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Reference Document - Geotechnical Investigation Report - DBW24/0090 & RAP24/0025
REFERENCE DOCUMENT
DBW24/0090 & RAP24/0025
NOOSA COUNCIL
27 AUGUST 2024
3 September 2020 Project No. 20248-001-Rev0
Rebecca Stead
26 Attunga Heights
Noosa Heads QLD 4567
Email: [email protected]
GEOTECHNICIAL INVESTIGATION FOR PROPOSED EXTENSIONS TO A RESIDENCE
26 ATTUNGA HEIGHTS, NOOSA HEADS
Dear Rebecca,
1.0 INTRODUCTION
As requested, Tectonic has undertaken a geotechnical investigation for proposed extensions to an existing
residence at 26 Attunga Heights, Noosa Heads. This report presents the results of our geotechnical
investigation, incorporating a slope stability assessment, together with geotechnical advice for the project.
In summary, subject to the implementation of the recommendations made herein, it is assessed that there
would be a Low Risk to the proposed extensions from slope instability in accordance with the Australian
Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007).
1.1 Details of the Site and Development
The site is described as Lot 14 on RP89506, covers an area of 533 m2, and has frontage to Attunga Heights
along the western boundary. At the time of the assessment, an existing residence occupied the central part
of the site. Access was provided by an asphalt driveway directly off the northern end of Attunga Heights.
Residential properties surrounded the site to the north, west and south, with vacant allotments to the east. A
2020 aerial image of the site and surrounds is shown in Text Figure 1 below. A more detailed description of
the site is given in Section 2.
Existing Residence
Subject Site
BH2
BH1
Inset
Text Figure 1: Aerial image of site and surrounds from 2020 (courtesy of Nearmap imagery)
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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Architectural drawings were supplied to us (Ref. Alexander Robinson Design, Project No. WD2026, Issue B
dated 28 July 2020) indicate that the proposed extensions are to comprise a new two-storey addition to the
eastern side of the existing residence, with additional timber decking to the northern, eastern and western
sides. The main extension is to comprise a new kitchen on the lower floor level, with a master bedroom and
ensuite for the upper. A carport is proposed to the west of the existing residence. It is understood that
internal renovations will also be required, which may include new columns and footings. A carport is
proposed to the western side.
Provided cross sections indicate that earthworks for the proposed extensions will primarily comprise cutting
into the existing ground surface up to about 2 m deep, with filling less than 1 m high proposed. It is
recommended that the extensions should comprise lightweight construction materials, and be suspended
above ground (wherever possible) to limit fill earthworks required.
The property has an existing connection to a mains potable water supply, and is sewered. Extracts from the
proposed layout plans and architectural elevations are shown in Text Figure 2 below.
Proposed extensions
(pale grey shading)
Text Figure 2: Extracts from Alexander Robinson Design architectural drawings
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1.2 Method and Scope of Investigation
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps, ground level contour information, and Noosa Council (NC) landslide hazard overlays.
An experienced geotechnical engineer from Tectonic undertook a site walk-over on 19 August 2020 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.
The walkover was incorporated with a subsurface investigation comprising the drilling of boreholes at two
locations (targeting the abovementioned main structural additions), using hang augering techniques to
depths between 2.4 m and 3.0 m below ground level (BGL). Dynamic cone penetrometer (DCP) testing was
carried out adjacent to each borehole location and downhole. The boreholes were backfilled on completion
with spoil, with excess material slightly mounded at surface. Borehole co-ordinates were obtained using a
hand-held GPS unit with a reported accuracy of +/- 3 m.
Laboratory testing was not warranted due to the sandy nature of the subsurface conditions encountered.
Reports of the boreholes and explanatory notes are attached. Test locations are shown in Text Figure 1
above. The results of the desk-top study, walk-over survey and subsurface investigation are included in
Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been prepared by Mr Scott Davis, an RPEQ with 12 years of experience in geotechnical
engineering, including a number of slope stability projects on the Sunshine Coast.
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology and Subsurface Conditions
Available geological information 1 indicates that the site is underlain by Pleistocene age dune sand (Ref: Text
Figure 3), and is near the contact with the older Noosa Quartz Diorite formation.
Noosa Quartz Diorite
Dune Sand
Approx. Site Location
Text Figure 3: Extract of Gympie Special geology map
1
Queensland Government, Department of Mines and Energy, 1:100,000 Gympie Special Geology Map, Sheet 9445 & Part 9545, 1999
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Materials encountered in the boreholes were consistent with the published geology, comprising dune sands.
