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Reference Document - MCU26 0001 & OPW26 0002 - AS PUBLISHED · MCU26/0001
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NOOSA COUNCIL
REFERENCE DOCUMENT
MCU26/0001 & OPW26/0002
Geotechnical Investigation
Proposed Residential Unit Development - 17 Bottlebrush
Avenue, Noosa Heads
Geotechnical Investigation 30 January 2026 | Ref. 25408-001-Rev0
30 January 2026 | 25408-001-Rev0
Document Set ID: 25158026
24528612
Version: 1,
2, Version Date: 10/09/2026
13/02/2026
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Page 2
Document Review
Doc No. Revision Prepared By Reviewed By Date Issued
001 0 Oliver De Silva Ashley Davey 30 January 2025
Doc No. Revision Method of Delivery Issued To
001 0 Electronic Josh Cappiello ([email protected])
Prepared By Reviewed By
Oliver De Silva BAppSc PMEG Ashley Davey BEng (Geological) RPEQ 8159
Engineering Geologist Director/Principal Geotechnical Engineer
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Executive Summary
This report details the results of a geotechnical investigation undertaken for the proposed residential unit
development at 17 Bottlebrush Avenue, Noosa Heads. The investigation was performed by Tectonic for
Made One Pty Ltd. Recor are the structural consultants and project managers for the build.
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 basement retention.
Preliminary architectural drawings indicate that the development will comprise a three-storey unit
building, over a single level basement carpark. Construction will include a combination of masonry and
lightweight materials, suspended concrete floors, and pools on both the second level and roof. No
earthworks details were provided at the time of reporting; however, the plans indicate that excavations
for the basement would likely be in the order of about 5 m deep to the rear, with the cut daylighting at
the front. Localised filling of 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 the lift overrun pit and other elements.
The subsurface conditions encountered in the boreholes are summarised as follows:
Topsoil – comprising very loose to loose organic sand, to 0.1 m below ground level (BGL); then
Fill – comprising loose sand, to depths of between 0.7 m and 1.1 m BGL; with underlying
Natural Sands – comprising poorly graded sands, mostly being loose to depths of between 1.2 m
and 1.7 m BGL; then becoming medium dense and increasing to dense below depths of between
3.2 m and 3.6 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 systems by a suitably qualified and experienced RPEQ
structural engineer, there should be a Low Risk to the proposed development from slope instability.
Excavations through the soils encountered should be achievable using conventional earthmoving plant
(e.g. 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 (e.g. sheet or secant piles).
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 of about 1.3 m
BGL (existing ground levels) in the boreholes. High-level footings or locally deepened pads could be
utilised in most areas of cut, whilst piles would be required elsewhere.
Given the sandy soil profile, where deepened footings are required away from cut areas, 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 piers or driven piles are not considered to be suitable
for this site.
