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Appendix G - Geotechnical Report
15 December 2020 Project No. 20332-001-Rev0 Total Concept Homes Attention: Mr Tim English Email: [email protected] Copy to: Mr John Gillespie, CadCon Surveying & Town Planning ([email protected]) SLOPE STABILITY ASSESSMENT PROPOSED SUBDIVISION 108 BLACK MOUNTAIN RANGE ROAD, BLACK MOUNTAIN Dear Tim, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed residential subdivision at 108 Black Mountain Range Road, Black Mountain. This report presents the results of our slope stability assessment, together with geotechnical advice for the development. In summary, subject to 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 Site and Development The subject site is described as Lot 2 on RP131369 and covers a total area of 59,774 m2. The site has frontage to Black Mountain Range Road to the western boundary. The surrounds feature similar rural residential blocks to the north, east and south. Text Figure 1 below illustrates the site position. A detailed description of the site features is given in Section 2. Approx. site of proposed residence Existing residence Existing Lot boundary Text Figure 1: 2019 Aerial Image of site and surrounds (courtesy Noosa Council [NC] Interactive Maps) 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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We have been supplied with a proposed layout plan for the subdivision by Alta Projects (Ref. Alta, Job No.
A6578/20, Dwg. No. A6578/20/P01 to P04), which indicates that it is proposed to retain an existing residence
on a larger portion of the existing allotment extending to the east and south-east. The north-western portion
of the allotment would be divided from the existing allotment, where a new residence would be located.
Drawings show that an existing access for the house would be upgraded, initially being shared between the
two future allotments off Black Mountain Range Road to the west; before splitting along the proposed
southern boundary of the new lot, extending to the position of the future residence.
Sections provided as part of the drawing package show that much of the driveway alignment would be at
grade with the existing track, with filling of up to 1.5 m height required towards the central area of the
alignment. The fill would be retained, and it is understood that the primary design option being considered is
gabion baskets. Some minor cutting is expected on the upslope side of the driveway to the eastern side of
the alignment.
The property does not have access to mains sewerage or a piped, potable water supply, and therefore
collection of rainwater in tanks and on-site disposal of effluent will be required.
An extract of the design drawings is given below in Text Figure 2.
Indicative area for proposed future residence
Proposed subdivision boundary
Driveway alignment
Existing Lot boundary
Text Figure 2: Extract of overall subdivision layout (LHS); and driveway alignment to future residence (courtesy Alta)
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, aerial photos, and a site classification report prepared by Ground Drill also
prepared a report to address on-site effluent disposal design and feasibility. Both reports are attached (Ref.
Job No. 20/373, dated 3 December 2020).
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 slope stability and existing
structures. The walk-over survey was carried out by an experienced engineering geologist from Tectonic
on 26 November 2020 concurrent with the drilling by Ground Drill.
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 reviewed by Mr Ashley Davey, an RPEQ with more than 25 years’ experience in
geotechnical engineering, including a number of slope stability projects.
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2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology and Subsurface Conditions
The 1:100,000 scale Gympie Special Geological Map 1 (Ref. Text Figure 3 below) indicates that the site is
underlain by materials derived of the early to middle Triassic age Kin Kin Beds (pale green blue shading),
which are described as “arenite, argillite and phyllite.”
Approx. site location
Text Figure 3: Extract of Gympie Special Geology Map
The Ground Drill investigation comprised drilling of two boreholes (designated BH1 and BH2) to the north
and south of the approximate proposed residence on the future divided allotment, with an additional borehole
(denoted borehole BH6) drilled to capture part of the driveway alignment to the east. Boreholes BH1 and
BH2 were terminated in the underlying weathered rock at a target depth of 2 m below ground level (BGL);
with borehole BH6 extending to auger drilling refusal at 2.5 m BGL.
Additional shallow boreholes were also drilled to the east of the prosed residence on the future allotment, to
provide information for the on-site effluent disposal design. These were taken to a target depth of 1.5 m BGL.
All boreholes were drilled using a 4WD utility mounted drill rig, with solid flight augering equipment. In situ
strength testing comprised the dynamic cone penetrometer (DCP) method, taken from surface adjacent and
at selected depths downhole.
The subsurface profile logged in each of the boreholes was relatively consistent, and comprised the profile
indicated on the following page:
1
State of Queensland, Department of Mines and Energy, Nambour Special 1:100,000 Geological Series, Sheet 9445 and Part 9545,
First Edition 1999
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An upper layer of residual, medium plasticity, stiff silty clays to between 0.6 m and 0.7 m BGL;
overlying;
Extremely weathered (XW) material comprising stiff to very stiff silty clays, to between 1 m and
1.4 m BGL; then grading to
Weathered Phyllite, inferred to be very low to low strength to the borehole termination/target
depths.
Groundwater seepage was not encountered during the drilling of either borehole BH1 or BH2, with all
soils being described as “dry.”
The site was classified by Ground Drill as Class P in accordance with AS2870-2011 due to the presence
of mature trees. In not for this factor the site would have been classified Class M with characteristic
surface movements (ys) of up to 35 mm reported by Ground Drill.
