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Reference Document - MCU25 0060 (As Published)
NOOSA COUNCIL
REFERENCE DOCUMENT
MCU25/0060
Proposed Residential
Development
22/512 David Low Way, Castaways Beach
Proposed Residential Development25 August 2025 | Ref. 25038-001-Rev0
25 August 2025 | 25038-001-Rev0 i
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Document Review
Doc No. Revision Prepared By Reviewed By Date Issued
001 Draft Ben Cole Geoff Hurley 6 March 2025
001 0 Ben Cole Geoff Hurley 25 August 2025
Doc No. Revision Method of Delivery Issued To
001 0 Electronic Antonio Cappiello
Shaun Lockyer Architects
([email protected])
Prepared By Reviewed By
Ben Cole BEng (Hons) Geoff Hurley BSc (Hons) MSc DIC C.Geol RPEQ 5716
Geotechnical Engineer Senior Consultant
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Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 22/512 David Low Way, within The Dunes estate at Castaways Beach. The investigation
comprised drilling of boreholes at three locations across accessible areas of the development footprint.
This report summarises the results of our investigation and provides geotechnical recommendations
regarding earthworks and footings, together with temporary and permanent basement retention.
Architectural design drawings provided indicate that the development will comprise a three-storey
building. Bulk excavations will be in the order of up to 3 m to 3.5 m deep and localised filling up to
about 1.5 m high is anticipated. Excavations are expected to extend close to the site boundaries and
therefore, some form of retention will be required.
The subsurface conditions encountered in the boreholes are summarised as follows:
▪ Topsoil – comprising very loose to loose sand to depths of 0.1 m to 0.2 m below ground level (BGL);
then
▪ Natural Sands – comprising poorly graded sands mostly being loose to medium dense to depths of
between 1.1 m and 1.3 m BGL; then becoming medium dense and increasing to dense below depths
of about 3.1 m to 3.8 m BGL in boreholes BH1 and BH2.
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 should 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 could be formed at
1V:2H in the very loose to loose sands, and at 1V:1H 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 retention system, such as sheet piles or contiguous reinforced concrete piles,
will be required. These retention systems, which may have to be temporarily braced during construction,
may also be designed to form the permanent basement walls.
Footings for the proposed development and any ancillary structures (e.g. retaining walls) must penetrate
through any 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 in the boreholes below depths of
between about 1.1 m and 1.3 m BGL (existing ground levels). High-level footings would be suitable in
excavation areas where medium dense (or denser) natural sands are exposed at the subgrade level. Piled
footings would be required if loose sand materials are found at or near foundation subgrade level. Given
the sandy soil profile, where deepened footings are required, 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 buildings, open bored piers or driven piles are not considered to be suitable for this site. Cast in
place continuous flight auger (CFA) piles may also be suitable for both foundation and shoring piles
provided a small rig capable of traversing the site can be sourced.
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Contents
1. Introduction ............................................................................................................................................................ 1
2. Proposed Development ....................................................................................................................................... 1
3. Method of Investigation ......................................................................................................................................2
4. Results of Investigation .......................................................................................................................................3
4.1 Site Description ......................................................................................................................................................... 3
4.2 Topography................................................................................................................................................................. 6
4.3 Geology ........................................................................................................................................................................ 6
4.4 Subsurface Conditions ............................................................................................................................................ 7
4.5 Groundwater............................................................................................................................................................... 8
4.6 Surface Drainage....................................................................................................................................................... 8
5. Slope Stability Assessment .................................................................................................................................9
5.1 Existing Conditions................................................................................................................................................... 9
5.2 Stability Assessment .............................................................................................................................................. 10
5.2.1 Quantitative Assessment .................................................................................................................... 11
6. Geotechnical Recommendations ..................................................................................................................... 13
6.1 Development Limitations..................................................................................................................................... 13