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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................................................................................................................................................................. 5
4.3 Geology ........................................................................................................................................................................ 5
4.4 Subsurface Conditions ............................................................................................................................................ 6
4.5 Groundwater............................................................................................................................................................... 7
4.6 Surface Drainage ....................................................................................................................................................... 7
5. Slope Stability Assessment .................................................................................................................................8
5.1 Existing Conditions................................................................................................................................................... 8
5.2 Stability Assessment ................................................................................................................................................ 8
6. Geotechnical Recommendations ..................................................................................................................... 11
6.1 Development Limitations..................................................................................................................................... 11
6.2 Earthworks ................................................................................................................................................................. 11
6.2.1 Site Preparation and Trafficability................................................................................................... 11
6.3 Excavations ................................................................................................................................................................ 12
6.3.1 Filling ......................................................................................................................................................... 12
6.4 Building Foundations ............................................................................................................................................ 13
6.4.1 Indicative Site Classification .............................................................................................................. 13
6.4.2 Footing Design....................................................................................................................................... 13
6.5 Temporary and Permanent Retention ............................................................................................................ 15
6.6 Drainage ..................................................................................................................................................................... 16
7. Conclusion ............................................................................................................................................................. 16
8. Limitations ............................................................................................................................................................. 17
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Text Figures
Text Figure 1: Layout plan (upper) and 3D Render, courtesy of Blackwood.............................................................. 1
Text Figure 2: Decemeber 2025 aerial image of the site and immediate surrounds (courtesy
Nearmap) ................................................................................................................................................................................. 3
Text Figure 3: View of Bottlebrush Avenue frontage, looking north ............................................................................ 4
Text Figure 4: View of distressed retaining walls at rear of existing residence......................................................... 4
Text Figure 5: Cracking in concrete steps (LHS) and foundation wall of existing residence (LHS) ................... 4
Text Figure 6: Plan showing ground surface contours (courtesy NC ‘Intramaps’)................................................... 5
Text Figure 7: Extract of 1:100,000 Laguna Bay Surface Geology Map ........................................................................ 6
Text Figure 8: Extract from NC landslide hazard overlay mapping ............................................................................... 8
Text Figure 9: Footing depth required to minimise risk of undermining ................................................................ 14
Tables
Table 1: Summary of Subsurface Conditions........................................................................................................................ 6
Table 2: Stability Risk Levels ........................................................................................................................................................ 9
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007) ............................................................... 10
Table 4: Retaining Structure Design Parameters............................................................................................................... 16
Figures
Figure 1: Site Plan
Appendices
Appendix A Borehole & DCP Reports and Explanatory Notes
Appendix B AGS Guidlines - Qualitative Terminology for Use in Assesing the Risk to Property
Appendix C Limitations
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1. Introduction
This report details the results of a geotechnical investigation undertaken for the proposed residential unit
development at 17 Bottlebrush Avenue, Noosa Heads. The investigation was performed by Tectonic for
Made One Pty Ltd, generally in accordance with Proposal P25626-Rev0, dated 16 December 2025. Recor
are the structural consultants and project managers for the build.
The investigation comprised drilling of boreholes at two locations and a single standalone dynamic cone
penetrometer (DCP) test, 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 basement retention. In summary, subject to the
implementation of the recommendations made herein, it is assessed that there should 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
Preliminary architectural drawings were supplied to Tectonic (Ref. Blackwood Architecture, Project No.
202612, Sheet Nos. DA1.01 to DA6.05 [not inclusive], Issue A, Dated 11 December 2025). These plans
indicate that the development will comprise a three-storey unit building, over a single level basement
carpark. Construction will include a combination of masonry and lightweight materials, suspended
concrete floors, and pools on both the second level and roof.
No earthworks details were provided at the time of reporting; however, the plans indicate that
excavations for the basement would likely be in the order of about 5 m deep to the rear, with the cut
daylighting at the front. Localised filling of less than 1 m high is anticipated. Excavations will extend
close to the site boundaries on the northern and southern sides, and therefore, some form of temporary
retention will likely be required. Isolated deeper excavations may be required for the lift overrun pit and
other elements. A basement layout plan and 3D render view are shown in Text Figure 1 below.
Text Figure 1: Basement level layout plan (LHS) and 3D render (RHS), courtesy of Blackwood
Architecture
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3. Method of Investigation
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps and slope stability studies, as well as Noosa Council (NC) planning overlays and site information.
An experienced engineering geologist from Tectonic undertook a site walk-over on 19 January 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.
The walk-over was followed by a subsurface investigation comprising the drilling of two hand augered
boreholes (designated BH1 and BH2) to depths of up to 4 m below ground level (BGL) and one
standalone DCP test (DCP3), in accessible areas across the site.
In-situ testing, using the 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 up to 4.8 m BGL. Boreholes were backfilled on completion using 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.