Along the southern edge of the existing access track, and in the cutting behind the existing residence
exposures of weathered rock were observed. These were noted to consist of moderately weathered, very
low to low strength, pale yellow brown phyllite; which had a well developed foliated fabric. To the south-
east of the existing residence this was interlayered with XW materials, although the lateral extent of this
was not clear, and such materials may be restricted to the near-surface of the exposed rock mass.
A view of the exposed weathered rock above the highest section of proposed earthworks filling is shown
below in Text Figure 4.
Text Figure 4: Exposed weathered phyllite in cutting above highest section of earthworks filling
2.2 Topography
As illustrated in Text Figure 5 on the following page, the wider area around the site consists of a series of
approximately parallel ridgelines and gullies, aligned east-west. The ridgelines within the subject allotment
lead east from Black Mountain Range Road, which traces a crest forming a drainage divide between
eastward and westward draining creek lines.
Just off Black Mountain Range Road, to the south-west of the overall existing allotment, a rounded gully
head was present, with sideslopes ranging from about 25° (47 %) at the site entrance to between 15° and
20° (27 % and 36 %) nearer the existing residence. Minor existing cut and fill earthworks had been
undertaken sidelong to the slope to facilitate the existing track, being 1 m to 1.5 m depth and around 1 m
height respectively. Below the existing residence, to the north, a series of cut terraces had been formed for
horticultural purposes. These were in the order of 2.5 m width, with cut batters of around 0.6 m depth or less.
Across the existing earthworks features on site, no signs of significant slumping were observed.
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The proposed subdivision boundary runs south-west to north-east across the site, just off the southern edge
of a spur. The proposed residence on the future allotment would be sited on this spur, with the top sloping at
around 5° (9 %) to the north and south, then extending to 9° to 10° (16 % to 18 %) around mid slope; and
steepening further to around 20° (36 %) around 30 m downslope of the residence. Slopes were noted to
increase consistently, with no irregular surface features observed which would indicate slope instability.
Initial gully forming off Black Mtn Range Rd
Overall existing lot boundary
Text Figure 5: Image showing ground surface contours (courtesy NC Interactive Maps)
Text Figures 6 and 7 below illustrate the site conditions at the time of our inspection.
Drill rig
Large trees to west of proposed residence
Text Figure 6: View looking south over area of proposed residence (drill rig at borehole BH1)
Ground sloping at 15° to 20°
XW to moderately weathered
phyllite exposed in cut
Text Figure 7: View looking west over existing access alignment, where fill is proposed highest on new driveway
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2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed during our walk-over, and neither was any
seepage encountered during borehole drilling. Groundwater conditions may vary seasonally, and could
rise to shallower depths, particularly following periods of prolonged or heavy rainfall.
2.4 Surface Drainage
Broadly, the natural surface drainage follows the ground surface contours, which over the proposed
development areas tended to fall to the north. Surface water flows affecting the proposed residence and
eastern part of the driveway alignment would be considered negligible, given the topographical position
towards the crest of a well developed spur. The natural ground does rise above the western part of the
driveway alignment, where a catchment over the area of an existing residence may proliferate.
No significant erosion was noted during our walkover inspection.
Surface water is expected to drain well from the site considering the moderate to steep natural gradients
and low permeability of the soils in the immediate subsurface, and relatively shallow depth to rockhead.
2.5 Vegetation
The immediate periphery of the proposed driveway consisted of periodically slashed grass, which was quite
long at the time of our inspection. Small to medium size mature trees were noted on the sloping ground to
the north of the proposed driveway, which were observed to be relatively ‘plumb’.
Around the area of the proposed residence, slashed grass was also present over most of the area, with
some large mature trees just to the west of the proposed building location. These were also noted to be
quite straight at the time of our inspection, with no tilting or distorted growth present, which can be an
indicator of slope instability.
2.6 Buildings and Other Structures
The existing residence comprised a one storey, low set, masonry walled structure, constructed over a cut
and fill earthworks pad. The residence was mostly on cut to the south, which ranged up to around 1.5 m
depth. Fill on the northern side of the residence was supported by a grouted rock wall and ranged up to 2 m
height. The grouted rock facing supporting the fill did not show any signs of global instability having affected
the structure; with the residence generally being in good structural condition, other than some minor cracking
to the concrete apron to the slab. It was noted that the cracking was coincident with the daylighting point for
underslab pipework (assumed to be for wastewater and water).
Although not within the subject site, a retaining wall was present along the south-western boundary, which
would be adjacent to the proposed driveway access. This was estimated to be in the order of about 4 m
height, and was constructed of large concrete blocks, spread over two tiers. This is shown below in Text
Figure 8.
Text Figure 8: Looking south-west over retaining wall on neighbouring allotment
It was presumed that this had been recently constructed, and it was not clear whether upslope surface water
flows were being captured by the structure, and if so, where discharge was being directed to.
