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Geotech Report - techtonic · RAL22/0021
GEOTECHNICAL INVESTIGATION
Proposed Residential Development
39 Picture Point Crescent, Noosa Heads
Report for: HM Property Pty Ltd
22511-001-Rev0
10 February 2023
Tectonic Geotechnical Pty Ltd
PO Box 899, Buderim QLD 4556
Office: 07 5478 9016
Email: [email protected]
www.tectonicgeo.com.au
A.B.N. 83 165 727 828
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
Document Review
Document No. Revision Prepared By Reviewed By Date Issued
001 0 Robert Gibb Ashley Davey 10 February 2023
Report Distribution
Document Revision Method of Delivery Issued To
Heidi Meyer
001 0 Electronic
([email protected])
Chris Camp
001 0 Electronic
([email protected])
Jack Lewis
001 0 Electronic
([email protected])
Prepared By: Reviewed By:
Robert Gibb BSc (Hons) Ashley Davey RPEQ 8159
Senior Engineering Geologist Managing Director
10 February 2023
Report No. 22511-001-Rev0 i
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PROPOSED RESIDENTIAL DEVELOPMENT
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Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 39 Picture Point Crescent, Noosa Heads. The investigation was performed by Tectonic for
HM Property Pty Ltd. The investigation comprised drilling of boreholes at two locations across accessible
areas of the development footprint, together with in-situ soil strength testing. One stand-alone in situ strength
test (using the dynamic cone penetrometer method) was also undertaken. This report summarises the
results of the investigation and provides geotechnical recommendations regarding earthworks and footings,
together with temporary and permanent retention.
Based on the provided information we understand that it is proposed to reconfigure a single existing lot
(Lot 2 on RP220514) in to two separate lots being an equitable 723 m2 in land area. Concept architectural
drawings provided to us with the project brief indicate that following subdivision, each new lot may be
developed with residences, reaching up to four storeys height. The extent of earthworks cuts or the nature
of the proposed structures has not been indicated on the drawings provided, and the findings of this
assessment should be reviewed once detailed architectural drawings are available (to confirm feasibility).
The subsurface conditions encountered in the boreholes, and inferred from deeper DCP testing at the
boreholes, are summarised as follows:
Fill – comprising very loose to loose sands (encountered in borehole BHT-2 only) to a depth of
0.7 m BGL; then, or from ground surface level in borehole BHT-1
Natural Sands:
o being very loose to depths of between 1 m and 2 m BGL; then
o loose to between 2 m and 3.6 m BGL; underlain by
o medium dense sands to between 3.8 m and 4.6 m BGL; followed by
o dense sands to the testing termination depths of between 4.8 m and 5.6 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 the investigation.
The sand dune slope to the rear of this site is close to the ‘limit of equilibrium’ (at an inclination of just over
30°), and thus relatively shallow, translational movements could be expected in such a situation. Deep seated,
global instability impacting the proposed residential structures at this site is unlikely to occur provided that
the footing system is adequately designed. It is assessed that where piles are taken to found a minimum of
7 m in to the underlying dense sands and below a line at 28° drawn to extend upwards from the toe of the
slope (taken to be from Noosa Drive approximately), a Low Risk of slope instability (to property) would apply.
In an adverse case, where ongoing near surface instability occurs on the dune face below the proposed
buildings, and retrogresses; it is possible that the piles may become exposed above the ground surface
temporarily. Thus, some form of lateral capacity must be allowed for in the pile design to address this.
A grout ‘curtain’ must be installed along the northern edge of the buildings (at the slope crest) to a depth of
no less than 6 m. The plane extending at 28° from the toe has also been used to inform the need for
protection to the side boundaries. It is possible that sand would be removed from neighbouring properties
with ongoing instability on the dune slope, which has the potential to affect the subject site where the building
envelopes do not extend as far north/downslope. The grout curtain should also extend from the northern
edge of the building envelopes, and south along each respective site boundary for about 10 m, tapering from
6 m deep at the northern end.
All excess stormwater collected around the residence (including overflow from rainwater tanks etc.) must be
directed by pipes or lined channels to the nearest legal point of discharge. Where possible, utilising a ‘wet’
or ‘charged’ system where necessary, stormwater collected from the roof and other elevated hard surfaces
should be piped to discharge into the kerb along Picture Point Crescent. Where this can only be designed to
take some flows; stormwater will need to be directed to the dune slope to the north (ultimately leading to the
road reserve for Noosa Drive), where it is suggested that an RPEQ designed stormwater dissipation and
infiltration device should be installed. The device must be designed and constructed to ensure a non-
worsening impact from stormwater runoff on the neighbouring properties/infrastructure down slope.
