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Slope Stability Assessment
25 July 2022 Project No. 22198-001-Rev1 Paul & Janelle DeSouza C/- Sarah Waller Architecture Attention: Sarah Waller Email: [email protected] SLOPE STABILITY ASSESSMENT PROPOSED RESIDENCE 178 DAVIS ROAD, COOTHARABA Dear Sarah, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed residence at 178 Davis Road, Cootharaba. This report presents the results of our slope stability assessment, together with geotechnical advice for the project. In summary, subject to implementation of the recommendations made herein, there would be a Low Risk of slope instability affecting the proposed residence in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). 1.1 Details of Site and Development The property is described as Lot 7 on SP115866 and covers an area of 11.25 ha, with frontage to Davis Road along the southern boundary. The proposed residence would be towards the northern end of the property. Surrounding the site are a mixture of bushland and rural residences in adjacent blocks. Text Figure 1 on the next page illustrates the site location. A detailed description of the site is given in Section 2. Approx. site of residence Lot boundary Text Figure 1: Aerial image of site and surrounds from 2019 (courtesy Noosa Council [NC} Developmenti) 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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Paul & Janelle DeSouza 22198-001-Rev1
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Architectural layout and design drawings were provided with the project brief (Ref. Sarah Waller
Architecture [SWA] Project No. 2103, Dwg. No. 000 to 701, Rev E, dated April 2022). The drawings
indicate that the residence would be sited towards the northern side of the allotment, just off the northern
crest of a well defined topographical knob.
The structure would be built over an extensive one storey footprint, with a mixture of indoor space and
semi-outdoor pavilions. The building would be of low set construction, indicated on the drawings to mostly
feature clad walls, with occasional feature stonework and glass.
To facilitate the construction, it is generally expected that cutting would be undertaken around 1 m to 2 m
from the current topographical crest. Filling is indicated on the drawings to range to 2 m to 3 m height to the
north-eastern and north-western corners of the house.
A swimming pool would be located downslope to the north, with landscaped gardens between requiring
filling of about 3 m height to achieve the level with the house floor level. The pool shell would be at the
finished level of the fill on the upslope side, and up to 2 m out of ground downslope.
A shed is also indicated on the drawings about 25 m to 30 m west of the main residence. Construction
details were not provided, however based on topographical information at that location, it is estimated that
cut/fill earthworks in the order of about 1 m depth/height would be required to form an level building
platform between.
Access to the proposed residence would following an existing track alignment extending from the site
frontage to Davis Road along the crestline of a spur ridge extend from the north-western corner of the
allotment and exploit an existing roughly formed track to the western boundary. The driveway leads
thereafter to the front (east) of the residence and across the slope to the north. It is assumed the access
driveway would be surfaced with concrete.
The site does not have access to a potable mains water supply or sewerage infrastructure, and therefore
rainwater collection tanks will require to provide drinking water with effluent disposal also to be
accommodated on site as part of the future design process. Water tanks are shown to the west of the
house, but it is not known if these would be above or below ground.
A layout and perspective view of the proposed residential construction works is presented below in Text
Figure 2.
Text Figure 2: Site layout and rendered architectural view of proposed residence (courtesy SWA)
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1.2 Method and Scope of Investigation
A site classification report prepared by Ground Drill was reviewed to gain an appreciation of the likely
ground conditions, which included a design for on-site effluent disposal (Ref. Ground Drill, Job No. 22/237,
dated 22 June 20222).
A walk-over survey was conducted by Tectonic to make an appraisal of the general condition of the site in
regard to topography, drainage, vegetation cover, geology, erosion, condition of existing structures and
slope stability. The walk-over survey was carried out by an experienced engineering geologist from
Tectonic on 17 June 2022. The results of the desk-top study and walk-over survey are included in Section
2 below.
1.3 Qualifications of Responsible Engineer
This report has been prepared by David Pollock, an RPEQ with more than 17 years of experience in
geotechnical engineering, including a number of slope stability projects.
