cocks BEng (Civil/Geotechnical) MIEAust Ashley Davey RPEQ 8159
Geotechnical Engineer Managing Director
10 September 2021
Report No. 21327-001-Rev0 i
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PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed multi-storey
residence at 34 Plover Street, Peregian Beach. The investigation was performed by Tectonic for
Nick and Katie Jorss.
The investigation comprised drilling of boreholes at two locations across accessible areas of the
development footprint. This report summarises the results of the investigation and provides geotechnical
recommendations regarding earthworks and footings, together with temporary and permanent retention.
Preliminary architectural drawings indicate that the development will comprise the construction of a two to
three storey split level building, with most of the lower levels excavated into the hillside. Construction will
likely include a combination of masonry and lightweight materials (e.g. weatherboard cladding to walls etc.)
and suspended concrete floors. A pool will be suspended above the central part of the lower level.
No earthworks details were provided at the time of reporting; however, provided plans indicate that
excavations for the lower level would likely be in the order of about 3 m deep in the western part of the site,
with the cut daylighting towards the east. Localised filling less than 1 m high is anticipated. Excavations will
extend close to the site boundaries and, therefore, some form of temporary retention will likely be required.
Isolated deeper excavations may be required for underground services and footings or other elements.
The subsurface conditions encountered in the boreholes are summarised as follows:
Topsoil – comprising loose silty sand to 0.1 m below ground level (BGL); then
Natural Sands – comprising loose, poorly graded sands to 2 m BGL, then becoming medium
dense, increasing to dense (or denser) below depths of about 4.5 m to 5.2 m BGL.
No signs of groundwater seepage (‘springs’) were observed during our walk-over. Groundwater was not
encountered in the boreholes at the time of investigation.
Provided the recommendations within this report are adhered to, and subject to detailed design and
construction of the footing and retention system by a suitably qualified and experienced RPEQ structural
engineer, there would be a Low Risk to property from slope instability.
Excavations through the existing soils should be achievable using conventional earthmoving plant (e.g. small
or larger excavators with toothed buckets). Temporary unsurcharged cuts in the very loose to loose sands
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,
provided groundwater seepage is not encountered and sands do not slough from the batter face as they dry
out. It is expected that space restrictions will limit battering of temporary cuts on the site and so some form of
temporary retention system will be required such as sheet, contiguous or secant pile walls.
Footings for the proposed development and any ancillary structures (e.g. retaining walls) must penetrate
through any fill materials and unsuitable natural soils (i.e. very loose to loose sands) to found at least 0.5 m
into medium dense (or denser) natural sands. Such materials were encountered at about 2 m BGL (existing
ground levels) in the boreholes. Thus, piled footings will be required unless excavations expose such
materials at or near foundation subgrade level. High-level footings would be suitable in excavation areas
where medium dense (or denser) natural sands are exposed at the subgrade level.
Given the sandy soil profile, where deepened footings are required (i.e. away from deeper excavations),
steel screw piles are considered to be a suitable foundation type. Due to the presence of loose sands,
together with the close proximity of adjacent residences, open bored or driven piles are not considered to be
suitable for this site.