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4. Results of Investigation
4.1 Site Description
The site is formally described as Lot 143 on RP88934, covering an area of 506 m2, with frontage to
Bottlebrush Avenue along the south-western boundary. At the time of the investigation a two-storey
house occupied most of the front half of the site, with a concrete driveway providing direct access off
Bottlebrush Avenue.
Based on a visual assessment, the existing building was noted to be in fair structural condition exhibiting
no obvious signs of major distress (e.g. cracking, misalignment, etc.). However, some
dilapidation/distress was evident across the existing driveway, retaining walls and fencing, which
appeared to be related to age, poor construction and possibly consolidation of the upper loose sands.
Residential unit developments and houses occupied properties to the north, east and west of the site. It
appeared that these surrounding unit buildings comprised mostly at grade carparking, with the lower
level partially excavated into the hillside.
A recent aerial image of the site and immediate surrounds is shown in Text Figure 2 below.
Subject site
Text Figure 2: Decemeber 2025 aerial image of the site and immediate surrounds (courtesy Nearmap)
Site conditions at the time of our investigation are shown overleaf in Text Figures 3 to 5.
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Text Figure 3: View of Bottlebrush Avenue frontage, looking north
Text Figure 4: View of distressed retaining walls at rear of existing residence
Text Figure 5: Cracking in steps (LHS) and condition of foundation wall of existing residence (LHS)
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4.2 Topography
As illustrated in Text Figure 6 below, the site is located on the south-westerly facing flank of an ancient
sand dune, which crests about 200 m to the east/north-east. During the walk-over, topographic features
specific to the site were noted, and ground slopes were recorded using a hand-held clinometer.
The natural surface profile had been interrupted by historic landscaping and residential construction;
with inferred unaltered ground slopes along the side boundaries measured at the time of our assessment
of about 10°. Based on our observations it appeared that cut and fill earthworks of up to about 1.5 m
depth/height had been undertaken across the site. To the rear of the residence the ground surface was
gently sloping, typically being less than 3°, and supported by timber sleeper walls about 0.5 m high. The
front of the property comprised a gently sloping carparking area, being less than 5°, with the driveway
continuing down to Bottlebrush Avenue at about 12°.
Generally, no irregular surface features were noted which would indicate global slope instability.
Subject Site
Text Figure 6: Plan showing ground surface contours (courtesy NC ‘Intramaps’)
4.3 Geology
Available geological information 1 indicates that the site is underlain by Pleistocene age blowout dunes
comprising “quartz sand.” An extract of the relevant geological mapping is shown in Text Figure 7
overleaf.
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 organic sand, to a depth of 0.1 m BGL; then
Fill – comprising loose sand, to depths of between 0.7 m and 1.1 m BGL; with underlying
Natural Sands – comprising poorly graded sands, mostly being loose to depths of between 1.2 m
and 1.7 m BGL; then becoming medium dense and increasing to dense below depths of between
3.2 m and 3.6 m BGL.
Table 1 below summarises the subsurface profile encountered.
Table 1: Summary of Subsurface Conditions
Medium Dense Dense (or denser)
Borehole No. Topsoil Fill Loose Sands
Sands Sands
BH1 0.0 – 0.1 0.1 – 1.1 1.1 – 1.3 1.3 – 3.3 3.3 – 4.8*DCP
BH2 0.0 – 0.1 0.1 – 0.7 0.7 – 1.2 1.2 – 3.2* 3.2 – 4.7*DCP
DCP3 0.0 – 1.7* 1.7 – 3.6* 3.6 – 3.9DCP
Notes: 1. Depths noted in m BGL at time of investigation (19 January 2026)
2. * inferred from DCP testing
3. DCP
denotes DCP termination depth
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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 to the south-west towards
Bottlebrush Avenue, thus draining into the roadside drainage system. It is noted that the site has
relatively large catchment; however, most runoff from upslope of the property is anticipated to be
intercepted by the existing stormwater infrastructure from the neighbouring structures and streets to the
north-east.
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.