Subsurface conditions are summarised as follows:
Very loose to loose sands (silty/poorly graded) from the ground surface to depths between 1.4 m and
1.6 m BGL; then
Medium dense, poorly graded sand to the borehole termination depths.
It should be noted that DCP testing extended below the borehole termination depth,s and indicates that the
sands become dense (or denser) below about 3 m to 3.5 m BGL.
Groundwater seepage was not encountered during the investigation, with the soils generally being moist.
Reports of boreholes are attached to this letter, along with explanatory notes.
2.2 Topography
As illustrated in Text Figure 4, the site is located on the upper, eastern flank of a large sand dune traced by
Attunga Heights. The subject site is located just off the dune crest, with the existing residence situated on
relatively level ground presumably constructed by cut and fill earthworks.
The natural surface profile has been interrupted by historic landscaping and residential construction at the
site; with surrounding ground slopes to the east and south of the residence measured at the time of our
assessment between about 10° and 12°. Steeper ground slopes ranging between about 20° and 22° were
recorded to the north and north-east of the existing residence, with these gradients continuing down the flank
of the sand dune beyond the property boundary. Based on our observations, the proposed extension areas
are to be situated on moderately sloping ground to the western and eastern sides of the existing residence.
Subject Site
Text Figure 4: Ground surface contours, 1 m spacing (courtesy NC interactive mapping)
Site conditions at the time of our investigation are shown in Text Figures 5 and 6 overleaf.
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Main Extension Area
Existing Residence
Text Figure 5: Panoramic view of the existing residence and main extension area, looking south
Proposed Carport Site
Text Figure 6: Panoramic view of the proposed carport area, looking west from the existing residence
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
recorded during borehole drilling.
2.4 Surface Drainage
Surface drainage would be expected to follow the contours to the north-east. Run-off would be minimal due
to the sandy nature of the soils at the site, and is currently interrupted by terracing and landscaping
features.
It is acknowledged that a surface water catchment for the site exists to the west. However, no significant
surface water run-off features were noted around the western side of the residence during our assessment
(e.g. deep rilling or scour/wash out scars) which would indicate that significant overland flow is generally
minimal during rainfall events. Some erosion protection (i.e. coir logs) was present to the west of the
residence.
Erosion scour was noted along the eastern edge of the residence, including some undermining of concrete
footings for the northern staircase and residence slab. This appeared to have been caused by overtopping
of the existing stormwater infrastructure (i.e. roof gutters), with outlet pipes discharging within close
proximity to the existing residence.
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Text Figure 7: Coir logs to the west of the residence (LHS) and scouring and undermining along the eastern edge of the
residence (RHS).
2.5 Vegetation
The ground surface to the east of the existing residence was primarily covered by bare sands and scattered
shrubs and mature trees. Dense understory was noted to the west, with scattered trees. Mature trees
around the proposed development areas were noted to be relatively straight, exhibiting no obvious signs of
tilting/leaning which could be attributed to slope instability. Some trees to the north-west of the residence
were noted to be leaning, which could be an indicator of minor soil creep.
2.6 Buildings and Other Structures
The house is a two storey structure primarily of blockwork construction. Timber decking extended from the
northern and western edges of the residence. The eastern side of the residence appeared to have settled
previously, with cracking and vertical separation of blockwork noted (Ref. Text Figure 8). It appeared that
this was related to poor/insufficient footing construction rather than global slope instability.
Timber sleeper walls ranging up to about 1 m high were present along the western edge of the house and
were in relatively good condition at the time of the assessment. A blockwork wall ranging up to about 1 m
high was noted along the southern boundary, and was overturning. A timber retaining wall dilapidated by
age was present to the north of the residence.
Text Figure 8: Cracking and separation noted along the eastern edge of the residence (LHS) and overturning block
retaining wall along the southern boundary (RHS).
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 9).
Mapping indicates that the site is within a High Landslide Hazard Area.
Subject site
Text Figure 9: Extract from NC landslide hazard overlay mapping
It should be noted that the Noosa landslide hazard map is a broad-scale indication of the potential
landslide susceptibility and is superseded by site specific investigation and analysis. No visual signs of
global slope instability were not observed in the location of the residence or proposed extensions. No
landslide back scarps or areas of naturally ‘hummocky’ ground were observed.
3.2 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 1 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached.
Table 1: 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
L Low Risk
to 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.
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Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°. The
overall slope across the proposed development area is generally in the order of 10˚ to 22˚, being steepest in
the north-eastern corner. 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).