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PROPOSED RESIDENTIAL DEVELOPMENT
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Table of Contents
1.0 INTRODUCTION ........................................................................................................................................................ 1
2.0 PROPOSED DEVELOPMENT ................................................................................................................................... 1
3.0 METHOD OF INVESTIGATION ................................................................................................................................. 2
4.0 RESULTS OF INVESTIGATION ................................................................................................................................ 2
4.1 Site Description............................................................................................................................................. 2
4.2 Topography................................................................................................................................................... 4
4.3 Geology ........................................................................................................................................................ 5
4.4 Subsurface Conditions.................................................................................................................................. 5
4.5 Groundwater ................................................................................................................................................. 6
4.6 Surface Drainage .......................................................................................................................................... 6
4.7 Vegetation .................................................................................................................................................... 7
4.8 Buildings and Other Structures ..................................................................................................................... 7
5.0 SLOPE STABILITY ASSESSMENT .......................................................................................................................... 8
5.1 Existing Conditions ....................................................................................................................................... 8
5.2 Stability Assessment..................................................................................................................................... 9
6.0 GEOTECHNICAL RECOMMENDATIONS .............................................................................................................. 11
6.1 Development Limitations & Stability Discussion ......................................................................................... 11
6.2 Earthworks .................................................................................................................................................. 12
6.2.1 Site Preparation and Trafficability ......................................................................................................... 12
6.2.2 Excavations ........................................................................................................................................... 13
6.2.3 Filling..................................................................................................................................................... 13
6.3 Building Foundations .................................................................................................................................. 14
6.3.1 Indicative Site Classification.................................................................................................................. 14
6.3.2 Footing Design ...................................................................................................................................... 14
6.4 Temporary and Permanent Retention......................................................................................................... 15
6.5 Drainage ..................................................................................................................................................... 16
7.0 SUMMARY & CONCLUSIONS................................................................................................................................ 17
8.0 LIMITATIONS .......................................................................................................................................................... 17
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TABLES
Table 1: Summary of Subsurface Profiles ........................................................................................................................... 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
TEXT FIGURES
Text Figure 1: Layout plan (LHS) and a cross-sectional view (RHS) of proposed development (courtesy Clout) ............... 1
Text Figure 2: Aerial image of the site and immediate surrounds (courtesy Nearmap, 24 November 2022) ...................... 3
Text Figure 3: View of site frontage looking north from Picture Point Crescent................................................................... 3
Text Figure 4: View looking north over pool and western side of existing residence........................................................... 3
Text Figure 5: Looking upslope (and south) from northern boundary towards existing residence....................................... 4
Text Figure 6: Site plan showing ground surface contours (courtesy NC Inframaps).......................................................... 4
Text Figure 7: Extract of Laguna Bay Surface Geology Map .............................................................................................. 5
Text Figure 8: Extract from NC landslide hazard overlay mapping ..................................................................................... 8
Text Figure 9: Illustrative depths for piled footings ........................................................................................................... 11
Text Figure 10: Footing depth recommended to minimise risk of undermining ................................................................ 15
FIGURES
Figure 1: Site Plan
APPENDICES
APPENDIX A
Borehole & DCP Reports & Explanatory Notes
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
APPENDIX C
Limitations
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1.0 INTRODUCTION
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 39 Picture Point Crescent, Noosa Heads. The investigation was performed by Tectonic for
HM Property Pty Ltd, generally in accordance with Proposal P22802-Rev0, dated 15 December 2022.
The investigation comprised drilling of boreholes at two locations across accessible areas of the
development footprint, together with in-situ soil strength testing. One stand-alone in situ strength test (using
the dynamic cone penetrometer [DCP] method) was also undertaken. This report summarises the results of
the investigation and provides geotechnical recommendations regarding earthworks and footings, together
with temporary and permanent retention.
In summary, subject to the implementation of the recommendations made herein, it is assessed that there
would be a Low Risk of slope instability affecting the proposed development in accordance with the
Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS
2007).
2.0 PROPOSED DEVELOPMENT
Based on the provided information we understand that it is proposed to reconfigure a single existing lot
(Lot 2 on RP220514) in to two separate lots being an equitable 723 m2 in land area. Concept architectural
drawings provided to us with the project brief (Ref. Chris Clout Design [Clout], C1.0 to C5.0, dated 4
November 2022) indicate that following subdivision, each new lot may be developed with residences
extending over most of the available area of the lots.
The residences would be of three storeys height to the Picture Point Crescent frontage (upslope/south), with
a partial lower level on the downslope side (north) extending the overall height to four storeys. Thus, it is
anticipated that bulk excavations for the lower storey would reach about 3 m to 4 m below the existing
ground levels in certain locations. Filling of 2 m to 3 m height is shown on the preliminary architectural
sections on the downslope side of the development to the north, but the extent of this is unclear. Filling to
this extent is not permissible from a slope stability perspective, as detailed in Section 6.1 herein.
Excavations will likely extend close to some of the site boundaries to site the lower level of the proposed
building and therefore, temporary retention will likely be required.
Isolated deeper excavations may be required for underground services and footings or other elements. An
existing sewer running through the building area will be realigned to the northern side of the proposed
building envelopes as part of the works.
Text Figure 1 below presents a layout plan and cross-sectional view of the proposed development.
Text Figure 1: Layout plan (LHS) and a cross-sectional view (RHS) of proposed development (courtesy Clout)
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3.0 METHOD OF INVESTIGATION
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps, slope stability studies as well as Noosa Council (NC) planning overlays and site information.
An experienced engineering geologist from Tectonic undertook a site walk-over on 16 January 2023 to
visually assess any indicators of past slope instability. The walk-over included assessment of soil
exposures, surface drainage conditions, topography, evidence of erosion or past instability, and vegetation
coverage/condition.
