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220328 - Edwards Street - Slope Stability Assessment
25 March 2022 Project No. 22073-001-Rev0 Bev Jenner Const. C/- Westera Partners Attention: Adam Ide Email: [email protected] SLOPE STABILITY ASSESSMENT PROPOSED DUAL OCCUPANCY DEVELOPMENT 168 EDWARDS STREET, SUNSHINE BEACH Dear Adam, 1.0 INTRODUCTION As requested, Tectonic has undertaken a geotechnical investigation for the proposed dual occupancy development at 168 Edwards Street, Sunshine Beach. 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 development 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 35 on RP48113, covers an area of about 607 m2, and has frontage to Edwards Street along the south-western boundary. Edwards Street runs parallel with David Low Way, which was present just below Edwards Street, with an earthworks cut between. The site was vacant at the time of our assessment, with similar sized developed residential blocks surrounding otherwise. A 2019 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. Lot boundary Text Figure 1: Aerial image of site and surrounds from 2019 (courtesy of Noosa Council ‘Intramaps’) 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 us (Gockel Architects, Ref. Dwg. No. 100.01 to 400.02, Rev A, dated 1
February 2021) indicate that the proposed development would consist of an offset, two storey dual
occupancy building, with a semi-basement, daylightling to the site frontage and being completely in ground
to the rear (north).
The basement would consist of a concrete ground slab and walls, with a suspended concrete slab then
forming the base level for the initial habitable floor above. Lightweight cladding would be used on the exterior
to the two upper storeys, with relatively lightweight sheetmetal roofing above. A masonry blockwork party
wall is anticipated through the centre of the structure. Some relatively small outstands from the first floor form
small cantilevered sections to the front, with a verandah covered by an extended and suspended upper
storey on each of the properties.
A small swimming pool would be located to the rear boundary of each of the properties within the dual
occupancy, being dug in to the upslope side, and left above ground to the front (facing the residences).
No specific earthworks details were available at the time of reporting; however, based on existing site
conditions and the proposed development it is anticipated that bulk cutting of up to about 3 m depth would be
required to reach the design floor level for the basement. The swimming pool would require cut of about
1.5 m depth. Isolated deeper excavations may be required for the pool and underground services/footings.
Filling is expected to the corners of the block either side of the entrance, of about 1 m to
2 m height, to form landscaped gardens and stairs up to the level of the first habitable floor of the buildings.
The property has existing connections to town water and sewerage. A proposed layout plan is provided in
Text Figure 2 below.
Text Figure 2: Proposed site layout plan and section through northernmost unit (courtesy Gockel Architects)
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, Noosa Council (NC) planning overlays and a site classification ‘soil test’ report by Unearthed
Geotechnical (Unearthed Ref. 022-020, dated February 2022).
An experienced engineering geologist from Tectonic undertook a site walk-over on 24 February 2022 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, vegetation
coverage and existing structures.
The Unearthed ‘soil test’ included three hand auger boreholes to a depth of 2 m below ground level (BGL),
which did not meet with the maximum excavation depth for the semi basement, or deeper information
sufficient to provide advice on footings and stability. Concurrent with the walk-over, Tectonic undertook
additional in situ testing using the dynamic cone penetrometer (DCP) method to depths of 3.9 m at the
maximum excavation depth, and a second test towards the front of the block to correlate the Unearthed
results with.
The results of the desk-top study, walk-over survey and subsurface investigation are included in Section 2
below.
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1.3 Qualifications of Responsible Engineer
This report has been reviewed by 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 information 1 indicates that the site is underlain by Pleistocene age parabolic dunes
comprising “quartz sand” (Ref. Text Figure 3 below).
Approx. site location
Text Figure 3: Extract from Laguna Bay Surface Geology Map
Materials encountered in the Unearthed boreholes were consistent with the published geology, comprising
an upper layer of silty sand, then relatively ‘clean’ dune sands below. Subsurface conditions are summarised
as follows, with the in situ strength information being inferred from Tectonic’s DCP testing:
Loose Silty Sand to between 0.6 m and 0.7 m BGL; then
Loose to Medium Dense Sand to depths of up to about 2.1 m BGL; overlying
Medium Dense Sand to the maximum depth of the Tectonic investigation (3.9 m BGL).