6.2 Earthworks ................................................................................................................................................................. 13
6.2.1 Site Preparation and Trafficability................................................................................................... 13
6.2.2 Excavations .............................................................................................................................................. 14
6.2.3 Filling ......................................................................................................................................................... 14
6.3 Building Foundations ............................................................................................................................................ 15
6.3.1 Indicative Site Classification .............................................................................................................. 15
6.3.2 Footing Design....................................................................................................................................... 15
6.4 Temporary and Permanent Retention ............................................................................................................ 17
6.5 Drainage ..................................................................................................................................................................... 18
7. Conclusion ............................................................................................................................................................. 19
8. Limitations ............................................................................................................................................................. 19
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Text Figures
Text Figure 1: Section through north of building (courtesy SLa) ................................................................................... 2
Text Figure 2: 2025 aerial image of the site and immediate surrounds (courtesy Nearmap imagery) ........... 4
Text Figure 3: Easterly facing overview of site....................................................................................................................... 4
Text Figure 4: Batter at southern boundary resulting from construction of the neighbouring
residence .................................................................................................................................................................................. 5
Text Figure 5: Timber sleeper retaining wall along northern boundary ...................................................................... 5
Text Figure 6: Plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’) ........................... 6
Text Figure 7: Extract of 1:100,000 Laguna Bay Surface Geology Map ........................................................................ 7
Text Figure 8: Extract from NC landslide hazard overlay mapping ............................................................................... 9
Text Figure 9: Footing depth required to minimise risk of undermining ................................................................ 16
Tables
Table 1: Summary of Subsurface Conditions........................................................................................................................ 8
Table 2: Stability Risk Levels ...................................................................................................................................................... 10
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007) ............................................................... 11
Table 4: Retaining Structure Design Parameters............................................................................................................... 18
Figures
Figure 1: Site Plan
Appendices
Appendix A Borehole Reports and Explanatory Notes
Appendix B AGS Guidlines - Qualitative Terminology for Use in Assesing the Risk to Property Some
Guidelines for Hillside Construction
Appendix C Limitations
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1. Introduction
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 22/512 David Low Way, residing on Marcus Dunes Court within The Dunes estate at
Castaways Beach. The investigation was performed by Tectonic for Kylie Collum, generally in accordance
with our Proposal P25025-Rev0, dated 29 January 2025. Shaun Lockyer Architects (SLa) are the design
consultants and project managers for the build.
The investigation comprised drilling of boreholes at three 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 advice for temporary
and permanent basement 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. Proposed Development
Architectural drawings were supplied to us with the project brief (Ref. SLa, Project No. 0448, Dwg. No.
000.00 to 400.07, Rev E & F, most recently dated 3 April 2025). Based on the drawings provided, it is
understood that the proposed residence will consist of a three level structure:
▪ The lower level would be ‘dug in’ to the ground completely on the upslope side, and ‘daylighting’
downslope to the east. The lower level footprint would be over most of the available area for
building within the site. An in-ground pool would also be located towards the eastern site boundary,
with the lip equitable with the lower level of the house.
▪ The main central living level would be present above, incorporating a garage to the west and
cross-over from the Marcus Dunes Court road frontage.
▪ The uppermost level would be limited in area, including enclosed living spaces to the west, and an
area of open roof terrace to the east.
▪ A landscaped batter slope would extend down from the north-western corner of the block towards
the main pedestrian and vehicular accesses.
Bulk excavations for the development will be in the order of up to about 3 m depth below ground level
(locally up to 3.5 m) to site to the lower level of the house, and up to about 2 m depth for the in-ground
pool. With the site works proposed, filling is expected to be minimal, however could range up to about
1 m in height and 0.5 m height for localised levelling or landscaping purposes over the remainder of the
site.
