Original source · versioned page text
RFI - slope stability reporting
26 October 2022 Project No. 22393-001-Rev0 RECOR Pty Ltd Attention: Nina Porteous Email: [email protected] GEOTECHNCIAL INVESTIGATION AND SLOPE STABILITY ASSESSMENT PROPOSED DUPLEX DEVELOPMENT 73 SOUTHERN CROSS PARADE, SUNRISE BEACH Dear Nina, 1.0 INTRODUCTION As requested, Tectonic has undertaken a geotechnical investigation and slope stability assessment for proposed duplex development at 73 Southern Cross Parade, Sunrise Beach. 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 672 on RP225123, covers an area of about 810 m2, and has frontage to Southern Cross Parade along the eastern boundary. At the time of the assessment, a single to double storey residence occupied most of the central part of the allotment. Similar residential allotments with houses and low-rise unit buildings surrounded the site, with a residence under construction directly to the north. 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
See the original
The page text is free to read. Viewing or downloading an original file needs an account.
Log in to see the originalLog in to download the original file
The download may take up to a minute.
Full text of the other 22 pages
Page 2
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
Architectural drawings supplied to Tectonic (Ref. Zest Building Design, Project No. 22004, Dwg No’s A.000
to A.400 [not inclusive] Rev A, dated 13 September 2022) indicate that the proposed development will
comprise the demolition of the existing structures and construction of a new two storey, slab on ground spilt
style duplex building. Construction is likely to comprise lightweight materials (e.g. weatherboard cladding
to walls, sheet metal roofing etc.) for the upper levels and rendered brick/masonry blockwork (or similar) for
the lower levels. Separate pools are proposed for each unit, one being in the north-western corner of the
site and the other along the southern boundary.
No earthworks detailed were available at the time of reporting; however, based on the provided sections,
cut and fill earthworks in the order of about 1.5 m depth/height will likely be required. Drawings indicate
that most of the significant earthworks are to be retained. No details were available for the proposed pools;
however, it is anticipated that they will be partially below ground with concrete ‘upstand’ walls along the
downslope edges.
The property has existing connections to town water and sewerage. A site layout plan and some cross-
sectional views of the proposed duplex development are shown in Text Figure 2 below.
Text Figure 2: Proposed site plan (upper) and some cross-sectional views (lower) courtesy of Zest Building Design
2/13
Page 3
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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
7 October 2022. The walk-over assessment was combined with a geotechnical investigation comprising two
boreholes (designated BH1 and BH2) undertaken by hand auger methods in accessible areas across the
site to 3 m below ground level (BGL). The approximate locations of the boreholes are shown in Figure 1
attached at the end of the report text. In situ testing, using the dynamic cone penetrometer (DCP) method,
was carried out from the ground surface and at select intervals downhole. DCP testing extended below the
borehole termination depths unit practical refusal was achieved.
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 information 1 indicates that the site is underlain by Pleistocene age parabolic sand dunes
comprising “quartz sand” (Ref. Text Figure 3).
Approx. Site Location
Text Figure 3: Extract from Laguna Bay Surface Geology Map
1
1:100,000 Laguna Bay Surface Geology, Sheet 9545, Compilation Map, DNRME, 2018
3/13
Page 4
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
The boreholes encountered relatively consistent subsurface conditions, summarised as follows:
Fill (borehole BH1 only) – comprising loose silty sand to 0.4 m BGL; then or from the ground
surface
Natural Sands – initially comprising loose silty sand and poorly graded sand to depths between
1 m and 1.2 m BGL, then becoming medium dense to the borehole termination depths.
DCP testing extended below the borehole termination depths, with inferred dense (or denser) sands
indicated at depths between 3.3 m and 4 m BGL.
Groundwater was not encountered during drilling of the boreholes, with the soils being moist.
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 northerly
facing flank of an ancient sand dune, which crests about 100 m to the south.
The natural surface profile at the site had been interrupted by historic landscaping and residential
construction. Based on a visual appraisal 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.
Inferred natural ground slopes surrounding the residence and along Southern Cross Parade typically were in
the order of about 8° to 12°, which is the assumed natural gradient across the site prior to residential
construction. The rear of the property had been terraced, with low height timber sleeper walls (<1 m high)
supporting relatively level platforms. At the time of the assessment an unretained cutting about 2 m high
was observed on the downslope property adjacent to the north-eastern corner of the site. This cutting was
battered at about 70° and exposed sand.
Subject Site
Text Figure 4: Ground surface contours, 1 m spacing (courtesy NC interactive mapping)
Site conditions at the time of our investigation are shown in Text Figures 5 and 6 overleaf.
4/13
Page 5
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
Text Figure 5: View of the Southern Cross Parade frontage, looking west
Text Figure 6: View of the western part of the site, looking east
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, nor was groundwater
encountered in the boreholes.
2.4 Surface Drainage
Surface drainage would be expected to follow the contours to the north. However, run-off would be minimal
due to the sandy nature of the soils at the site, which is currently interrupted by the existing residence,
terracing and landscaping features.