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
Available geological information 1 indicates that the site is at or close to, an inferred geological boundary, with
the older middle Triassic age Kin Kin Beds to the west (described as “strongly cleaved and commonly kinked
mudstone and thin siltstone laminae…”); and the late Triassic to middle Jurassic age Tiaro Coal Measures
shown to the east (described as “…quartzose sandstone, siltstone, shale…”).
Text Figure 3 presents an extract of the relevant geological map.
Kin Kin Beds
Tiaro Coal Measures
Approx. site location
Text Figure 3: Extract from Gympie Surface Geology Map
1
1:100,000 Gympie Surface Geology Map, Sheet 9445, Dept. Natural Resources, Mines & Energy, 2018
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During our walk-over assessment at the site, a number of exposed, relatively flat topped corestones were
noted to be present at the topographical high point forming the site of the residence. These were of
boulder size, with the material described as coarse grained, pale grey stained yellow brown, moderately
weathered sandstone. There were extremely weathered sections around the corestones, which enabled
the excavation of test pit TP2 (Ref. below).
A view of the exposed sandstone corestones is given below in Text Figure 4.
Exposed corestones
Text Figure 4: General view of exposed corestones (looking south), and detail view inset
2.2 Subsurface Conditions
The Ground Drill soil test included subsurface investigation within the proposed building envelopes via.
test pits using a 5.5 tonne excavator with toothed bucket and ripping attachments. Test pits TP1 to TP3
were dug progressively from the south-eastern corner of the residence, the central/highest point and the
north-western corner, respectively.
Detailed summaries of each test pit are given below:
TP1 encountered a surficial layer of loose, silty sand to 0.2 m below ground level (BGL), then stiff to
very stiff, sandy and silty clays (residual soils) to 1.5 m BGL, where extremely weathered (XW)
materials were encountered as very stiff to hard sandy clays. Distinctly weathered, low strength (or
stronger) sandstone was logged at 2 m BGL, where bucket refusal occurred.
Test pit TP2 was excavated between corestones (Ref. Section 2.1 above), with a relatively deep silty
sand and clayey sand profile, being loose to 1.5 m BGL, where distinctly weathered sandstone
resulted in bucket refusal.
Test pit TP3 on the downslope, northern side of the residence, encountered loose silty sand to 0.5 m
BGL, then stiff to very stiff sandy clays to 1.6 m BGL, then becoming dry and hard to 2.3 m BGL,
where distinctly weathered sandstone was encountered. It was possible to penetrate the sandstone to
2.5 m BGL under firm ripping conditions in the confined test pit base.
Test pit TP4 was excavated approximately within the expected footprint of the shed and encountered
loose silty sand to 0.5 m BGL, then stiff, sandy clay to 1.5 m BGL, where distinctly weathered
sandstone was logged, resulting in bucket digging refusal.
Groundwater seepage was not encountered during the digging of the test pits, however “very moist” soils
were logged by Ground Drill in test pit TP2 just above the inferred sandstone layer at a depth of
1.2 m BGL and below the upper sand at 0.5 m BGL in test pit TP3.
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Ground Drill has classified the site as Class P strictly in accordance with AS2870-2011 due to poor
bearing capacity and trees. However, they go on to state that were it not for this, the site would exhibit the
characteristics of a Class H1 site, with estimated characteristic surface movements (ys) ranging from 41
mm to 50 mm.
2.3 Topography
On the wider scale, the site is located within an area of relatively high relief, moderately steep, rounded
spurs and knobs. These are cut sporadically by relatively wide and flat bottomed creek valleys.
The eastern allotment boundary generally follows a rounded ridgeline (over which the access traverses),
rising to meet the knob upon which the residence is proposed in the north of the block. Ground generally
slopes to the west through the allotment, towards a creek valley as mentioned above.
