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Approved Plan - EXE230029
Plans and drawings: the text below has words only, no drawings.
REPORT TO
NICK AND JULIETTE CUSACK
ON
GEOTECHNICAL ASSESSMENT
FOR
PROPOSED RESIDENCE
AT
20 WOONGAR STREET, BOREEN POINT, QLD
Date: 16 May 2023
Ref: 35891URrpt Rev1
www.jkgeotechnics.com.au
T: +61 2 9888 5000
JK Geotechnics Pty Ltd
ABN 17 003 550 801
NOOSA COUNCIL
APPROVED PLAN
EXE23/0029 6 NOV 2023
Document Set ID: 22532435
Version: 1, Version Date: 07/08/2023
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Report prepared by:
Paul Roberts CPEng, RPEQ
Principal Associate | Engineering Geologist
For and on behalf of
JK GEOTECHNICS
PO Box 503,
Cooroy, QLD, 4563
Head Office:
PO BOX 976
NORTH RYDE BC NSW 1670
DOCUMENT REVISION RECORD
Report Reference Report Status Report Date
35891URrpt Final Report 24 April 2023
35891URrpt Rev1 Revised report following review by 16 May 2023
architect
© Document copyright of JK Geotechnics
This report (which includes all attachments and annexures) has been prepared by JK Geotechnics (JKG) for its Client, and is
intended for the use only by that Client.
This Report has been prepared pursuant to a contract between JKG and its Client and is therefore subject to:
a) JKG’s proposal in respect of the work covered by the Report;
b) The limitations defined in the Client’s brief to JKG;
c) The terms of contract between JKG and the Client, including terms limiting the liability of JKG.
If the Client, or any person, provides a copy of this Report to any third party, such third party must not rely on this Report, except
with the express written consent of JKG which, if given, will be deemed to be upon the same terms, conditions, restrictions and
limitations as apply by virtue of (a), (b), and (c) above.
Any third party who seeks to rely on this Report without the express written consent of JKG does so entirely at their own risk and
to the fullest extent permitted by law, JKG accepts no liability whatsoever, in respect of any loss or damage suffered by any such
third party.
At the Company’s discretion, JKG may send a paper copy of this report for confirmation. In the event of any discrepancy between
paper and electronic versions, the paper version is to take precedence. The USER shall ascertain the accuracy and the suitability
of this information for the purpose intended; reasonable effort is made at the time of assembling this information to ensure its
integrity. The recipient is not authorised to modify the content of the information supplied without the prior written consent of
JKG.
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Table of Contents
1 INTRODUCTION 1
2 DETAILS OF THE SITE AND THE PROPOSED DEVELOPMENT 1
3 ASSESSMENT PROCEDURE 2
4 DESCRIPTION OF EXISTING CONDITIONS 2
4.1 Geology 2
4.2 Subsurface Conditions 3
4.3 Summary of Observations 3
4.3.1 Topography 3
4.3.2 Groundwater and Surface Drainage 4
4.3.3 Vegetation 4
4.3.4 Buildings and Other Structures 5
5 GEOTECHNICAL STABILITY ASSESSMENT 5
5.1 Existing Conditions 5
5.2 Risk Assessment 6
5.2.1 Potential Geotechnical Hazards 6
5.2.2 Risk Analysis 7
5.2.3 Additional Comments 7
5.3 Geotechnical Constraints to Development 7
6 COMMENTS AND RECOMMENDATIONS 8
6.1 Site Layout and Development Details 8
6.2 Site Classification 8
6.3 Demolition and Excavation 8
6.3.1 Dilapidation Surveys 8
6.3.2 Demolition and Excavation Methods 8
6.3.3 Seepage 9
6.4 Temporary Excavation Support and Retention 9
6.5 Retention Design Parameters 10
6.6 Footing Design 10
6.7 Floor Slabs 12
6.8 Earthworks 12
6.8.1 Subgrade Preparation 13
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6.8.2 Drainage During Construction 13
6.8.3 Engineered Fill 13
6.9 Wastewater Treatment and Disposal 14
6.10 Overall Effect of Development on Stability 14
7 CONCLUSION 14
8 GENERAL COMMENTS 15
ATTACHMENTS
Table A: Summary Of Risk Assessment To Property
Figure 1: Site Location Plan
Figure 2: Geotechnical Site Plan
Figure 3: Geotechnical Mapping Symbols
Appendix A: Landslide Risk Management Terminology
Report Explanation Notes
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1 INTRODUCTION
This report presents the results of our geotechnical assessment for the proposed residence at 20 Woongar
Street, Boreen Point, QLD. The assessment was commissioned by Nick Cusack by return of a signed
‘Acceptance of Proposal’ form dated 2 March 2023. The commission was on the basis of our proposal
(Ref. P58265UR) dated 2 March 2023.