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5. Slope Stability Assessment
5.1 Existing Conditions
The NC Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 8),
which indicates that parts of the site are within a Moderate Landslide Hazard Area.
Subject site
Text Figure 8: Extract from NC landslide hazard overlay mapping
It should be noted that the NC landslide hazard map is a broad-scale indication of the potential landslide
susceptibility and is superseded by this site-specific assessment. No visual signs of global slope instability
were observed across the allotment. No landslide back scarps, groundwater springs, or areas of naturally
‘hummocky’ ground were observed. Additionally, overall natural ground slopes were gentle to moderate,
with medium dense natural sands encountered at relatively shallow depths.
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 overleaf
and the Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
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Table 2: Stability Risk Levels
Risk Level Example Implications(1)
VH Very High Unacceptable without treatment. Extensive detailed investigation and research, planning and
Risk 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 requires investigation,
Risk 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 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’ for slope stability
where the surface gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands.
Our measurements indicate that natural overall gradients ranged up to about 10°. 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, particularly when combined with soil suction. 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. The potential for significant erosion of the dune
immediately down slope from the site is considered negligible, provided that future disturbances (e.g.
excavations etc.) are managed with sufficient engineering consideration. Given the setback of the
property from the coastline, the risk from erosion of the sand dune by coastal process leading to
instability at the site is also assessed to be negligible.
Considering the existing site conditions (Ref. Section 4), and subject to the implementation of
recommendations given below, it is assessed that there should be a Low 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
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Table 3 below is our qualitative assessment of landslide risk for the development site. A summary of
qualitative terminology for use in assessing risk to property is attached (taken from AGS 2007).
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
Hazard Likelihood Consequence Assessed Risk Comments
Given the gentle to moderate natural gradients
Failure through through the site, it is considered Unlikely that a
very loose to landslide would develop in these materials, subject to
loose natural Unlikely Medium Low implementation of the recommendations in Section 6.
sands above the It is assessed that there could be a potential Medium
foundation depth impact from such instability so the resultant risk would
be Low as per AGS 2007.
The likelihood of a deep failure through the medium
Deep failure dense (or denser) natural sand below the proposed
through the foundation depths is assessed as Rare due to the lack
medium dense (or of precedence of such movements locally, no signs of
Rare Major Low
denser) sands any existing slope instability and apparent lack of
below the groundwater. Although the consequence of such a
foundation depth failure would be Major, the resultant risk is Low as
per AGS 2007.
The likelihood of a slide/failure through any temporary
retention system (sheet pile wall, contiguous pile wall
Temporary
or similar) as outlined in Section 6.4, is assessed as
instability through
Rare, subject to detailed design by a suitable qualified
temporary
Rare Major Low RPEQ engineer and construction by a specialist
retention system,
contractor which accounts for the required factors of
as discussed in
safety against slope instability. Although the
Section 6.4.
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 6 have been designed to mitigate those impacts.
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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
Should any revisions be made to the extent of the proposed development as described in Section 2,
Tectonic must review these prior to construction to assess if there are any impacts on slope stability.
To achieve a Low Risk of instability, the following is recommended:
Unretained filling should be limited to no more than 1 m above the existing ground surface, unless
assessed and approved by a geotechnical RPEQ.
All footings must be founded below the zone of influence of the surrounding structures and
infrastructure (including underground services).
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be RPEQ designed
and certified, which must include an assessment of global and internal stability.
Design and construction of footings shall be undertaken in accordance with Section 6.3.2.
Formal drainage should be constructed to ensure all stormwater is directed away from the existing
and proposed structures.
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 and uncontrolled
fill should then be stripped and set aside. Based on the boreholes, topsoil stripping depths should
generally range between 0.1 m and 0.2 m BGL, with uncontrolled fill encountered to depths 1.1 m BGL
(existing ground levels) in borehole BH1. 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 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.
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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)
Construction areas and surrounds should be graded to shed surface water runoff.