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Table of Contents
1.0 INTRODUCTION ........................................................................................................................................................ 1
2.0 PROPOSED DEVELOPMENT ................................................................................................................................... 1
3.0 METHOD OF INVESTIGATION ................................................................................................................................. 2
4.0 RESULTS OF INVESTIGATION ................................................................................................................................ 2
4.1 Site Description............................................................................................................................................. 2
4.2 Topography................................................................................................................................................... 3
4.3 Geology ........................................................................................................................................................ 4
4.4 Subsurface Conditions.................................................................................................................................. 4
4.5 Groundwater ................................................................................................................................................. 5
4.6 Surface Drainage .......................................................................................................................................... 5
5.0 SLOPE STABILITY ASSESSMENT .......................................................................................................................... 5
5.1 Existing Conditions ....................................................................................................................................... 5
5.2 Stability Assessment..................................................................................................................................... 6
5.2.1 Quantitative Assessment ........................................................................................................................ 7
6.0 GEOTECHNICAL RECOMMENDATIONS ................................................................................................................ 9
6.1 Development Limitations .............................................................................................................................. 9
6.2 Earthworks – General ................................................................................................................................... 9
6.2.1 Site Preparation and Trafficability ........................................................................................................... 9
6.2.2 Excavations ............................................................................................................................................. 9
6.2.3 Filling..................................................................................................................................................... 10
6.3 Building Foundations .................................................................................................................................. 10
6.3.1 Indicative Site Classification.................................................................................................................. 10
6.3.2 Footing Design ...................................................................................................................................... 11
6.4 Temporary and Permanent Retention......................................................................................................... 12
6.5 Drainage ..................................................................................................................................................... 13
7.0 SUMMARY & CONCLUSIONS................................................................................................................................ 14
8.0 LIMITATIONS .......................................................................................................................................................... 14
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TABLES
Table 1: Summary of Subsurface Profiles ........................................................................................................................... 4
Table 2: Stability Risk Levels .............................................................................................................................................. 6
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007) ......................................................................... 7
Table 4: Retaining Structure Design Parameters .............................................................................................................. 13
TEXT FIGURES
Text Figure 1: Layout plan (upper) and a cross-sectional view (lower) of proposed development, courtesy of
Sparks ................................................................................................................................................................ 1
Text Figure 2: 2021 aerial image of the site and immediate surrounds (courtesy Nearmap imagery)................................. 2
Text Figure 3: Panoramic view of the site looking south from northern boundary ............................................................... 3
Text Figure 4: View of the site looking west (LHS) and minor cracking and shallow depressions in Plover Street
(RHS) ................................................................................................................................................................. 3
Text Figure 5: Site plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’) ...................................... 3
Text Figure 6: Extract of Gympie Special 1:100,000 Geology Map ..................................................................................... 4
Text Figure 7: Extract from NSC landslide hazard overlay mapping ................................................................................... 5
Text Figure 8: Slope/W output for long term conditions...................................................................................................... 8
Text Figure 9: Footing depth required to prevent undermining......................................................................................... 12
FIGURES
Figure 1: Site Plan
APPENDICES
APPENDIX A
Borehole & DCP Reports & Explanatory Notes
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
APPENDIX C
Limitations
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1.0 INTRODUCTION
This report details the results of a geotechnical investigation undertaken for a proposed multi-storey
residence at 34 Plover Street, Peregian Beach. The investigation was performed by Tectonic for
Katie and Nick Jorss, generally in accordance with Proposal P21473-Rev0, dated 12 August 2021.
The investigation comprised drilling of boreholes at two locations across accessible areas of the
development footprint, together with in-situ strength testing. This report summarises the results of the
investigation and provides geotechnical recommendations regarding earthworks and footings, together with
temporary and permanent retention.
In summary, subject to the implementation of the recommendations made herein, it is assessed that there
would be a Low Risk of slope instability affecting the proposed development in accordance with the
Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS
2007).
2.0 PROPOSED DEVELOPMENT
Preliminary architectural drawings were supplied to us (Ref. Sparks Architects, Job No. 178, Drawing Nos.
DD 0.0 to DD 7.12 [not inclusive], dated 4 August 2021). These plans indicate that the development will
comprise the construction of a two to three storey split level building, with most of the lower levels
excavated into the hillside. Construction will likely include a combination of masonry and lightweight
materials (e.g. weatherboard cladding to walls etc.) and suspended concrete floors. A pool will be
suspended above the central part of the lower level.
No earthworks details were provided at the time of reporting; however, provided plans indicate that
excavations for the lower level would likely be in the order of about 3 m deep in the western part of the site,
with the cut daylighting towards the east. Localised filling less than 1 m high anticipated. Excavations will
extend close to the site boundaries and, therefore, some form of temporary retention will likely be required.
Isolated deeper excavations may be required for underground services and footings or other elements.
Text Figure 1 below presents a layout plan and cross-sectional view of the proposed development.