The site was inspected by our Principal Associate engineering geologist, in order to assess the stability of the
site and the effect on stability as a result of the proposed development. This report presents a summary of
our site observations, a review of the subsurface conditions, the results of our geotechnical assessment and
geotechnical comments and recommendations with regard to the proposed development.
2 DETAILS OF THE SITE AND THE PROPOSED DEVELOPMENT
The site comprises Lot 1 on Plan Number RP112348. The coordinates of the site are approximately
Latitude -26.282555 and Longitude 152.993253. A site location plan is presented as Figure 1.
We have been provided with the following relevant information:
Survey plan (Drawing Number 210809CD, dated 31 August 2021) prepared by Geo Surveying.
Architectural drawings (Drawing Numbers CD 01 01, CD 01 02 and CD 02 03 Issue 6 and CD 02 01 and 02
Issue 5, dated 27 February 2023) prepared by Morrison Architecture.
A “Soil Test Report” (Ref. 210450(S), dated 15 December 2021) prepared by Australian Soil Investigations
(ASI).
A “Site and Soil Evaluation” (Ref. 210450(A), dated 14 December 2021) prepared by ASI.
Based on a review of the provided information and additional information provided by Morrison
Architecture, we understand that the existing shed and car port over the south-eastern portion of the site
will remain and a new one and two storey residence will be constructed to the north of the existing shed.
The residence will extend over the existing slope and the structure will be supported by steel columns and
the suspended floors will be timber framed. An effluent disposal system is proposed over the south-western
portion of the site. The proposed finished floor reduced levels (RL’s) of the Lower and Ground Floor Levels
will be RL10.4m and RL13.2m, respectively. A limited amount of localised excavation at the southern
(upslope) end of the proposed lower level in the order of a maximum of 2.2m will be required to achieve the
design surface levels. At a later date, a southern extension including an additional bedroom and new car
port is proposed; the bedroom will be provided with a suspended timber floor and the car port will be provide
with an on ground concrete slab.
Structural loads have not been provided and typical loadings for this type of development have been
assumed.
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3 ASSESSMENT PROCEDURE
This stability assessment is based upon:
A detailed inspection of the topographic, surface drainage and geological conditions of the site and its
immediate environs which was completed by our Principal Associate engineering geologist on
15 March 2023. These features were compared to those of other similar lots in neighbouring locations
to provide a comparative basis for assessing the risk of instability affecting the proposed development.
The attached Appendix A defines the terminology adopted for the risk assessment together with a
flowchart illustrating the Risk Management Process based on the guidelines given in AGS 2007c
(Reference 1).
A review of the subsurface conditions presented in the ASI “Soil Test Report” which was separately
commissioned by Nick Cusack.
The attached Figure 2 presents a geotechnical site plan showing the principal geotechnical features present
at the site and is based on the provided survey plan with the approximate outline of the proposed Ground
Floor level superimposed. Additional features on Figure 2 have been measured by hand held clinometer and
tape measure techniques and hence are only approximate. Should any of the features be critical to the
proposed development, we recommend they be located more accurately using instrument survey
techniques. Figure 3 presents an explanation of the geotechnical mapping symbols used on Figure 2.
4 DESCRIPTION OF EXISTING CONDITIONS
4.1 Geology
The 1:100,000 Gympie Special geological map (First Edition 1999) prepared by the Queensland Department
of Mines and Energy indicates that the site is underlain by Late Triassic – Jurassic age Myrtle Creek Sandstone;
(see Plate 1).
Approximate Site Location
Plate 1: Extract of Gympie Special geology map
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4.2 Subsurface Conditions
Based on our review of the provided ASI “Soil Test Report” and our site observations, the pertinent aspects
of the subsurface conditions are summarised below:
Sandy clay fill assessed to be of medium to high plasticity was encountered in BH2 and was 0.7m thick.
Based on the location of BH2, the fill is likely backfill to the timber sleeper retaining wall. Based on the
DCP test results the fill has been assessed to be poorly compacted. We also note that in the absence of
formal records of placement and density test results, the fill is regarded as ‘uncontrolled’ as defined in
AS2870-2011 ‘Residential slabs and footings’.