Given the sandy nature of the site and surrounds, rainfall would normally be anticipated to infiltrate the
ground surface. However, as the proposed development will occupy a large part of the block, significant
run-off from hard surfaces will occur. Nevertheless, where directed into swales rather than pipes, such run-
off would be expected to infiltrate into the natural sands relatively rapidly. No evidence of significant erosion
was observed across the subject site.
2.5 Vegetation
The ground surface around the existing residence primarily comprised maintained grass and bare sand
which were fringed by scattered shrubs, garden beds and trees.
2.6 Buildings and Other Structures
The existing residence was a single to double structure, with a partially formalised slab on ground undercroft
area. Landscaping and terracing earthworks surrounding the residence were primarily supported by low
height (<1 m high) timber sleeper walls. Access was provided directly off Southern Cross Parade via an
exposed aggregate driveway.
Based on an external assessment, visible areas of the existing residence 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. Some tilting/leaning was noted in the surrounding timber sleeper walls, with
‘hairline’ cracking observed in the driveway and asphalt seal along Southern Cross Parade; however, this
5/13
Page 6
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
distress appeared to be related to age, poor construction and possibly settlement of the upper sands rather
than global slope instability.
As mentioned previously an unretained cutting was observed adjacent to the north-eastern corner of the site
(on the neighbouring property). At the time of the assessment, the footings of a carport were exposed by the
excavation (Ref. Text Figure 7). Further to the west, remaining areas of the cutting were supported by a
concrete block retaining wall.
Text Figure 7: View of unretained cutting near north-eastern boundary (LHS) and tilting sleeper wall (RHS)
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 8), which indicates that most of the site is within a Moderate (pale brown shading) landslide
hazard zone.
Subject Site
Text Figure 8: Extract from NC landslide hazard overlay mapping
It should be noted that the NC landslide hazard map is a broad-scale indication of the potential landslide
susceptibility and is superseded by this site-specific assessment. No visual signs of global slope
instability were observed across the allotment. No landslide back scarps, groundwater springs, or areas
of naturally ‘hummocky’ ground were observed.
6/13
Page 7
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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
Very High
VH implementation of treatment options essential to reduce risk to Low; may be too expensive and not practical.
Risk
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
M Moderate Risk and 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,
L Low Risk
ongoing 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 natural gradients ranged up to about 12°. 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. As the downslope allotment is occupied by an existing
residential structure, the potential for significant erosion of the toe of the dune is considered negligible,
provided that future disturbances (e.g. excavations etc.) are managed with sufficient engineering
consideration by the downslope owner. It should be noted that this assessment excludes geotechnical risk
associated with the unretained cutting adjacent to the north-eastern corner of the site, which should be
retained with an engineered designed and certified structure as soon as possible by the downslope property
owner.
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
7/13
Page 8
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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 that a landslip would
1: Failure through very loose develop in these materials, subject to
to loose natural sands above Unlikely Medium Low implementation of the recommendations in
the 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
2: Deep failure through the
Rare due to the lack of precedence of such
medium dense (or denser)
Rare Major Low movements locally, no signs of any existing slope
sands below the foundation
instability and apparent lack of groundwater.
depth
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. Subject to
implementation of these measures during construction, it is assessed that there would be a factor of safety of
at least 1.5 against slope instability.
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 development as described in Section 1.1,
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 footings must found below the zone of influence of the surrounding structures and infrastructure
(including retaining walls).
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.
Design and construction of footings shall be undertaken in accordance with Section 4.4.
Formal drainage should be constructed to ensure all stormwater is directed away from the existing
and proposed structures.
4.2 Earthworks
No earthworks details were available at the time of reporting; however, provided drawings indicate that cut
and fill earthworks in the order of about 1.5 m depth/height will be required. Isolated deeper excavations may
be required for underground footings and service trenches. Should proposed cuts/fills exceed these
quantities, Tectonic must assess the impacts on slope stability before construction.
8/13
Page 9
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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 sand subgrade following stripping will likely be in a loose condition. Where slab on ground construction
is proposed (e.g. driveway) 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.
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 Noosa Council and Unitywater infrastructure
along the site boundaries). The impacts from ground vibration must be closely monitored and managed
during construction. It would be prudent to conduct a dilapidation survey of neighbouring structures prior to
commencement of construction.
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 loose sands
could be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up 2 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, or otherwise
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
9/13
Page 10
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
and infrastructure, including on neighbouring properties, will need to be specifically designed to prevent
undermining. Based upon our experience with similar sites, Unitywater requirements for sewer/water
infrastructure may restrict the use of sheet piles (if required).
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 Site Classification
Site classification for footing design is determined in accordance with AS 2870-2011 Residential Slabs and
Footings, and depend on the depth of cut / fill undertaken at a site, variations in the natural soil/rock depths,
and the reactivity of the in situ soil / rock and any imported fill materials.