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Text Figure 1: Section through north of building (courtesy SLa) 3. 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 Council (NC) planning overlays and site information. An experienced geotechnical engineer from Tectonic undertook a site walk-over on 20 February 2025 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 followed by a subsurface investigation comprising the drilling of three boreholes (designated BH1 to BH3) in accessible areas across the site. Borehole BH1 was drilled toward the north-western corner of the site using Tectonic’s 4WD mounted solid flight auger rig to a depth of 6 m below ground level (BGL). Due to access restrictions boreholes BH2 and BH3 were drilled using hand augering techniques to 3 m BGL (limited due to hole collapse) further to the east of the site. In-situ testing, using the dynamic cone penetrometer (DCP) method, was carried out from the ground surface and at selected depths within each borehole. DCP testing was extended below the base of the hand augered boreholes to depths of 3.9 m BGL. Boreholes were backfilled on completion with drilling spoil, with excess material slightly mounded at surface. Laboratory classification 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. Subsurface conditions are discussed in Section 4.4. Borehole coordinates were recorded using a handheld global positioning system (GPS). The results of the desk-top study and walk-over survey are included in Section 4 below. Proposed Residential Development 25 August 2025 | 25038-001-Rev0 2
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4. Results of Investigation 4.1 Site Description The site is formally described as Lot 22 SP111721, covering an area of 648 m2, with frontage to Marcus Dunes Court to the west. At the time of our investigation the site was vacant and was mostly occupied by small trees and a coverage of grass. Neighbouring properties to the north and south comprised two to three storey residential buildings. Densely vegetated dunes separated the property from Marcus Beach to the east, over a distance of about 190 m. Historical imagery of the site suggests that some stripping of vegetation and possibly some minor earthworks has taken place across the south of the site during construction of the neighbouring property. The neighbouring property to the north, consisted of two main levels, with the lower level slightly below the street level. A partial basement level appeared to have been constructed towards the centre of the building. The house was of rendered blockwork construction. Terraced timber retaining walls in the order of about 0.8 m to 1 m height were present in the back garden. Along the adjoining boundary with the subject site, a timber sleeper retaining wall was present, ranging from 0.2 m to 1.2 m height, retaining landscaping fill above. The house to the north was offset about 2 m from the common property boundary. The existing residence to the south also had a reduced basement level which ‘daylighted’ fully to the eastern side, and two full levels above. An above ground pool was observed downslope, to the eastern side of the house. The house was set back around 2 m from the adjoining boundary fence, with the ground continuing at grade from the subject site. The buildings adjacent and all fences appeared to be relatively straight and in good condition. A sewer manhole and inspection opening was observed along the downslope, eastern lot boundary. It is understood from service plans obtained prior to the fieldwork that this relates to a sewer rising main/gravity main interface. A 2025 aerial image of the site and surrounds is shown in Text Figure 2 overleaf. Proposed Residential Development 25 August 2025 | 25038-001-Rev0 3
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Subject Site
Text Figure 2: 2025 aerial image of the site and immediate surrounds (courtesy Nearmap imagery)
Site conditions at the time of our investigation are shown overleaf in Text Figures 3 to 5.
Text Figure 3: Easterly facing overview of site
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Text Figure 4: Batter at southern boundary resulting from construction of the neighbouring residence
Text Figure 5: Timber sleeper retaining wall along northern boundary
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4.2 Topography
As illustrated in Text Figure 6, the site is located on the south-easterly facing flank of an ancient sand
dune which crests locally about 60 m to the north-west. The ground slope is therefore oblique across the
site. During our site walk-over survey, topographic features specific to the site were noted, and ground
slopes were recorded using a hand-held clinometer.
The inferred natural slopes across the site measured at the time of our assessment mostly ranged from
about 8° to the west, up to about 14° towards the south-eastern corner of the site. An area of slightly
steeper ground was observed along the southern site boundary, with slopes ranging from 20° up to
about 24°. Historical imagery suggests that this batter could possibly have been formed during
construction of the neighbouring property. Dune slopes were estimated to range up to about 15°
immediately beyond the eastern boundary of the site, extending down over the stoss slope over a
distance of about 70 m.
Generally, no irregular surface features were noted which would indicate global slope instability within
the site or on slopes surrounding nearby.
Subject Site
Text Figure 6: Plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’)
4.3 Geology
Available geological information1 indicates that the site is underlain by Holocene age blowout dunes
comprising “quartz sand.” An extract of the relevant geological mapping is shown in Text Figure 7 on the
following page.
1
Queensland Government, Department of Mines and Energy, 1:100,000 Laguna Bay Surface Geology Map, Sheet 9545, 11 July 2018
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Approx. Site Location
Text Figure 7: Extract of 1:100,000 Laguna Bay Surface 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. The subsurface conditions encountered in the boreholes are
summarised as follows:
▪ Topsoil – comprising very loose to loose sand to depths of 0.1 m to 0.2 m BGL; then
▪ Natural Sands – comprising poorly graded sands mostly being loose to medium dense to depths of
between 1.1 m and 1.3 m BGL; then becoming medium dense and increasing to dense below depths
of about 3.3 m to 3.8 m BGL in boreholes BH1 and BH2.
Table 1 below summarises the subsurface profile in each borehole.
