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GEOTECHNICAL INVESTIGATION
Proposed Residential Unit Development
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27 Bottlebrush Avenue, Noosa Heads
Report for: Novato Group Pty Ltd
ATF Bottlebrush Unit Trust
21286-001-Rev0
10 November 2021
Tectonic Geotechnical Pty Ltd
PO Box 899, Buderim QLD 4556
Office: 07 5478 9016
Email: [email protected]
www.tectonicgeo.com.au
A.B.N. 83 165 727 828
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Page 2
PROPOSED UNIT DEVELOPMENT
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Document Review
Document No. Revision Prepared By Reviewed By Date Issued
001 0 Michael Cocks Ashley Davey 10 November 2021
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Report Distribution
Document Revision Method of Delivery Issued To
001 0
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Luke Bourne
([email protected])
Prepared By: Reviewed By:
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Michael Cocks BEng (Civil/Geotechnical) MIEAust Ashley Davey RPEQ 8159
Geotechnical Engineer Managing Director
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Report No. 21286-001-Rev0 i
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Executive Summary
This report details the results of a geotechnical investigation undertaken for the proposed unit development
at 27 Bottlebrush Avenue, Noosa Heads. The investigation was performed by Tectonic for Novato Group
Pty Ltd ATF Bottlebrush Unit Trust.
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 temporary and permanent basement
retention.
Preliminary architectural drawings indicate that the development will comprise a three-storey building, over
a single level basement carpark. Construction will likely include a combination of masonry and lightweight
materials (e.g. weatherboard cladding to walls etc.), and suspended concrete floors. No earthworks details
were provided at the time of reporting; however, provided plans indicate that excavations for the basement
would likely be in the order of about 6 m deep to the rear, daylighting at the front. Localised filling less than
0.5 m high is anticipated to form finished levels. Excavations will extend close to the site boundaries and
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therefore, some form of temporary retention will 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:
Fill – comprising very loose to loose sandy gravel and silty sand to depths between 0.6 m and
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1.3 m below ground level (BGL, boreholes BH1 and BH2 only); then or from the ground surface
Natural Sands – comprising silty materials to about 1 m to 1.2 m BGL, then poorly graded sands
initially, being loose to medium dense to depths between 2.2 m and 5.2 m BGL, then becoming
medium dense to the borehole termination depths.
DCP testing was continued beneath the borehole termination depths, which indicates that the sands
become dense (or denser) at depths between 4.2 m and 6.3 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
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construction of the footing and retention system by a suitably qualified and experienced structural engineer,
there would be a Low Risk to property from slope instability.
Excavations through the existing soils should be achievable using conventional earthmoving plant (e.g. small
or larger excavators with toothed buckets). Temporary unsurcharged cuts in the very loose to loose sands
could be formed at 1V:2H and at 1V:1H in medium dense (or denser) sand, for batters up to 3 m in height,
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provided groundwater seepage is not encountered and sands do not slough from the batter face as they dry
out. Space restrictions will limit battering of temporary cuts on the site and so some form of temporary
retention system will be required such as sheet, contiguous or secant pile walls.
Footings for the proposed development and any ancillary structures (e.g. retaining walls) must penetrate
through any fill materials and unsuitable natural soils (i.e. very loose to loose sands) to found at least 0.5 m
into medium dense (or denser) natural sands. Such materials were encountered below depths between 2.2 m
and 5.2 m BGL (existing ground levels). Thus, piled footings will be required unless excavations expose such
materials at or near foundation subgrade level. High-level footings would be suitable in excavation areas
where medium dense (or denser) natural sands are exposed at the subgrade level.