This was incorporated with a subsurface investigation comprising the drilling of two boreholes (designated
BHT-1 and BHT-2) in accessible areas across the site. Due to access constraints, boreholes were drilled
using hand augering equipment. The boreholes were taken to a target depth of 3 m below ground level
(BGL).
In-situ DCP testing 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.
Additionally, a single in situ DCP test (denoted DCPT-3) was completed in a steeper part of the site
downslope to a depth of 3.6 m BGL.
Laboratory testing was not warranted due to the sandy nature of the subsurface conditions encountered.
A ‘soil test’ undertaken by Global Testing (Ref. Job No. 22-0439, dated 12 April 2022) was also reviewed as
part of this investigation, which included two hand auger boreholes taken to 1.5 m BGL towards the southern
corner of the existing residence.
Tectonic 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 hand-held 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 2 on RP220514, covering an area of about 1447 m2, with frontage to
Picture Point Crescent along the southern boundary. At the time of the investigation the site featured an
existing residence, positioned over central and southern portions.
Similar sized residential blocks were present to the east and west of the site, which also contained
residences of between one and three storeys (generally increasing in height downslope/away from Picture
Point Crescent).
A recent aerial image of the site and surrounds is shown in Text Figure 2 overleaf.
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Subject site
Text Figure 2: Aerial image of the site and immediate surrounds (courtesy Nearmap, 24 November 2022)
Site conditions at the time of our investigation are shown below and overleaf in Text Figures 3 to 5.
Text Figure 3: View of site frontage looking north from Picture Point Crescent
Text Figure 4: View looking north over pool and western side of existing residence
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Text Figure 5: Looking upslope (and south) from northern boundary towards existing residence
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 just off the
crest of an ancient back dune, over the upper flank which slopes down to the north-west. The dune flank
continues off site to the north-west, across Noosa Drive, and thence to the lower reaches of the Noosa River
about 75 m downslope.
The natural ground slope around the existing residence had been altered as a result of historic earthworks
and construction. Based on the available topographical information and site observations, the natural ground
slope appears to be inclined at around 30° around the existing residence, increasing to a maximum of 34°
towards the northern site boundary. This relatively steep sloping ground continued consistently off site to the
north towards Noosa Drive, and was generally quite linear. No irregular surface features were noted which
would indicate global slope instability.
Subject site
Text Figure 6: Site plan showing ground surface contours (courtesy NC Inframaps)
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4.3 Geology
Available geological information 1 indicates that the site is at a mapped boundary between recent reclamation
(denoted Qhh, green striped shading) and Pleistocene age “quartz sand, parabolic sand dunes.” Given the
elevation and nature of the site, the former is unlikely and the boundaries are possibly as a result of the
mapping being at a relatively large scale (Ref. Text Figure 7).
Approx. site location
Text Figure 7: Extract of Laguna Bay Surface Geology Map
4.4 Subsurface Conditions
Detailed descriptions of the subsurface conditions encountered in the Tectonic 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, and inferred from
deeper DCP testing at the boreholes, are summarised as follows:
Fill – comprising very loose to loose sands (encountered in borehole BHT-2 only) to a depth of
0.7 m BGL; then or from the ground surface in borehole BHT-1
Natural Sands:
o being very loose to depths of between 1 m and 2 m BGL; then
o loose to between 2 m and 3.6 m BGL; underlain by
o medium dense sands to between 3.8 m and 4.6 m BGL; followed by
o dense sands to the testing termination depths of between 4.8 m and 5.6 m BGL.
Table 1 on the following page provides a detailed summary of the subsurface profiles encountered in each
Tectonic and Global borehole, and inferred from deeper DCP testing at each location.
1
Queensland Dept. of Natural Resources, Mines and Energy, 1:100,000 Laguna Bay Surface Geology, Sheet 9545, 2018
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Table 1: Summary of Subsurface Profiles
Very Loose Very Loose
Medium Dense (or
Borehole No. Sands (Fill) Sands Loose Sands
Dense Sands denser) Sands
(Natural)
BH1
(Global 0.0 – 0.4 0.4 – 1.9 1.9 – 2.8 2.8 – 3.8 3.8 – 4.8TD
Testing)
BH2
(Global 0.0 – 0.3 0.3 – 2.0 2.0 – 3.6 3.6 – 4.6 4.6 – 5.3TD
Testing)
BHT-1 - 0.0 – 1.0 1.0 – 2.1 2.1 – 4.6 4.6 – 5.6TD
BHT-2 0.0 – 0.7 - 0.7 – 2.0 2.0 – 4.0 4.0 – 5.7 TD
DCPT-3 - 0.0 – 1.2 1.2 – 2.0 2.0 – 3.6 TD -
Notes: 1. Depths noted in m BGL at time of investigation (BH1 & BH2, 11 April 2022, BHT-1, T-2 and DCPT-3,
16 January 2023)
2. - denotes not encountered
3. TD denotes borehole terminated at test target depth
4.5 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over. Groundwater was not
encountered in the boreholes at the time of investigation.
Groundwater conditions are expected to be reflective of prevailing weather conditions and seepage may
develop, particularly following periods of prolonged or heavy rainfall.