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Table 1: Summary of Subsurface Conditions
Loose to Medium Medium Dense Dense (or denser)
Borehole No. Topsoil
Dense Sands Sands Sands
0.0 – 0.2 0.7 – 1.1 0.2 – 0.7 3.3 – 6.0TD
BH1 1.1 – 3.3
0.0 – 0.1 0.1 – 1.3 1.3 – 3.0TD 3.8 – 3.9*DCP
BH2 3.0 – 3.8*
0.0 – 0.1 0.1 – 1.2 1.2 – 3.0TD NE
BH3
Notes: 1. Depths noted in m BGL at time of investigation (20 February 2025)
2. NE denotes not encountered
3. TD denotes borehole terminated at target depth
4. * inferred from DCP testing
5. DCP denotes DCP termination depth
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 downslope to the south-east. It
is noted that the site has a relatively large theoretically catchment. However, it is noted that while no
kerbage was present along Marcus Dunes Court, the road camber was away from the site frontage, with
formal stormwater infrastructure in place (i.e catch pits and culverts). The upslope catchment to the north
would also be disrupted by the presence of developed residential allotments.
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.
Recommendations to improve surface drainage as part of the development works are provided in
Section 6.5.
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5. Slope Stability Assessment
5.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 8).
Mapping indicates that part of the site is within a Moderate (pale yellow shading) Landslide Hazard Area.
Lot boundary
Text Figure 8: Extract from NC 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. Our observations of surface
features relevant to this slope stability assessment are summarised as follows:
No indicators of recent global slope instability were observed, i.e. no landslide scarps or ‘hummocky’
ground was evident across the proposed building area. Overall natural ground slopes were moderate.
No signs of groundwater seepage ‘springs’ were observed across the proposed development area.
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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)
V Very High Unacceptable without treatment. Extensive detailed investigation and research,
H Risk planning and implementation of treatment options essential to reduce risk to Low;
may be too expensive and not practical. Work likely to cost more than value of
property.
H High Risk Unacceptable without treatment. Detailed investigation, planning and
implementation of treatment options required to reduce risk to Low. Work would
cost a substantial sum in relation to the value of the property.
M Moderate May be tolerated in certain circumstances (subject to regulators’ approval) but
Risk requires investigation, planning and implementation of treatment options to reduce
risk to Low. Treatment options to reduce to Low risk should be implemented as soon
as practicable.
L Low Risk Usually acceptable to regulators. Where treatment has been required to reduce the
risk to this level, ongoing maintenance required.
VL Very Low Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend
on the nature of the property at risk; these are only given as a general guide.
Given the geology, topography and particularly the relatively moderately sloping terrain across the
proposed development area, it is considered rare that a landslide could affect the proposed development.
Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°,
although possibly lower in the order of 26˚ to 28˚ for very loose sands. 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). Natural slopes around this site are between 8°
and 14°, with a slightly steeper area parallel with the southern boundary extending to no more than 24°.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there should be a Low (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 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
Failure through very loose to Unlikely Medium Low Given the moderate natural gradients through the
loose natural sands above the proposed building area and limited depth of loose or
foundation depth very loose sands, it is considered Unlikely that a landslip
would develop in these materials, subject to
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.
Deep failure through the Rare Major Low The likelihood of a deep failure through the medium
medium dense (or denser) dense (or denser) natural sand below the proposed
sands below the foundation foundation depth is assessed as Rare due to the lack of
depth precedence of such movements locally, no signs of any
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.
Temporary instability through Rare Major Low The likelihood of a slide/failure through any temporary
temporary retention system, as retention system (sheet pile wall, contiguous pile wall or
discussed in Section 6.4. similar) as outlined in Section 6.4, is assessed as Rare,
subject to detailed design by a suitable qualified RPEQ
engineer and construction by a specialist contractor
which accounts for 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 factor of safety (FOS) against slope instability for the proposed development,
numerical analysis has been conducted using Slope/W including consideration of groundwater
conditions. As per the advice in the recommendations below, it is expected that footings would be taken
to found in the underlying medium dense (or denser) sands, and therefore a surcharge on the subgrade
would only occur where these materials are expected to daylight following bulk excavation. A surcharge
load of 10 kPa per residential storey has been adopted in these locations. A 5 kPa uniform surface has
been applied in all proposed external landscaped areas otherwise. Groundwater levels were assumed at
2 m BGL, which is noted to be conservative, given the sandy subsurface profile and the ground
conditions encountered during our investigation.
The approximate extent of the section line analysed as part of numerical analysis (Section A-A’) is
indicated in Figure 1. The key points from our analyses for the proposed development are summarised as
follows (given overleaf):
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▪ Unit weight and shear strength parameters adopted for the soil strata were estimated based on our
interpretation of the geotechnical investigation data and were typically within the mid range of
values recommended for similar materials in Appendix D of Australian Standard AS4678-2002 Earth
Retaining Structures.