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The crest area of the spur running south-west to north-east through the site, which incorporates the area of
the existing residence, and the proposed residence on the future subdivided allotment allotment has been
mapped by NC as presenting a Low/Very Low landslide susceptibility (clear/no shading). However the initial
flanks of the crest are mapped as a Moderate hazard area (pale yellow brown shading), which does cover a
part of the proposed building envelope for the future residence, and the driveway alignment. Towards the
inverts of gullies away from construction areas, High risk areas have been assigned (pale orange shading).
An extract of the NC mapping is given below in Text Figure 9.
Overall existing Lot boundary
Text Figure 9: Extract from NC Landslide Hazard Mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility only,
largely based on ground slope, and is superseded by this site specific assessment.
The site does not exhibit any indicators of global, natural slope instability. No landslide back scarps or
areas of natural ‘hummocky’ ground were observed, the existing structures on site were in good condition,
with no significant erosion or slumping observed over the existing access track alignment, or other minor
earthworks features (e.g. horticultural terraces). In addition the subsurface conditions were favourable
with stiff to very stiff soils present from surface, and weathered phyllite rock logged at relatively shallow
depth in both the boreholes and exposed in earthworks cuttings.
3.2 Qualitative 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 on the
following page 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
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 (Ref. Section 2), and proposed development works, it is assessed that
there would be a Low (L) Risk of global slope instability affecting the proposed development subject to the
implementation of recommendations given below. 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 on the page 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 driveway alignment exploits an existing track in
some parts, and although the central portion of the
driveway is proposed sidelong to a moderately
steep slope; given the presence of stiff to very stiff
residual soils and XW materials from surface, no
1: Proposed driveway - evidence of existing instability (including relatively
shallow failure through existing good performance of existing fill embankments),
natural soil materials (fill Unlikely Minor Low apparent lack of shallow groundwater, and the
subgrades or earthworks cuts); proposed engineer designed retention of future fill
or future fill materials for the driveway, it is considered Unlikely that a
landslip would develop, subject to implementation of
the recommendations in Section 4 (e.g. foundations
levels and fill quality). Given a potential Medium
impact from such shallow instability the resultant risk
would be Low as per AGS 2007.
Given the presence of stiff residual soils at surface,
and stiff to very stiff XW materials below at shallow
depth, the rounded nature of the spur crest where
construction is proposed, and negligible catchment
2: Proposed residence on
for water above the site and apparent lack of
subdivided Lot - Failure
groundwater, and no evidence of existing instability
through natural soils or XW Unlikely Medium Low
in the vicinity of the proposed residence, it is
materials above the foundation
considered Unlikely that a landslip would develop,
depth
subject to implementation of the recommendations
in Section 4. With a potential Medium impact from
such shallow instability the resultant risk would be
Low as per AGS 2007.
The likelihood of a deep failure through the weathered
phyllite rock below the foundation depth is assessed
3: Proposed residence on
as Barely Credible due to the strength of these
subdivided Lot, or proposed
Barely materials, no evidence of existing instability, and
driveway alignment - Deep Major Very Low
Credible relatively consistent nature of the sloping ground
seated failure in weathered rock
across the development area. Although the
below the foundation depth
consequence of such a failure could be Major, 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.
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4.0 RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached. This document should be referred to by those
preparing the concept and designs for future residential development at the site.
4.1 Site Layout and Geotechnical Constraints
Preliminary operational works plans (Ref. Section 2) were available for review at the time of this assessment,
however no bulk earthworks or building design details were are available regarding the proposed residence.
We understand that future building works would be located proximal to the area indicated in Text Figure 2
(not including areas for effluent disposal).
There should generally be no limitations on the methodology adopted for the future residence e.g. low set or
high set construction, or a split level structure built ‘in to’ the existing slope profile, however earthworks
should be limited to cut and fill of up to 1.5 m high/depth.
The general building location denoted in Text Figure 2 on the future subdivided allotment are considered
suitable from a slope stability viewpoint, although this should not encroach on the break of slope present to
the north and south, where ground slopes around 20° (36 %) were noted. Tectonic must be consulted to
assess the impacts on slope stability if development is proposed elsewhere on the site or if cut or fill
earthworks are to exceed 1.5 m in height/depth for the residence.
For the proposed driveway alignment, given the moderately steep to steep ground slopes present over
ground to the northern edge, all fill embankments >0.5 m height should be retained, and this is shown as the
intent on the supplied drawings (Ref. Section 2).
4.2 Earthworks
In construction areas, all vegetation, tree stumps and large roots must be ‘grubbed out’, existing uncontrolled
fill (e.g. along the proposed driveway alignment) with any organic rich topsoil stripped from the site. It is
recommended that subgrades be proof rolled using a minimum 4 tonne rubber tyred vehicle or drum roller to
identify any ‘soft spots’. Any such soft spots should be overexcavated and replaced with compacted select
fill.
Although trafficability should generally be good in ‘dry’ conditions, the soils could be subject to strength loss
from repetitive vehicle loading (particularly from rubber tyred plant) or if they become wet. Due to the
potential for poor trafficability during and after wet periods, placement of a minimum 150 mm thick granular
working platform may be required, particularly along access ways.