6.2.2 Excavations
Excavations through the soils encountered in the boreholes should be achievable using conventional
earthmoving plant (e.g. excavators with toothed buckets). Temporary unsurcharged 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 unretained cuts,
if required, should be specifically assessed by Tectonic, unless supported by temporary retention
measures.
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).
It is likely that above temporary batter slopes will not be able to be achieved, due to spatial restraints,
and so temporary shoring will be required (e.g. sheet, contiguous or secant piles), as per the
recommendations in Section 6.4 below. Excavations in close proximity to 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 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).
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 natural 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
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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 half the width of the compaction equipment (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 from shallow depths. If
not for this factor, 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, footings 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 of about 1.3 m BGL (existing ground levels) in the boreholes.
High-level footings or locally deepened pads could be utilised in most areas of cut, whilst piles would be
required elsewhere.
High level footings, where achievable in 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. Excavations
would likely loosen the sands, so densification of high-level footing bases should be allowed for, by
compaction with a vibrating plate, prior to placement of steel reinforcement.
Piles founded in medium dense (or denser) sands may be designed using an allowable end bearing
pressure (ABP) of 400 kPa. Should higher bearing pressure be required, deeper piles founded at least 1 m
into dense (or denser) sand (about 5 m BGL) could be designed using an ABP of 1 MPa.
Given the sandy soil profile, where piles are required, steel screw piles would commonly be adopted.
Due to the presence of loose sands, together with the close proximity to surrounding infrastructure, open
bored or driven piles are not considered to be appropriate for this site. Steel reinforced concrete bored
piles could be feasible; however, the use of sacrificial liners (e.g. ‘formatube’) would be required to
prevent the caving of upper loose sands. Grout-injected, continuous flight auger (CFA) piles could also
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be considered, particularly if necessary to achieve sufficient shear and bending resistance for global
stability (e.g. for basement walls).
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 as fill (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 below. 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
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 footing excavations 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.
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6.4 Temporary and Permanent Retention
Excavations are expected to range up to around 5 m BGL (existing ground levels) and, given that the
planned excavations extend close to some of the site boundaries and underground infrastructure (e.g.
stormwater to north-west), some form of temporary excavation shoring will likely be required.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by
use of sheet piles, however Unitywater and NC requirements for existing infrastructure, and the
sensitivity of neighbouring structures, 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. However, the proximity to neighbouring properties would likely 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 to loosen sands to the design foundations levels to prevent adverse
vibrations during sheet pile installation. Ground vibration monitoring and management would also be
necessary.
Other shoring methods that could be considered include bored contiguous or secant piles, micropiles or
CFA 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 structural
facing.
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 associated 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 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 pressure
distribution applicable to the wall type (e.g. trapezoidal for propped or anchored walls) and based on the
parameters listed in Table 4 overleaf. These values do not include allowance for hydrostatic pressures
nor transient (e.g. construction) or adjacent footing loadings.
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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 Sands 17 27 0.38 0.55 2.66
Medium Dense Sands 18 33 0.29 0.46 3.39
Dense (or Denser) Sands 19 36 0.26 0.41 3.85
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
Permanent 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.
All excess stormwater collected around the building (including overflow from rainwater tanks, etc.) must
be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Bottlebrush Avenue. Subsurface disposal of stormwater must not be undertaken.
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.
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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.