Ground slopes were recorded using a hand-held clinometer during our site walk-over. The residence is
proposed to be sited just to the north of the crest of a well rounded knob, with ground slopes extending
north-east, north and north-west. The rounded crest featured slopes of about 10°, increasing downslope to
12° to 16° to the north and north-west; although in excess of 22° just off the proposed south-eastern edge of
the building footprint.
At the proposed shed location, ground slopes extended to the north-west at about 10° to 14°.
While the knob was rounded, the slopes were generally quite consistent, with slightly divergent to linear
ground observed. No irregular or ‘hummocky’ ground was seen at the time of our field assessment.
Text Figure 4 below gives an overview of the topography of the site.
Lot boundary
Approx. house site
Approx. access route
Low lying ground
associated with larger
creek drainage
Text Figure 5: Image showing ground surface contours at site and surrounds (courtesy NC)
Text Figures 6 and 7 on the following page give an indication of the ground conditions at the site at the time
of our walkover inspection.
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Text Figure 6: Looking north from southern edge of building envelope
Text Figure 7: Looking west across knob, showing transition from crest to moderately steep flanks
Approx. house site
Approx. shed site
Text Figure 8: View from shed site looking east across spur to house site
2.4 Groundwater
No signs of surface groundwater seepage (‘springs’) were observed during our walk-over, and no standing
groundwater table was recorded in the Ground Drill test pits. Slightly higher moisture content was logged in
the Ground Drill test pits at isolated locations below surface sands, and close to the interface to the
underlying weathered sandstone. This indicates that there is potential for the development of periodic
groundwater seepage in these zones.
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2.5 Surface Drainage
The topographical position of the house means there is a negligible upslope catchment, however the crest
of the spur leading west would present a drainage path towards the shed. It is noted that the ground surface
does have positive fall away from both structures however, with the ground form generally expected to aid
shedding.
Surface water should drain well from the site considering the moderately steep natural gradients, and
relatively shallow depth to apparent ‘rockhead.’
2.6 Vegetation
The site was mostly cleared with a surface consisting of periodically slashed grass. Large mature trees were
present to the east, west and south of the proposed house; and to the south of the proposed shed.
The large mature trees observed in the vicinity of the proposed house and shed were noted to be relatively
‘plumb.’ Where trees are tilted or distorted in their growth, this can be (but is not necessarily always) a sign
of instability.
2.7 Buildings and Other Structures
No formal structures were present on site at the time of our assessment.
3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
NC mapping (Ref. Text Figure 9 below) indicates that the site is considered to be within an area of Moderate
landslide hazard.
Lot boundary
Text Figure 9: Extract from NC Landslide Hazard Overlay
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It should be noted that the NC map is a broad-scale indication of the potential landslide hazard only, is
largely based on ground slope; and is superseded by this site specific assessment. At the time of our
visit, the site did not exhibit any indicators of significant slope instability.
While the natural ground slope inclinations at the site were moderately steep, no landslide back scarps,
groundwater springs, or areas of naturally ‘hummocky’ ground were observed. Shallow rock was found
consistently in the test pit excavations.
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 attached.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
Treatment options to reduce to Low risk should be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk
L Low Risk to this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Considering the existing site conditions (Refer 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 residence and shed. Regulators (NC) normally require that a Very Low or Low Risk of
landslide affecting property must be demonstrated to enable development approval. Summarised in Table 2
below is our qualitative assessment of landslide risk for the site. A summary of qualitative terminology for
use in assessing risk to property is attached (taken from AGS 2007).
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Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk
Comments
The likelihood of a relatively shallow failure through
the fill embankment or otherwise in stiff natural
residual soils is assessed as Unlikely due to the
site preparation having removed all loose/softened
upper soils; the limitations on fill height and
1: Shallow landslide proposed
recommended quality, consistency of the residual
fill or through stiff natural Unlikely Medium Low
clay soils, lack of permanent groundwater, and
residual soils
anticipated minimal surface water ingress to the site.