Steep sand dune slopes generally remain marginally stable (i.e. FOS just 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
when the ‘toe’ of the dune is not disturbed.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Areas outside of structures and concrete ground slabs are generally
covered by vegetation being either grass, shrubs or trees. Discharge of stormwater onto dune slopes may
cause channel erosion, undermining and disturbance of vegetation, resulting in development of generally
localised instability. Stormwater runoff must be collected (or re-directed) and discharged from the site as
discussed in Section 4.5 below.
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. Sand dune instability due to undercutting of the toe generally
occurs as a translational-type slide where a ‘slab’ of sand on the dune face slides down slope. The depth of
such sliding may range up to a few metres during one event, but on-going erosion can cause retrogressive
failure of dune slopes leading to multiple sliding events, if left unchecked. The risk such an occurrence at this
site is minimal, considering the location of the site being away from features such as the ocean or high flow
natural drainage line that could cause toe erosion.
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 extensions. Regulators (Noosa Shire Council) normally require that a Very
Low or Low Risk of landslide affecting the property must be demonstrated to enable development approval.
Summarised in Table 2 is our qualitative assessment of landslide risk for the development site. A summary
of qualitative terminology for use in assessing risk to property is attached (taken from AGS 2007).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
Given the position of the site near a dune crest, and
moderate natural gradients (up to 20˚ to 22˚) on the
downslope side of the residence, relatively limited
depth of very loose/loose material (about 1.3 m to
1: Shallow landslide through
1.6 m BGL), and depth of footings recommended, it
very loose to loose natural soils Unlikely Medium Low
is considered Unlikely that a landslip would develop
above the foundation depth
in these materials, subject to implementation of the
recommendations in Section 4. Given a potential
Medium impact from such instability the resultant
risk would be Low as per AGS 2007.
The likelihood of a deep landslide through the medium
dense (or denser) natural sand below the foundation
depth recommended is assessed as Rare due to the
2: Deep landslide through the
moderate overall ground slopes, strength of the
natural medium dense (or
Rare Major Low materials, lack of groundwater, and no evidence of
denser) sand below the
similar large scale previous instability slope instability
foundation depth
on the site. Although the consequence of such a
failure would be Major, the resultant risk is Low as per
AGS 2007.
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.
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4.0 RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.
4.1 Site Layout and Development Limitations
The plan extents of the proposed extensions shown in Text Figure 2 above are considered suitable from a
slope stability viewpoint. Should any revisions be made to the proposed extent of the extensions, Tectonic
must review these prior to construction to assess if there are any impacts on slope stability.
It is anticipated that cut earthworks for the proposed extensions will be in the order of about 2 m depth. To
maintain slope stability filling must not exceed 0.5 m in height above natural ground levels on the northern and
eastern sides (i.e. downslope) of the residence, and up to 1 m depth on the western and southern sides. It is
noted that a retaining wall is shown in the architectural design drawings (Section A) that is 1.5 m high with a
floor slab adjacent. Due to the limits on fill heights, the structure will need to be designed as a suspended slab
with void space behind the wall.
The existing distressed retaining walls (to the northern and southern sides of the residence), should be
removed and replaced with new engineer designed retaining walls.
4.2 Earthworks
4.2.1 Site Preparation and Trafficability
In construction areas, any vegetation, tree stumps and large roots should be stripped and ‘grubbed out’. Any
organic rich topsoil should also be stripped and removed from site. Organic topsoils and garden soils must
not be re-used as structural fill; however, small volumes may be retained on site for later use across
landscaping areas, if required.
The natural subgrade following stripping will likely be in a loose condition. Where slab on ground
construction is proposed (i.e. carport) the ground surface should be compacted by several passes of a
vibrating plate compactor or small smooth drum roller (or similar), in conjunction with surface application of
ample water.
Due to the presence of loose sands trafficability will be poor for wheeled vehicles/plant. Placement of a
minimum 150 mm thick granular working platform (roadbase/rock fill) may be required at access locations
and across laydown areas.
Care must be taken with the use of vibratory equipment to ensure that there are no adverse impacts on
surrounding structures. The impacts from ground vibration must be closely monitored and managed during
construction.
4.2.2 Filling
As noted above, fill heights must not exceed 0.5 m to 1 m above existing ground level. The existing sandy
soils (excluding topsoil) 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 vibrating plate). Care must be taken with the use of vibratory equipment to ensure that there
are no adverse impacts on surrounding structures. The impacts from ground vibration must be closely
monitored and managed during construction.
Should importation of fill be required, it is recommended that such fill comprise a clean sand (<10% fines).