▪ Analysis was undertaken for specified slip entry and exit zones to assess global slope instability
allowing for the full extent of the building envelope. The analysis returned an acceptable minimum
FOS value in excess of 1.5 for the longer term conditions anticipated (including allowance for a
consistent groundwater table at 1.5 m BGL).
▪ This result exceeds the industry standard minimum FOS requirement of 1.5.
Our analysis indicates that an acceptable FOS against instability could be achieved for proposed
residence at 22/512 David Low Way and that there would be a Low Risk of slope instability affecting the
proposed development, subject to implementation of the recommendations outlined in Section 6.
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6. 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 that the main bulk earthworks feature would be the cutting across the site to place the lower level of the main residence on. This is anticipated to be in the order of 3 m to 3.5 m below existing levels, and possibly slightly deeper to allow for footings, slab beams and underground services/drainage. Filling for the ground level is expected to range up to about 1.5 m height on the downslope side (to be battered); with earthworks elsewhere for localised levelling and landscaping not expected to exceed 0.5 m depth/height. Should bulk earthworks, or the proposed methodology/extent of works change, Tectonic should be consulted to re-assess the recommendations given in this report. 6.2 Earthworks 6.2.1 Site Preparation and Trafficability Following removal of the existing buildings and associated construction elements (e.g. footings, services, etc.), any tree stumps and large roots should be ‘grubbed out’. Any organic rich topsoil should then be stripped and set aside. Based on site observations and boreholes, topsoil stripping depths should generally range between 0.1 m and 0.2 m BGL. It is noted that deeper pockets of topsoil may exist. Topsoil materials must not be used in structural fill; however, may be placed across landscaping areas. Unsuitable natural material such as any saturated, loose or soft materials encountered should also be removed during subgrade preparation for bulk earthworks. 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. 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 poor trafficability during placement of a minimum 150 mm thick granular working platform may be required on soil subgrades along haul roads and across laydown areas. Specific platform design would be required prior to mobilisation and trafficking of any heavy mobile plant on the site (e.g. cranes, concrete pumps or piling rigs). Construction areas and surrounds should be graded to shed surface water runoff. Swale drains should be constructed along the crest and toe of any cuts, and above fill batters, where possible. Drains are to be graded to discharge collected waters to a control point(s) before release into existing stormwater infrastructure. Proposed Residential Development 25 August 2025 | 25038-001-Rev0 13
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It should be noted that during wet weather, the natural soils may cause sediment run-off. Appropriate erosion and sediment control measures must be installed prior to commencement of earthworks at this site, particularly given the relatively sensitive eco-receptor downslope. 6.2.2 Excavations 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 sand could be formed at 1V:2H and at 1V:1H 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. If filling or permanent cuts exceed 1 m, then these 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). Given the limited extent of the lower semi-basement level, and offset to properties to the south, temporary battering may be an option for the southern and western sides of the excavation. The proximity to the residence to the north will likely result in the need for temporary shoring. Excavations in close proximity to such existing structures, including on neighbouring properties, will need to be specifically designed to prevent undermining. It is recommended that footing exposures or confirmation of 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 as well as 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). 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). 6.2.3 Filling The existing sandy soils where free of deleterious materials (e.g. vegetation, building debris 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 Proposed Residential Development 25 August 2025 | 25038-001-Rev0 14
Page 20
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. It should be noted that the proposed works are outside of the scope of AS2870. 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. However, 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. 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 slab, footing and plumbing design. 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, etc.) must penetrate through any 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 below depths between about 1.1 m and 1.3 m BGL (existing ground levels) in the boreholes. Thus, it is expected that a combination of high-level footings (e.g. in areas of deeper cut) and locally deepened pads, possibly ranging to piles in uncut or filled areas. 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. Piles founded in medium dense (or denser) sands may be designed using an allowable end bearing pressure of 600 kPa. Settlement under such loads is expected to be in the order of up to 20 mm or 1% of footing width. It should be noted that excavation for concrete strip footings (or similar) may loosen the base. Therefore, the use of a vibrating plate and ample water would be required to densify any high-level footing bases prior to the placement of reinforcing steel. Due to the presence of loose sands, open bored piers or driven piles are not considered to be suitable for this site. Steel reinforced concrete bore piles may be possible; however, the use of casings or Proposed Residential Development 25 August 2025 | 25038-001-Rev0 15
Page 21
sacrificial liners (e.g. formatube, etc.) would be required. Cast in place continuous flight auger (CFA) piles
may also be suitable for foundation piles (and shoring piles) provided a small rig capable of traversing
the site can be sourced. It is generally expected however, that helical steel screw piles would be the most
suitable solution for the parts of the building requiring deeper footings from a cost and constructability
perspective.