Given footings are to be founded in sandstone rock,
a potential Medium impact from such instability is
assessed, with the resultant risk to be Low as per
AGS 2007.
The likelihood of a deep failure through the more
competent weathered sandstone (below bucket refusal
depths, based on Ground Drill excavation logs) is
assessed as Barely Credible because of the strength
2: Deep failure below
Barely of these materials and no evidence of such instability
proposed foundation depth in Major Very Low
Credible within this geology in similar conditions nearby.
weathered sandstone
Although the consequence of such a failure could be
Major, given that the entire residence could be
affected, the resultant risk is Very 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.
It is noted that potentially significant fill earthworks would be proposed as per the current drawings (Ref.
Section 1.1), with fill up to 3 m possibly indicated to the north-western, northern and north-eastern fringes of
the footprint. The ground slope on the flanks of the knob range from about 10° to 20° in areas affecting the
building envelope.
From a stability perspective, no fill greater than 1 m height on this ground would be permitted without full
face support with an engineer designed retaining wall. It is not anticipated that the extensive retaining works
required would be acceptable from a cost or aesthetic perspective, and thus optimisation of the earthworks
should be undertaken as part of the detailed design as the project progresses. It is envisaged that
undertaking a greater depth of cut earthworks may provide some gains (including cost savings on footings),
possibly in conjunction with tiered retaining walls in area of slightly deeper fill as part of landscaping in
deeper fill areas.
Finalised plans including proposed earthworks should be reviewed by Tectonic prior to construction.
4.1 Earthworks
All fill greater than 1 m height should be fully supported by an engineer designed and certified retaining wall.
Any loose or soft soils, organic rich topsoil and severely root affected soils, must be removed from the
proposed construction area. Any fill materials utilised should be free of organic or otherwise unsuitable
materials and compacted at moisture contents within the range of -2 % to +2 % of optimum moisture content
for Standard Compaction.
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Confirmatory testing must be carried out at regular intervals and further details for control and testing of fill are
given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and Residential
Developments. Because of the depth of fill, testing under a Level 1 overview as per AS3798 is
recommended.
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.
All fill must be ‘benched’ into the slope, which would apply whether the fill subgrade consists of soil or rock.
Fill should be over-built by 0.5 m and then trimmed back to the well compacted material.
Excavation through the natural soils should be achievable using conventional earthmoving plant (e.g. 5 to 8
tonne excavators with toothed buckets). For efficient excavations, below this, in weathered sandstone and
for the removal and handling of large corestones, larger plant (e.g. 20 to 30 tonne excavators) with ripper
attachments should be allowed for, particularly for any confined excavations. Due to the variable nature of
weathering within the rock, it is possible that higher strength zones could be encountered in deeper
excavations. If such materials are continuous with minimal defects (relatively massive ‘bedrock’), heavy
breaking with a hydraulic breaker may be required.
Permanent batter slopes should be constructed at a maximum grade of 1V:2H in medium dense (or
denser)/stiff (or stiffer) natural soils or controlled fill up to 1 m high. Batters in low strength or stronger
weathered sandstone may be steepened to 1V:1H, or steeper if assessed by an experienced geotechnical
engineer. Cuts above 2 m height will require further assessment as to permanent battering arrangements,
where not retained. Permanent soil or fill batters will require erosion protection (e.g. revegetation or surface
protection).
4.2 Retaining Structures
Retaining structures must be founded as described in Section 4.3 below. Retaining structures greater than
1 m high or supporting backslopes greater than 5% will require engineer design and certification of
construction. We suggest the parameters given in Table 3 may be adopted for retaining wall design.