4.6 Surface Drainage
Surface drainage would be expected to follow natural surface contours to the north/north-west towards
Noosa Drive (vegetated reserve, or indeed ultimately the road corridor). It is noted that the site has a limited
catchment, being close to the localised back dune crest. While there is a positive change in elevation to the
south of the site to residences on the opposite side of Picture Point Crescent, some protection may be
afforded to the site by the presence of kerb and channel drainage over the frontage. It also noted that
Picture Point Crescent tends to fall slightly south and north-east, away from the site in both directions.
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. The road reserve between Noosa Drive and
the subject site is also well vegetated, and about 30 m width.
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 not significant during rainfall events,
Recommendations to improve surface drainage as part of the development works are provided in
Section 6.5.
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4.7 Vegetation
To the south-eastern corner of the site off Picture Point Crescent, landscaped gardens with small shrubs,
small to medium size trees, and grasses were present. Downslope of the existing residence and within the
subject site, mainly vines and ferns were present with sporadic trees. A medium size tree to the north-
eastern corner of the pool was noted to be leaning slightly. This can be, but is not necessarily always, an
indication of slope instability. On adjacent lots to the east and west, and on ground downslope, moderately
densely planted, larger mature trees were observed.
4.8 Buildings and Other Structures
The existing residence consisted of a split level upper section to the south, ranging from one to two storeys;
then extending over a suspended single level downslope to the north. To the south, the one to two storey
split-level portion of the house featured a mixture of masonry brick and lightweight clad exterior walls. The
suspended portion to the north was timber framed and clad, with steel posts connected to the ground. The
suspended portion of the residence is shown in Text Figure 5.
A pool was present to the south-western corner of the block, which had a concrete shell, and some fill to the
surrounds supported on the downslope side by a masonry block wall. This was estimated to be fully in-
ground to the southern side, extending to about 1 m to 1.5 m out of ground on the downslope side. Along
the northern edge of the pool a suspended timber deck was present (over steel posts), reaching a maximum
height of 3.5 m above ground.
A view of the pool and split-level upper portion to the residence can be seen in Text Figure 4 above.
Based on an external assessment, the structures on the site appeared to be in relatively good structural
condition, with no obvious signs of distress which could easily be attributed to slope instability. It was noted
that an external set of the stairs to the north-western side of the house were slightly tilted, however it was
noted that the posts supporting the stairs were not connected to the concrete footing, which in turn was
founded very close to surface.
A number of small timber retaining walls were present to the north-east, and immediately downslope of, the
existing residence. These ranged to about 1 m height, and were in a poor state of repair, with a number of
rotting and broken rails and posts.
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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 the site is within a Moderate Landslide Hazard Area. It is noted that the Noosa Drive
reserve and road corridor below the site to the north is not classified in the overlay mapping.
Lot boundary
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. The allotment did not exhibit any visible
surface features which would indicate global slope instability during our walkover: no landslide back
scarps, noteworthy groundwater seepage or areas of naturally ‘hummocky’ ground were observed.
Medium dense sands were encountered at all the investigation locations, at depths ranging from about
2.8 m to 3.6 m BGL (existing), with dense sands below.
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5.2 Stability Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 2 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
Table 2: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to
L Low Risk
this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface
gradient is about 30°, although often lower (26˚ to 28˚) for very loose sands. Our measurements indicate that
the overall natural gradient is around 30°, although ranging to about 34° downslope around the northern site
boundary. 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 layer more resistance to erosion, 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 toe of the dune is over 30 m from the northern
boundary, and is protected to a large degree by the presence of the Noosa Drive road corridor. The sealed
surface of the road, and formalised drainage conditions would make the likelihood of water erosion or
significant disturbance of the dune by natural forces, in the vicinity of the site 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 residential development. Regulators (NC) normally require that a Very Low or Low
Risk of landslide affecting the property must be demonstrated to enable development approval. Summarised
in Table 3 overleaf is our qualitative assessment of landslide risk for the development site. A summary of
qualitative terminology for use in assessing risk to property is attached (taken from AGS 2007).
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PROPOSED RESIDENTIAL DEVELOPMENT
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Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence Risk Comments
Given the slightly gentler gradients through the
proposed building area and retention system proposed
1: Building Envelope - Failure it is considered Unlikely that a landslip would develop
through very loose to loose or in these materials, subject to implementation of the
Unlikely Medium Low
medium dense natural sands recommendations in Section 6 (especially footing
above the foundation depth. depth and type). 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 dense (or
denser) natural sands below the proposed foundation
2: Building Envelope - Deep
depth is assessed as Rare due to the lack of precedence
failure through the dense (or
Rare Major Low of such movements locally, no signs of any existing slope
denser) sands below the
instability and a lack of groundwater. Although the
foundation depth.
consequence of such a failure would be Major, the
resultant risk is Low as per AGS 2007.
The likelihood of a slip/failure through the temporary
retention system (sheet pile wall, or similar) as outlined in
3: Instability through temporary Section 6.4, is assessed as Rare, subject to detailed
sheet pile retention system, or design by a suitable qualified RPEQ engineer and
Rare Major Low
similar, as discussed in Section construction by a specialist contractor which accounts for
6.4. the required factors of safety against slope instability.
Although the consequence of such a failure would be
Major, the resultant risk is Low as per AGS 2007.
