Council archive · source document
slope stability · MCU22/0172
13 September 2022 Project No. 22339-001-Rev0 Integra Partnerships Attention: Alex Smith Email: [email protected] GEOTECHNCIAL INVESTIGATION AND SLOPE STABILITY ASSESSMENT PROPOSED RESIDENCES 22 GRANT STREET, NOOSA HEADS Dear Alex, 1.0 INTRODUCTION As requested, Tectonic has undertaken a geotechnical investigation and slope stability assessment for proposed residences at 22 Grant Street, Noosa. This report presents the results of our 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 of slope instability affecting the proposed development 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 90 on RP92518, covers an area of about 594 m2, and has frontage to Grant Street to the south-west. At the time of the assessment, the existing single-storey residence was yet to be demolished. Similar residential allotments with houses surrounded the site in all directions. A 2022 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. Subject Site Text Figure 1: Aerial image of site and surrounds from 2022 (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 supplied to Tectonic (Blackwood Architecture and Design, Project No. 202307, Dwg
No’s DA1.01 to DA6.10, dated 8 August 2022) indicate that the proposed residential development will
comprise the following:
Demolition of the existing residence.
Earthworks (majority cut) to provide terraced level building pads.
Construction of a terraced three-storey duplex including a swimming pool.
Construction is likely to comprise a combination of lightweight materials (e.g. timber, sheet metal roof etc.),
rendered brick/masonry blockwork (or similar) and suspended concrete slabs. No earthworks details were
available at the time of reporting; however, provided drawings indicate that excavations up to 2.5 m depth
will be required, and minimal fill earthworks (less than 1.0 m height).
The property has existing connections to town water and sewerage. A site layout plan and an elevation
view of the proposed duplex development are shown in Text Figure 2 below.
Text Figure 2: Proposed site layout (upper) and an elevation view of the proposed residences (lower)
1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of published
geological maps, slope stability studies and available geotechnical data. A walk-over survey was conducted
in order to make an appraisal of the general condition of the site in regard to topography, drainage,
vegetation cover, geology, erosion, and condition of existing structures.
The walk-over survey was carried out by an experienced geotechnical engineer from Tectonic on
29 August 2022. The walk-over inspection was combined with a geotechnical investigation comprising
drilling of two boreholes (BH1 and BH2) around the existing residence to a depth of 3.0 m below ground level
(BGL). The approximate locations of the boreholes are shown in Figure 1 attached to this report. In situ
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testing, using the dynamic cone penetrometer (DCP) method, was carried out from the ground surface and
extending from the borehole termination depth to depths of 4.9 m to 5.5 m BGL.
The borehole logs and explanatory notes are attached to this report. The results of the desk-top study, walk-
over survey and borehole drilling are included in Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been reviewed by Mr Ashley Davey, an RPEQ with over 27 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 information1 indicates that the site is located at a contact between Anthropocene age
man-mad deposits (“Qhh”) comprising “gravel, sand and silt” and Pleistocene age parabolic dunes (“Qpd/2”)
comprising “quartz sand” (Ref. Text Figure 3).
Approx. site location
Text Figure 3: Extract from Laguna Bay Geology Map
The boreholes encountered relatively consistent subsurface conditions, summarised as follows:
Natural Sands – initially comprising very loose to loose silty sand/sand to depths between 1.5 m
and 2.2 m BGL, then becoming medium dense to the borehole termination depths (3 m BGL).
Results of the DCP testing completed below the borehole termination depths indicated inferred dense
sands at depths of 3.4 m BGL, then extending to the DCP termination depths of 4.9 m to 5.5 m BGL.
Groundwater was not encountered during drilling of the boreholes, with the soils being dry to moist.
1
1:100,000 Laguna Bay Surface Geology, Sheet 9545, Compilation Map, DNRME, 2018
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2.2 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 4, the site is located on the south-
westerly facing flank of an ancient sand dune, which crests about 300 m to the east.
The natural surface profile at the site had been interrupted by historic landscaping and residential
construction. Based on a visual inspection it appeared that cut and fill earthworks in the order of about 1.5 m
depth/height had been previously undertaken, most of which were supported by retaining walls or structural
residential walls. The existing residence was a single storey building terraced into the natural slope.
The natural ground slope was measured to be about 9° at the rear of the property. The slope from the front
of the existing residence to the road was measured at the time of our assessment to be about 15°. A brick
retaining wall up to 0.7 m height was located immediately upslope of the footpath and a blockwork retaining
wall up to 1.3 m height was observed in the courtyard just behind the existing building that extended across
the width of the property. To the west of the property on Grant Street, splitting the north and southbound
lanes, a grouted rock retaining wall was observed, approximately 3 m height and sloped at 1 in 1.
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 to 8 below and on the following
page.
