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Attachment 3 - Revised Slope Stability Assessment
17 November 2022 Project No. 22328-001-Rev1 Brett Hage Email: [email protected] Copy to: Mr John Gillespie, CadCon Surveying & Town Planning ([email protected]) SLOPE STABILITY ASSESSMENT PROPOSED SUBDIVISION 96 BLACK MOUNTAIN RANGE ROAD, BLACK MOUNTAIN Dear Brett, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed residential subdivision at 96 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 3 on RP131369 and covers a total area of 66,274 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. Existing residence Existing overall Lot boundary Approx. site of proposed residence on proposed subdivided lot Text Figure 1: 2019 Aerial Image of site and surrounds (courtesy Near Map, 14 August 2022) 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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Brett Hage 22328-001-Rev1
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We have been supplied with a proposed layout plan for the subdivision by CadCon Surveying & Town
Planning (CadCon, Ref. Dwg. No. 218806-02-02, dated 25 May 2022), and site and roadwork layout plans
by Alta Projects (Ref. A6682/22/P01 to P04, dated July 2022) which indicated that it is proposed to retain an
existing residence on a slightly smaller lot to the north (proposed Lot 1) which would total 1.882 ha.
Proposed Lot 2 to the south would be of 4.754 ha, with a proposed building area along the western
boundary.
A separate driveway access would be expected for each lot, with a cross-over shown to the south-western
corner of proposed Lot 1, and north-western corner of proposed Lot 2.
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 proposed layout plan is given below in Text Figure 2.
Proposed subdivision boundary
Proposed Lot 2 building area
Overall existing lot boundary
Text Figure 2: Extract of overall subdivision layout plans, including roadworks (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, stability studies and aerial photos.
Tectonic undertook a site investigation to inform this assessment, which comprised the following:
Two boreholes drilled to depths of between 4.3 m and 4.5 m below ground level (BGL) using a
4WD utility mounted, solid flight auger drill rig owned and operated by Tectonic. In situ testing
included dynamic cone penetrometer (DCP) testing from surface adjacent to each location and at
selected depths downhole. Pocket penetrometer tests were also carried out on suitable recovered
cohesive materials.
Laboratory testing at a NATA accredited geotechnical laboratory comprising one Atterberg Limits
and one Particle Size Distribution test.
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 19 August 2022 concurrent with the borehole drilling. 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 Ashley Davey, an RPEQ with more than 27 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 Surface Geology 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 (green blue shading),
which are described as “strongly cleaved and commonly kinked mudstone with thin siltstone laminae.”
Approx. site location
Text Figure 3: Extract of Gympie Surface Geology Map
The investigation comprised drilling of two boreholes (designated BH1 and BH2) from an earthworks building
pad level (already in situ at the time of the fieldwork) and on the natural sloping ground to the east. Borehole
BH1 was taken to a target depth of 4.3 m BGL, and borehole BH2 was terminated at auger drilling refusal in
weathered rock at 4.5 m BGL. The borehole locations are shown indicatively in Text Figure 4.
Approx. borehole BH2
location
Drill rig at borehole
BH1
Text Figure 4: View looking south across building area, showing indicative borehole locations
1
Dept. of Natural Resources, Mines & Energy, Nambour Surface Geology Map (1:100,000 scale) Sheet 9445, 2018
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The boreholes encountered slightly different subsurface profiles as follows:
Borehole BH1 was drilled from the crest of the earthworks cut/fill pad present on site at the time of our
fieldwork, and the following profile was logged:
Fill: Firm to stiff, high plasticity, silty clay to 0.4 m BGL, then becoming stiff to 0.8 m BGL;
underlain by
Residual soil: stiff, silty clay, high plasticity to 1.3 m BGL; overlying
Extremely weathered (XW) material: described as very stiff to hard silty clay, with fine to medium
size angular gravel fragments; then below 2.3 m BGL grading to
Meta-Mudstone, being distinctly weathered and very low strength; recovered during augering as
grey, silty ‘rock flour’, tending to be dry below 3 m depth.
Borehole BH1 was terminated at 4.3 m BGL (target depth).
Borehole BH2, drilled downslope from the natural ground surface, featured a subsurface profile described as
follows:
Residual soil: stiff to very stiff, high plasticity, silty clay with medium to coarse size gravel and fine
grained sand, to 1.3 m BGL; then
XW material: of very stiff to hard, gravelly silty clay to 3 m BGL; overlying
Meta-Mudstone, being distinctly weathered and very low strength; possibly becoming slightly
stronger (up to low strength below 4.3 m BGL), where auger drilling refusal then occurred at
4.5 m BGL.
Groundwater seepage was not encountered during the drilling of either borehole BH1 or BH2, with all fill
and residual soils being moist; and XW materials generally slightly drier.
Along the western edge of the existing building pad, a cut of between about 0.5 m to 0.6 m depth below
the original ground surface was present. This exposed an XW material comprising pale yellow brown to
grey, hard, silty clay with meta-mudstone gravels. The nature of material in the cutting tended to allow the
gravel particles to fritter from the face, with some minor erosion debris located at the toe.