As discussed further below, it is Likely that near surface
translational failures may occur over the face of the
natural dune slope within the site, and in the slope that
continues down to Noosa Drive to the north, over the
4: Dune Slope Below Building
design life of the project. However, given the extent of the
Envelope –shallow translational
Likely Insignificant Low recommendations below, it is considered that this will
movements in very loose to loose
affect future landscaping/garden areas only, and when
natural sands near surface.
considered against the value of the property and that the
likelihood for adverse effects to the building is limited, an
Insignificant consequence is assessed. Thus the
resultant risk is Low as per AGS 2007.
Our assessment indicates that a Low Risk of slope instability could be achieved for proposed residential
development at 39 Picture Point Crescent, and that there would be a Low Risk of slope instability, subject to
implementation of the recommendations outlined in Section 6 (especially around permanent retention of
cuts, adoption of relatively deep piles, and protection from erosion from the dune face).
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
6.0 GEOTECHNICAL RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.
6.1 Development Limitations & Stability Discussion
It is anticipated earthworks will include cuts for the lower level of up to about 3 m to 4 m deep, in order to site
the lower level of the proposed residential structures. The extent of earthworks cuts has not been indicated
on the drawings provided, and the findings of this assessment should be reviewed once detailed
architectural drawings are available (to confirm feasibility).
Filling of up to 3 m to 4 m height is shown on the preliminary architectural concept drawings, however such
filling would result in an unacceptable surcharge over the sensitive dune slope, and resulting batter angle of
around 45° to be contained within the subject site. Localised levelling and grading may be permissible
elsewhere on the site, however fill heights should be limited to no more than 0.5 m above the existing ground
surface.
As mentioned above, the sand dune at this site is close to the ‘limit of equilibrium,’ where the FOS of the slope
is approximately 1.0. The dune slope at the site was at an inclination of just over 30°, and thus relatively
shallow, translational movements could be expected in such a situation.
Deep seated, global instability impacting the residential structures on the slopes at this site is unlikely to
occur provided that the footing system is adequately designed. It is assessed that where piles are taken to
found a minimum of 7 m in to the underlying dense sands and below a line at 28° drawn to extend upwards
from the toe (taken to be from Noosa Drive approximately), a Low Risk of slope instability would apply.
This is indicated conceptually in Text Figure 9 below.
Piles to found below plane at 28° and with 7 m embedment in to dense sand
Line at 28° from toe
Text Figure 9: Illustrative depths for piled footings
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
In an adverse case, where ongoing near surface instability occurs on the dune face below the proposed
buildings, and retrogresses; it is possible that the piles may become exposed above the ground surface
temporarily. Thus, some form of lateral capacity must be allowed for in the pile design to address this.
A grout ‘curtain’ must be installed along the along the front (northern) edge of the buildings (at the slope
crest) to a depth of no less than 6 m. The plane extending at 28° from the toe has also been used to inform
the need for protection to the side boundaries. It is possible that sand would be removed from neighbouring
properties with instability on the dune slope, which has the potential to affect the subject site where the
building envelopes do not extend as far north/downslope. The grout curtain should also extend from the
northern edge of the building envelopes, and south along each respective site boundary for about 10 m,
tapering from 6 m deep at the northern end.
It would be prudent to consider the following for the final design and construction of the development
regarding the dune slope below the site.
One of the primary triggers for large-scale instability of sand dunes is when the toe of the dune is
undermined. This may be caused by excavations or natural erosion. The toe of the dune must be
protected from any such disturbance.
In order to minimise future erosion (and initial surficial slipping) it is recommended that the faces of all
dune and batter slopes are protected using with topsoil/mulching of the surface and revegetation, in
combination with a cellular confinement system, such as GeoWeb (or similar), in critical areas such as
stormwater outlet zones. Any erosion or instability will need to be appropriately remediated as soon
as possible after it is identified. If ongoing instability occurs, the slope may need to be retained to
prevent further movement.
6.2 Earthworks
6.2.1 Site Preparation and Trafficability
Following clearing of existing vegetation, any tree stumps, large roots and remnant foundation elements from
the existing structure should be ‘grubbed out’. Any organic rich topsoil should then be stripped and set
aside. Based on the boreholes, topsoil stripping depths are expected to be minimal with pockets of
uncontrolled fill being around 0.7 m depth locally; deeper areas of unsuitable materials may be present on
site.
Topsoil materials must not be used in structural fill; however, may be placed across landscaping areas.
Unsuitable saturated or loose materials encountered should also be removed during subgrade preparation.
It should be noted that compaction of such relatively ‘clean’ sands as structural fill requires the addition of
substantial volumes of water (‘flooding’) combined with the use of vibratory compaction equipment (typically
a vibrating plate). Care must be taken with the use of vibratory equipment (e.g. compactors or sheet piling)
to ensure that there are no adverse impacts on surrounding structures. The impacts from ground vibration
must be closely monitored and managed during construction, to be detailed in a construction management
plan. It would be prudent to conduct a dilapidation survey of neighbouring structures prior to commencement
of construction.
The exposed natural subgrade following excavations will likely be in a loose condition and must be
compacted by several passes of a vibrating plate or minimum 4 tonne vibrating smooth drum roller, in
conjunction with surface application of ample water, to achieve the required compaction.