Silty sand topsoil (0.03m thick) encountered at surface level (BH1) and below the fill (BH2).
From below the topsoil, BH1 encountered gravelly sand over residual sandy clay assessed to be of low to
medium plasticity to the termination depth of 3.0m and BH2 initially encountered a 0.5m thick band of
residual gravelly sand overlying residual sandy clay assessed to be of medium to high plasticity which
extends to the termination depth of 3.0m. Based on our review of the DCP blow counts we interpret:
o The relative density of the gravelly sand in BH1 to be very loose and the clay strength to be
initially soft then soft to firm, improving to stiff to very stiff below about 2.2m depth and hard
strength below about 2.4m depth.
o The relative density of the gravelly sand in BH2 to be loose to medium dense and the underlying
clays to be generally of hard strength.
At the termination depth of BH1, the ASI log notes “approaching weathered sandstone” and DCP refusal
was at about 2.0m depth. In BH2, DCP refusal was at about 2.2m depth but there was no similar note
regarding “approaching weathered sandstone”. Based on our experience in this area and the results of
the ASI investigation, we have interpreted the DCP refusal depths to indicate the surface of extremely
weathered sandstone bedrock surface at depths of about 2.8m (BH1) and 2.2m (BH2), although the
presence of bands of weathered bedrock or ‘floaters’ within the residual soil profile (indicated by the
high DCP blow counts) cannot be discounted. We note that the transition of hard residual clay to
extremely weathered sandstone represents the weathering profile of the bedrock and the actual soil-
rock interface can vary across the site. However, based on the topographic setting of the site overall the
bedrock surface would be expected to step or slope down to the north.
No groundwater was encountered within the depth of the investigation.
4.3 Summary of Observations
We recommend that the summary of observations which follows in Sections 4.3.1 to 4.3.4 be read in
conjunction with the attached Figure 2.
4.3.1 Topography
The site is located on the crest of a hillside above the western foreshore of Lake Cootharaba and is
situated on the northern side of a small promontory (orientated east-west) in the foreshore area. The
upper (southern) portion of the site is relatively flat and has a southern frontage on to Woongar Street.
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Across the central portion of the site the site surface levels sloped down to the north at between about
25o and 40o (see Plate 2) to a gently sloping lower portion that extended northwards into neighbouring
bushland. The slope also extended north-west and south-east beyond the site boundaries.
Plate 2: steep slope crossing the central portion of the site (looing south-east)
The steeper (south-eastern) portion of the central sloping area was overgrown and the “rock RW”
indicated on the survey plan was not visible. Upslope of the central sloping area a timber sleeper
retaining wall supported the relative flat yard area to the south. For more details on the retaining walls
see Section 4.3.4 below.
4.3.2 Groundwater and Surface Drainage
As noted in Section 4.2 above, no groundwater was encountered within the depth of the investigation.
Based on the topographic setting of the site, surface run-off from the south not captured by the roads
stormwater drainage system would be directed down to the north and north-east through the site to the
bushland downslope.
Locally, some run-off from the upper section of the site may be captured by drainage behind the retaining
walls within the upper yard area.
4.3.3 Vegetation
The site contained a number of medium to large sized trees which also occupied the neighbouring
properties. The northern portion of the neighbouring property to the east was grass covered.
The bushland to the north extends to the lake foreshore and contained numerous trees (maximum height
about 20m).
Based on our site observations there were occasional trees that were leaning and/or had curved bases
over the steeper section of the central sloping area of the site (see Plate 2 above).
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4.3.4 Buildings and Other Structures
The south-eastern portion of the site was occupied by a brick shed, metal clad amenities shed and a
timber car port with a brick paved floor.
The relatively flat upper (southern) portion of the site was supported by a timber sleeper retaining wall
about 0.6m height (see Plate 3) and as noted above there is a boulder retaining wall indicated on the
survey plan that was overgrown and corresponds to the steeper portion of the sloping central portion of
the site. There were no obvious signs of significant timber retaining wall damage.
Plate 3: timber sleeper retaining wall supporting the southern portion of the site
The neighbouring two storey timber house to the west was set-back about 1.5 from the common site
boundary. Surface levels were similar across the site boundary and the neighbouring surface levels in
the rear yard stepped and sloped down to the north with a rock facing supporting a steeper section just
beyond the neighbouring northern site boundary. The northern portion of the neighbouring house was
suspended between timber posts supported by concrete footings.