Groundwater seepage was not encountered during the Unearthed ‘soil test’ fieldwork, with the soils generally
being described as moist.
Unearthed has classified the site as Class S in accordance with AS2870, with a characteristic surface
movement due to normal seasonal moisture variation only (ys), inferred to be ≤20 mm.
1
1:100,000 Laguna Bay Surface Geology Map, Sheet 9545, Dept. of Natural Resources, Mines and Energy, 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, on a wider scale, the site was located within an undulating dune area, around
650 m inland of Sunshine Beach (present to the east). The site was positioned on a south-westward facing
lee side dune face. The dune crested on a rounded knob on Weyba Street to the east, sloping down to
Heathland Drive in the west. Edwards Street and David Low Way were both formed sidelong to the
predominant slope.
An overall ground slope of about 8° to 9° was present towards the site frontage with Edwards Street, which
was inferred as the natural slope. The natural surface profile had however been interrupted by historic
landscaping features, with a timber retaining wall around 1 m high along the rear boundary (north-east); and
a dug out area to the southern corner. The sideslopes of the dug out area were in the order of about 25°,
with the base being around 5°.
Localised knob
Subject site
Sunshine Beach
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 below.
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Text Figure 5: View looking south over site from upslope area
Text Figure 6: Looking north-east from frontage, showing dug out area to RHS of photo
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
recorded during ‘soil test’ fieldwork by Unearthed.
2.4 Surface Drainage
Surface drainage would be expected to follow the contours to the south-west. Given the sandy nature of the
site and surrounds, rainfall would normally be anticipated to infiltrate the ground surface. The existing
retaining wall on the upslope side of the block, and the planned pool may help intercept flows from above,
however a potentially large upslope catchment does exist at this site. That being said, no evidence of
significant erosion was observed across the subject site.
As the proposed dual occupancy buildings will occupy a large part of the block, significant run-off from hard
surfaces is anticipated to occur. Nevertheless, where directed into swales rather than pipes, such run-off
would be expected to infiltrate into the natural sands relatively rapidly.
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2.5 Vegetation
The majority of the site featured periodically slashed grass, with small to medium size trees and shrubs to
the peripheries. Some palms at the crest of the 25° slope above the dug out area to the southern corner
were noted to be tilting slightly.
2.6 Buildings and Other Structures
The subject site was vacant at the time of our walk-over. As mentioned above, a timber retaining wall
ranging to about 1 m height was present along the north-eastern boundary of the lot, retaining landscaped
garden areas on the adjacent lot above.
Towards the southern corner of the property a wall of a mixture of masonry blockwork and concrete was
present, ranging to about 1 m height, with a sheetmetal fence atop. On the neighbouring property to the
south (166 Edwards Street), a swimming pool was present above this wall; with a shed on around 1 m of fill
to the street frontage. A dual occupancy development also existed on 166 Edwards Street, with the buildings
being of one storey, with rendered exterior walls. A view of these features is shown below in Text Figure 7.
Text Figure 7: View of blockwork and concrete wall to southern corner, and view along adjoining southern boundary
To the north, on 170 Edwards Street, a two storey residence was present, being of lightweight clad walls and
sheetmetal roofing. Part of the downslope front of the building had been cut in to the natural ground, with the
rest being suspended on relatively short steel posts.
Based on an external visual assessment, retaining walls along the site boundaries and buildings on adjacent
blocks appeared to be in good structural condition and did not exhibit any signs of distress (e.g. cracking,
misalignment, etc.) which could easily be attributed to global slope instability.
Broken asphalt surfacing, presumed to be a former site access, extended form the site frontage around one
third of the way in to the site.