Given the sandy soil profile, where deepened footings are required (i.e. away from deeper excavations),
steel screw piles are considered to be an appropriate footing 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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Table of Contents
1.0 INTRODUCTION ........................................................................................................................................................ 1
2.0 PROPOSED DEVELOPMENT ................................................................................................................................... 1
3.0 METHOD OF INVESTIGATION ................................................................................................................................. 1
4.0 RESULTS OF INVESTIGATION ................................................................................................................................ 2
4.1 Site Description............................................................................................................................................. 2
4.2 Topography................................................................................................................................................... 4
4.3 Geology ........................................................................................................................................................ 4
4.4 Subsurface Conditions.................................................................................................................................. 5
4.5 Groundwater ................................................................................................................................................. 5
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4.6 Surface Drainage .......................................................................................................................................... 5
5.0 SLOPE STABILITY ASSESSMENT .......................................................................................................................... 6
5.1 Existing Conditions ....................................................................................................................................... 6
5.2
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Stability Assessment..................................................................................................................................... 7
5.2.1 Quantitative Assessment ........................................................................................................................ 8
6.0 GEOTECHNICAL RECOMMENDATIONS ................................................................................................................ 9
6.1 Additional Investigation ................................................................................................................................. 9
6.2 Development Limitations .............................................................................................................................. 9
6.3 Earthworks – General ................................................................................................................................. 10
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6.3.1 Site Preparation and Trafficability ......................................................................................................... 10
6.3.2 Excavations ........................................................................................................................................... 10
6.3.3 Filling..................................................................................................................................................... 11
6.4 Building Foundations .................................................................................................................................. 11
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6.4.1 Indicative Site Classification.................................................................................................................. 11
6.4.2 Footing Design ...................................................................................................................................... 11
6.5 Temporary and Permanent Retention......................................................................................................... 13
6.6 Drainage ..................................................................................................................................................... 14
7.0 SUMMARY & CONCLUSIONS................................................................................................................................ 14
8.0 LIMITATIONS .......................................................................................................................................................... 14
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TABLES
Table 1: Summary of Subsurface Profiles ........................................................................................................................... 5
Table 2: Stability Risk Levels .............................................................................................................................................. 7
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007) ......................................................................... 8
Table 4: Retaining Structure Design Parameters .............................................................................................................. 13
TEXT FIGURES
Text Figure 1: Layout plan (LHS) and cross-sectional view (RHS) of proposed development, courtesy of Zest ................. 1
Text Figure 2: 2021 aerial image of the site and immediate surrounds (courtesy Nearmap imagery)................................. 2
Text Figure 3: View of the site looking east from Bottlebrush Avenue ................................................................................ 3
Text Figure 4: Panoramic view of the Bottlebrush Avenue street frontage, looking east..................................................... 3
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Text Figure 5: Dilapidation in existing blockwork walls surrounding the residence ............................................................. 3
Text Figure 6: Site plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’) ...................................... 4
Text Figure 7: Extract of Gympie Special 1:100,000 Geology Map ..................................................................................... 4
Text Figure 8: Extract from NSC landslide hazard overlay mapping ................................................................................... 6
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Text Figure 9: Slope/W output for long term conditions...................................................................................................... 9
Text Figure 10: Footing depth required to prevent undermining....................................................................................... 12
FIGURES
Figure 1: Site Plan
APPENDICES
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APPENDIX A
Borehole & DCP Reports & Explanatory Notes
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
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APPENDIX C
Limitations
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1.0 INTRODUCTION
This report details the results of a geotechnical investigation undertaken for the proposed residential unit
development at 27 Bottlebrush Avenue, Noosa Heads. The investigation was performed by Tectonic for
Novato Group Pty Ltd ATF Bottlebrush Unit Trust, generally in accordance with Proposal P21365-Rev0,
dated 29 June 2021.
The investigation comprised drilling of boreholes at three locations across accessible areas of the
development footprint, together with in-situ strength testing. This report summarises the results of the
investigation and provides geotechnical recommendations regarding earthworks and footings, together with
temporary and permanent 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).
2.0 PROPOSED DEVELOPMENT
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Preliminary architectural drawings were supplied to us (Ref. Zest Building Design, Project No. 21031,
Sheet Nos. A01.001 to A03.101 [not inclusive], dated 9 November 2021). These plans indicate that the
development will comprise a three-storey building, over a single level basement carpark. Construction will
likely include a combination of masonry and lightweight materials (e.g. weatherboard cladding to walls etc.),
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and suspended concrete floors.
No earthworks details were provided at the time of reporting; however, provided plans indicate that
excavations for the basement would likely be in the order of about 6 m deep to the rear, daylighting at the
front. Localised filling less than 0.5 m high is anticipated. Excavations will extend close to the site
boundaries and therefore, some form of temporary retention will be required. Isolated deeper excavations
may be required for the lift overrun pit and other elements.
Text Figure 1 below presents a basement layout plan and cross-sectional view of the proposed
development.
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Text Figure 1: Layout plan (LHS) and cross-sectional view (RHS) of proposed development, courtesy of Zest
3.0 METHOD OF INVESTIGATION
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps, slope stability studies as well as Noosa Shire Council (NSC) planning overlays and site information.