Text Figure 1: Layout plan (upper) and a cross-sectional view (lower) of proposed development, courtesy of Sparks
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3.0 METHOD OF INVESTIGATION
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps, slope stability studies as well as Noosa Shire Council (NSC) planning overlays and site information.
An experienced geotechnical professional from Tectonic undertook a site walk-over on 26 August 2021 to
visually assess any 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.
This was incorporated with a subsurface investigation comprising the drilling of two boreholes (designated
BH1 to BH2) in accessible areas across the site. Boreholes were drilled using Tectonic’s 4WD mounted
solid flight auger rig to 6 m below ground level (BGL).
In-situ testing, using the dynamic cone penetrometer (DCP) method, was carried out from the ground surface
at each borehole location. Boreholes were backfilled on completion using spoil arising during drilling, with
excess material slightly mounded at surface.
Laboratory testing was not warranted due to the sandy nature of the subsurface conditions encountered.
Borehole locations are shown in Figure 1 attached at the end of this report, with borehole and DCP logs
presented in Appendix A, together with explanatory notes.
The results of the desk-top study and walk-over survey are included in Section 4 below.
4.0 RESULTS OF INVESTIGATION
4.1 Site Description
The site is formally described as Lot 10 on P9313, covers an area of about 506 m2, with frontage to Plover
Street along the western boundary. At the time of the investigation the site was vacant, with a variably
dense vegetation cover comprising long grass, bushes and scattered mature trees. Surrounding properties
included a vacant allotment to the south, easement to the north and the Peregian Beach foredune to the
east.
Multi-storey residences occupied properties further to the north and south. Minor cracking and shallow
depressions were noted within the asphalt seal along Plover Street; however, this was likely related to
factors other than slope instability (e.g. consolidation of the upper loose sand materials). A 2021 aerial
image of the site and surrounds is shown in Text Figure 2 below.
Subject Site
Peregian Beach Foredune
Text Figure 2: 2021 aerial image of the site and immediate surrounds (courtesy Nearmap imagery)
Site conditions at the time of our investigation are shown in Text Figures 3 and 4 overleaf.
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Text Figure 3: Panoramic view of the site looking south from northern boundary
Text Figure 4: View of the site looking west (LHS) and minor cracking and shallow depressions in Plover
Street (RHS)
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 Text Figure 5 below, the site is located on the
easterly facing flank of an ancient sand dune which crests about 40 m to the west. The toe of the dune
traces the eastern property boundary, with the main Peregian Beach foredune cresting about 60 m further to
the east.
The ground slope across the site was primarily waning (reducing) and relatively planar. Ground surface
gradients typically varied between about 16° across the western edge of the site, flattening to less than 6°
along the eastern boundary. No irregular surface features were noted which would indicate global slope
instability.
Subject Site
Text Figure 5: Site plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’)
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4.3 Geology
Available geological information 1 indicates that the site is near the boundary of Quaternary age, mainly
parabolic dunes comprising ‘quartz sand’ and Holocene age coastal swamp deposits comprising ‘quartz
sand and peaty quartz sand’ (Ref. Text Figure 6).
Approx. Site Location
Text Figure 6: Extract of Gympie Special 1:100,000 Geology Map
4.4 Subsurface Conditions
Detailed descriptions of the subsurface conditions encountered in the boreholes are provided in the
borehole reports attached in Appendix A, along with definition sheets which describe terms and
abbreviations used on the reports. Subsurface conditions encountered in the boreholes are summarised as
follows:
Topsoil – comprising loose silty sand to 0.1 m BGL; then
Natural Sands – comprising loose poorly graded sands to 2 m BGL, then becoming medium
dense, increasing to dense (or denser) below depths of about 4.5 m to 5.2 m BGL.
Table 1 below, summarises the subsurface profile in each borehole.