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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 8),
which indicates that isolated parts of the site, including parts of the proposed development areas, are within
a Moderate (pale yellow-brown shading) landslide hazard area.
Lot boundary
Text Figure 8: Extract from NC landslide hazard overlay mapping
It should be noted that the Noosa Council (NC) landslide hazard map is a broad-scale indication of the
potential landslide susceptibility and is superseded by site specific investigation and assessment (this
report). No visual signs of global slope instability were not observed across the subject site. Tilting palms
to the southern corner of the block were likely as a result of them being planted at the crest of a sharp
break in slope (from about 9° to 25°). No landslide back scarps or areas of naturally ‘hummocky’ ground
were observed.
Furthermore, it is inferred that the isolated Moderate hazard areas within the NC overlay are more likely
because of localised steepenings as a result of landscaping features (i.e. the dug out area to the southern
corner of the subject block, and sloping, retained garden beds on 166 Edwards Street).
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 overleaf and
the Qualitative Terminology for Use in Assessing Risk to Property is also attached.
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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 to
L Low Risk
this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
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 was generally in the order of 8˚ to 9˚. 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 supported surface 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 (or intermediate
steepenings over the face) 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, particularly considering the moderate ground
surface gradients and the protection afforded by the road infrastructure downslope of the site.
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 deples. Regulators (NC) normally require that a Very Low or Low Risk of landslide
affecting the property must be demonstrated to enable development approval. Summarised in Table 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).
This slope stability assessment excludes geotechnical risk that may be associated with settlement due to
consolidation of loose sands over time which could affect non-piled structures (e.g. pavements, driveways,
landscaping structures).
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Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence Risk Comments
Given the moderate natural gradients through the
proposed building area (≤9° whereas limit of
equilibrium for stability for loose sands typically
≥30°), lack of groundwater, it is considered Unlikely
1: Failure through loose soils
Unlikely Medium Low that a landslip would develop in these materials,
above the foundation depth.
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 failure through the medium
dense (or denser) natural sands below the proposed
2: Deep failure through the foundation depth is assessed as Rare due to the
medium dense (or denser) strength of the materials, lack of precedence of such
Rare Major Low
natural soils below the movements locally and no signs of any existing deep
foundation depth. seated slope instability. Although the consequence of
such a failure would be Major, the resultant risk would
be 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
The planned extents of the proposed development shown in Text Figure 2 are acceptable from a slope
stability viewpoint. Should any revisions be made to the proposed extent of the development, Tectonic must
review these prior to construction to assess if there are any impacts on slope stability.
To achieve a Low Risk of instability, the following is recommended:
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
and certified, which must include an assessment of global and internal stability.
Any proposed fill exceeding 1 m must be fully supported by an engineer designed and certified
retaining structure, unless assessed and approved by a suitably qualified and experienced
geotechnical RPEQ.
Deepened, piled footings will be required with design and construction of footings to be undertaken
in accordance with Section 4.3.
No specific earthworks details were available at the time of reporting; however, based on existing site
conditions and the proposed development it is anticipated that bulk cutting of up to about 3 m depth would be
required to reach the design floor level for the basement. The swimming pool would require cut of about
1.5 m depth. Filling is expected to the corners of the block either side of the entrance, of about 1 m to
2 m height, to form landscaped gardens and exterior stairs.
Should proposed cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability
before construction.
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The existing timber retaining wall to the north-eastern boundary should be removed and replaced with a new
structure preferably of more durable materials (e.g. concrete) behind the pool shells and pool terraces. While
only landscaping works appear to the proposed in the southern corner adjoining 166 Edwards Street, an
assessment should be made of the existing masonry blockwork/concrete wall and surcharge from features
above (e.g. the swimming pool) such that a non-worsening condition eventuates from the proposed
construction, and especially that no ‘undermining’ through cut earthworks occurs.