An experienced geotechnical engineer from Tectonic undertook a site walk-over on 27 July 2021 to visually
assess any indicators of past slope instability. The walk-over included assessment of soil exposures,
surface drainage conditions, topography, evidence of erosion or past instability, and vegetation
coverage/condition.
This was incorporated with a subsurface investigation comprising the drilling of three boreholes (designated
BH1 to BH3) in accessible areas across the site. Borehole BH1 was drilled to the front of the site using
Tectonics’ 4WD mounted solid flight auger rig to 6 m below ground level (BGL). Due to access restrictions
boreholes BH2 and BH3 were drilled along the southern boundary and to the rear, respectively, to 3 m BGL
using hand augering techniques.
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In-situ testing, using the dynamic cone penetrometer (DCP) method, was carried out from the ground surface
and within each borehole. Boreholes were backfilled on completion using spoil arising during drilling, with
excess material slightly mounded at surface.
Laboratory testing was not warranted due to the sandy nature of the subsurface conditions encountered.
Borehole locations are shown in Figure 1 attached at the end of this report, with borehole and DCP logs
presented in Appendix A, together with explanatory notes. 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.
4.0 RESULTS OF INVESTIGATION
4.1 Site Description
The site is formally described as Lot 138 on RP88934, covering an area of about 506 m2, with frontage to
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Bottlebrush Avenue along the south-western boundary. At the time of the investigation a single storey
residence occupied most of the eastern part of the site, with an asphalt and concrete driveway providing
direct access off Bottlebrush Avenue. Most of the ground surface around the existing residence was
terraced by timber sleeper, concrete block and loosely stacked boulder walls typically ranging between about
1 m and 1.5 m high.
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Based on a visual assessment the existing building was noted to be in good structural condition exhibiting no
obvious signs of distress (e.g. cracking, misalignment, etc.). Dilapidation was evident across the existing
driveway and retaining walls (cracking, bulging, misalignment), which appeared to be related to age, poor
construction and possibly consolidation of the upper loose sands.
Similar residential unit developments and houses occupied properties to the north, south/east and west of
the site. It appeared that the surrounding buildings comprised mostly at grade carparking, with the lower
level partially excavated into the hillside. Based on a visual assessment, it appeared that excavations up to
about 3 m deep had been undertaken on neighbouring properties. Observable walls appeared to be in
relatively good structural condition, based on an external assessment. It was noted that part of a concrete
kerb and channel on the opposing (southern) side of Bottlebrush Avenue displayed misalignment, with
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cracking evident in the Bottlebrush Avenue asphalt seal. It appeared that this distressed was due to
consolidation of fill materials, which are inferred to have been placed during the construction of the road.
A 2021 aerial image of the site and surrounds is shown in Text Figure 2.
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Subject Site
Text Figure 2: 2021 aerial image of the site and immediate surrounds (courtesy Nearmap imagery)
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Site conditions at the time of our investigation are shown below in Text Figures 3 to 5.
Text Figure 3: View of the site looking east from Bottlebrush Avenue
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Text Figure 4: Panoramic view of the Bottlebrush Avenue street frontage, looking east
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Text Figure 5: Dilapidation in existing blockwork walls surrounding the residence
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4.2 Topography
During the site walk-over, topographic features specific to the site were noted, and ground slopes were
recorded using a hand-held clinometer. As illustrated in 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.
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
between about 12° and 14°. Based on our observations it appeared that cut and fill earthworks in the order
of about 1.5 m deep and 1 m high had been undertaken across the site. Most areas across the site were
terraced and gently sloping with cut/fill embankments supported by retaining walls. No irregular surface
features were noted which would indicate global slope instability.
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Subject Site
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Text Figure 6: Site plan showing ground surface contours (courtesy Noosa Council ‘Intramaps’)
4.3 Geology
Available geological information 1 indicates that the site is underlain by Pleistocene age dune sand
(Ref: Text Figure 7) and is near a contact with the older Noosa Quartz Diorite formation.
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Approx. Site Location
Text Figure 7: Extract of Gympie Special 1:100,000 Geology Map
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Queensland Government, Department of Mines and Energy, 1:100,000 Gympie Special Geology Map, Sheet 9445 & Part 9545, 1999
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4.4 Subsurface Conditions
Detailed descriptions of the subsurface conditions encountered in the boreholes are provided in the
borehole reports attached in Appendix A, along with definition sheets which describe terms and
abbreviations used on the reports. Subsurface conditions encountered in the boreholes are summarised as
follows:
Fill – comprising very loose to loose sandy gravel and silty sand to depths between 0.6 m and
1.3 m BGL (boreholes BH1 and BH2 only); then or from the ground surface
Natural Sands – comprising silty materials to about 1 m to 1.2 m BGL, then poorly graded sands
initially, being loose to medium dense to depths between 2.2 m and 5.2 m BGL, then becoming
medium dense to the borehole termination depths.