Fill materials should be compacted at moisture contents within the range of -2 % to +2 % of optimum
moisture content for Standard Compaction. Confirmatory compaction 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.
Fill (site won and imported) 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 roller 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. Topsoil must not be re-used as structural fill, but may be used for landscaping.
Any imported fill should comprise a good quality material meeting the following specifications:
Soaked CBR ≥ 10 %
Shrink-swell index (Iss) ≤1 %
Maximum particle size ≤ 100 mm
Following stripping of topsoil and any organic laden surface soils, fill must be ‘benched’ into the natural soil,
with batters over filled by 0.5 m (horizontally) and then trimmed back to the well compacted material.
It is anticipated that site earthworks could generally be completed by medium size excavators (8 tonne to 10
tonne) with toothed bucket attachments. However, the ground profile indicated by the Ground Drill boreholes
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suggests that weathered rock could be expected within 1 m to 1.5 m of the ground surface. Such materials
may require the use of ripping attachments, or selected pneumatic breaking.
Temporary batters in natural soils of stiffer (or stiffer) consistency, and future controlled fill may be formed at
1V:1H; with permanent batters formed no steeper than 1V:2H. Flatter batters may be required if
groundwater seepage is encountered. Permanent batters will require erosion protection such as covering
with erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
batter slopes, cut or fill batters would need to be supported by retaining walls.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
Any excavations required adjacent to existing structures, will need to be specifically designed to ensure that
the structures are not adversely affected (e.g. 'undermined').
Ground surfaces should be graded to shed surface water runoff away from construction areas. It should be
noted that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion
and sediment control measures must be installed prior to commencement of earthworks at this site,
especially given the presence of natural gullies and dams within the site boundary downslope of the
proposed construction areas.
4.3 Footing Design
Footings for the proposed building and any ancillary structures (e.g. retaining walls) must penetrate through
any future fill to ‘socket’ at least 0.5 m into natural residual soil comprising stiff (or stiffer) clay or a minimum
of 0.3 m in to weathered rock. Based on the Ground Drill boreholes, depending on the construction
methodology adopted, normally dimensioned, conventional high-level footings would be appropriate, or
possibly extended locally via the use of steel reinforced concrete bored piles (or ‘bucket piles’).
Footings could be designed with an allowable bearing capacity of 100 kPa when founded in stiff (or stiffer)
natural residual clays or 400 kPa in weathered rock.
Footing design should also account for the characteristic surface movement (ys), indicated by the site
classification report by Ground Drill to be equivalent to Class M. The re-use of the site-won clay materials for
filling works could result in greater estimates of characteristic ground surface movement and should be
considered by the design engineer once earthworks details have been confirmed.
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/excavations/retaining walls, then
rising up at 1V:1H, as illustrated in Text Figure 10 overleaf.
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Text Figure 10: Footing depth required to minimise the risk of undermining and surcharge loading
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. Should bored piles be adopted, it is recommended that the piling contractor have a ‘mud
bucket’ or similar device to enable clean out of the pile bases. ‘Spinning’ of the augers is not acceptable
and will not sufficiently remove drill spoil from pile bases. A suitably qualified and experienced engineer
should inspect the footing excavations to confirm appropriate foundation materials and cleanliness prior to
pouring concrete.
If any soil conditions encountered during construction are found to differ from those noted above, Tectonic
must be notified immediately and an inspection carried out to determine if changes to footing design are
required.
4.4 Retaining Structures
Any retaining walls must be founded as described in Section 4.3 above. Engineer design and certification of
retaining walls is required where such walls:
Exceed 1 m height.
Are subjected to surcharge loads (e.g. from backslope, adjacent structures, etc.).
Are located within close proximity to existing structures.
Retaining structures should be engineer designed in accordance with AS4678-2002 Earth-retaining
structures. We suggest the following parameters may be adopted for retaining wall design:
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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)
Stiff to Very Stiff
18 26 0.39 0.56 2.56
Natural Clays
Weathered Phyllite
21 35 0.27 0.43 3.69
Rock
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).
4.5 Drainage
Adequate site drainage should be installed to ensure that stormwater runoff is directed away from the
building walls and footings (e.g. swales in landscaped areas or grated drains across pavements up slope of
structures including retaining walls for the driveway). Diversion drainage should be installed above the crest
of all cut/fill batters, and toe of all cut batters. Uncontrolled flow of stormwater over embankment batters (e.g.
from gutter overflow during high intensity storms, or if formal arrangements are not in place for the retaining
wall on the neighbouring property to the south-west) has the potential to saturate and weaken the
embankment materials, potentially leading to slumping failure. Future building and landscaping must be
carried out to maintain good drainage.
Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).
All excess stormwater collected on site must be directed by pipes to the legal point of discharge, at least 5 m
down slope from the proposed or existing structures. Surface dissipation, such as a rock weir or similar,
must be installed at the stormwater outlet(s) to minimse the risk of soil erosion at the discharge point..
Subsurface disposal of stormwater must not be undertaken.