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CLIENT MADE ONE PTY LTD PROJECT GEOTECHNICAL INVESTIGATION FOR CANOPY RESIDENCES 17 BOTTLEBRUSH AVENUE, NOOSA HEADS DRAWN BY DATE MM 29/01/2026 CHECKED BY DATE OD 29/01/2026 SCALE SHEET SIZE AS SHOWN A3 DRAWING TITLE NOTES SITE PLAN BASEMAP COURTESY OF NEARMAP, DATED 6 DECEMBER 2026 PROJECT NO DOC NO. FIGURE NO REVISION 25408 001 ane APPROX. BOREHOLE LOCATIONS APPROX. DCP locations Document Set ID: 28528628 Version: 2, Version Date: 18/08/2026
Page 24
Appendix A
Borehole & DCP Reports and Explanatory
Notes
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Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 25408
Client: Made One Pty Ltd Commenced: 19/1/2026
Project Name: Proposed Residential Unit Development Completed: 19/1/2026
Hole Location: 17 Bottlebrush Avenue, Noosa Heads Logged By: MM
Hole Position: 509074.0 m E 7080441.0 m N MGA2020 Zone 56 Checked By: OD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: MM
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 coarse grained,
dark grey, with organics
SAND (FILL), fine to medium grained, grey
L
1
SW SAND, fine to medium grained, pale grey,
(AEOLIAN)
Not Encountered
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 25408.GPJ <<DrawingFile>> 29/1/2026 13:55 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
D to
2
HA
M
MD
3
D
4
Hole Terminated at 4.00 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
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Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 25408
Client: Made One Pty Ltd Commenced: 19/1/2026
Project Name: Proposed Residential Unit Development Completed: 19/1/2026
Hole Location: 17 Bottlebrush Avenue, Noosa Heads Logged By: MM
Hole Position: 509098.0 m E 7080452.0 m N MGA2020 Zone 56 Checked By: OD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: MM
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 coarse grained, VL
dark grey, with organics
SAND (FILL), fine to medium grained, dark
grey
L
SW SAND, fine to medium grained, pale grey,
(AEOLIAN)
1
Not Encountered
D to
HA
M
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 25408.GPJ <<DrawingFile>> 29/1/2026 13:55 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
2
MD
3
Hole Terminated at 3.00 m
Target depth
4
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
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CLIENT : Made One Pty Ltd SHEET : 1 OF 1
CONTRACTOR :
PROJECT : Proposed Residential Unit Development
LOCATION : 17 Bottlebrush Avenue, Noosa Heads
PROJECT No. : 25408
TEST: DCP3
LOGGED: OD 28/1/2026 CHECKED: OD
EASTING: 509088.6 m NORTHING: 7080451.0 m
MGA2020 Zone 56
Depth (m)
Blows per 100 mm
0 5 10 15 20 25
0.5
1.0
1.5
2.0
TECTONIC 3.00.2 LIB.GLB Log IS DCP 3 PER PAGE 25408.GPJ <<DrawingFile>> 29/1/2026 13:55 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
2.5
3.0
3.5
4.0
4.5
Comments
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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
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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.
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METHOD OF ROCK DESCRIPTION
USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Rock 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.
STRENGTH
Point Load
Symbol Term Index, Is(50) Field Guide
(MPa)
VL Very 0.03 to 0.1 Material crumbles under firm blows with sharp end of pick; can be peeled
Low with knife; too hard to cut a triaxial sample by hand. Pieces up to 30 mm
can be broken by finger pressure.
L Low 0.1 to 0.3 Easily scored with a knife; indentations 1 mm to 3 mm show in the specimen
with firm blows of pick point; has dull sound under hammer. A piece of core
150 mm long by 50 mm diameter may be broken by hand. Sharp edges of core
may be friable and break during handling.
M Medium 0.3 to 1 Readily scored with a knife; a piece of core 150 mm long by 50 mm diameter
can be broken by hand with difficulty.
H High 1 to 3 A piece of core 150 mm long by 50 mm diameter cannot be broken by hand but
can be broken with pick with a single firm blow; rock rings under hammer.
VH Very 3 to 10 Hand specimen breaks with pick after more than one blow; rock rings under
High hammer.
EH Extremely >10 Specimen requires many blows with geological pick to break through intact
High material; rock rings under hammer.
Relationship between Is(50) and UCS (unconfined compressive strength) will vary with rock type and strength, and
should be determined on a site-specific basis. UCS is typically 10 to 30 x Is(50), but can be as low as 5.