Soil samples were dispatched to Construction Sciences, a NATA accredited laboratory located in Kunda
Park for geotechnical testing. Laboratory testing was undertaken in accordance with Australian Standard
AS1289 Method for Testing Soils for Engineering Purposes and comprised one Atterberg Limits and Particle
Size Distribution test to allow an assessment of soil classification. The laboratory test certificate is attached.
The following Table 1 summarises the results of laboratory testing conducted on a selected soil sample
recovered from borehole BH1.
Table 1: Summary of Laboratory Results
Atterberg Limits
Depth Fines
TP No. Liquid Limit Plasticity Index Linear Shrinkage
(m) Content (%)
(%) (%) (%)
BH1 0.4 – 0.6 82 52 20 9.5
‘Fines Content’ is the percentage of clay/silt fraction in the soil sample
Laboratory testing indicates that the sample tested from borehole BH1 (residual soil) may be described as a
high plasticity, silty clay with trace gravel and sand.
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 surface water regimes.
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Immediately off Black Mountain Range Road to the west of proposed Lot 2, a road embankment was present
being <1 m height and inclined at about 25°. Natural ground below this sloped down at between 10° and 18°,
with the cut behind the earthworks pad in the Lot 2 building area being about 25° to 30°.
The earthworks pad was about 5 m to 6 m width, with the borehole drilling information indicating < 1 m height
of fill had been placed on the downslope side. This was battered at about 32°, with some evidence of rilling
(about 100 mm depth) over the face noted.
Natural ground below the proposed building area was sloping at between 13° and 15°, down to the east.
Below the centre of the proposed Lot 2 building area a very gently defined, rounded spur was present, with
the head of a larger (dry) gully present further to the north/north-east. The gully head was sloping at about
20°. To the south-east of the rounded spur, an intermediate drainage line proliferated. Some filling appeared
to have taken place across the top of this historically (possibly for horticultural purposes), leaving a number
of ‘benches’ with an overall slope of about 30° (the benches were about 2.5 m to 3 m wide, with the face of
fill at about 35° to 40°). At the base of the historic fill terraces, lusher ground was noted, with seepage
observed.
Despite the above, no irregular surface features were observed which would indicate wider scale global
slope instability (e.g. ‘hummocky’ ground, tension cracks).
Broad gully head developing north-
east of proposed Lot 2 building area
Overall existing lot boundary
Main drainage lie
Intermediate drainage line;
seepage at head
Text Figure 5: Image showing ground surface contours (courtesy Noosa Council [NC] Interactive Maps)
Text Figures 6 and 7 below and overleaf illustrate the site conditions at the time of our inspection.
Drill rig at borehole BH1
Text Figure 6: View looking south over building area on proposed Lot 2
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Drill rig at borehole BH1
Historic terraces
Area of wet ground
Text Figure 7: View looking west over proposed Lot 2 building area, from downslope
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 shallow 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 building
area on Lot 2 fall towards the main drainage line to the east. Surface water flows affecting the proposed
building locations on Lot 2 may be sourced from surface water overtopping the unprotected edge of Black
Mountain Range Road directly west and above. The intermediate drainage line to the south is largely
contained in areas downslope of the proposed building area. Ground below the building area forms a gently
rounded spur, which may also aid in dispersing, rather than concentrating flows.
No significant erosion was noted during our walkover inspection, however rilling of about 100 mm depth was
noted over the face of the existing fill material.
Surface water is expected to drain well away from the level building pad, considering the moderately steep
natural gradients and low permeability of the soils in the immediate subsurface, and relatively shallow depth
to rockhead.
The drilling of borehole BH2 just upslope and west of the area of possible seepage noted in Text Figure 7
indicated no proliferation of ‘groundwater’ laterally. With the topography observed, it is more likely that
this is as a result of surface water being retained in soils upslope in the intermediate drainage line, and
fed out at the transition to the natural soils on the sloping ground below.
2.5 Vegetation
To the west of the site area was a mixture of periodically slashed grass and sporadic small to medium size
trees; which were observed to be relatively ‘plumb’.
Further downslope to the east and south-east, slightly larger mature trees were present in a forested area.
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 located on the main spur to the north of the site area comprised a lightweight
structure, formed of two shipping containers. A larger, sheetmetal shed was present along the crest of the
spur to the east. The driveway servicing the existing residence had a gravel surface. While these structures
appeared to be in relatively good condition, it is acknowledged that with the nature of their construction,
signs of structural distress as a result of global slope instability may not always be apparent based on an
external, visual assessment.
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
In the proposed Lot 2 area, a High landslide hazard has been allotted in NC landslide hazard overlay
mapping for much of the main drainage line running west-east through the overall lot; and its associated
flanks. The area currently proposed for building on proposed Lot 2 generally has been mapped as having a
Low (no shading) to Moderate hazard rating (Ref. extract in Text Figure 8).