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
impacting the proposed development. This advice is subject to implementation of the recommendations
given in this report.
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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 BEng (Geological) RPEQ 8159
Engineering Geologist Principal Geotechnical Engineer
Attachments: Ground Drill Site Classification Report & Effluent Disposal Report (Ref. Job No. 20/373, dated
3 December 2020)
Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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Ground Drill Pty Ltd ATF The Bennett
Family Trust ABN - 15204820396
M – 0407035220
W- www.grounddrill.com.au
E [email protected]
QBCC Lic – 15034580
Pomona QLD 4568
Site Classification Report
Job No: 20/373 Client Ref: N/A
Date: 03/12/20
Client: Tectonic Geotechnical
Site Address: Proposed Lot 2, 108 Black Mountain Range Road, Black Mountain
Proposed Structure: Proposed Residence
Information Provided by Client: No house plans were available
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Introduction
The purpose of this report is to classify the site in accordance with AS2870-2011. This report
is based on the field investigation/lab testing and information if any provided by the client.
This Report relates exclusively to the tested locations and the above-mentioned structure.
Further investigation may be required. Further investigation is recommended when deeper
depths of fill or marine clays are thought to be present, by way of deeper boreholes/other
test methods/use of prior investigation within the area. Bore logging in accordance with
AS1726-2017 Geotechnical Site Investigations.
Site Classification
Class “P”
(Due to large mature trees)
Based on the soil profiles the site would have the characteristics of an
'M' site with an approximate Ys of 21-35mm if not a “P”.
Important Notes Relating to the classification
This report does not address slope stability, if a slope stability report is required the services of a
suitably qualified Geotechnical Engineer should be sought.
Allowable Bearing Capacity
An approximate bearing capacity of 100kpa was encountered at a depth of 100mm
below existing ground surface level in the SILTY CLAY at location BH1 and location
BH2.
This is a suggested minimum founding depth; greater depth may be required due to the
following. Bearing capacity can be highly variable over the allotment these depths are
specific to the test locations. Actual founding depth should be nominated by the designing
engineer whilst taking construction and footing type, fill, underground services, retaining
walls, settlement, proposed cut/fill and any trees into account. Soft and/or settling soils may
present within the investigated depths or at deeper depths, further investigation may be
required to establish this.
Page 17
Lab Results/Predicted Surface Movement Bore Hole BH 1 200-500 Shrink-Swell (Indicative) 1.0% Ys N/A Site Description Future Site Works: Unknown Existing Earthworks: Nil Retaining Walls: Nil Vegetation: Heavily grassed, scattered large mature trees Slope: 11-25% Allotment Type: Acreage Surrounding Allotments: Acreage Surface Water: Surface water from adjoining allotments may traverse this site Water Table: Not encountered Rock: encountered Watercourses: Nil nearby Road Status: Sealed By using this report, you are agreeing to the statements and limitations mentioned in this report. Authorised Signatory Matthew Bennett on behalf of Ground Drill Pty Ltd QBCC Lic # 15034580
Page 18
Limitations of Report & Notes
This report is based on information gathered from the field investigation and information provided by
the client if any. Ground Drill should be notified immediately if ground conditions are found to differ
from this report during initial earthworks.
Unless otherwise stated this report does not address the following issues;
• Mining Subsidence.
• Slope Stability (If the site is in a known slip area or is steep, it is recommended that a
Geotechnical Engineer be engaged to conduct a Slope Stability assessment).
• Deep soft or settling soils
• Soils within investigated depths or deeper may be subject to settlement with additional load of
structure and any fill placed, the designing engineer is to allow for this potential settlement.
• Deep settling marine clays may be present.
• Soil Contamination, hazardous materials and or acid sulphate.
• Any slopes, dimensions or plans in this report are not to scale and only show approximate
testing locations.
• Flood levels (It is the client’s responsibility to inform us of any flood levels on the allotment).
• If water is found in the boreholes this depth may vary greatly over time. This report does not
address the influence any natural springs may have on the structure.
• It is the client’s responsibility to inform us of any nearby underground services that have been
or will be installed near the proposed structure.
• Earthworks could affect the site classification and or founding depth.
• If any fill is placed on the allotment post investigation the founding depth will change,
furthermore placement of reactive fill may change the sites classification.
• All topsoil and vegetation is to be removed.
• Surface water is to be diverted away from structure.
• Do not plant trees near the structure which could interfere with the footing’s performance.
• It is the owner’s responsibility to read and understand the CSIRO BTF 18 Foundation
Maintenance and Footing Performance: A Home-owner’s Guide.
• Strength of founding material can be highly variable dependent upon moisture content.
• This investigation undertaken consisted of the included boreholes/test pits, given the size of
the site it is possible that a problem could have been missed. eg. Filled dam/gully, soft soils,
fill, springs, boulders, rock etc. We cannot give a definitive assurance that none of these
problems exist on the site unless we did a greater number of boreholes which would be
uneconomical and impractical.
• This report may not provide all necessary information to design an adequate footing system,
further investigation may be required using larger drill rigs and/or different testing methods.