Based on the results of the investigation, the site is classified as Class P in accordance with AS 2870-2011
Residential Slabs and Footings, due to the presence of poor bearing capacity soils at shallow depths, and
trees. If not for these factors, the site would have the characteristics of Class S, with the estimated
characteristic surface movements (‘ys’) ranging up to about 20 mm. However, due to the potential variability
in subgrade density a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footings 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. Footings for the proposed duplex buildings and
ancillary structures (e.g. pool, 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 m and 1.2 m BGL (existing ground levels) in the boreholes.
Thus, piled footings will be required unless excavations expose medium dense sands at or near foundation
subgrade level.
Piles so founded may be designed using an allowable end bearing pressure of 300 kPa. 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.
Given the sandy soil profile and depths of footings required, steel screw piles are an appropriate footing type
for the proposed development generally. However, in areas of cut exposing medium dense (or denser)
sands at the subgrade level high-level footings may be adopted. It should be noted that excavations for
concrete strip footings (or similar) may loosen the base. Therefore, the use of a vibrating plate would be
required to densify any high-level footing bases prior to the placement of reinforcing steel.
Due to the presence of loose sands, together with the close proximity to surrounding infrastructure, open
bored piers or driven piles are not considered to be appropriate 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.
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.
10/13
Page 11
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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 and cutting/retaining wall on neighbouring block). 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.
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 footings be inspected by an RPEQ prior to
placement of steel reinforcement and pouring of concrete to confirm allowable bearing pressures and
cleanliness of excavations.
11/13
Page 12
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
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 3 may be adopted for retaining wall design. The values in Table 3 assume
that the walls are fully drained and no allowance is included for hydrostatic pressures, construction loading,
up-slope footing loadings, the effects of sloping ground and backfill compaction stresses. Any temporary
shoring required to support excavations during construction may also be designed using the parameters in
Table 3.
Table 3: Retaining Structure Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle Ø Ko
Ka
(kN/m3) (°) (Non-yielding Kp
(Cantilever wall)
wall)
Loose Sand 17 30 0.33 0.50 3.00
Medium Dense (or
18 34 0.28 0.44 3.54
Denser) Sands
Future 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.
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. It is assumed that the pool floors will include
hydrostatic pressure relief valves, which must daylight at the ground surface.
All excess stormwater collected around the residence (including overflow from rainwater tanks, pools etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Southern Cross Parade. Subsurface disposal of stormwater, such as by an infiltration pit, must not be
undertaken.
12/13
Page 13
RECOR Pty Ltd 22393-001-Rev0
26 October 2022
5.0 CONCLUSION
It is assessed that there would be a Low Risk to the proposed development 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
Michael Cocks BEng NER RPEQ 27540 Ashley Davey BEng (Geological) RPEQ 8159
Senior Geotechnical Engineer Managing Director
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
13/13
Page 14
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Ph: 07 5478 9016
PROJECT:
PROPOSED UNIT DEVELOPMENT
73 SOUTHERN CROSS PARADE,
SUNRISE BEACH
TITLE:
SITE PLAN
CLIENT:
RECOR PTY LTD
BH1
DRAWN: DATE:
MCC 25/10/2022
BH2 CHECKED: DATE:
ACD 25/10/2022
SCALE:
AS SHOWN (APPROX.)
BH1 Subject Site
Subject Site LEGEND:
APPROX. BOREHOLE
LOCATIONS
Notes:
Base plan adopted from Nearmap imagery dated 18 September 2022 &
Zest Building Design Project No. 22004, Drawing No. A.002, Rev A, dated 13 September 2022 0m 20 40m
PROJECT NO.: FIGURE: REV:
22393-001 1 0
Page 15
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 22393
Client: RECOR Pty Ltd Commenced: 7/10/2022
Project Name: Proposed Unit Development Completed: 7/10/2022
Hole Location: 73 Southern Cross Parade, Sunrise Beach Logged By: LM
Hole Position: 510275.0 m E 7079137.0 m N MGA2020 Zone 56 Checked By: MCC
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty SAND (FILL), fine to medium grained, dark
grey, trace of fine to medium sized gravel
SM Silty SAND, fine to medium grained, grey L
SP SAND, fine to medium grained, pale grey
Not Encountered
1
M
HA
2 MD
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22393.GPJ <<DrawingFile>> 25/10/2022 15:13 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
3 Augered to 3 m BGL
Hole Terminated at 3.00 m
Target depth
4
5
6
7
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.: 22393
Client: RECOR Pty Ltd Commenced: 7/10/2022
Project Name: Proposed Unit Development Completed: 7/10/2022
Hole Location: 73 Southern Cross Parade, Sunrise Beach Logged By: LM
Hole Position: 510251.0 m E 7079131.0 m N MGA2020 Zone 56 Checked By: MCC
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
SM Silty SAND, fine to medium grained, dark grey
SM Silty SAND, fine to medium grained, grey
L
Not Encountered
1
SP SAND, fine to medium grained, pale grey
M
HA
2
MD
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22393.GPJ <<DrawingFile>> 25/10/2022 15:13 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
Augered to 2.9 m BGL
3
Hole Terminated at 3.00 m
Target depth
4
5
6
7
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)