Overall existing Lot boundary
Text Figure 8: 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 fill earthworks pad, or other minor
earthworks features (e.g. horticultural terraces). In addition, the natural subsurface conditions were
favourable with stiff to very stiff soils present from near surface, and weathered meta-mudstone rock
logged at relatively shallow depth in both the boreholes; and XW material 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 2 on the
following page and the Qualitative Terminology for Use in Assessing Risk to Property is also attached.
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Table 2: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation
VH Very High Risk of treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost
more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
H High Risk
required to reduce risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires investigation, planning
M Moderate Risk and implementation of treatment options to reduce risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing
L Low Risk
maintenance required.
VL Very Low Risk Acceptable. Manage by normal slope maintenance procedures.
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the
property at risk; these are only given as a general guide.
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 3 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 3: Details of Qualitative Risk Assessment for Property (AGS 2007
Assessed
Hazard Likelihood Consequence
Risk
Comments
Given the presence of stiff residual soils at the
natural surface, and stiff to very stiff XW materials
below at shallow depth, the rounded nature of the
spur crest where construction is proposed, and
minimal catchment for water above the site and
1: Proposed residence on
apparent lack of groundwater, and no evidence of
subdivided Lot 2- Failure
existing instability in the vicinity of the proposed
through fill materials natural Unlikely Medium Low
residence, it is considered Unlikely that a landslip
soils above the foundation
would develop, subject to implementation of the
depth
recommendations in Section 4 (particularly with
attention to remediation of existing fill). 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 XW
materials or weathered meta-mudstone rock below the
2: Proposed residence on foundation depth is assessed as Rare due to the
subdivided Lot 2 - Deep strength of these materials, no evidence of existing
seated failure in XW material or Rare Major Low instability, and relatively consistent nature of the
weathered meta-mudstone sloping ground across the development area.
below the foundation depth Although the 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.
3.3 Quantitative Assessment
In order to calculate the factor of safety (FOS) against slope instability for the proposed building works for
proposed Lot 2, numerical analysis has been conducted using Slope/W. Based upon the results of our
investigation, an adverse ground profile was adopted, considering relatively high groundwater table
conditions (1 m BGL, noting that this was not recorded during our investigation), and larger scale, relatively
deep seated failure modes.
A 20 kPa surcharge load has been applied across the building envelope to represent a residence of up to
two storeys height.
Results of the numerical slope stability analysis are illustrated in Text Figure 9 overleaf.
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Text Figure 9: Slope/W Model output, groundwater at 1 m BGL (Ref. Figure 1 for approx. section line)
The key points from our analysis are summarised as follows:
Unit weight and shear strength parameters adopted for the soil strata were taken from the mid to
lower range of values recommended for similar materials in Appendix D of Australian Standard
AS4678-2002 ‘Earth Retaining Structures’.
Analyses were undertaken for a specified range of slip circle centroids and radii, with a minimum
FOS value of 1.5 recorded for the longer term case, even where groundwater is assumed to
proliferate at shallow depth (1 m BGL).
This result meets the industry standard minimum FOS (i.e. ≥1.5) for long term conditions.
Our analysis indicates that the proposed development will achieve at least the required minimum FOS.
Subject to implementation of the recommendations made in Section 4 during building works, there would be
a Low Risk of slope instability affecting the proposed development.
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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 layout plans (Ref. Section 2) were available for review at the time of this assessment, however
no bulk earthworks or building design details were available regarding the proposed residence, and would be
subject to review and possibly site specific investigation and assessment prior to construction. We
understand that future building works would be located proximal to the area indicated in Text Figure 2 (not
including areas for effluent disposal).
The general building location denoted in Text Figure 2 on the future subdivided allotment is considered
suitable from a slope stability viewpoint, although it is recommended that the building envelope is truncated
such that it remains on the rounded spur running through the centre of the building area. This is shown
indicatively in Text Figure 10 below.
Building envelope on northern
side of this line only
Approx. ≥12 m from boundary
Text Figure 10: Indicative restrictions on building area (base plan courtesy NC Interactive Mapping)
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 (original natural ground). All existing fill should be
removed within the proposed building areas prior to construction, and cut-off drainage installed upslope (Ref.
Sections 4.2 and 4.6, respectively). The cross-overs are expected to be generally at-grade with the existing
ground (running adjacent to the slope contours); however any future internal driveways on proposed Lot 2
should also be constructed as per the earthworks and drainage recommendations given in this report.
4.2 Earthworks
In construction areas, all vegetation, tree stumps and large roots must be ‘grubbed out’, existing uncontrolled
fill (e.g. existing earthworks pad) with any organic rich topsoil stripped from the site. It is not expected that
the horticultural fill terraces to the south-east would be disturbed. 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.
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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.
Given the inclination of the slopes around the vicinity of the proposed building area, filling should be limited
to no more than 1.5 m height above the natural ground.