DCP testing was continued beneath the borehole termination depths, which indicates that the sands
become dense (or denser) at depths between 4.2 m and 6.3 m BGL.
Table 1 below, summarises the subsurface profile in each borehole.
Table 1: Summary of Subsurface Profiles
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Very Loose/ Medium Dense Dense (or
Borehole No. Fill
Loose Sands Sands denser) Sands
BH1 0 – 1.3 1.3 – 5.2* 5.2 – 6.0TD 6.2* – 6.6DCP
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BH2 0 – 0.6 0.6 – 2.3 2.3 – 3.0TD 4.1* – 4.7DCP
BH3 NE 0 – 2.2 2.2 – 3.0TD 4.3* – 4.7DCP
Notes: 1. Depths noted in m BGL at time of investigation (27 July 2021)
2. NE denotes not encountered
3. TD denotes borehole terminated at target depth
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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.
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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.
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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5.0 SLOPE STABILITY ASSESSMENT
5.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 8)
which indicates that most of the site is within a Moderate (pale yellow shading) Landslide Hazard Area. A
small section of the central part of the site is mapped within a Very Low/Low (no shading) Landslide Hazard
Area.
Subject Site
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Text Figure 8: Extract from NSC landslide hazard overlay mapping
It should be noted that the Noosa landslide hazard map is a broad-scale indication of the potential
landslide susceptibility and is superseded by this site-specific assessment. The allotment did not exhibit
any visible surface features which would indicate global slope instability during our walkover: no landslide
back scarps, noteworthy groundwater seepage or areas of naturally ‘hummocky’ ground were observed.
Distress observed in the existing retaining walls and driveway is likely related to factors other than slope
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instability (e.g. poor construction, age etc.).
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5.2 Stability Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 2 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
Table 2: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
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Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to
L Low Risk
this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
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Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface
gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands. Our measurements
indicate that the overall natural gradient is up to 14° (21%). 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).
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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.
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Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation, and development of generally localised instability.
The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion. At this site, the dune around and below the site is developed
with neighbouring residences and Bottlebrush Avenue; and therefore water erosion or significant disturbance
of the dune by natural forces, in the vicinity of the site, is considered extremely unlikely.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of global slope instability
affecting the proposed development. Regulators (NSC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in Table
3 overleaf is our qualitative assessment of landslide risk for the development site. A summary of qualitative
terminology for use in assessing risk to property is attached (taken from AGS 2007).
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Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence Risk Comments
Given the low to moderate natural gradients through
the proposed building area and retention system
1: Failure through very loose to proposed it is considered Unlikely that a landslip
loose natural sands above the Unlikely Medium Low would develop in these materials, subject to
foundation depth implementation of the recommendations in Section 4.
Given a potential Medium impact from such instability
the resultant risk would be Low as per AGS 2007.
The likelihood of a deep failure through the medium
dense (or denser) natural sand below the proposed
2: Deep failure through the foundation depth is assessed as Rare due to the lack of
medium dense (or denser) sands Rare Major Low precedence of such movements locally, no signs of any
below the foundation depth existing slope instability and a lack of groundwater.
Although the consequence of such a failure would be
Major, the resultant risk is Low as per AGS 2007.
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The likelihood of a slip/failure through the temporary
retention system (sheet pile wall, or similar) as outlined in
3: Instability through temporary Section 6.4, is assessed as Rare, subject to detailed
sheet pile retention system, or design by a suitable qualified RPEQ engineer and
Rare Major Low
6.4.
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similar, as discussed in Section 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 FOS against slope instability for the proposed development, numerical analysis has
been conducted using the software Slope/W. The ground surface profile has been modelled using a
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combination of preliminary design drawings and slope angles measured in the field by Tectonic. The
analysed subsurface profile was compiled based on the results of our subsurface investigation. Some
inferences regarding subsurface conditions were made between test locations. A minimum FOS value of 1.5
is the commonly adopted industry standard for long term conditions.
No groundwater was encountered at the time of investigation. Given the gradients across the site and highly
permeable nature of the sandy soil, development of shallow groundwater is considered unlikely. Should
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groundwater seepage be encountered during construction Tectonic must be notified to assess the impacts
on temporary instability.