Due to the potential for development of poor trafficability during construction, placement of a minimum
150 mm thick granular working platform may be required across laydown areas. Specific platform design
would be required prior to mobilisation and trafficking of any heavy mobile plant on the site (e.g. mobile
cranes, concrete pumps or piling rigs).
Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
6.2.2 Excavations
Excavations through the soils should be achievable using conventional earthmoving plant (e.g. small or
larger excavators with toothed buckets). Temporary unsurchaged cuts in the very loose to loose sand could
be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to 4 m in height. Flatter
batters or temporary retention may be required if groundwater seepage is encountered or if sloughing of dry
sand occurs at the batter surface. Deeper cuts, if required, should be specifically assessed by Tectonic.
Permanent cuts or fills greater than 1 m must be retained. Permanent cut batters up to 1 m depth; and fill
batters up to 0.5 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 the above temporary batter slopes will not be able to be achieved, due to spatial restraints,
and so temporary shoring may be required (e.g. sheet piles) as per the recommendations in Section 6.4
below. Excavations in close proximity to existing structures and infrastructure (e.g. sewer main within the
site), including on neighbouring properties, will need to be specifically designed to prevent undermining. It is
recommended that footing exposures or confirmation of existing foundations via certified building plans be
undertaken to assess the possibility of undermining. If undermining is a potential issue the existing
structures may need to be underpinned or excavations may require temporary shoring.
The advice of a suitably qualified and experienced RPEQ structural engineer must be sought regarding any
potential undermining and design and construction of any underpins/temporary shoring (if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
6.2.3 Filling
The natural sandy soils where free of deleterious materials (e.g. vegetation, organics etc.) may be considered
for re-use off site as structural fill, subject to appropriate moisture adjustment. It should be noted that
compaction of such relatively ‘clean’ sands as structural fill requires the addition of substantial volumes of
water (‘flooding’) combined with the use of vibratory compaction equipment (typically a smooth drum roller or
heavy vibrating plate).
Should importation of fill be required, it is recommended that such fill should comply with the following:
Minimum CBR of 10%
Maximum plasticity index of 15%
Shrink-swell index ≤ 1%
Maximum particle size of 100 mm
Any fill should be compacted at moisture contents within the range of ±2 % of optimum moisture content for
Standard Compaction. Fill material should be placed in 250 mm thick (loose thickness) layers and be
compacted by repeated rolling using a minimum 4 tonne smooth drum roller (or equivalent, with vibration on
if safe to do so) to achieve a dry density ratio of at least 95 % of the maximum Dry Density Ratio (DDR),
based upon the Standard Compaction method, or a Dry Density Index of at least 70 %.
Confirmatory compaction testing must be carried out at regular intervals. A minimum of two tests per vertical
lift is recommended where structural fill heights are to exceed 300 mm. Further details for control and testing
of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and
Residential Developments.
Fill embankment edges must be over-filled by 0.5 m (horizontally) and then trimmed back to well compacted
material.
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
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 undertaken at a site, variations in the natural soil depths, and the
reactivity of the in situ materials and any imported fill materials.
Based on the results of the investigation, the site is classified as Class P in accordance with AS 2870-2011
Residential Slabs and Footings, due to the presence of poor bearing capacity soils at shallow depths and
trees. If not for these factors, the site would have the characteristics of Class S, with the estimated
characteristic surface movements (‘ys’) ranging up to about 20 mm. However, due to the potential variability
in subgrade density a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footing and plumbing design.
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 fill materials and unsuitable natural
soils (i.e. very loose to loose and medium dense sands). Footings should be taken to found at least 7 m in to
the underlying dense sands. This would result in relatively long piles ranging up to the order of 10 m BGL as
shown indicatively in Text Figure 9.
Concrete piles so founded may be designed using an ultimate end bearing pressure of 3 MPa, and ultimate
side adhesion (in dense sand) of 60 kPa. Settlement under such loads is expected to be in the order of up to
20 mm or 1% of footing width. In addition, piles on the downslope edge of the buildings (at least) will likely be
required to have resistance to normal shear forces.
A review of the design plans should be undertaken, once available, such that the footing design for the
structures can be optimised.
Given the sandy soil profile and depths of footings required, steel screw piles could be considered a suitable
foundation type, where more onerous shear resistance is not required. The use of driven piles would be
precluded with the proximity to neighbouring residences, and the presence of sewer infrastructure within the
site. Steel reinforced, concrete bored piles could be considered, however installation may not be
straightforward, given the presence of very loose to loose sands in the upper soil profile. The use of a
continuous flight auger, grout-injected pile system would be required for bored piles. 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.
Depending on the detailed design of the footing system, once access becomes available (following building
demolition) it is recommended that additional geotechnical investigation is undertaken to confirm parameters
for piling. This should comprise two boreholes drilled to at least 2 m beyond the design pile toe depths (say
about 12 m BGL) with standard penetration testing at regular, 1.5 m intervals.
If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling may
not encounter sufficient drilling resistance (from torque measurement) in sands at the suggested founding
depth, and piles could extend to much greater depth than necessary.
It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
deflections under tension and compression cycles) should also be undertaken prior to construction.
Kentledge for static testing, or simulated cyclic loads, should prove the piles to a minimum of two times the
expected working load within tolerable deflections.