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 200 mm thick (loose thickness) layers and be
compacted by repeated rolling using a roller and/or repeated passes with a vibrating plate compactor, with
vibration on if safe to do so to achieve a Density Index of at least 70%.
Confirmatory compaction testing should 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
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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.
4.2.3 Excavations
Excavations through the existing soils should be achievable using conventional earthmoving plant (e.g. small
to medium excavators in the 4 tonne to 12 tonne range). Unsurcharged temporary cuts in the very loose to
loose sands could be formed at 1V:2H, for batters up to 2 m in height. If groundwater seepage is encountered,
flatter batters may be required. Deeper cuts, if required, should be specifically assessed by Tectonic. All
permanent cut/fill batters must be supported by engineer designed retaining walls.
If the above temporary batter slopes cannot be achieved, due to spatial restraints, temporary shoring will be
required. Excavations in close proximity to existing footings will need to be specifically designed to prevent
undermining. It is recommended that footing exposures or confirmation of foundations via certified building
plans be undertaken to assess the possibility of undermining. If undermining is a potential issue the existing
structures may need to be underpinned or the excavation may require temporary shoring such as sheet
piles.
The advice of a suitably qualified and experienced RPEQ structural engineer must be sought regarding any
potential undermining and design and construction of any underpins/temporary shoring (if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).
4.3 Footings
Due to the very loose to loose density of the upper sands, moderate slope of the site and slope stability
constraints, the site is classified Class P in accordance with AS2870-2011. However, the natural soil
profile at the site would exhibit the characteristics of a Class S site with an approximate characteristic
surface movement (ys) of up to 20 mm.
No design loadings were available at the time of reporting, however it is noted that a suspended first floor
slab is proposed, and in some areas structure loads may be relatively high. Footings for the proposed
extensions and carport (including retaining walls and pool) must penetrate through any fill and unsuitable
soils to be founded at least 1 m into medium dense (or denser) sands and be founded a minimum of 3 m
BGL (existing ground levels). Steel screw piles would be a suitable footing option for this site.
It was noted that the eastern side of the residence appeared to have settled previously with cracking and
vertical separation noted in the blockwork. Based on this evidence it is recommended that the existing
footings should not be re-used for additional loads imposed by the extensions. Furthermore, it is also
recommended that the eastern edge of the residence be underpinned using screw piles founded at least 1 m
into medium dense (or denser) natural sand. Any new internal footings within the existing structure footprint
(i.e. for new columns, etc.), will need to extend below the existing concrete slab to be founded at least 0.5 m
into medium dense (or denser) sands at about 2 m depth. Screw piles so founded may be designed using an
allowable end bearing pressure of 300 kPa.
Due to the presence of loose sands, together with the close proximity of adjacent structures, open bored
piles or driven piles are not considered to be suitable for this site. 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.
All footings should be founded such that they are not adversely affected by any adjacent excavations, batter
slopes, trenches, retaining walls, or in ground structures (e.g. swimming pool) that are not designed to
support building loads. Footings should be founded at least below a plane extending 1 m horizontally from
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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 required to prevent undermining
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.
4.4 Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.3 above. 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.
The parameters outlined in Table 3 may be adopted for retaining wall design. The values in Table 3 assume
that the walls are fully drained and no allowance is included for hydrostatic pressures, construction loading,
up-slope footing loadings, the effects of sloping ground and backfill compaction stresses. Any temporary
shoring required to support excavations during construction may also be designed using the parameters in
Table 3.
Table 1: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle Ø
Ka Ko
(kN/m3) (°) Kp
(Cantilever wall) (Non-yielding wall)
Very Loose/ Loose
17 30 0.33 0.50 3.00
Sands
Medium Dense (or
18 34 0.28 0.44 3.54
Denser) Sands
Future Sand Fill(Note 1) 17 32 0.31 0.47 3.25
Note:
1) For sand fill placed and compacted as recommended in Section 4.2 above.
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4.5 Drainage
Surface diversion drainage (e.g. grassed or landscaped swales or diversion mounds) should be constructed
on the upslope (western) side of the residence to intercept any surface runoff and direct it away from the
residence. Adequate site drainage must be installed to ensure that stormwater runoff is directed away from
house 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 retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).
All collected surface water, roof water, and overflow from the any tanks/pools should be directed by pipes or
surface lined channels (with erosion protection) to the Council stormwater system or to the north-eastern
corner of the property. A rock fill weir must be constructed at the drainage discharge point.
Subsurface disposal of stormwater (i.e. infiltration pits) must not be undertaken.
5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed extensions from slope instability subject to
implementation of the recommendations given in this report.