Text Figure 5: Panoramic view west across the existing residence
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Text Figure 6: General condition of existing residence, to rear and front
Text Figure 7: View east towards the existing residence
Text Figure 8: Looking north-west along Grant Street, showing the grouted rock retaining wall
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2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
encountered in the boreholes, with soils being moist.
2.4 Surface Drainage
Most of the stormwater catchment (to the north-east of the property) is anticipated to be intercepted by the
existing infrastructure along Katharina Street and existing grated stormwater inlet pits within the upslope unit
complex driveway. Given the sandy nature of the site and surrounds, rainfall would normally be anticipated
to infiltrate the ground surface. No evidence of significant erosion was observed across the subject site.
2.5 Vegetation
Landscaping was present around the perimeter of the residence, comprising shrubbery and several mature
trees. The mature trees were noted to be relatively straight, exhibiting no obvious signs of distortion which
could be indicative of global slope instability.
2.6 Buildings and Other Structures
The existing residence was a single storey, cut in, slab on ground, brick veneer structure. Terracing by
previous earthworks had occurred during the original construction, supported by masonry blockwork (1.3 m
height) and brick retaining walls (0.7 m height). Landscaping timber sleeper walls approx. 0.6 m in height
were noted beside the existing house midway along the southern boundary.
Based on an external assessment, visible areas of the existing structures and retaining walls appeared to be
in good structural condition and did not exhibit any signs of distress (e.g. cracking, misalignment, etc.) which
could be attributed to slope instability. ‘Hairline’ cracking was observed within the concrete driveway and
within the asphalt along Grant Street.
The timber fences surrounding the property were observed to be slightly ‘wavy’, however this could be
attributed to founding within the very loose to loose sands encountered in the boreholes. The grouted rock
retaining wall on Grant Street (Text Figure 8 above) was observed to be relatively straight and in generally
good condition.
3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The Noosa Council (NC) Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in
Text Figure 9), which indicates that parts of the site (including the proposed development area) are within a
Moderate (pale brown shading) landslide hazard zone.
Subject Site
Text Figure 9: Extract from NC landslide hazard overlay mapping
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It should be noted that the NC landslide hazard map is a broad-scale indication of the potential landslide
susceptibility and is superseded by this site specific assessment. No visual signs of global slope
instability were observed in the location of the proposed residences. No landslide back scarps,
groundwater springs, 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 in Appendix B.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of
VH Very High Risk treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost
more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
H High Risk
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 investigation, planning and
M Moderate Risk implementation of treatment options to reduce 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
L Low Risk
maintenance required.
VL Very Low Risk Acceptable. Manage by normal slope maintenance procedures.
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ for slope stability where
the surface gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands. Our
measurements indicate that gradients ranged up to about 15°. The limit of equilibrium is the ‘tipping point’ to
instability, where the factor of safety (FOS) of the slope is approximately 1.0. The FOS of a slope is the
balance (ratio) between the forces driving instability (e.g. soil weight, groundwater pressure) and the resisting
forces (e.g. soil friction and cohesion).
Sand dune slopes generally remain stable (i.e. FOS greater than 1.0) when they are:
well vegetated (mix of ground cover, shrubs and small trees);
not subjected to concentrated stormwater flows;
not surcharged with additional loads (e.g. fill); and
not undercut by excavation or erosion.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation, and development of generally localised instability.
The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion. At this site, the dune around and below the development area is
covered by existing structures, retaining walls and a road (Grant Street); and therefore water erosion or
significant disturbance of the dune by natural forces, in the vicinity of the site, is considered extremely unlikely.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of global slope instability
affecting the proposed development. Regulators (NC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in
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Table 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 low to moderate natural gradients through
the proposed building area it is considered Unlikely
1: Failure through very loose to that a landslip would develop in these materials,
loose natural sands above the Unlikely Medium Low subject to implementation of the recommendations in
foundation depth Section 4. It is assessed that there could be a
potential Medium impact from such instability so the
resultant risk would be Low as per AGS 2007.
The likelihood of a deep failure through the medium
dense (or denser) natural sand below the proposed
foundation depth is assessed as Rare due to the lack of
2: Deep failure through the
precedence of such movements locally, no signs of any
medium dense (or denser) sands Rare Major Low
existing slope instability and apparent lack of
below the foundation depth
groundwater. Although the consequence of such a
failure could 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.
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
Should any revisions be made to the extent of the proposed residences described in Section 1.1, Tectonic
must review these prior to construction to assess if there are any impacts on slope stability.
No earthworks details were available at the time of reporting; however, provided drawings indicate that
excavations up to 2.5 m depth and fill earthworks less than 1.0 m height will be required. Should proposed
cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability before construction.