The neighbouring one and two storey steel framed concrete block and timber clad house and a metal
clad single level northern extension were suspended over the slope (see Plate 2 above) and a metal clad
garage was located over the south-western portion of the neighbouring site. The buildings were set-back
between about 1m and 3m from the common site boundary. Surface levels were similar across the site
boundary
There were no obvious signs of damage to buildings within or neighbouring the site that indicated slope
instability.
5 GEOTECHNICAL STABILITY ASSESSMENT
5.1 Existing Conditions
Reference to the Noosa Shire Council Landslide Hazard Overlay Map [Ref. OM-LS-14A] dated 2019 indicates
that the site contains areas of terrain that are designated as “Moderate Hazard“ and “High Hazard”. An
extract of the Landslide Hazard Overlay Map OM-LS-14B is presented as Plate 4.
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Site Location
Plate 4: Extract of the Landslide Hazard Overlay Map OM-LS-14B
Based on our site observations:
There were signs of near surface creep of the steeper central portion of the site indicated by leaning
trees and trees with curved bases. However, there were no obvious signs of potential deeper slope
instability such as tension cracks in the slope surfaces.
There were no obvious signs of retaining wall instability such as tension cracks behind the wall crests,
bulging faces etc although our observations were limited by the overgrown nature of some areas of the
site. However, we have assumed that the retaining walls within the site were engineer designed.
We have no information regarding the placement of the fill on the site and as such it has been assessed as
being ‘uncontrolled’ as defined in AS2870-2011. As noted in Section 4.2 above we also assessed the fill to be
poorly compacted based on the DCP test results.
5.2 Risk Assessment
5.2.1 Potential Geotechnical Hazards
Based on our site observations, assessment of existing conditions provided above, and the form of the
proposed residence, we consider that the potential geotechnical hazards for the site, may be summarised as
follows:
1. Deep seated instability.
2. Slow moving near surface creep of the steeper section of the site.
3. Instability of existing retaining walls.
4. Instability of temporary excavation batters.
5. Instability of new retaining walls.
It is important to be mindful that slope instability can occur at any time and it would be difficult or impossible
to predict when the identified potential hazards will occur. Also, we cannot predict when an extreme or
unusual event may occur (such as an earthquake or 1 in 100 year rainfall event etc) and what impact it would
have on the stability of the identified potential hazards described above.
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5.2.2 Risk Analysis
Our geotechnical assessment of the risk of instability is based on the methodology proposed by Australian
Geomechanics Society (2007c) ‘Practice Note Guidelines for Landslide Risk Management’ (Reference 1) and
the relevant site features described above.
Risk To Property
The attached Table A summarises our assessment of risk to property under existing conditions and during
and following construction.
Under existing conditions, during and following construction, Table A indicates that the assessed risk to
property is “Low” and “Very Low”. In accordance with the criteria given in Reference 1, “Low” and “Very
Low” risk levels are considered to be “Acceptable”.
Risk To Life
We have not completed a comprehensive assessment of risk to life. However, based on the indicative
probabilities associated with the assessed likelihood of instability and assuming typical temporal,
vulnerability, evacuation and spatial factors for this type of site, we consider that the levels of risk to life
under existing condition, during and following construction to be “Acceptable” in relation to the criteria given
in Reference 1, i.e. less than 1 x10-6. We note that this assumes the recommendations outlined in Section 6
below are adopted.
5.2.3 Additional Comments
It must be recognised that, due to the many complex factors that can affect a site, the subjective nature of a
risk analysis, and the imprecise nature of the science of geotechnical engineering, the risk of instability for a
site cannot be completely removed. It is, however, essential that risk be reduced to at least that which could
be reasonably anticipated by the community in everyday life and that landowners be made aware of
reasonable and practical measures available to reduce risk as far as possible. Hence, risk cannot be
completely removed, only reduced, as removing risk is not currently scientifically achievable.
In preparing our recommendations given below we have assumed that no activities on surrounding land
which may affect the risk on the subject site would be carried out. We have further assumed that all Council
buried services and other buried services within and neighbouring the site are, and will be regularly
maintained to remain, in good condition.