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 (which is not recommended) 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. This would also be
applicable when considering possible adverse interactions with buildings on adjacent properties.
Text Figure 9: Footing depth required to minimise risk of undermining
Proposed Residential Development
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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 for the basement may extend to depths of around 3 m BGL (existing ground levels) and the semi-basement footprint will extend close to the site boundaries, particularly on the northern side. Therefore, some form of temporary shoring will be required to support the basement excavation in that area at least. Isolated excavations elsewhere may also require temporary shoring. Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use of sheet piles, however Unitywater requirements for sewer/water infrastructure 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) in sands, although the proximity to neighbouring properties may 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. Sheet piles may require pre-drilling of dense sands (generally anticipated to be below about 3.5 m to 4 m depth BGL) 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 the sands would ‘flow’ through gaps between the piles before application of facing. An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the permanent walls.Sheet piles can provide temporary to retention during construction only, therefore requiring a permanent structural wall to be constructed. Sheet pile walls can also be designed to provide permanent support. Bored pile walls generally require greater width (horizontally) for installation as compared to sheet piles and are often more expensive. Advice of a specialist piling 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 to optimise the retention system. Detailed design will also need to account for relevant factors of safety. 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. Proposed Residential Development 25 August 2025 | 25038-001-Rev0 17
Page 23
Table 4: Retaining Structure Design Parameters
Unit Lateral earth pressure coefficients
Friction
Retained material weight Ka Ko Kp
angle (°)
(kN/m3) (Cantilever wall) (Non-yielding wall)
Loose or Loose to
17 27 0.38 0.55 2.66
Medium Dense Sands
Medium Dense Sands 18 33 0.29 0.46 3.39
Dense (or denser)
18 36 0.26 0.41 3.85
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 piled
footings, if required, also designed to resist lateral loads imposed by the retaining wall. Detailed design
must include an assessment of internal and global stability.
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.
All excess stormwater collected around the residence from hard surfaces (including overflow from
rainwater tanks etc.) must be directed by pipes or lined channels to discharge into the existing
stormwater infrastructure along Marcus Dunes Court. A stormwater management report was prepared
by Westera Partners for the development (Ref. S25-137, dated 18 August 2025) which allows for
management and discharge of run-off from roof and hardstand areas around the house to be discharged
as per this recommendation. The report states that run-off from garden areas around the residence will
continue to discharge per the existing conditions, with a non-worsening condition to the south/
south-east. This would be considered the minimum recommendation from a slope stability perspective.
Subsurface disposal of stormwater must not be undertaken.
Proposed Residential Development
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Page 24
7. Conclusion 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. 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. Proposed Residential Development 25 August 2025 | 25038-001-Rev0 19
Page 25
LEGEND PROJECT APPROX. BOREHOLE LOCATIONS PROPOSED RESIDENCE 22/512 DAVID LOW WAY. ‘THE DUNES’, CASTAWAYS BEACH mms SECTION A-A' er es DRAWING TITLE NOTES. f BASEMAP COURTESY OF NEARMAP, DATED 21 JANUARY 2025 SITE PLAN OVERLAY IMAGE COURTESY SHAUN LOCKYER ARCHITECTS (PROJECT NO. 0448 DWG NO. 100.01 REV C PHASE SD) PROJECT NO. DOC NO. FIGURE NO. REVISION SCALE EET SIZE
Page 26
Appendix A
Borehole Reports and Explanatory Notes
Proposed Residential Development
25 August 2025 | 25038-001-Rev0 1
Page 27
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 25038
Client: Kylie Collum C/- Shaun Lockyer Architects Commenced: 20/2/2025
Project Name: Proposed Residence Completed: 20/2/2025
Hole Location: 22/512 David Low Way, 'The Dunes', Castaways Beach Logged By: KAB
Hole Position: 510249.0 m E 7075353.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: Hand 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 DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SAND, (TOPSOIL), fine to medium grained, L