To replicate future landscaping, a surcharge load of 5 kPa has been applied upslope of the proposed
building. Although building loads were not available at the time of reporting; a gross building load of 30 kPa
has been applied to areas of the building likely founded using high-level footings. Other areas will be
supported by piled foundations taken into the underlying medium dense (or denser) natural sands.
Soil shear strength was based upon effective parameters which were estimated based on our interpretation
of the subsurface investigation results, while also considering the values recommended for similar materials
in Appendix D of Australian Standard AS4678-2002 Earth Retaining Structures. Adopted soil parameters
are shown on the Slope/W output presented in Text Figure 9 on the following page. Analysis was
undertaken for a specified range of entry and exit points that could impact the proposed development.
The analysed profile represents the critical section (i.e. ‘worst case’ scenario) for the proposed development,
which covers the deepest cut. Our analysis indicates that the proposed development would achieve at least
the required minimum FOS for long-term stability (i.e. 1.5), subject to permanent retention of cuts using
engineer designed structures.
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It should be noted that this modelling addresses the global stability of the proposed works; however, the
stability of the retaining walls (modelled as high strength structures along the northern side of the building)
must be addressed by others as part of detailed design.
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Text Figure 9: Slope/W output for long term conditions
6.0 GEOTECHNICAL RECOMMENDATIONS
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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 Additional Investigation
Due to inaccessibility at the time of investigation, a deeper borehole may be required in the north-eastern
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corner of the site depending on the chosen temporary retention system. Should a cantilevered temporary
retention system be adopted, the required ‘toe in’ depth beneath the proposed excavation level may be
relatively deep (e.g. in excess of 10 m). As the current investigation in this area was limited to handheld
equipment, the subsurface conditions below about 3 m BGL are somewhat unknown. Previous experience
in the greater Noosa area indicates that relatively high strength weathered rock of the Noosa Quartz Diorite
formation underlies the ancient dune at depth, which could pose a risk to the constructability of the chosen
temporary retention system. The deeper borehole should be drilled at least to the depth of the proposed
temporary retention system ‘toe in’ to confirm the deeper subsurface conditions. Tectonic could provide a
quote for the deeper investigation, following confirmation of the temporary retention system and removal of
the existing residence.
6.2 Development Limitations
It is anticipated earthworks will include cuts for the lower level of up to about 6 m deep to the rear,
daylighting to the front. Localised filling less than 0.5 m high is anticipated to form finished surface levels.
Should proposed cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability
before construction.
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6.3 Earthworks – General
6.3.1 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 1.3 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)
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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
AF
conjunction with surface application of ample water, to achieve the required compaction.
Due to the potential for development of poor trafficability during construction, placement of a minimum
150 mm thick granular working platform may be required across laydown areas. Specific platform design
would be required prior to mobilisation and trafficking of any heavy mobile plant on the site (e.g. mobile
cranes, concrete pumps or piling rigs).
Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).
6.3.2 Excavations
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Excavations through the soils should be achievable using conventional earthmoving plant (e.g. small or
larger excavators with toothed buckets). Temporary unsurchaged cuts in the very loose to loose sand could
be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to 3 m in height. Flatter
batters or temporary retention may be required if groundwater seepage is encountered or if sloughing of dry
sand occurs at the batter surface. Deeper cuts, if required, should be specifically assessed by Tectonic.
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Permanent cuts or fills greater than 1 m must be retained. Permanent cut and fill batters up to 1 m high shall
not exceed 1V:2.5H. Any permanent soil and fill batters will require erosion protection (e.g. revegetation or
surface protection).
Due to spatial restraints and the depth of excavations proposed, temporary shoring will be required (e.g.
sheet piles) as per the recommendations in Section 6.5 below. Excavations in close proximity to existing
structures and infrastructure (e.g. sewer main along the northern and eastern boundaries), including on
neighbouring properties, will need to be specifically designed to prevent undermining. It is recommended
that footing exposures or confirmation of existing foundations via certified building plans be undertaken to
assess the possibility of undermining. If undermining is a potential issue the existing structures may need to
be underpinned or excavations may require temporary shoring.