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 boreholes suggests
that harder digging conditions in very low strength (or stronger) rock could be expected within 2.3 m to 3 m of
the ground surface. Such materials may require the use of ripping attachments, under slow digging
conditions.
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 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 within the site boundary downslope of the proposed
construction area.
4.3 Site Classification
Based on the laboratory test results, site classification strictly of Class P would apply to the site, given the
presence of historic uncontrolled fill. Were it not for this, the subsurface profile encountered in the
boreholes would be equivalent to Class M as per AS2870, providing an estimated surface movement due
to seasonal moisture variation (ys) of up to 40 mm.
Designers should therefore also take account of the potential surface movement due to tree induced
suction change (yt) additional to the normal design suction change. An understanding of the full mature
height of the trees in the vicinity of the proposed residence, and finalisation of which trees would be
retained and which be removed would be required. Appendix H of AS2870-2011 comprises a Guide to
the Design of Footings for Trees.
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4.4 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 1 m into natural residual soil comprising stiff (or stiffer) clay or a minimum of
0.3 m in to low strength (or stronger) weathered rock. Based on the boreholes and our site observations,
depending on the construction methodology adopted, normally dimensioned, conventional high-level footings
would be appropriate in upslope areas where bulk earthworks cuts are undertaken, and extended on the
downslope side via the use of steel reinforced concrete bored piles (or ‘bucket piles’).
Footings could be designed with a maximum allowable bearing pressure of 100 kPa when founded in stiff
residual clays, 200 kPa in very stiff to hard residual soils; or 400 kPa in very low strength (or stronger)
weathered rock.
Footing design should also account for the characteristic surface movement (ys), indicated by the site
classification given above. 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 11.
Text Figure 11: 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 RPEQ
should inspect the footing excavations to confirm appropriate foundation materials and cleanliness prior to
pouring concrete.
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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.
4.5 Retaining Structures
Any retaining walls must be founded, at a minimum, as described in Section 4.4 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:
Table 4: 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 Natural Clays 18 25 0.41 0.58 2.46
Very Stiff to Hard
19 27 0.38 0.55 2.66
Natural Clays
Very Low Strength,
23 35 0.27 0.43 3.69
Weathered Rock
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Lateral earth pressure coefficients and material properties for fill materials would be dependent on the type
of material used, and the quality of compaction achieved.
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.
4.6 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
proposed construction area prior to commencement of Building Works (to intercept flows from Black
Mountain Range Road), 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) 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 (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.
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All excess stormwater collected on site must be directed by pipes to discharge at least 10 m down slope
from the proposed structures. Surface dissipation, such as a rock weir or similar, must be installed at the
stormwater outlet(s) to minimise the risk of soil erosion at the discharge point. Discharge on to, or directly
above, the horticultural terraces to the south-east of the proposed building envelope would not be
permissible.
Subsurface disposal of stormwater must not be undertaken.
5.0 WASTEWATER TREATMENT AND DISPOSAL
An assessment of the type and configuration of wastewater system(s) that may be suitable for a proposed
development requires:
Assumptions to be made regarding the typical building/worker configuration and probable
wastewater flow; and
Evaluation of the terrain-soil combinations to identify distinct units.
5.1 Wastewater Flow Design Allowance
For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy of
six persons. This assumption is consistent with current NC development standards and has been used to
demonstrate feasibility of on-site effluent disposal. Should the proposed residence exceed this size, it is the
owner’s responsibility to demonstrate feasibility of the design, meeting all relevant regulations.
For each occupant, a typical wastewater flow allowance of 120 L/person/day has been taken from
Appendix H of AS1547:2012. This represents the flow allowance for households with standard water
reduction fixtures (including duel flush toilets, shower flow restrictors, aerator faucets and water conserving
automatic washing machines) with reticulated town water supply.
We therefore estimate a typical daily household wastewater flow rate of 720 L/day (6 x 120 L/person/day).
It should be noted that wastewater flow rates may be further reduced by the adoption of full water reduction
facilities (e.g. reduced flush 6/3 L toilets, shower flow restrictors, aerator faucets, front-load washing
machines and flow/pressure control valves on all water-use outlets, etc.). Conversely, the use of extra
wastewater producing fixtures (e.g. spa baths) has the potential to increase the household wastewater flow
rate. Final wastewater flow rates must be calculated at the time of detailed design of the wastewater system,
at building stage once the house design is known.
5.2 Terrain-Soil Assessment
The terrain-soil combination(s) encountered at a site are assessed so that an Expected Available Area (EAA)
can be delineated for suitable methods of wastewater treatment and land application.
Several possible areas would be available within proposed Lot 2 for effluent disposal, and it is assumed that
it would be preferable to maintain this on the downslope side of the residence. The terrain below the Lot 2
house site has been categorised as Waxing Convergent in accordance with the slope configurations
described in Figure C2 of AS1547:2012, which also describes the relationship between the shape of the
slope and surface drainage. Such slopes tend to concentrate and accelerate run-off as it moves down slope.