4.2 Earthworks
Care should be taken to ensure construction works do not cause excessive ground vibrations that could
result in damage to existing nearby structures. Should vibratory techniques be proposed (e.g. vibratory fill
compaction, sheet piling, etc.), ground vibrations should be monitored, and construction methods may need
to be altered should adverse impacts be anticipated or encountered. It would be prudent to conduct a
dilapidation survey of nearby structures before commencement of construction.
4.2.1 Site Preparation and Trafficability
Following removal of the existing structures (where required) and associated construction elements (e.g.
footings, services, etc.), any tree stumps and large roots should be ‘grubbed out’. Any organic rich topsoil
and uncontrolled fill should then be stripped and set aside. Any organic affected topsoil materials must not
be used in structural fill; however, may be placed across landscaping areas.
The natural subgrade following stripping will likely be in a loose condition. Where slab on ground
construction is proposed 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.
After initial stripping the subgrade is expected to comprise a mixture of silty sands and relatively ‘clean’
beach ridge sands. Although relatively good trafficability should initially be encountered, these materials
could be subject to strength loss from repetitive vehicle loading (particularly from heavy rubber tyred plant)
and if they become excessively dry. Due to the potential for poor trafficability, placement of a minimum
150 mm thick granular working platform will likely be required, at least across access and laydown areas.
Groundwater seepage was not encountered during the Unearthed ‘soil test’ fieldwork; however, depending
on the prevailing weather conditions at the time of construction, groundwater seepage may develop at
relatively shallow depths.
The site should be graded to shed surface water runoff away from construction areas. Swale drains should
be constructed along the toes of any cuts, and above cuts where possible, with the drains graded to
discharge collected waters to a control point before release off-site. Appropriate erosion and sediment
control measures must be installed prior to commencement of earthworks at this site.
4.2.2 Filling
The existing sandy soils (excluding any topsoil) may be reused as structural fill, subject to appropriate
moisture adjustment. It should be noted that compaction of relatively ‘clean’ sands (expected to form most of
the semi-basement excavation spoil) 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).
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.
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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 existing soils should be achievable using conventional earthmoving plant (e.g. small
excavators) with toothed bucket attachments. It is understood that a cut of up to about 3 m depth would be
required to site the garages proposed beneath the dual occupancy buildings. We would recommend that non
surcharged temporary batters for the excavation do not exceed 1V:1.5H in the loose sands (to about 2 m
depth at the location of the cut); and up to 1V:1H in the medium dense materials below. This assumes that
groundwater, or other seepage, is not encountered and allows for a maximum batter height of 3 m. Although
unlikely, if groundwater seepage is encountered, flatter batters may be required. Deeper cuts, if required,
should be specifically assessed by Tectonic. Permanent cut and fill batters (e.g. for landscaped areas) up to
1 m high (vertical height) may be battered at no steeper than 1V:3H in loose sands or 1V:2H in medium dense
sands. 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 will be
required, as per the recommendations in Section 4.4 below. 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 where close interaction is anticipated (possibly the pool on 166 Edwards Street). If
undermining is a potential issue the existing structures may need to be underpinned or the excavation 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).
4.3 Footings
No design loadings were available at the time of reporting. Footings for the proposed dual occupancy
buildings and ancillary structures (e.g. retaining walls etc.) must penetrate through any fill materials and
unsuitable natural soils (i.e. loose sands) to found at least 0.5 m into medium dense (or denser) sands which
were encountered/inferred at depths below about 2 m BGL (existing ground levels). With the scope of the
basement excavation, it is likely that a mixture of normally dimensioned, conventional high-level footings and
relatively short piles would be utilised for the structure.
Footings founded as described above could be designed with a maximum allowable bearing pressure of
100 kPa (high level), or 300 kPa (maximum end bearing pressure for piles). Settlement under such loads is
expected to be in the order of up to 20 mm or 1 % of footing width.
It should be noted that excavation for high level footings may loosen the foundation subgrade. Therefore,
the use of a vibrating plate would be required to densify any high-level footing bases prior to the placement
of reinforcing steel.