Table 1: Summary of Subsurface Profiles
Medium Dense Dense (or
Borehole No. Topsoil Loose Sands
Sands denser) Sands
BH1 0 – 0.1 0.1 – 2.0 2.0 – 4.5 4.5 – 6.0TD
BH2 0 – 0.1 0.1 – 2.0 2.0 – 5.5 5.2 – 6.0TD
Notes: 1. Depths noted in m BGL at time of investigation (26 August 2021)
2. TD denotes borehole terminated at target depth
1
Queensland Government, Department of Mines and Energy, 1:100,000 Gympie Special Geology Map, Sheet 9445 & Part 9545, 1999
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4.5 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over. Groundwater was not
encountered in the boreholes at the time of investigation. Groundwater conditions are expected to be
reflective of prevailing weather conditions and seepage may develop, particularly following periods of
prolonged or heavy rainfall.
4.6 Surface Drainage
Surface drainage would be expected to follow natural surface contours towards the eastern boundary. It is
noted that the site has a relatively large catchment; however, most runoff from upslope of the property is
anticipated to be intercepted by the existing stormwater infrastructure along Plover Street.
Given the sandy nature of the site, rainfall would normally be anticipated to infiltrate the ground surface.
However, as the proposed development will occupy a large part of the block, significant run-off from hard
surfaces will occur. Nevertheless, where directed into swales rather than pipes, such run-off would be
expected to infiltrate into the natural sands relatively rapidly.
No significant surface run-off features were noted during our assessment (e.g. deep rilling or scour/wash out
scars). This indicates that significant overland flow is generally absent during rainfall events.
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 7)
which indicates that most of the site is within a Very Low/Low (no shading) Landslide Hazard Area. Isolated
areas in the north-western corner and along the eastern boundary are mapped as being within a Moderate
(pale yellow shading) Landslide Hazard Area.
Approx. Site Location
Text Figure 7: Extract from NSC landslide hazard overlay mapping
It should be noted that the Noosa 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 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 encountered at relatively shallow depth.
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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 2 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
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 to
L Low Risk
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.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface
gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands. Our measurements
indicate that the overall natural gradient is up to 16°. The limit of equilibrium is the ‘tipping point’ to instability,
where the factor of safety (FOS) of the slope is approximately 1.0. The FOS of a slope is the balance (ratio)
between the forces driving instability (e.g. soil weight, groundwater pressure) and the resisting forces (e.g. soil
friction and cohesion).
Sand dune slopes generally remain stable (i.e. FOS greater than 1.0) when they are:
well vegetated (mix of ground cover, shrubs and small trees);
not subjected to concentrated stormwater flows;
not surcharged with additional loads (e.g. fill); and
not undercut by excavation or erosion.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation, and development of generally localised instability.
The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion. At this site, the area below the site is protected by the relatively
wide Peregian Beach foredune; and therefore, water erosion or significant disturbance of the dune by natural
forces, in the vicinity of the site, is considered unlikely over the design life of the house.
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 (NSC) 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 overleaf 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).
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Table 3: 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 and retention system
1: Failure through very loose to proposed it is considered Unlikely that a landslip
loose natural sands above the Unlikely Medium Low would develop in these materials, subject to
foundation depth implementation of the recommendations in Section 4.
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.
The likelihood of a slip/failure through the temporary
retention system (sheet pile wall, or similar) as outlined in
3: Instability through temporary Section 6.4, is assessed as Rare, subject to detailed
sheet pile retention system, or design by a suitable qualified RPEQ engineer and
Rare Major Low
similar, as discussed in Section construction by a specialist contractor which accounts for
6.4. the required factors of safety against slope instability.
Although the consequence of such a failure would be
Major, the resultant risk is Low as per AGS2007.
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 FOS against slope instability for the proposed development, numerical analysis has
been conducted using the software Slope/W. The ground surface profile has been modelled using a
combination of preliminary design drawings and slope angles measured in the field by Tectonic. The
analysed subsurface profile was compiled based on the results of our subsurface investigation. Some
inferences regarding subsurface conditions were made between test locations. A minimum FOS value of 1.5
is the commonly adopted industry standard for long term conditions.
No groundwater was encountered at the time of investigation. Given the gradients across the site and highly
permeable nature of the sandy soil, development of shallow groundwater is considered unlikely. Should
groundwater seepage be encountered during construction Tectonic must be notified to assess the impacts
on temporary instability.