It has been assessed from the results of the investigation that the soil profiles across the likely effluent
disposal area on Lot 2 is equivalent to Light Clays (Soil Category 5), based upon the soil materials
encountered in the boreholes. This soil classification has been derived via the method described in Table
M1 of AS1547:2012. The indicative soil permeability rate (Ksat) for a soil of this category is 0.06 m/d.
15/19
Page 16
Brett Hage 22328-001-Rev1
17 November 2022
5.3 Recommendations
5.3.1 System Type
It is recommended that wastewater from Lot 2 be directed to a suitably sized Household Sewage Treatment
Plant (HSTP) connected to shallow subsurface or surface irrigation systems. The HSTP for proposed Lot 2
must be licensed to produce at least Secondary standard effluent, with Chief Executive approval from the
Department of Housing and Public Works; however to maximise available residential space, it may also be
desirable to use an Advanced Secondary treatment system.
Shallow subsurface drip irrigation is proposed due to the presence of slopes greater than 10 % (6°) away
from the gently sloping crest areas, and certainly within possible land application areas.
Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 2.5 mm/day for Category 5 soil.
Therefore, minimum irrigation areas of at least 435 m² for the proposed lot will be required. This includes a
50 % reduction allowance to the DIR, which has been applied due to slopes of around >20 % being present
within the proposed development areas. The nominal 500 m2 land application areas, as shown in Figure 1
would therefore be appropriate, and it is assessed that that there is enough room to site the wastewater
irrigation areas with the necessary setbacks.
The need for a reserve area is generally applied to septic tank systems, producing low quality (Primary
Standard) effluent, due to increased potential for fowling and failure of the trenched land application area to
occur. As the use of an aerated wastewater treatment system is proposed, which are capable of delivering
high quality (Secondary Standard or better) effluent, the risk of fouling and system failure is negligible. As
shallow subsurface drip irrigation is proposed which can be easily repaired in-place as required, the addition of
a separate reserve area is not warranted.
5.3.2 System Positioning
Positioning (‘setback’) of the land application system must be in accordance with the horizontal and vertical
setback distances recommended in Tables T2, T3 and T5 of the Queensland Plumbing and Wastewater
Code 2019, as summarised in Tables 5, 6 and 7 below and on the following page.
Table 1: Setbacks for subsurface land application area for on-site sewage treatment plant
Horizontal separation distance
Site Feature
Upslope Downslope Level
Property boundaries, pedestrian paths, walkways, recreation areas,
2m 4m 2m
retaining wall, and footings for buildings and other structures.
Inground swimming pools. 6m
Inground potable water tank.
6m
(Secondary standard effluent or better)
Note: Distances measured from the edge of trench/bed excavation or subsurface irrigation distribution
pipework to the nearest point of the feature.
16/19
Page 17
Brett Hage 22328-001-Rev1
17 November 2022
Table 2: Setbacks for surface irrigated land application area for on-site sewage treatment plant
Site Feature Horizontal separation distance
Property boundaries, pedestrian paths and walkways 2m
Water edge of a swimming pool 6m
Dwellings, recreation areas 10 m
Note: Distances measured from the edge of the irrigated wetted area to any point of the feature.
Table 3: Setback distances for on-site sewerage facilities (Protection of surface and groundwater)
Separation Distance
Site Feature
Advanced Secondary Secondary Effluent
Effluent
Surface Waters:
Top of bank of permanent or intermittent water course, lake,
bay or estuary;
Top water level of a surface water source used for agriculture, 10 m 30 m
aquaculture or stock purposes;
Open stormwater drainage channel or drain;
Bore or a dam.
Unsaturated soil depth to a permanent water table (vertically) 0.3 m 0.6 m
Note: Distances measured from the edge of the irrigated wetted area to any point of the feature.
Unsaturated soil depths of 0.6 m (i.e. depth to rock or wet soil) and 0.3 m are recommended for Secondary
and Advanced Secondary effluent, respectively. Boreholes indicate adequate unsaturated soil depths are
available.
Individual site layout shall be decided at building stage, in conjunction with a design provided by a NC
registered evaluator.
5.3.3 System Construction
Construction of wastewater land application areas must be carried out in accordance with Appendix M of
AS1547:2012 and NC requirements. In order to prevent any adverse impacts on the water quality from
effluent disposal, the elements given below shall be implemented:
Shallow subsurface drip systems must be installed at 100 mm to 150 mm depth into 150 mm to
250 mm of good quality sandy topsoil, in grassed of other suitably vegetated areas. Any sandy
topsoil material won from site works may be used to supplement the soil within the respective
disposal fields or alternatively shall be imported to the site.
Receiving native soils will be prepared by undertaking deep ripping / shallow cultivation combined
with the addition of organic matter (10 kg/m² raw mulch) and gypsum (1 kg/m²) to ‘break up’ any
relatively hard clay subsoils and improve overall drainage characteristics.