• This report relates exclusively to single or two storey residential structures, or structures with
similar loads. Larger commercial or greater than 2 storey structures may need further
investigation and this report is to be used as an initial guide only.
• If trees/palms are present, or have been removed from, within or near to the proposed
structure footprint, it is the responsibility of the designing engineer to use the principals of
AS2870-2011 Appendix H “Guide to Design of Footings for Trees”. The classification therefore
excludes the effect of trees on the site unless otherwise stated.
Distance from proposed structure that trees can affect footings:
0.75 x 'h' 'M' sites
1.0 x 'h' 'H' sites
1.5 x 'h' 'E' Sites
'h' is the mature height of the tree. Where there are rows of trees the distance should be
increased.
• It is the designing engineer’s responsibility to calculate the yt and add it to the ys.
• As per the Plumbing and Drainage Regulation 2019, all sites classified H1, H2, E or P must
have an articulating plumbing joint where connected to sanitary drainage. This should be
designed by a competent person (E.g. RPEQ or similar).
Page 19
Borelogs
BH 1 Soil Description DCP BH 2 Soil Description DCP
0 SILTY CLAY (CI) – Medium plasticity, 0 SILTY CLAY (CI) – Medium plasticity,
pale brown/orange, dry, stiff. pale brown/orange, dry, stiff.
100 3 100 3
200 4 200 4
300 5 300 5
400 4 400 4
500 5 500 5
600 EXTREMELY WEATHERED MATERIAL 7 600 EXTREMELY WEATHERED MATERIAL 9
SILTY CLAY (CI) – Medium plasticity, SILTY CLAY (CI) – Medium plasticity,
pale brown, dry, stiff-very stiff. pale brown, dry, stiff-very stiff.
700 7 700 8
800 12 800 7
900 12 900 12
1000 14 1000 11
1100 1100 11
1200 1200
1300 1300
1400 DISTINCTLY WEATHERED PHYLLITE 1400 DISTINCTLY WEATHERED PHYLLITE
(DW) – pale brown, dry, very low (DW) – pale brown, dry, very low
strength rock. strength rock.
1500 1500
1600 1600
1700 1700
1800 1800
1900 1900
2000 Hole Terminated. 2000 Hole Terminated.
*- DCP restarted down the borehole
SWL -Standing Water Level
PP- Pocket Penetrometer
ISS-Indicative Shrink-Swell
SS – Shrink-Swell
Page 20
Proposed Driveway Borehole
(This borehole does not form part of the Site Classification Report)
BH 6 Soil Description DCP
0 SILTY CLAY (CI) – Medium plasticity,
brown, dry, stiff.
100 3
200 4
300 5
400 4
500 4
600 5
700 EXTREMELY WEATHERED MATERIAL 5
SILTY CLAY (CI) – Medium plasticity,
pale brown, dry, stiff-very stiff.
800 5
900 8
1000 4
1100 11
1200 10
1300 DISTINCTLY WEATHERED PHYLLITE 18
(DW) – pale brown, dry, very low
strength rock.
1400
1500
1600
1700
1800
1900
2000
2100 DISTINCTLY WEATHERED PHYLLITE
(DW) – pale brown, dry, low strength
rock.
2200
2300
2400
2500 Auger Refusal
Page 21
Site Photos [OCR of page images] Site Photos
Page 22
Ground Drill Pty Ltd PH - 0407035220
ATF The Bennett Family Trust W - www.grounddrill.com.au
ABN - 15204820396 E - [email protected]
QBCC Lic – 15034580 Pomona QLD 4568
Effluent Disposal Report
Job No: 20/373
Date: 03/12/20
Client: Tectonic Geotechnical
Site Address: Proposed Lot 2, 108 Black Mountain Range Road, Black Mountain
Proposed Structure: Proposed 4-bedroom residence on tank water
Information Provided by the Client: No house plans were available
Local Authority: Noosa Shire Council
Page 23
Introduction
The purpose of this report is to evaluate and design a suitable on-site treatment system and disposal
area in accordance with AS 1547 – 2012, the Queensland Plumbing and Wastewater Code and the
Plumbing and Drainage Act. The report relates exclusively to the above site and the proposed and or
existing buildings shown on the plan only. All measurements and levels shown are as accurate as
possible with the information provided these are to be used for locating the onsite effluent disposal
system only.