4.2 Earthworks
4.2.1 Site Preparation and Trafficability
In construction areas, any relict construction elements, vegetation, tree stumps and large roots should be
stripped and ‘grubbed out’. Any organic rich topsoil should also be stripped and removed from site, based
on the boreholes, depths should generally range up to about 0.2 m depth. 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, the 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 to ensure that there are no adverse impacts on
surrounding structures and underground services (e.g. existing NC sewer infrastructure along the front
boundary). The impacts from ground vibration must be closely monitored and managed during construction.
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It would be prudent to conduct a dilapidation survey of neighbouring structures prior to commencement of
construction.
4.2.2 Filling
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 minimum 4 tonne vibrating smooth drum roller or vibrating plate) to achieve required
compaction.
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. Select fill should have a maximum particle size of 100 mm for an uncompacted layer
thickness of 200 mm and shall be compacted by repeated rolling with a small compactor to achieve a dry
density ratio of at least 95% of the Standard Maximum Dry Density for cohesive soils, or 70% Dry Density
Index for any imported cohesionless soils.
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
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 fill and natural soils encountered in the boreholes should be achievable using
conventional earthmoving plant (e.g. small excavators). Non-surcharged temporary cuts in the very loose to
loose sands could be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to
3 m in height. Although unlikely, if groundwater seepage is encountered or if sloughing of dry sand occurs at
the batter surface, flatter batters or temporary retention may be required. Deeper cuts, if required, should be
specifically assessed by Tectonic.
Permanent batters up to 1 m high (vertical height) may be battered no steeper than 1V:2.5H. All remaining
permanent cut/fill batters must be supported by engineer designed retaining walls. Any permanent soil and fill
batters will require erosion protection (e.g. revegetation or surface protection).
If the above temporary batter slopes cannot be achieved, due to spatial restraints, temporary shoring would
be required (e.g. sheet piles, contiguous pile wall etc.). Excavations in close proximity to existing structures
and infrastructure, 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 structural RPEQ 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).
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4.3 Site Classification
Due to the insufficient bearing capacity of the very loose to loose sand and probability of ‘uncontrolled’ fill
associated with the demolished residence, the site is classified as Class P in accordance with AS2870-
2011 Residential Slabs and Footings. However, it is assessed that the existing soil profile would exhibit
the characteristics of Class S, with the estimated characteristic surface movements (‘ys’) ranging up to
about 20 mm. Due to the potential variability in subgrade density, and allowing for some settlement of the
loose sands, a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footing and plumbing design).
The use of reactive fill materials (clays) can adversely affect the characteristic ground surface movement,
and must be considered during design if fill material other than sand is proposed to be used on the site.
4.4 Footings
No design loadings were available at the time of reporting. High level footings should not be utilised due to
insufficient bearing capacity and the potential for excessive settlements in the loose upper sands
encountered in the boreholes. Footings for the proposed residence and ancillary structures (e.g. retaining
walls etc.) must penetrate through any fill materials and unsuitable natural soils (i.e. very loose to loose
sands) to found at least 0.5 m into medium dense (or denser) sands which were encountered at depths
between 1.5 m and 2.2 m BGL in the boreholes. A minimum founding depth of 3 m BGL (existing ground
levels) is recommended.
In areas of cut exposing medium dense (or denser) sands at the subgrade level high-level footings may be
adopted. Elsewhere, piles would be required. It should be noted that excavations for concrete strip/pad
footings (or similar) may loosen the base. Therefore, the use of a vibrating plate would be required to
densify any high-level footing bases prior to the placement of reinforcing steel. High level footings, where
achievable in deeper cut areas, may be designed using an allowable bearing pressure of 100 kPa provided
that they are founded in the natural medium dense (or denser) sands. Settlement under such loads is
expected to be in the order of up to 20 mm or 1% of footing width.
Given the sandy soil profile and depth of footings required below existing ground levels, steel screw piles
could be adopted outside of deeper cut areas. Due to the presence of loose sands, together with the close
proximity to surrounding infrastructure, open bored piers or driven piles are not considered to be suitable for
this site. Steel reinforced concrete bored piles could be feasible; however, the use of sacrificial liners (e.g.
‘formatube’) would be required to prevent the caving of upper loose sands.
Steel reinforced concrete bored piles founded as noted above may be designed using an allowable end
bearing pressure of 300 kPa. Parameters for screw pile design are given in Table 3 below. Settlements
under such pressures should be no more than about 25 mm.