Shallow subsurface irrigation lines will run parallel to slope contours. To evenly distribute
wastewater throughout the irrigation areas, pressure equalisation systems (i.e. pressure
compensating drip emitters) should be installed.
17/19
Page 18
Brett Hage 22328-001-Rev1
17 November 2022
Surface diversion drainage shall be installed on the up-slope sides of the land application areas to
divert surface water and shallow subsurface seepage flows away. Such drainage should penetrate
to a minimum depth of 0.3 m BGL.
A low earthen bund shall be constructed, down slope of the effluent disposal areas up to 0.3 m high
to retain run-off and promote sheet flow.
Plant and maintain grasses, reeds and/or small shrubs within the land application areas. Preference
should be given to native plant species endemic to the local area, which are suitable for mostly wet
soils and are tolerant of nutrient loads (e.g. native riparian vegetation)
Erect signage at the boundaries of the area in at least two places in accordance with Council
requirements to warn people regarding the presence of wastewater pipes (e.g. “Sewage Effluent
Pipework Installed Below – DO NOT DIG” or “Recycled Water – Avoid Contact - DO NOT DRINK”).
Erect barriers where necessary to restrict access to public which may interfere or damage the
irrigation system.
Detailed design of the effluent disposal systems must be carried out by a NC registered evaluator to
accompany the plumbing application at the building stage, once the building designs are finalised.
5.3.4 Maintenance and Inspection
Operation and maintenance of a HSTP is the responsibility of the property owner. A licensed service agent
must be appointed to undertake routine servicing works in line with regulatory authority approval conditions.
To comply with NC requirements, the following management and inspection shall be implemented once the
effluent disposal systems are established:
Continuous monitoring by the property owner for any signs of system failure such as septic odour,
seepage, soil and/or plant discoloration.
Should any signs of system failure be observed by the property owner, including the in-built HSTP
failure alarm, the manufacturer shall be contacted immediately to arrange for the necessary repairs.
Quarterly inspection, testing and repair (if necessary) to the HSTP and irrigation area by a Licensed
Inspector (normally a service provided by the HSTP manufacturer).
6.0 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.
This assessment relates only to the proposed subdivision for the development application
process. Existing structures, batter slopes and road connections may be subject to risk of slope
instability but assessment of those matters was beyond the scope of our commission. Also excluded
from this assessment are any future building works, which would be subject to site specific assessment
and review by a geotechnical RPEQ prior to construction.
18/19
Page 19
Brett Hage 22328-001-Rev1
17 November 2022
7.0 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: Figure 1 – Site Plan
Borehole Logs & Explanatory Notes
Laboratory Test Certificate
Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
19/19
Page 20
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Office: 07 5478 9016
Email: [email protected]
PROJECT:
GEOTECHNICAL INVESTIGATION
PROPOSED SUBDIVISION
Indicative 435 m2 LAA 96 BLACK MOUNTAIN RANGE ROAD,
BLACK MOUNTAIN
TITLE:
SITE PLAN
BH1
CLIENT:
BH2 BRETT HAGE
DRAWN: DATE:
RKG 13/09/2022
CHECKED: DATE:
ACD 13/09/2022
SCALE:
AS SHOWN
LEGEND:
APPROX. BOREHOLE
LOCATION
0 20 40 m
SECTION A-A
PROJECT NO.: FIGURE: REV:
Notes
1. Base image courtesy Near Map (14 August 2022) 22328 1 0
Page 21
Borehole No.
BH1
Page 1 of 1
Engineering Log - Borehole Project No.: 22328
Client: Brett Hage Commenced: 19/8/2022
Project Name: Proposed Subdivision Completed: 19/8/2022
Hole Location: 96 Black Mountain Range Road, Black Mountain Logged By: LM
Hole Position: 486514.2 m E 7079389.7 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty CLAY (FILL), high plasticity, yellow brown,
with fine to coarse sized gravel F to
St
M
CH Silty CLAY, high plasticity, pale brown, trace of St 160
fine grained sand (RESIDUAL)
1
Silty CLAY, high plasticity, grey and pale brown,
with fine to medium size gravel, trace fine grained Augered to 1.6m
sand (XW MATERIAL)
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22328.GPJ <<DrawingFile>> 13/9/2022 16:40 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
D to
VSt
Not Encountered
M
2
AD/T
20/100mm
META-MUDSTONE, distinctly weathered, very Refusal
low strength (recovered as dry, grey silt) Augered to 2.5m
20/40mm
Refusal
3
4
Hole Terminated at 4.30 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 22
Borehole No.