To replicate future landscaping, a surcharge load of 5 kPa has been applied upslope of the proposed
building. Most of the building will be supported by piled foundations taken into the underlying medium dense
(or denser) natural sands and therefore, building surcharge loads have not been included in the model.
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 8 on the following page. Analysis was
undertaken for a specified range of entry and exit points that could impact the proposed development.
The analysed profile represents the critical section (i.e. ‘worst case’ scenario) for the proposed development,
which covers the deepest cut. Our analysis indicates that the proposed development would achieve at least
the required minimum FOS for long-term stability (i.e. 1.5), subject to permanent retention of cuts using
engineer designed structures.
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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.
Text Figure 8: Slope/W output for long term conditions
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6.0 GEOTECHNICAL RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.
6.1 Development Limitations
It is anticipated earthworks will include cuts for the lower level of up to about 3 m deep in the western part of
the site, with filling expected to be required up to about 1 m in height to form finished surface levels. Should
proposed cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability before
construction.
6.2 Earthworks – General
6.2.1 Site Preparation and Trafficability
Following clearing of existing vegetation, any tree stumps and large roots should be ‘grubbed out’. Any
organic rich topsoil should then be stripped and set aside. Based on the boreholes, topsoil stripping depths
should generally range up to about 0.1 m deep; however, deeper areas of unsuitable materials may be
present across the site. Although not encountered in the boreholes any existing uncontrolled fill must be
stripped from construction areas. Therefore, it is possible that deeper pockets of topsoil or uncontrolled fill
may exist and deeper, localised grubbing out of tree roots may be required.
Topsoil materials must not be used in structural fill; however, may be placed across landscaping areas.
Unsuitable saturated or loose materials encountered should also be removed during subgrade preparation.
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 and infrastructure (e.g. sewer main
along the northern boundary). 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.
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).
6.2.2 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, should be specifically assessed by Tectonic.
Permanent cuts or fills on the dune slope 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). Higher unretained fill batters may be permissible along the eastern
boundary (dune toe) subject to an assessment by an RPEQ geotechnical engineer.
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It is likely that the above temporary batter slopes will not be able to be achieved, due to spatial restraints,
and so temporary shoring may be required (e.g. sheet piles) as per the recommendations in Section 6.4
below. Excavations in close proximity to existing structures and infrastructure (e.g. sewer main along the
northern boundary), 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 RPEQ structural engineer 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).
6.2.3 Filling
The natural sandy soils where free of deleterious materials (e.g. vegetation, organics etc.) may be reused as
structural fill, 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
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.
Fill embankment edges must be over-filled by 0.5 m (horizontally) and then trimmed back to well compacted
material.
6.3 Building Foundations
6.3.1 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.
10 September 2021
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PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
6.3.2 Footing Design
No design loadings were available at the time of reporting. Footings for the proposed development 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 0.5 m into medium dense (or denser) natural
sands. Such materials were encountered at about 2 m BGL (existing ground levels) in the boreholes. Thus,
piled footings will be required unless excavations expose such materials at or near foundation subgrade level.
Piles so founded may be designed using an allowable end bearing pressure of 300 kPa. High level footings,
where achievable in deeper cut areas, may be designed using an allowable bearing pressure of 100 kPa
provided that they are founded in the natural medium dense (or denser) sands. Settlement under such loads
is expected to be in the order of up to 20 mm or 1% of footing width.
Given the sandy soil profile and depths of footings required, steel screw piles are considered to be a suitable
foundation type for the proposed development generally. However, in areas of cut exposing medium dense
(or denser) sands at the subgrade level high-level footings may be adopted. It should be noted that
excavations for concrete strip footings (or similar) may loosen the base. Therefore, the use of a vibrating
plate would be required to densify any high-level footing bases prior to the placement of reinforcing steel.
Due to the presence of loose sands and nearby structures, open bored piers or driven piles are not
considered to be suitable for this site and it is expected that steel screw piles could be utilised. Steel
reinforced concrete bore piles may be possible; however, the use of sacrificial liners (e.g. Formatube) would
be required. The advice of a specialist piling contractor must be sought regarding feasibility, design and
installation of the piling system for this site, including an assessment of potential vibration effects on nearby
structures, and monitoring requirements.