Table 3: Screw Pile Design Parameters
Ultimate Allowable
Bearing
Unit weight Friction angle, Bearing Bearing
Founding Material Capacity
(kN/m3) Ø (°) Capacity fbult Capacity fball
Factor, Nq
(kPa) (kPa)
Medium dense natural
18 32 25 1200 400 (Note 1)
sand
Dense to very dense (or
19 36 35 1800 600
denser) natural sand
Notes:
1) Allowable pressure based on a working stress factor of safety value of 3 on the ultimate pressure. If a limit state
design methodology is adopted the ultimate bearing capacity may be used and must be multiplied by a
geotechnical reduction factor Øg determined in accordance with AS2159-2009 to obtain the Design
Geotechnical Strength (Rd,g).
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The advice of a specialist piling contractor must be sought regarding feasibility, design and installation of the
piling system for this site, including an assessment of potential vibration effects on nearby structures, and
monitoring requirements.
If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling may
not encounter sufficient drilling resistance (from torque measurement) in sands at the suggested founding
depth, and piles could extend to much greater depth than necessary.
It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
deflections under tension and compression cycles) should also be undertaken prior to construction.
Kentledge for static testing, or simulated cyclic loads, should prove the piles to a minimum of two times the
expected working load within tolerable deflections.
All footings should be founded such that they are not adversely affected by or affect any adjacent
excavations, trenches, retaining walls, or in ground structures that are not designed to support building loads
(e.g. sewer along front boundary). Footings should be founded 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 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.
Should bored piles be adopted, it is essential that the piling contractor have a ‘mud bucket’ or similar to
enable clean out of the pile bases. It is recommended that bored pile sockets 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.
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4.5 Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.4 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 4 may be adopted for retaining wall design. The values in Table 4 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 4.
Table 4: 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 28 0.36 0.53 2.77
Sands
Medium Dense (or
18 32 0.31 0.47 3.25
Denser) Sands
Future Fill * * * * *
Note: * depends on fill material type and level of compaction
4.6 Drainage
Given the sandy nature of the site, rainfall is generally expected to rapidly infiltrate any unsealed surfaces. If
necessary, diversion drainage should be constructed to divert seepage and/or runoff away from the building
pad areas and batter slopes to prevent saturation and erosion.
Permanent site drainage must be installed to ensure that stormwater runoff is directed away from proposed
building walls, footings and retaining walls (e.g. positive gradients and/or channel drains). Future building
and landscaping must be carried out to maintain good drainage. Surface diversion drainage should be
constructed above the crest of any new cut or fill embankments (e.g. grassed or landscaped swales or
diversion mounds).
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, pool etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Grant Street.
Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.
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5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed residential duplex 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
Jarrad McVey BEng (Civil) MIEAust Ashley Davey BEng (Geological) RPEQ 8159
Geotechnical Engineer Principal Geotechnical Engineer
Attachments: Figure 1 – Site Plan
Borehole Reports and Explanatory Notes
Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Office: 07 5478 9016
Email: [email protected]
PROJECT:
PROPOSED RESIDENCES
22 GRANT STREET
NOOSA HEADS
BH2
TITLE:
BH1 SITE PLAN
CLIENT:
INTEGRA PARTNERSHIPS
DRAWN: DATE:
JPM 12/09/2022
CHECKED: DATE:
ACD 13/09/2022
SCALE:
AS SHOWN
LEGEND:
Approx. Borehole Locations
Notes: PROJECT NO.: FIGURE: REV:
Aerial imagery courtesy of Nearmap (14 August 2022) 0 5 10 15 20 m
22339-001 1 0
Page 15
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 22339
Client: Integra Partnerships Commenced: 29/8/2022
Project Name: Proposed Residence Completed: 29/8/2022
Hole Location: 22 Grant Street, Noosa Logged By: LM
Hole Position: 508818.0 m E 7080680.0 m N MGA2020 Zone 56 Checked By: ACD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: WD
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 to medium grained, dark grey, trace HW
of silt HW
becoming grey and dark grey
DS
VL to
0.6-0.8m
L
1
Not Encountered
without silt, pale grey
M
HA
DS
1.5-1.8m
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22339.GPJ <<DrawingFile>> 9/9/2022 11:30 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
Augered to 2.9m
3
Hole Terminated at 3.00 m
Target depth
4
5
15/90mm
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 16
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 22339
Client: Integra Partnerships Commenced: 29/8/2022
Project Name: Proposed Residence Completed: 29/8/2022
Hole Location: 22 Grant Street, Noosa Logged By: LM
Hole Position: 508839.0 m E 7080686.0 m N MGA2020 Zone 56 Checked By: ACD
Drill Model and Mounting: Hand Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: WD
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND, fine to medium grained, dark grey
VL
becoming grey to pale grey
1
Not Encountered
SP SAND, fine to medium grained, pale grey
M
HA
L
Augered to 2.0m
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22339.GPJ <<DrawingFile>> 9/9/2022 11:30 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
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 17
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 18
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 19
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 20
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 21
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 22
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 23
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)
Used in these records
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