BH2
Page 1 of 1
Engineering Log - Borehole Project No.: 22328
Client: Brett Hage Commenced: 19/8/2022
Project Name: Proposed Subdivision Completed: 19/8/2022
Hole Location: 96 Black Mountain Range Road, Black Mountain Logged By: LM
Hole Position: 486544.9 m E 7079377.0 m N MGA2020 Zone 56 Checked By: RKG
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
CH Silty CLAY, high plasticity, pale brown, with
medium to coarse sized gravel and fine grained
sand (RESIDUAL)
CH Silty CLAY, high plasticity, pale brown 180
St to
becoming friable M
VSt
1
Silty Gravelly CLAY, medium to high plasticity,
pale grey brown, fine to coarse size gravel, trace
fine grained sand (XW MATERIAL) Augered to 1.6m
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 22328.GPJ <<DrawingFile>> 13/9/2022 16:40 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
Not Encountered
2
CH becoming grey brown mottled orange
D to VSt to Augered to 2.7m
M H
AD/T
20/100mm
3
META-MUDSTONE, distinctly weathered, very Refusal
low strength (recovered as dry, grey silt)
4
grading to low stregth
Hole Terminated at 4.50 m
Auger Drilling Refusal
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 23
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RA Rotary Air BH Tractor Mounted Backhoe
ADH Hollow Auger RD Rotary Blade or Drag bit EX Tracked Hydraulic Excavator
HA Hand Auger RT Rotary Tri-cone bit HMLC Core – 63mm
*T TC-Bit HQ Core – 63mm
*V V-Bit NMLC Core – 47mm
NQ Core – 52mm
R Ripper
RH Rock Hammer
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
5,4,10 N=14 5,4,10 = Blows per 150mm. N = Blows per 300mm penetration following 150mm seating
30/65mm Where practical refusal occurs, the blows and penetration for that interval are reported
RW Penetration occurred under the rod weight only
HW Penetration occurred under the hammer and rod weight only
HB Hammer double bouncing on anvil
DS Disturbed sample
BDS Bulk disturbed sample
SV Field shear vane test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev1
February 2021
Page 24
METHOD OF SOIL DESCRIPTION
USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Soil is classified and described in borehole and test pit logs using the preferred method given in AS1726 – 2017.
The material properties are assessed in the field by visual/tactile methods.
Particle Size Plasticity Properties
Major Sub Division Particle Size
Division
BOULDERS > 200 mm
COBBLES 63 to 200 mm
Coarse 19 to 63 mm
GRAVEL Medium 6.7 to 19 mm
Fine 2.36 to 6.7 mm
Coarse 0.6 to 2.36 mm
SAND Medium 0.21 to 0.6 mm
Fine 0.075 to 0.21
SILT 0.002 to 0.075
CLAY < 0.002 mm
NOTE: The U line is an approximate upper bound for most materials.
Data which plot above the U line may represent unusual/problem soil
behavior, or unreliable data and should be considered carefully.
MOISTURE CONDITION AS1726 - 2017
Symbol Term Description
D Dry Sands and gravels are free flowing. Clays & Silts may be brittle or friable and powdery.
M Moist Soils are darker than in the dry condition & may feel cool. Sands and gravels tend to cohere.
W Wet Soils exude free water. Sands and gravels tend to cohere.
CONSISTENCY AND DENSITY AS1726 - 2017
Symbol Term Undrained Shear Symbol Term Density Index % SPT “N” #
Strength
VS Very Soft 0 to 12 kPa VL Very Loose Less than 15 0 to 4
S Soft 12 to 25 kPa L Loose 15 to 35 4 to 10
F Firm 25 to 50 kPa MD Medium Dense 35 to 65 10 to 30
St Stiff 50 to 100 kPa D Dense 65 to 85 30 to 50
VSt Very Stiff 100 to 200 kPa VD Very Dense Above 85 Above 50
H Hard Above 200 kPa
In the absence of test results, consistency and density may be assessed from correlations with the observed behaviour of
the material.
# SPT correlations are not stated in AS1726 – 2017, and may be subject to corrections for overburden pressure and
equipment type.
Rev1
October 2017
Page 25
METHOD OF ROCK DESCRIPTION USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Rock is classified and described in borehole and test pit logs using the preferred method given in AS1726 – 2017.
The material properties are assessed in the field by visual/tactile methods.
STRENGTH
Point Load
Symbol Term Index, Is(50) Field Guide
(MPa)
VL Very 0.03 to 0.1 Material crumbles under firm blows with sharp end of pick; can be peeled
Low with knife; too hard to cut a triaxial sample by hand. Pieces up to 30 mm
can be broken by finger pressure.
L Low 0.1 to 0.3 Easily scored with a knife; indentations 1 mm to 3 mm show in the specimen
with firm blows of pick point; has dull sound under hammer. A piece of core
150 mm long by 50 mm diameter may be broken by hand. Sharp edges of core
may be friable and break during handling.
M Medium 0.3 to 1 Readily scored with a knife; a piece of core 150 mm long by 50 mm diameter
can be broken by hand with difficulty.
H High 1 to 3 A piece of core 150 mm long by 50 mm diameter cannot be broken by hand but
can be broken with pick with a single firm blow; rock rings under hammer.
VH Very 3 to 10 Hand specimen breaks with pick after more than one blow; rock rings under
High hammer.