If steel screw piles are adopted, 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.
Footing design should also account for potential soil reactivity to be equivalent to Class M. It should be noted
that placement of reactive clay may result in a more severe equivalent site classification, such as H1 or
greater.
All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
retaining walls, or in ground structures that are not designed to support building loads. Footings should
found at least below a plane extending 1 m horizontally from the base of trenches/excavations/retaining
walls/swimming pool, then rising up at 1V:1H, as illustrated in Text Figure 9 overleaf.
10 September 2021
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PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
Text Figure 9: 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 engineer 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 should be notified immediately and an inspection carried out to determine if changes
to footing design are required.
6.4 Temporary and Permanent Retention
Excavations are expected to range up to around 3 m BGL (existing ground levels) and, given that the
planned excavation extends close to some of the site boundaries and underground infrastructure (e.g.
sewer), some form of temporary shoring will likely be required to support the excavation.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, however Unitywater requirements for the sewer may restrict the use of sheet piles. Shoring of
deeper cuts would likely 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 and subsurface drainage system. It will be necessary to conduct numerical analysis
10 September 2021
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PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
to optimise the retention 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 4. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.
Table 4: 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 28 0.36 0.53 2.77
Sands
Medium Dense Sands 1.9 32 0.31 0.47 3.25
Dense (or Denser)
1.9 34 0.28 0.44 3.54
Sands
Future Controlled Fill* * * * * *
Note: * Depends on fill material type and level of compaction.
Retaining structures must be founded as per the recommendations in Section 6.3.2 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. A solution using screw piles to support retaining walls, possibly
including raked piles, may be feasible although bending and shear capacities would need to be carefully
checked in design from a soil-structure interaction viewpoint. Additional lateral capacity may be achieved by
incorporating a deeper pile cap that also acts as a shear key attracting passive resistance from the sands in
front of the wall.
6.5 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 excess stormwater collected around the residence (including overflow from rainwater tanks etc.) must be
directed by pipes or lined channels to discharge into the existing stormwater infrastructure along Plover
Street (where possible), or at least 5 m downslope of any structure or embankment.
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PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
7.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment it is assessed that there would be a Low Risk of slope instability
affecting the proposed development subject to implementation of the recommendations given in this report.
8.0 LIMITATIONS
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.
10 September 2021
Report No. 21327-001-Rev0 14
Page 20
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Ph: 07 5478 9016
PROJECT:
PROPOSED RESIDENCE
34 PLOVER STREET
PEREGIAN BEACH
TITLE:
SITE PLAN
BH1
Peregian Beach Foredune CLIENT:
BH2
NICK & KATIE JORSS
DRAWN: DATE:
MCC 9/09/2021
Subject Site
CHECKED: DATE:
ACD 9/09/2021
SCALE:
AS SHOWN (APPROX.)