6.0 LIMITATIONS
Your attention is drawn to the document Limitations, which is attached to this letter report.
Please contact the undersigned should you wish to discuss any of the above matters.
Yours faithfully
TECTONIC GEOTECHNICAL PTY LTD
Scott Davis BEng CPEng RPEQ 16439 Ashley Davey BEng (Geological) RPEQ 8159
Senior Geotechnical Engineer Principal Geotechnical Engineer
Attachments: Borehole Reports and Explanatory Notes
Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 20248
Client: Rebecca Stead Commenced: 19/08/2020
Project Name: Proposed Extensions Completed: 19/08/2020
Hole Location: 26 Attunga Heights, Noosa Heads Logged By: MCC
Hole Position: 509180.0 m E 7080869.0 m N MGA94 Zone 56 Checked By: ACD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 70 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Soil Description 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
roots to 0.1m depth
VL
0.5
D
Not Encountered
1.0
L
HA
TECTONIC 2.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 20248.GPJ <<DrawingFile>> 20/08/2020 12:57 10.01.00.01 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 2.00.2 2016-10-13 Prj: Tectonic 2.00 2016-02-16
1.5
SP SAND, fine to medium grained, pale grey to white
M Augered to 2.0m
2.0 MD
Hole Terminated at 2.40 m
2.5 Collapsing sands
3.0
3.5
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit No resistance Level (Date) U - Undisturbed Sample D - Dry VS - Very Soft
RR - Rock Roller ranging to D - Disturbed Sample M - Moist S - Soft
RD - Rotary Drilling refusal Inflow SPT - Standard Penetration Test W - Wet F - Firm
NMLC -Rock Core Partial Loss VSt - Very Stiff
HA - Hand Auger Complete Loss H - Hard
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
Classification System
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 14
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 20248
Client: Rebecca Stead Commenced: 19/08/2020
Project Name: Proposed Extensions Completed: 19/08/2020
Hole Location: 26 Attunga Heights, Noosa Heads Logged By: MCC
Hole Position: 509165.0 m E 7080870.0 m N MGA94 Zone 56 Checked By: ACD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 70 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Soil Description Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND (TOPSOIL), fine to medium grained,
dark grey, with roots
VL
SM Silty SAND, fine to medium grained, grey
0.5
L
1.0
Not Encountered
TECTONIC 2.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 20248.GPJ <<DrawingFile>> 20/08/2020 12:57 10.01.00.01 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 2.00.2 2016-10-13 Prj: Tectonic 2.00 2016-02-16
SP SAND, fine to medium grained, pale grey to white
Augered to 1.5m
1.5 M
HA
2.0
MD
2.5
Augered to 2.7m
3.0
Hole Terminated at 3.00 m
Target depth
3.5
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit No resistance Level (Date) U - Undisturbed Sample D - Dry VS - Very Soft
RR - Rock Roller ranging to D - Disturbed Sample M - Moist S - Soft
RD - Rotary Drilling refusal Inflow SPT - Standard Penetration Test W - Wet F - Firm
NMLC -Rock Core Partial Loss VSt - Very Stiff
HA - Hand Auger Complete Loss H - Hard
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
Classification System
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 15
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RD Rotary Blade or Drag bit NQ Diamond Core - 47 mm
*V V-Bit RT Rotary Tri-cone bit NMLC Diamond Core - 52 mm
*T TC-Bit RA Rotary Air HQ Diamond Core - 63 mm
HA Hand Auger HMLC Diamond Core – 63mm
ADH Hollow Auger BH Tractor Mounted Backhoe
HA Hand Auger EX Tracked Hydraulic Excavator
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
5,4,10 N=14 5,4,10 = Blows per 150mm. N = Blows per 300mm penetration following 150mm seating
30/65mm Where practical refusal occurs, the blows and penetration for that interval are reported
RW Penetration occurred under the rod weight only
HW Penetration occurred under the hammer and rod weight only
HB Hammer double bouncing on anvil
DS Disturbed sample
BDS Bulk disturbed sample
FV Field vane shear test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev0
October 2013
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 16
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
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 17
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
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 18
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
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Page 19
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
Document No.
Set ID:68029 RL2 August 2010
23080488
Version: 1, Version Date: 27/06/2024
Page 20
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Document Set No.
Form ID: 23080488
68029 RL2 August 2010
Version: 1, Version Date: 27/06/2024
Page 21
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)
Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024
Used in these records
Reference Document - Geotechnical Investigation Report - DBW24/0090 & RAP24/0025Related documentApplication case desk ↗
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