Site Description
Weather Conditions Fine
Slope 5-15%
Slope Stability N/a
Vegetation Heavily Grassed
Fill Nil
Flooding Nil
Watercourses Nil
Site exposure Sun & wind
Salinity Nil
Surface condition Nil cracks, seepage or compaction
Boulders & Rock outcrops Nil
Will water from uphill traverse the area No
Water table Nil
Soil Category (AS1547-2012 Table M1)
Borehole 0-0.2m 0.2m-0.6m
BH3-BH5 Cat 4 Cat 5
Clay Loams Light Clays
Weakly Structured Moderately Structured
0.12-0.5m/d 0.06-12m/d
Recommended Design Irrigation Rate
Test Result 0.10
Recommended Design Irrigation Rate 3mm/day
Adopted Rate 3/mm/day
Calculations
Bedrooms 4
Persons 6
DIR (mm/day) 3
Litres per person per day (Tank supply) 120
Calculations 6x120=720 /3= 240
Disposal Area Required 240m2 Covered surface pressure
compensating irrigation
Page 24
Risk Assessment
Risk Risk Mitigation
Surface Runoff Mounds
Human Contact Signs and mound to identify area. Apply by
setbacks
Overloading of system and LAA Use correct DIR, flow allowances and
maximum occupancy
Recommendations
• An approved Advanced Secondary ASTP is to be installed. (Installer to ensure tank
position does not damage to existing or proposed structures)
• A covered surface irrigation area of 240m2 consisting of pressure compensating lines
laid atop of the existing ground surface level and spaced approximately 800-1000mm
apart and covered with 150mm of mulch.
• The tank must be installed and anchored if necessary, to prevent flotation.
• Installer to confirm sufficient fall is available to the treatment plant prior to installation once
the floor height is known.
• Top of treatment plant shall be at least 150mm above any flood levels that are present.
• A filter must be installed in between treatment plant and irrigation area.
• Pump must have sufficient power to pressurise irrigation lines.
• All pipework and fittings used must meet relevant Australian Standards be buried, withstand
150% of pump shut-off-head and be fit for use.
• The supply line from the treatment plant to the irrigation area shall be buried 25-32mm
purple line poly within a 200mm layer sand bed to prevent damage.
• A flush out valve should be installed on the end of the irrigation area and used regularly.
• Flexible joints should be used from the treatment to the proposed/existing structure where
the site is “H1” or higher.
• A reserve area equal to the above area is to be kept free of development and vehicle traffic
as per plan.
• The area shall be surrounded by a 150mm high earth mound.
• The area shall be mulched to a depth of 150mm and evergreen plants planted to maximise
evapotranspiration.
• Two signs are to be erected stating ‘Recycled water-buried pipes-Do not dig’
• All Storm water from the residence and uphill slopes is to be diverted away from the area.
• Set back distances as per local council policies and AS1547 -2012 are to be met some of
these setbacks when Advanced Secondary ASTP with Surface Irrigation are;
10M from a watercourse, dam, table drain, bore etc.
10M from a dwelling or habitable building.
2M from a boundary/driveway if it slopes away or is level with that boundary/driveway.
4M from boundary/driveway if it slopes towards that boundary/driveway
6M from an inground water tank
6M from pool edge.
Authorised Signatory
Matthew Bennett
on behalf of Ground Drill Pty Ltd
QBCC Lic # 15034580
Page 25
Limitations of Report & Notes
This report is based on information gathered from the field investigation and information provided by the
client if any. Ground Drill should be notified immediately if ground conditions are found to differ from this
report.
Unless otherwise stated this report does not address the following issues;
• Mining Subsidence.
• Slope Stability (If the site is in a known slip area or is steep, it is recommended that a Geotechnical
Engineer be engaged to conduct a Slope Stability assessment).
• Deep soft or settling soils
• Soil Contamination, hazardous materials and or acid sulphate.
• Any slopes, dimensions, levels, site features or plans in this report are approximate only show
approximate testing locations.
• Flood levels (It is the client’s responsibility to inform us of any flood levels on the allotment).
• If water is found in the boreholes this depth may vary greatly over time. This report does not address
the influence any natural springs may have on the structure.
• It is the client’s responsibility to inform us of any nearby underground services that have been or will
be installed near the proposed area.
• Earthworks could affect the design.
• If any fill is placed on the allotment post investigation this could change the design.
• Surface water is to be diverted away from the area.
• This investigation undertaken consisted of the occluded boreholes/test pits, given the size of the site
it is possible that a problem could have been missed. eg. Filled dam/gully, soft soils, fill, springs,
boulders, rock etc. We cannot give a definitive assurance that none of these problems exist on the site
unless we did a greater number of boreholes which would be uneconomical and impractical.
Maintenance and Operation of System
• Scrape all dishes to remove fats, grease, etc. before washing
• Don't use a garbage grinder
• Use biodegradable soaps
• Use a low-phosphorus detergent
• Don't use powerful bleaches
• Don't put sanitary napkins and other hygiene products into the system
• Spray or irrigation areas should have access restricted
• Divert surface water away from land application area
• No vehicles or stock should be allowed on trenches, beds or irrigation area.
• Deep rooting trees or shrubs should not be grown over absorption trenches or pipes
• Vegetable gardens and/or fruit trees should not be planted within or near area, where there is a
possibility of airborne spray coming into contact with the plant.
• Structures should not be constructed near areas that encourage people to congregate.
• Play areas and equipment should be well away from areas where there is no possibility of spray
coming into contact with humans.
• Treatment plants and septic tanks are to be desludged every 3-5 years.
• Ground Drill will not be liable for any Council Inspection, submission or reviewing fees due to
changes in report or plan requested by client or Council.