The advice of a suitably qualified and experienced RPEQ structural engineer must be sought regarding any
potential undermining and design and construction of any underpins/temporary shoring (if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
10 November 2021
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27 BOTTLEBRUSH AVENUE, NOOSA HEADS
6.3.3 Filling
The natural sandy soils where free of deleterious materials (e.g. vegetation, organics etc.) may be reused as
structural fill, subject to appropriate moisture adjustment. It should be noted that compaction of such
relatively ‘clean’ sands as structural fill requires the addition of substantial volumes of water (‘flooding’)
combined with the use of vibratory compaction equipment (typically a smooth drum roller or heavy vibrating
plate).
Should importation of fill be required, it is recommended that such fill should comply with the following:
Minimum CBR of 10%
Maximum plasticity index of 15%
Shrink-swell index ≤ 1%
Maximum particle size of 100 mm
Any fill should be compacted at moisture contents within the range of ±2% of optimum moisture content for
Standard Compaction. Fill material should be placed in 250 mm thick (loose thickness) layers and be
compacted by repeated rolling using a minimum 4 tonne smooth drum roller (or equivalent, with vibration on
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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.
AF
Fill embankment edges must be over-filled by 0.5 m (horizontally) and then trimmed back to well compacted
material.
6.4 Building Foundations
6.4.1 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
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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,
uncontrolled fill and trees. If not for these factors, the site would have the characteristics of Class S, with the
estimated characteristic surface movements (‘ys’) ranging up to about 20 mm. However, due to the potential
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variability in subgrade density a site classification of Class M is recommended in relation to assessment of
flexibility requirements for ground slabs, footing and plumbing design.
6.4.2 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 fill materials and unsuitable natural
soils (i.e. very loose to loose sands) to found at least 0.5 m into medium dense (or denser) natural sands.
Such materials were encountered from depths of between 2.2 m and 5.2 m BGL (existing ground levels) in the
boreholes. Thus, piled footings will be required unless excavations expose such materials at or near
foundation subgrade level.
Piles so founded may be designed using an allowable end bearing pressure of 300 kPa, or 1 MPa if founded
in dense sands at about 5 m to 6 m BGL. High level footings, where achievable in deeper cut areas, may be
designed using an allowable bearing pressure of 100 kPa provided that they are founded in the natural
medium dense (or denser) sands. Settlement under such loads is expected to be in the order of up to 20 mm
or 1% of footing width.
Given the sandy soil profile and depths of footings required, steel screw piles are an appropriate footing type
for the proposed development generally. However, in areas of cut exposing medium dense (or denser)
10 November 2021
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PROPOSED UNIT DEVELOPMENT
27 BOTTLEBRUSH AVENUE, NOOSA HEADS
sands at the subgrade level high-level footings may be adopted. It should be noted that excavations for
concrete strip footings (or similar) may loosen the base. Therefore, the use of a vibrating plate would be
required to densify any high-level footing bases prior to the placement of reinforcing steel.
Due to the presence of loose sands and nearby structures, open bored piers or driven piles are not
recommended for this site and it is expected that steel screw piles could be utilised. Steel reinforced
concrete bore piles may be possible; however, the use of sacrificial liners (e.g. Formatube) would be
required. The advice of a specialist piling contractor must be sought regarding feasibility, design and
installation of the piling system for this site, including an assessment of potential vibration effects on nearby
structures, and monitoring requirements.
If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling may
not encounter sufficient drilling resistance (from torque measurement) in sands at the suggested founding
depth, and piles could extend to much greater depth than necessary.
It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
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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
AF
that placement of reactive clay may result in a more severe equivalent site classification, such as H1 or
greater.
All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
retaining walls, or in ground structures that are not designed to support building loads. Footings should
found at least below a plane extending 1 m horizontally from the base of trenches/excavations/retaining
walls/swimming pool, then rising up at 1V:1H, as illustrated in Text Figure 10.
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Text Figure 10: Footing depth required to prevent undermining
Site drainage should be designed to readily remove surface water and to prevent ponding of water on
subgrade areas during construction and also adjacent to foundations and perimeter slabs once construction
is completed.
It is recommended that footings be inspected by a suitably qualified and experienced engineer prior to
placement of steel reinforcement and pouring of concrete to confirm allowable bearing pressures and
cleanliness of excavations.
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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.
6.5 Temporary and Permanent Retention
Excavations are expected to range up to around 6 m BGL (existing ground levels) and, given that the
planned excavation extends close to some of the site boundaries and underground infrastructure (e.g.
sewer), some form of temporary shoring will be required to support the excavation.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, however Unitywater requirements for the sewer may restrict the use of sheet piles. Shoring of
deeper cuts would likely require lateral restraint, and this is conventionally achieved by helical steel anchors
(similar to screw piles). However, the proximity to neighbouring properties would require permissions to be
obtained for subterranean access. Therefore, where lateral support by cantilever only is not feasible, it is
suggested that internal struts/propping be considered to provide lateral restraint for the shoring system.