Table 3: Retaining Wall Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle
Ka Ko
(kN/m3) (Degrees) Kp
(Cantilever wall) (Non-yielding wall)
Loose upper sands 17 28 0.40 0.56 2.70
Stiff (or stiffer)
residual clay soils and 18 26 0.39 0.56 2.56
XW material
Low strength (or
stronger) weathered 21 36 0.26 0.41 3.85
sandstone
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Retaining wall designers should include rigorous checks such that sufficient factors of safety to limit the risk
of bearing failure, internal instability and global instability to acceptable levels are allowed for in the design.
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4.3 Footing Design
All footings supporting the proposed residence (and ancillary structures, e.g. retaining walls) must extend
through any existing or future fill and natural soils, to found a minimum 300 mm into the underlying competent
weathered sandstone (i.e. equivalent to 300 mm deeper than test pit bucket ‘refusal’ depths, Ref. Section 2.2
above), or at shallower drilling/excavation refusal in consistent low strength (or stronger) material (i.e. not
corestones or ‘floaters’).
Footings so founded may be designed using an allowable bearing capacity of 400 kPa.
With the moderately steeply sloping nature of the site and depth to rockhead, depending on the final
configuration of the house, it is expected that a combination of the following could be adopted:
Concrete reinforced bored piles, in areas of proposed future retained filling.
Normally dimensioned, conventional slab on ground beams, in deepest areas of earthworks cut,
where low strength (or stronger) sandstone is exposed at surface.
Mass concrete pad footings, socketed into the underlying weathered sandstone may be a preferred
where residual soils and XW material are present at surface, but the underlying sandstone is present
within 500 mm of the ground level.
Footing design should also account for potential soil reactivity, which has been determined by Ground Drill to
be Class H1, with an expected characteristic surface movement (ys) of 41 mm to 50 mm. Filling in building
areas would have an adverse effect on the site classification (particularly with site-won clays) with more
onerous requirements likely (i.e. Class H2 or Class E); and should be considered as part of the earthworks
optimisation at this site.
All footings should found such that they are not adversely affected by any adjacent excavations, batter slopes,
trenches, or retaining walls that are not designed to support building loads. Footings should found at least
below a plane extending 1 m horizontally from the base of trenches/batter slopes/excavations/retaining walls,
then rising up at 1V:1H, as illustrated in Text Figure 10 on the following page. Interaction with the existing
retaining wall to the west of the site area should also be considered.
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Text Figure 10: Footing depth required to minimise risk of undermining and surcharging
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. Due to the depth of footing excavations required, the piling contractor must have a ‘mud
bucket’, or similar, to enable clean out of pile sockets. With the expectation that socketing to weathered
sandstone is required, contractors should also allow for a suitable rock auger for such material penetration.
A suitably qualified and experienced engineer should inspect the footing excavations to confirm appropriate
foundation materials and cleanliness prior to pouring concrete.
If any ground conditions encountered during construction are found to differ from those noted above,
Tectonic and Ground Drill must be notified immediately and an inspection carried out to determine if changes
to footing design are required.
4.4 Drainage
Surface diversion drainage should be constructed above the crest of any cut or fill embankments and
retaining walls (e.g. grassed or landscaped swales or diversion mounds). Adequate site drainage must be
installed to ensure that stormwater is directed away from building walls and footings, and also not allowed to
flow over fill embankments in an uncontrolled fashion (e.g. as overflow run-off from roof gutters, along the
surface of any fill embankments beneath the house, etc.).
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.
All excess stormwater collected around the residence (including overflow from rainwater tanks and the
swimming pool) should be directed downslope by pipes or lined channels to discharge into a rock-lined
contour drain, stacked rock weir, or similar, located at least 10 m down slope of any fill or structures,
including the land application area proposed to the north of the residential structures (generally to the north-
east or north-west). Subsurface disposal of stormwater must not be undertaken (including infiltration
pits).
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4.5 Effluent Disposal
Effluent disposal areas must be located downslope by not less than 10m from any structures, retaining walls
or cut or fill batters.
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of 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) David Pollock BE(Civil) RPEQ 14121
Engineering Geologist Senior Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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