Footing design should also account for potential soil reactivity to be equivalent to Class M. It should be noted
that placement of reactive clay may result in a more severe equivalent site classification, such as H1 or
greater.
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
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.
Text Figure 10: Footing depth recommended 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 footings be inspected by a suitably qualified and experienced RPEQ prior to
placement of steel reinforcement and pouring of concrete to confirm allowable bearing pressures and
cleanliness of excavations.
If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic should be notified immediately and an inspection carried out to determine if changes
to footing design are required.
6.4 Temporary and Permanent Retention
Excavations are expected to range up to around 3 m to 4 m BGL (existing ground levels) and, given that the
planned excavation extends close to some of the site boundaries and underground infrastructure (e.g.
sewer), some form of temporary shoring will likely be required to support the excavation(s).
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 in dense sand materials to prevent adverse
vibrations during sheet pile installation.
Other shoring methods that could be considered include bored contiguous or secant piles. Non-contiguous
or soldier pile walls are not feasible for the proposed development due to the likelihood that loose sands
would ‘flow’ through gaps between the piles.
An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the
permanent walls, whereas sheet piles would be temporary to provide retention during construction only,
therefore requiring a permanent structural wall to be constructed. Bored pile walls generally require greater
width (horizontally) for installation as compared to sheet piles and are often more expensive.
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
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,
will require engineer design and certification of construction. Retaining wall design should be carried out in
accordance with AS4678-2002 Earth retaining structures.
Design for temporary and permanent retaining structures could be carried out using a trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type and based on the parameters
listed in Table 4. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.
Table 4: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Very Loose/ Loose
1.7 28 0.36 0.53 2.77
Sands
Medium Dense Sands 1.8 34 0.28 0.44 3.54
Dense (or Denser)
1.9 36 0.26 0.41 3.85
Sands
Retaining structures must be founded as per the recommendations in Section 6.3.2 above with the footings
also designed to account for lateral loads imposed by the retaining wall. Detailed design must include an
assessment of internal and global stability. A solution using screw piles to support retaining walls, possibly
including raked piles, may be feasible although bending and shear capacities would need to be carefully
checked in design from a soil-structure interaction viewpoint. Additional lateral capacity may be achieved by
incorporating a deeper pile cap that also acts as a shear key attracting passive resistance from the sands in
front of the wall.
6.5 Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment or
retaining walls (e.g. grassed or landscaped swales or diversion mounds). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building walls, footings, batters and retaining
walls.
Although unlikely, if groundwater seepage is encountered during excavations, Tectonic must be notified to
assess the impact on stability.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.
All excess stormwater collected around the residence (including overflow from rainwater tanks etc.) must be
directed by pipes or lined channels to the nearest legal point of discharge.
Where possible, excess stormwater collected from the roof and other elevated hard surfaces should be
piped to discharge into the kerb along Picture Point Crescent. Given the relative elevations involved, the use
of a ‘wet’ or ‘charged’ drainage system may be required. Where this can only be designed to take some
flows; smaller volumes of excess stormwater will need to be directed to the dune slope to the north
10 February 2023
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PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
(ultimately leading to the road reserve for Noosa Drive), where it is suggested that a stormwater dissipation
and infiltration device should be installed.
Such a device should be designed and certified by a suitably qualified and experienced hydraulic or civil
RPEQ. The device must be designed and constructed to ensure a non-worsening impact from stormwater
runoff on the neighbouring properties/infrastructure down slope.
7.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment it is assessed that there would be a Low Risk of slope instability
affecting the proposed development subject to implementation of the recommendations given in this report.
8.0 LIMITATIONS
Your attention is drawn to the document Limitations, which is included in Appendix C of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of this
report should be, and to present you with recommendations on how to minimise the risks associated with the
services provided by Tectonic for this project.
10 February 2023
Report No. 22511-001-Rev0 17
Page 23
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Office: 07 5478 9016
Email: [email protected]
PROJECT:
GEOTECHNICAL INVESTIGATION,
Subject site 39 PICTURE POINT CRESCENT,
NOOSA HEADS
TITLE:
SITE PLAN
DCPT-3
CLIENT:
BHT-2
A’ HM PROPERTY PTY LTD
A DRAWN: DATE:
RKG 08/02/2023
BHT-1 CHECKED: DATE:
DJP 08/02/2023
SCALE:
AS SHOWN (APPROX.)
LEGEND:
APPROX. HAND AUGER
LOCATION
APPROX. STAND-ALONE
DCP LOCATION
0 15 30 m
Notes PROJECT NO.: FIGURE: REV:
1. Based plan courtesy of Noosa Council Interactive Mapping 22511 1 0
Page 24
PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
APPENDIX A
Borehole & DCP Reports & Explanatory Notes
10 February 2023
Report No. 22511-001-Rev0
Page 25
Borehole No.
BHT-1
Page 1 of 1
Engineering Log - Borehole Project No.: 22511
Client: HM Property Pty Ltd Commenced: 16/1/2023
Project Name: Proposed Residential Development Completed: 16/1/2023
Hole Location: 39 Picture Point Crescent Logged By: MS
Hole Position: 509063.0 m E 7081074.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SP SAND, fine grained, grey HW
HW
HW
HW
VL
becoming pale grey
1
Not Encountered
M
HA
L
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22511.GPJ <<DrawingFile>> 6/2/2023 12:26 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
becoming pale grey to pale brown grey
MD
Augered to 3.0m
3
Hole Terminated at 3.00 m
Target depth
4
5
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 26
Borehole No.