Anchored or propped sheet piles may require pre-drilling to prevent adverse vibrations during sheet pile
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installation.
Other shoring methods that could be considered include bored contiguous or secant piles. Non-contiguous
or soldier pile walls are not feasible for the proposed development due to the likelihood that loose sands
would ‘flow’ through gaps between the piles.
AF
An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the
permanent walls, whereas sheet piles would be temporary to provide retention during construction only,
therefore requiring a permanent structural wall to be constructed. Bored pile walls generally require greater
width (horizontally) for installation as compared to sheet piles and are often more expensive.
The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation of
the proposed retention and subsurface drainage system. It will be necessary to conduct numerical analysis
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,
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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.
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construction) or adjacent footing loadings.
Table 4: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Very Loose/ Loose
1.8 28 0.36 0.53 2.77
Sands
Medium Dense (or
1.9 34 0.28 0.44 3.54
Denser) Sands
Future Controlled Fill* * * * * *
Note: * Depends on fill material type and level of compaction.
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Retaining structures must be founded as per the recommendations in Section 6.4.2 above with the footings
also designed to account for lateral loads imposed by the retaining wall. Detailed design must include an
assessment of internal and global stability. A solution using screw piles to support retaining walls, possibly
including raked piles, may be feasible although bending and shear capacities would need to be carefully
checked in design from a soil-structure interaction viewpoint. Additional lateral capacity may be achieved by
incorporating a deeper pile cap that also acts as a shear key attracting passive resistance from the sands in
front of the wall.
6.6 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.
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Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure. It is assumed that any pool floors will include a
hydrostatic pressure relief valve. The outlet pipe from this valve must be taken to ‘daylight’ at the ground
surface.
AF
All excess stormwater collected around the residence (including overflow from rainwater tanks etc.) must be
directed by pipes or lined channels to discharge into the existing stormwater infrastructure along Bottlebrush
Avenue. Subsurface disposal of stormwater must not be undertaken.
7.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment it is assessed that there would be a Low Risk of slope instability
affecting the proposed development subject to implementation of the recommendations given in this report.
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8.0 LIMITATIONS
Your attention is drawn to the document Limitations, which is included in Appendix C of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of this
report should be, and to present you with recommendations on how to minimise the risks associated with the
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services provided by Tectonic for this project.
10 November 2021
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Page 20
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Ph: 07 5478 9016
PROJECT:
PROPOSED UNIT DEVELOPMENT
27 BOTTLEBRUSH AVENUE
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NOOSA HEADS
TITLE:
SITE PLAN
BH3
AF
CLIENT:
NOVATO GROUP PTY LTD ATF
BOTTLEBRUSH UNIT TRUST
DRAWN: DATE:
BH2 MCC 13/09/2021
Subject Site DATE:
CHECKED:
R ACD 13/09/2021
SCALE:
BH1
AS SHOWN (APPROX.)
LEGEND:
APPROX. BOREHOLE
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LOCATIONS
Notes:
Base plan adopted from Nearmap imagery dated 5 July 2021
0m 10 30m
PROJECT NO.: FIGURE: REV:
21286-001 1 0
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PROPOSED UNIT DEVELOPMENT
27 BOTTLEBRUSH AVENUE, NOOSA HEADS
APPENDIX A
Borehole & DCP Reports & Explanatory Notes
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AF
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10 November 2021
Report No. 21286-001-Rev0
Page 22
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 21286
Client: Novato Group Pty Ltd Commenced: 27/7/2021
Project Name: Proposed Unit Development Completed: 27/7/2021
Hole Location: 27 Bottlebrush Avenue, Noosa Heads Logged By: MCC
Hole Position: 509122.0 m E 7080376.0 m N MGA2020 Zone 56 Checked By: ACD
Drill Model and Mounting: Hand Auger/4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Sandy GRAVEL (FILL), fine to medium sized, D to
brown to pale brown, fine to coarse grained sand M
Silty SAND (FILL), fine to medium grained, grey
T
and brown, trace of roots
L
HA
1
SP SAND, fine to medium grained, pale grey to white
AF
Augered to 1.9m
2
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 21286.GPJ <<DrawingFile>> 13/9/2021 14:12 10.00.01.07 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
Not Encountered
3
M L to
MD
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AD/T
4
D
5
Augered to 5.2m
MD
6
Hole Terminated at 6.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 23
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 21286
Client: Novato Group Pty Ltd Commenced: 27/7/2021
Project Name: Proposed Unit Development Completed: 27/7/2021
Hole Location: 27 Bottlebrush Avenue, Noosa Heads Logged By: MCC
Hole Position: 509143.0 m E 7080385.0 m N MGA2020 Zone 56 Checked By: ACD
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
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Sandy GRAVEL (FILL), fine to coarse sized,
sub-rounded, trace of plastic
Silty SAND (FILL), fine to medium grained, grey,
T
trace of fine to coarse sized gravel
VL to
SM Silty SAND, fine to medium grained, grey, trace of L
roots
1
Not Encountered
SP SAND, fine to medium grained, pale grey to white
AF M
HA
Augered to 1.6m
L
2
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 21286.GPJ <<DrawingFile>> 13/9/2021 14:12 10.00.01.07 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
MD
Augered to 3.0m
3
Hole Terminated at 3.00 m
Target depth
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4
D
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 and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 24
Borehole No.