ROCK MATERIAL WEATHERING
Symbol Term Field Guide
Soil developed on extremely weathered rock; the mass structure and
Residual
RS substance fabric are no longer evident; there is a large change in volume
Soil but the soil has not been significantly transported.
Extremely Rock is weathered to such an extent that it has soil properties - i.e. it either
EW
Weathered disintegrates or can be remoulded, in water.
Whole of rock is discoloured, usually by iron staining or bleaching to the
extent that the colour of the original rock is not recognizable. Rock strength
Highly
HW is significantly changed by weathering. Porosity may be increased by
Weathered
leaching, or may be decreased due to deposition of weathering products in
Distinctly
DW pores.
Weathered
Whole of rock is discoloured, usually by iron staining or bleaching to the
Moderately extent that the colour of the original rock is not recognizable; but shows little
MW
Weathered or no change of strength from fresh rock.
Slightly Rock is slightly discoloured but shows little or no change of strength relative
SW
Weathered to fresh rock.
FR Fresh Rock shows no sign of decomposition or staining.
ABBREVIATIONS FOR DEFECT TYPES AND DESCRIPTIONS
Defect Type Coating or Infilling Roughness
CS/CZ Crushed seam/zone (Fault) Ca Calcite Po Polished
DB Drilling break Clay Clay Sm Smooth
EW Extremely weathered seam Cn Clean Sl Slickensided
HB Handling break Co Coating ( > 1mm) Ro Rough
IS/IZ Infilled seam/zone Fe Iron oxide VRo Very rough
Op Open
J Joint
Qz Quartz
Pt Parting St Stain Planarity
SS/SZ Sheared seam/zone (Fault) Vn Veneer (<1mm) Cu Curved
Vn Vein X Carbonaceous Dis Discontinuous
Ir Irregular
Pl Planar
St Stepped
Un Undulating
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Appendix B
AGS Guidlines - Qualitative Terminology for
Use in Assesing the Risk to Property
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
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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.
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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.
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Appendix C
Limitations
Geotechnical Investigation
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Limitations
This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is
accepted for the use of this document, in whole or in part, for any other purpose.
The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions
and limitations. Tectonic did not perform a complete assessment of all possible conditions or
circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do
not assume it has been provided. If a matter is not addressed, do not assume that any determination has
been made by Tectonic in regards to it.
Conditions may exist which were undetectable given that economic and time constraints limit the
practical extent of geotechnical investigation. Variations in conditions may occur between investigation
locations, and there may be special conditions pertaining to the site which have not been revealed by the
investigation and which have not therefore been taken into account in the document. Where variations
exist on site, additional studies and actions may be required.
Tectonic’s opinions are based upon information that existed at the time that the work was performed.
The passage of time, man-made or natural events, may alter the site conditions. It is understood that the
Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time
the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of
the site, or its surroundings, or any laws or regulations.
Any assessments made in the preparation of this document are based on the conditions indicated from
published sources and the findings of the investigation described. Actual subsurface conditions may
differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is
included, either express or implied, that the actual conditions will conform exactly to the assessments
contained in this document.
Where data supplied by the client or other external sources, including previous site investigation data,
have been used, it has been assumed that the information is correct unless otherwise stated. No
responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others.
This document is provided for the sole use by the Client and its professional advisers. No responsibility
whatsoever for the contents of this document will be accepted to any person other than the Client. Any
use which a third party makes of this document, or any reliance on or decisions to be made based on it,
is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered
by any third party as a result of decisions made or actions based on this document.
Limitations
August 2024 | Rev2
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PO Box 899 tectonicgeo.com.au [email protected]
Buderim
Geotechnical QLD 4555
Investigation
30 January 2026 | 25408-001-Rev0 21
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Used in these records
Reference Document - MCU26 0001 & OPW26 0002 - AS PUBLISHEDRelated documentApplication case desk ↗
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