BHT-2
Page 1 of 1
Engineering Log - Borehole Project No.: 22511
Client: HM Property Pty Ltd Commenced: 16/1/2023
Project Name: Proposed Residential Development Completed: 16/1/2023
Hole Location: 39 Picture Point Crescent Logged By: MS
Hole Position: 509075.0 m E 7081090.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SAND (FILL), fine grained, grey, trace of fine to HW
medium sized sub-angular gravel, trace of root HW
VL HW
fragments
HW
SP SAND, fine grained, grey
(NATURAL)
1 becoming pale grey
Not Encountered
L
M
HA
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22511.GPJ <<DrawingFile>> 6/2/2023 12:26 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
becoming fine to medium grained, poorly graded
Augered to 3.0m
3
Hole Terminated at 3.00 m
Target depth
4
5
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 27
CLIENT : HM Property Pty Ltd SHEET : 1 OF 1
CONTRACTOR :
PROJECT : Proposed Residential Development
LOCATION : 39 Picture Point Crescent
PROJECT No. : 22511
TEST: DCPT-3
LOGGED: MS 16/1/2023 CHECKED: RKG
EASTING: 509066.0 m NORTHING: 7081095.0 m
MGA2020 Zone 56
Depth (m)
Blows per 100 mm (AS 1289.6.3.2-1997)
0 5 10 15 20 25
HW
1
2
TECTONIC 3.00.2 LIB.GLB Log IS DCP 3 PER PAGE 22511.GPJ <<DrawingFile>> 6/2/2023 12:26 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
3
4
5
Comments
Page 28
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RA Rotary Air BH Tractor Mounted Backhoe
ADH Hollow Auger RD Rotary Blade or Drag bit EX Tracked Hydraulic Excavator
HA Hand Auger RT Rotary Tri-cone bit HMLC Core – 63mm
*T TC-Bit HQ Core – 63mm
*V V-Bit NMLC Core – 47mm
NQ Core – 52mm
R Ripper
RH Rock Hammer
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
5,4,10 N=14 5,4,10 = Blows per 150mm. N = Blows per 300mm penetration following 150mm seating
30/65mm Where practical refusal occurs, the blows and penetration for that interval are reported
RW Penetration occurred under the rod weight only
HW Penetration occurred under the hammer and rod weight only
HB Hammer double bouncing on anvil
DS Disturbed sample
BDS Bulk disturbed sample
SV Field shear vane test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev1
February 2021
Page 29
METHOD OF SOIL DESCRIPTION
USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Soil is classified and described in borehole and test pit logs using the preferred method given in AS1726 – 2017.
The material properties are assessed in the field by visual/tactile methods.
Particle Size Plasticity Properties
Major Sub Division Particle Size
Division
BOULDERS > 200 mm
COBBLES 63 to 200 mm
Coarse 19 to 63 mm
GRAVEL Medium 6.7 to 19 mm
Fine 2.36 to 6.7 mm
Coarse 0.6 to 2.36 mm
SAND Medium 0.21 to 0.6 mm
Fine 0.075 to 0.21
SILT 0.002 to 0.075
CLAY < 0.002 mm
NOTE: The U line is an approximate upper bound for most materials.
Data which plot above the U line may represent unusual/problem soil
behavior, or unreliable data and should be considered carefully.
MOISTURE CONDITION AS1726 - 2017
Symbol Term Description
D Dry Sands and gravels are free flowing. Clays & Silts may be brittle or friable and powdery.
M Moist Soils are darker than in the dry condition & may feel cool. Sands and gravels tend to cohere.
W Wet Soils exude free water. Sands and gravels tend to cohere.
CONSISTENCY AND DENSITY AS1726 - 2017
Symbol Term Undrained Shear Symbol Term Density Index % SPT “N” #
Strength
VS Very Soft 0 to 12 kPa VL Very Loose Less than 15 0 to 4
S Soft 12 to 25 kPa L Loose 15 to 35 4 to 10
F Firm 25 to 50 kPa MD Medium Dense 35 to 65 10 to 30
St Stiff 50 to 100 kPa D Dense 65 to 85 30 to 50
VSt Very Stiff 100 to 200 kPa VD Very Dense Above 85 Above 50
H Hard Above 200 kPa
In the absence of test results, consistency and density may be assessed from correlations with the observed behaviour of
the material.
# SPT correlations are not stated in AS1726 – 2017, and may be subject to corrections for overburden pressure and
equipment type.
Rev1
October 2017
Page 30
PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
APPENDIX B
AGS Guidelines - Qualitative Terminology for Use in
Assessing Risk to Property
Some Guidelines for Hillside Construction
10 February 2023
Report No. 22511-001-Rev0
Page 31
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 32
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 33
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
Page 34
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 35
PROPOSED RESIDENTIAL DEVELOPMENT
39 PICTURE POINT CRESCENT, NOOSA HEADS
APPENDIX C
Limitations
10 February 2023
Report No. 22511-001-Rev0
Page 36
LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)
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