BH3
Page 1 of 1
Engineering Log - Borehole Project No.: 21286
Client: Novato Group Pty Ltd Commenced: 27/7/2021
Project Name: Proposed Unit Development Completed: 27/7/2021
Hole Location: 27 Bottlebrush Avenue, Noosa Heads Logged By: MCC
Hole Position: 509217.0 m E 7080402.0 m N MGA2020 Zone 56 Checked By: ACD
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
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND, fine to medium grained, grey, trace of VL
roots D
becoming pale brown
T
1
SP SAND, fine to medium grained, pale grey to white
L
AF Augered to 1.5m
Not Encountered
2
HA
M
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 21286.GPJ <<DrawingFile>> 13/9/2021 14:12 10.00.01.07 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
Augered to 3.0m
3
MD
R
4
Hole Terminated at 4.00 m
Target depth
D
>15
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 and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
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TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RA Rotary Air BH Tractor Mounted Backhoe
ADH Hollow Auger RD Rotary Blade or Drag bit EX Tracked Hydraulic Excavator
HA Hand Auger RT Rotary Tri-cone bit HMLC Core – 63mm
*T TC-Bit HQ Core – 63mm
*V V-Bit NMLC Core – 47mm
NQ Core – 52mm
R Ripper
RH Rock Hammer
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
T
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
SV
AF
Bulk disturbed sample
Field shear vane test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev1
February 2021
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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
T
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
MOISTURE CONDITION
Symbol
D
Term
AF
Description
Dry
behavior, or unreliable data and should be considered carefully.
AS1726 - 2017
Sands and gravels are free flowing. Clays & Silts may be brittle or friable and powdery.
M Moist Soils are darker than in the dry condition & may feel cool. Sands and gravels tend to cohere.
W Wet Soils exude free water. Sands and gravels tend to cohere.
CONSISTENCY AND DENSITY AS1726 - 2017
Symbol Term Undrained Shear Symbol Term Density Index % SPT “N” #
Strength
VS Very Soft 0 to 12 kPa VL Very Loose Less than 15 0 to 4
R
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
D
In the absence of test results, consistency and density may be assessed from correlations with the observed behaviour of
the material.
# SPT correlations are not stated in AS1726 – 2017, and may be subject to corrections for overburden pressure and
equipment type.
Rev1
October 2017
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PROPOSED UNIT DEVELOPMENT
27 BOTTLEBRUSH AVENUE, NOOSA HEADS
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in
Assessing Risk to Property
Some Guidelines for Hillside Construction
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Report No. 21286-001-Rev0
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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
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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
AF
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
R 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
D
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
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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%
10-1
T
A – ALMOST CERTAIN 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
AF
-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)
R 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.
D
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 30
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.
T
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.
ROCK OUTCROPS
AF
Remove or stabilise boulders which may have unacceptable risk.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
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.
R
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.
D
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
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AF
R
D
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 32
PROPOSED UNIT DEVELOPMENT
27 BOTTLEBRUSH AVENUE, NOOSA HEADS
APPENDIX C
Limitations
T
AF
R
D
10 November 2021
Report No. 21286-001-Rev0
Page 33
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
T
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
AF
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.
R
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
D
which a third party makes of this document, or any reliance on or decisions to be made based on it, is the
responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any
third party as a result of decisions made or actions based on this document.
Rev 1 (March 2016)