grey, with organics
SP SAND, fine to medium grained, grey,
(AEOLIAN) MD
L to
1 MD
becoming pale grey
2
MD
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 25038.GPJ <<DrawingFile>> 6/3/2025 13:10 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
Not Encountered
becoming pale grey and grey
Augered to 3.0m
3 M
HA
D
4
Augered to 4.3m
>20
5
VD
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 < PL MD - Medium Dense
Core recovered (hatching and Soil Descriptions D - Dense
indicates material) = PL
Based on Unified Soil < PL VD - Very Dense
Core loss Classification System Fr - Friable
Page 28
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 25038
Client: Kylie Collum C/- Shaun Lockyer Architects Commenced: 20/2/2025
Project Name: Proposed Residence Completed: 20/2/2025
Hole Location: 22/512 David Low Way, 'The Dunes', Castaways Beach Logged By: KAB
Hole Position: 510263.0 m E 7075351.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: Hand 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 DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SAND, (TOPSOIL), fine to medium grained, VL HW
SP grey, with organics
L
SAND, fine to medium grained, grey,
(AEOLIAN)
L to
MD
1
Not Encountered
becoming pale grey
D to
HA
M
Augered to 1.9m
2
MD
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 25038.GPJ <<DrawingFile>> 6/3/2025 13:10 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
3
Hole Terminated at 3.00 m
Target depth
4
5
6
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 < PL MD - Medium Dense
Core recovered (hatching and Soil Descriptions D - Dense
indicates material) = PL
Based on Unified Soil < PL VD - Very Dense
Core loss Classification System Fr - Friable
Page 29
Borehole No.
BH3
Page 1 of 1
Engineering Log - Borehole Project No.: 25038
Client: Kylie Collum C/- Shaun Lockyer Architects Commenced: 20/2/2025
Project Name: Proposed Residence Completed: 20/2/2025
Hole Location: 22/512 David Low Way, 'The Dunes', Castaways Beach Logged By: KAB
Hole Position: 510277.0 m E 7075351.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: Hand 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 DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SAND, (TOPSOIL), fine to medium grained, VL HW
SP grey, with organics
SAND, fine to medium grained, grey,
(AEOLIAN)
becoming pale grey L
1
Not Encountered
D to
HA
M
Augered to 2.0m
2
MD
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 25038.GPJ <<DrawingFile>> 6/3/2025 13:10 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
3
Hole Terminated at 3.00 m
Target depth
4
5
6
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 < PL MD - Medium Dense
Core recovered (hatching and Soil Descriptions D - Dense
indicates material) = PL
Based on Unified Soil < PL VD - Very Dense
Core loss Classification System Fr - Friable
Page 30
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RD Rotary Blade or Drag bit NQ Diamond Core - 47 mm
*V V-Bit RT Rotary Tri-cone bit NMLC Diamond Core - 52 mm
*T TC-Bit RA Rotary Air HQ Diamond Core - 63 mm
HA Hand Auger HMLC Diamond Core – 63mm
ADH Hollow Auger BH Tractor Mounted Backhoe
EX Tracked Hydraulic Excavator
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
FV Field vane shear 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
August 2024
Page 31
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
Coarse Grained Soils
Symbol Term Description
D Dry Sands and gravels are free flowing.
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
Fine Grained Soils
Symbol Description
w < PL Moist, dry of plastic limit, (hard and friable or powdery)
w = PL Moist, near plastic limit, (soils can be moulded at a moisture content approximately equal to the plastic limit)
w > PL Moist, wet of plastic limit, (soils usually weakened and free water forms on hands when handling)
w = LL Wet, near liquid limit
w > LL Wet, wet of liquid limit
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.
Rev3
November 2024
Page 32
Appendix B AGS Guidlines - Qualitative Terminology for Use in Assesing the Risk to Property Some Guidelines for Hillside Construction Proposed Residential Development 25 August 2025 | 25038-001-Rev0 2
Page 33
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
68039 Landslide Risk Management Issued 10/7/07
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007 114 Australian Geomechanics Vol 42 No 1 March 2007
Page 37
a e €nic ‘e Appendix C Limitations
Page 38
Limitations This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Limitations August 2024 | Rev2
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PO Box 899 tectonicgeo.com.au [email protected]
Buderim QLD
Proposed Residential 4555
Development
25 August 2025 | 25038-001-Rev0 4