EH Extremely >10 Specimen requires many blows with geological pick to break through intact
High material; rock rings under hammer.
Relationship between Is(50) and UCS (unconfined compressive strength) will vary with rock type and strength, and
should be determined on a site-specific basis. UCS is typically 10 to 30 x Is(50), but can be as low as 5.
ROCK MATERIAL WEATHERING
Symbol Term Field Guide
Soil developed on extremely weathered rock; the mass structure and
Residual
RS substance fabric are no longer evident; there is a large change in volume
Soil but the soil has not been significantly transported.
Extremely Rock is weathered to such an extent that it has soil properties - i.e. it either
EW
Weathered disintegrates or can be remoulded, in water.
Whole of rock is discoloured, usually by iron staining or bleaching to the
extent that the colour of the original rock is not recognizable. Rock strength
Highly
HW is significantly changed by weathering. Porosity may be increased by
Weathered
leaching, or may be decreased due to deposition of weathering products in
Distinctly
DW pores.
Weathered
Whole of rock is discoloured, usually by iron staining or bleaching to the
Moderately extent that the colour of the original rock is not recognizable; but shows little
MW
Weathered or no change of strength from fresh rock.
Slightly Rock is slightly discoloured but shows little or no change of strength relative
SW
Weathered to fresh rock.
FR Fresh Rock shows no sign of decomposition or staining.
ABBREVIATIONS FOR DEFECT TYPES AND DESCRIPTIONS
Defect Type Coating or Infilling Roughness
CS/CZ Crushed seam/zone (Fault) Ca Calcite Po Polished
DB Drilling break Clay Clay Sm Smooth
EW Extremely weathered seam Cn Clean Sl Slickensided
HB Handling break Co Coating ( > 1mm) Ro Rough
IS/IZ Infilled seam/zone Fe Iron oxide VRo Very rough
Op Open
J Joint
Qz Quartz
Pt Parting St Stain Planarity
SS/SZ Sheared seam/zone (Fault) Vn Veneer (<1mm) Cu Curved
Vn Vein X Carbonaceous Dis Discontinuous
Ir Irregular
Pl Planar
St Stepped
Un Undulating
Rev4
February 2021
Page 26
Construction Sciences Pty Ltd Laboratory: Sunshine Coast Laboratory
ABN: 74 128 806 735 Phone: 07 5452 0100
Fax: 07 5452 0133
Address: Email: [email protected]
32 Hi-Tech Drive,
Kunda Park QLD 4556
QUALITY OF MATERIALS REPORT
Client: Tectonic Geotechnical Pty Ltd Report Number: 3740/R/55917-1
Client Address: 924 David Low Way, Marcoola Project Number: 3740/P/2437
Project: Black Mtn Range Road Lot Number:
Location: Various Internal Test Request: 3740/T/42557
Supplied To: Tectonic Geotechnical Pty Ltd Client Reference/s: O/N 22328
Area Description: Report Date / Page: 8/09/2022 Page 1 of 1
Test Procedures AS1289.3.6.1, AS1289.3.1.2, AS1289.3.2.1, AS1289.3.4.1, AS1289.2.1.1, AS 1289.3.3.1
Sample Number 3740/S/227750 Borehole name BH 1
Sampling Method Tested As Received Start Sample Depth (m) 0.4
Date Sampled 19/08/2022 End Sample Depth (m) 0.6
Sampled By Client Sampled (m)
Date Tested 29/08/2022 Material Source Insitu
PSD Preparation Material Type -
Atterberg Preparation Dry Sieved / Oven Dried Material Description MH Silt, high plasticity, trace of sand, trace of gravel
Specification Percent Specification PARTICLE SIZE DISTRIBUTION GRAPH
AS Sieve (mm)
Minimum (%) Passing (%) Maximum (%)
100
6.7 100
4.75 92 90
2.36 88 80
1.18 86
70
Percent Passing (%)
0.600 84
60
0.425 83
0.300 83 50
0.150 82 40
0.075 80
30
20
10
0
6.7
1.18
2.36
4.75
0.075
0.150
0.300
0.425
0.600
AS Sieve Size (mm)
Specification Specification Specification Specification
Test Result Result Test Result Result
Minimum (%) Maximum (%) Minimum (%) Maximum (%)
Liquid Limit (%) 52 0.075/0.425 Fines Ratio 0.97
Plastic Limit (%) 32 PI x 0.425 Ratio (%) 1666.0
Plastic Index (%) 20 LS x 0.425 Ratio (%) 791.4
Linear Shrinkage (%) 9.5 Linear Shrinkage Defects -
Remarks Results apply to the sample/s as received.,
Accredited for compliance with ISO/IEC 17025 – Testing
Accreditation Number: 1986
Corporate Site Number: 3740
Approved Signatory: Thomas Hewitt
Form ID: W85Rep Rev 2
Page 27
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
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68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
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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
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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.
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114 Australian Geomechanics Vol 42 No 1 March 2007
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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)