• If any part of this design is unclear, please call before buying products or installing.
• This report must be approved by the local authority and appropriate permits issued before any works
can be undertaken.
• Installer is to contact Noosa Shire Council prior to burying the system for a site inspection and
certification.
• Follow the manufacturer's instructions, relevant codes, standards and guidelines for installation,
servicing, maintaining and cleaning the system and irrigation area/trenches/beds
Page 26
Soil Profiles
BH 3 Soil Description BH 4 Soil Description BH 5 Soil Description
0 SILTY CLAY (CI) – Medium 0 SILTY CLAY (CI) – Medium 0 SILTY CLAY (CI) – Medium
plasticity, orange, dry, stiff. plasticity, orange, dry, stiff. plasticity, orange, dry, stiff.
100 100 100
200 200 200
300 300 300
400 400 400
500 500 500
600 EXTRMELY WEATHERED 600 EXTRMELY WEATHERED 600 EXTRMELY WEATHERED
MATERIAL (XW) – SILTY CLAY MATERIAL (XW) – SILTY CLAY MATERIAL (XW) – SILTY CLAY
(CI) – Medium plasticity, red/brown, (CI) – Medium plasticity, red/brown, (CI) – Medium plasticity,
dry, very stiff. dry, very stiff. red/brown, dry, very stiff.
700 700 700
800 800 800
900 900 900
1000 DISTINCTLY WEATHERED 1000 DISTINCTLY WEATHERED 1000 DISTINCTLY WEATHERED
PHYLLITE (DW) – Pale brown, dry, PHYLLITE (DW) – Pale brown, dry, PHYLLITE (DW) – Pale brown,
very low strength rock. very low strength rock. dry, very low strength rock.
1100 1100 1100
1200 1200 1200
1300 1300 1300
1400 1400 1400
1500 Hole Terminated. 1500 Hole Terminated. 1500 Hole Terminated.
Page 27
Covered Surface Irrigation Diagram
150mm cover of mulch
Pressure
compensating
150m earth mound dripper lines
Native nitrogen
tolerant shrubs with
non-invasive root
systems
Flush out valve
Covered Surface layout Ground Drill Pty Ltd
NTS Sunshine Coast, QLD.
QBCC # 150 345 80 C:\Users\Ground Drill Desktop\Documents\Advertising\print\logo.jpg
E: [email protected]
W: www.grounddrill.com.au
P: 0407 035 220
Page 28
Notes
Levels shown are approximate
No plans were available for the proposed residence at the time of investigation
and are sourced from local
council mapping or information N
provided by the client. These
levels are solely to demonstrate
the functionality of the system.
Actual floor height is to be
confirmed with the builder prior
to installation.
All pipe work from the structure
to the proposed system should
be covered and adequately
vented and be vermin/insect
proof.
All work carried out should meet Proposed Covered Surface
relevant codes/standards and be Irrigation Area - 240sqm
done by an appropriately (20 x 12m)
220m A (Approx RL 187-186)
qualified and licensed person. pprox.
Proposed system should be
constructed in a way to
maximize evapotranspiration
and minimize potential pooling
and runoff.
If plans were not available at
the time of investigation further
reporting and design work may
be needed prior to council Proposed Proposed Lot 2
submission and installation. Residence
26m
Care is to be taken not to (Approx RL 188.5) Slope
sompact,smear or damage the 37%
Reserve Area
Blac
soil structure during installation
(This can lead to premature
k
failure). BH4
Mou
BH2 Slope
If structures have been built Slope PT2 5-15%
BH5
nt
since our initial investigation 25%
ain
PT3
(Including neighboring
15m
Ran
structures) please contact us to
confirm this will not adversely
ge R
effect the design. BH3
Final landscaping and installation PT1
oad
should discourage and limit BH1 Slope
2m
causal pedestrian access.(By 11-13%
way of plantings, border, Trees 10-25m
mounds, visual and or physical
deterrents) BH6
The owner should obtain
knowledge on how to use and Proposed Advanced
maintain the proposed system. Proposed Lot 1 Secondary ASTP
Most of this is available from Existing (Approx RL 188)
your chosen supplier/installer. Residence
This report must be approved by
the local council prior to any
work commencing.
The effluent disposal report
must be accompanied by a
Site Classification Report. If
the Site Classification is
H1,H2, E or P an articulation
joint will need to be designed
by a person licensed to do
so.
Client- Tectonic Geotechnical
Site - Proposed Lot 2, 108 Black Mountain Range Road, Black Mountain
Ground Drill Pty Ltd
Date - 07/12/20
Abbreviations Sunshine Coast, QLD.
Job# - 20/373
BH : Borehole DCP : Dynamic Cone Penetrometer QBCC # 150 345 80
Scale - 1 : 1100 @ A4 (Site features are approximate-Do not scale)
PT : Percolation Test HD : Heavy Droplet Lilac Sprinkler E: [email protected]
TV : Turf Valve RWT : Rain Water Tank W: www.grounddrill.com.au
P: 0407 035 220
Page 29
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 30
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 31
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 32
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 33
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)