LEGEND:
APPROX. BOREHOLE
LOCATIONS
Notes:
Base plan adopted from Nearmap imagery dated 5 July 2021 0m 20 40m PROJECT NO.: FIGURE: REV:
21327-001 1 0
Page 21
PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
APPENDIX A
Borehole & DCP Reports & Explanatory Notes
10 September 2021
Report No. 21327-001-Rev0
Page 22
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 21327
Client: Nick & Kate Jorss Commenced: 26/8/2021
Project Name: Proposed Residence Completed: 26/8/2021
Hole Location: 34 Plover Street, Peregian Beach Logged By: MT
Hole Position: 509572.0 m E 7070811.0 m N MGA2020 Zone 56 Checked By: ACD
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND (TOPSOIL), fine to medium grained,
SP grey, with organics
SAND, fine to medium grained, pale grey
becoming pale brown to white
1 L
2
becoming white
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 21327.GPJ <<DrawingFile>> 10/9/2021 09:12 10.00.01.07 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
Not Encountered
D to
AD/T
3
M
MD
4
5
D to
VD 15/80mm
Refusal
6
Hole Terminated at 6.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 23
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 21327
Client: Nick & Kate Jorss Commenced: 26/8/2021
Project Name: Proposed Residence Completed: 26/8/2021
Hole Location: 34 Plover Street, Peregian Beach Logged By: MT
Hole Position: 509589.0 m E 7070804.0 m N MGA2020 Zone 56 Checked By: ACD
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND (TOPSOIL), fine to medium grained,
SP grey, with organics
SAND, fine to medium grained, pale grey
becoming pale brown to white
1 L
becoming white
2
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 21327.GPJ <<DrawingFile>> 10/9/2021 09:12 10.00.01.07 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
D to
M
Not Encountered
AD/T
3
MD
4
becoming dark brown, trace of silt fines
5
M
D to
VD
15/50mm
Refusal
6
Hole Terminated at 6.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 24
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RA Rotary Air BH Tractor Mounted Backhoe
ADH Hollow Auger RD Rotary Blade or Drag bit EX Tracked Hydraulic Excavator
HA Hand Auger RT Rotary Tri-cone bit HMLC Core – 63mm
*T TC-Bit HQ Core – 63mm
*V V-Bit NMLC Core – 47mm
NQ Core – 52mm
R Ripper
RH Rock Hammer
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
5,4,10 N=14 5,4,10 = Blows per 150mm. N = Blows per 300mm penetration following 150mm seating
30/65mm Where practical refusal occurs, the blows and penetration for that interval are reported
RW Penetration occurred under the rod weight only
HW Penetration occurred under the hammer and rod weight only
HB Hammer double bouncing on anvil
DS Disturbed sample
BDS Bulk disturbed sample
SV Field shear vane test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev1
February 2021
Page 25
METHOD OF SOIL DESCRIPTION
USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Soil is classified and described in borehole and test pit logs using the preferred method given in AS1726 – 2017.
The material properties are assessed in the field by visual/tactile methods.
Particle Size Plasticity Properties
Major Sub Division Particle Size
Division
BOULDERS > 200 mm
COBBLES 63 to 200 mm
Coarse 19 to 63 mm
GRAVEL Medium 6.7 to 19 mm
Fine 2.36 to 6.7 mm
Coarse 0.6 to 2.36 mm
SAND Medium 0.21 to 0.6 mm
Fine 0.075 to 0.21
SILT 0.002 to 0.075
CLAY < 0.002 mm
NOTE: The U line is an approximate upper bound for most materials.
Data which plot above the U line may represent unusual/problem soil
behavior, or unreliable data and should be considered carefully.
MOISTURE CONDITION AS1726 - 2017
Symbol Term Description
D Dry Sands and gravels are free flowing. Clays & Silts may be brittle or friable and powdery.
M Moist Soils are darker than in the dry condition & may feel cool. Sands and gravels tend to cohere.
W Wet Soils exude free water. Sands and gravels tend to cohere.
CONSISTENCY AND DENSITY AS1726 - 2017
Symbol Term Undrained Shear Symbol Term Density Index % SPT “N” #
Strength
VS Very Soft 0 to 12 kPa VL Very Loose Less than 15 0 to 4
S Soft 12 to 25 kPa L Loose 15 to 35 4 to 10
F Firm 25 to 50 kPa MD Medium Dense 35 to 65 10 to 30
St Stiff 50 to 100 kPa D Dense 65 to 85 30 to 50
VSt Very Stiff 100 to 200 kPa VD Very Dense Above 85 Above 50
H Hard Above 200 kPa
In the absence of test results, consistency and density may be assessed from correlations with the observed behaviour of
the material.
# SPT correlations are not stated in AS1726 – 2017, and may be subject to corrections for overburden pressure and
equipment type.
Rev1
October 2017
Page 26
PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in
Assessing Risk to Property
Some Guidelines for Hillside Construction
10 September 2021
Report No. 21327-001-Rev0
Page 27
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 28
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 29
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 31
PROPOSED RESIDENCE
34 PLOVER STREET, PEREGIAN BEACH
APPENDIX C
Limitations
10 September 2021
Report No. 21327-001-Rev0
Page 32
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)