Due to close proximity to the surrounding properties, driven piles are not considered to be suitable for this
site, and the presence of loose sands in the near subsurface would preclude the use of open bored piles
unless the construction methodology can address sloughing (sidewall collapse) of sands by use of liners or
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similar means. Given the sandy soil profile and depths of footings required in some areas, steel screw piles
are considered to be a suitable foundation medium.
If steel screw piles are adopted, it is recommended that site specific static load testing be conducted, 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.
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.
Footing design should also account for potential soil reactivity advised by Unearthed to be equivalent to
Class S.
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. 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 9.
Text Figure 9: 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.
It will be important to ensure close supervision of screw pile installation, as piling will likely not encounter
drilling refusal or ‘high’ torque readings at the recommended founding depth, and piles could extend to much
greater depth than necessary.
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 engineer prior to placement of steel reinforcement and pouring of concrete to
confirm allowable bearing pressures and cleanliness.
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4.4 Temporary and Permanent Retention
Excavations are expected to range up to around 3 m BGL (existing ground levels) and, given that the
planned extent of the excavations extends close to some of the site boundaries, some form of temporary
shoring may be required for part of the excavations.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, subject to vibratory assessment and parameters regarding adjacent properties. Shoring of
deeper cuts would likely require lateral restraint, and this is conventionally achieved by helical steel anchors
(similar to screw piles). However, the proximity to neighbouring properties would require permissions to be
obtained for subterranean access. Therefore, where lateral support by cantilever only is not feasible, it is
suggested that internal struts/propping be considered to provide lateral restraint for the shoring system.
Sheet piles may require pre-drilling to prevent adverse vibrations during sheet pile installation. Other shoring
methods that could be considered include bored contiguous or secant piles. Non-contiguous or soldier pile
walls are not feasible for the proposed development due to the likelihood that loose sands would ‘flow’
through gaps between the piles.
The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation of
the proposed retention system. It will be necessary to conduct numerical analysis to optimise the retention
system. Detailed design will also need to account for relevant factors of safety to maintain the temporary
and long-term stability of the retention system.
Retaining structures greater than 1 m high, or those required to support surcharge loads or sloping backfill,
will require engineer design and certification of construction. Retaining wall design should be carried out in
accordance with AS4678-2002 Earth retaining structures.
Design for temporary and permanent retaining structures could be carried out using a trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type and based on the parameters
listed in Table 3. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.
Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit
Friction
Retained material weight
angle (°) Ka Ko
(t/m3) Kp
(Cantilever wall) (Non-yielding wall)
Loose Upper Sands
1.8 30 0.33 0.5 3.0
(<2 m BGL)
Medium Dense Lower
1.9 33 0.29 0.46 3.39
Sands (>2 m BGL)
Future Controlled Fill* * * * * *
Note: * Depends on fill material type and level of compaction.
As a minimum, retaining structures must be founded as per the recommendations in Section 4.4 above with
the footings also designed to account for lateral loads imposed by the retaining wall. Detailed design must
include an assessment of internal and global stability, together with required bearing capacity and founding
depth, by the retaining wall design engineer.
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Bev Jenner Const. 22073-001-Rev0
C/- Westera Partners 25 March 2022
4.5 Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment (e.g.
grassed or landscaped swales or diversion mounds). Adequate site drainage must be installed to ensure
that stormwater runoff is directed away from building walls and footings (including overflow form the
proposed pools affecting the main dual occupancy buildings).
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. It is assumed that the pool floors will include a
hydrostatic pressure relief valve. The outlet pipe from this valve must be taken to ‘daylight’ at the ground
surface.
All excess stormwater collected around the site (including overflow from rainwater tanks and pool) should be
directed by pipes or lined channels to discharge into the existing stormwater infrastructure along Edwards
Street. Subsurface disposal of stormwater must not be undertaken.
5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed residence 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
Robert Gibb BSc (Hons) Ashley Davey RPEQ 8159
Engineering Geologist Principal Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 16
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 17
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
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 19
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)