5.3 Geotechnical Constraints to Development
Based on the contents of Sections 5.1 and 5.2 we consider that there are no geotechnical constraints to the
proposed development. Provided the proposed development is designed and constructed in accordance
with the advice presented in Section 6, we consider that following construction, the levels of risk to life and
property will remain at “Acceptable” levels.
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6 COMMENTS AND RECOMMENDATIONS
6.1 Site Layout and Development Details
Based on a review of the provided information and additional information provided by Morrison
Architecture, we understand that the existing shed and car port over the south-eastern portion of the site
will remain and a new one and two storey residence will be constructed to the north of the existing shed.
The residence will extend over the existing slope and the structure will be supported by steel columns and
the suspended floors will be timber framed. An effluent disposal system is proposed over the south-western
portion of the site. The proposed finished floor reduced levels (RL’s) of the Lower and Ground Floor Levels
will be RL10.4m and RL13.2m, respectively. A limited amount of localised excavation at the southern
(upslope) end of the proposed lower level in the order of a maximum of 2.2m will be required to achieve the
design surface levels. At a later date, a southern extension including an additional bedroom and new car
port is proposed; the bedroom will be provided with a suspended timber floor and the car port will be provide
with an on ground concrete slab.
6.2 Site Classification
The ASI “Soil Test Report” has identified the site as being Class P in accordance with AS2870-2011 due to the
presence of uncontrolled fill, “poor” allowable bearing pressure of the upper soil profile and the presence of
trees, some of which will be removed. We concur with the Class P site classification.
6.3 Demolition and Excavation
6.3.1 Dilapidation Surveys
Prior to the commencement of excavation, detailed dilapidation reports should be compiled on the
neighbouring properties to the east and west. In addition, Council may require that dilapidation surveys
report be completed on the Council assets lining the street frontage, i.e. the roadway and kerb and gutter.
The property owners should be asked to confirm that the reports present a fair record of existing conditions
as the reports may assist the client in defending themselves from unfair damage claims.
6.3.2 Demolition and Excavation Methods
The excavation recommendations provided below should be complemented by reference to the current Safe
Work Australia Code of Practice ‘Excavation Work’.
The outline of the proposed Ground Floor level is indicated on the attached Figure 2. To achieve design
surface levels excavations are expected to extend to a maximum depth of about 1m below existing levels.
The excavations will remove the overgrown rock retaining wall indicated by the survey plan over the eastern
end of the steep central portion of the site. The rock wall may extend into the neighbouring site.
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The bulk excavations must be carefully completed and prior to the excavations commencing the overgrown
rock wall should be cleared of vegetation and inspected by the structural and geotechnical engineers in order
to assess the current condition of the wall. In addition, if the rock wall extends across the site boundary,
then the inspection will assist in confirming the most appropriate excavation strategy in order to maintain
stability of the portion of wall that will remain, including any temporary propping or permanent stabilisation
measures, if applicable.
Based on the ASI investigation results, the proposed excavations will encounter the soil profile. Any topsoil
or root affected soils should be stripped and separately stockpiled for re-use in landscape areas or
appropriately disposed of off site.
Due to the small size and sloping nature of the central portion of the site, we expect the excavations will be
completed using small to medium sized tracked excavators, and excavation of the soils and any sections of
the overgrown rock wall will be readily completed with buckets of such equipment. Locally a ripping tyne
attachment to the excavator may be required over the deepest section of the excavation in order to remove
weathered rock. If rock breakers are required then further advice will need to be sought as potentially
damaging vibrations can be generated by rock breakers.
The poorly compacted surficial fill encountered in the investigation may extend across the site boundaries.
We therefore advise that sudden stop/start movements of any tracked equipment should be avoided in order
to reduce transmission of ground vibrations to the adjacent sections of buildings and structures within and
neighbouring the site.
6.3.3 Seepage
Groundwater seepage was not encountered in the boreholes completed by ASI. Some ephemeral seepage
inflows may be encountered in any excavations particularly after periods of heavy or prolonged rainfall. In
general, we expect that inflows, if any, to be of very small volume and managed by gravity drainage to the
lower gently sloping portion of the site. Inspection and monitoring of groundwater seepage during
excavations is recommended, so that any unexpected conditions, which may be revealed can be incorporated
into the drainage design.
6.4 Temporary Excavation Support and Retention
Temporary batter slopes through the sandy soil profile no steeper than 1 Vertical (V) in 1.5 Horizontal (H) are
considered to be appropriate, steeper batters of 1V in 1H will be suitable for clay soils and any extremely
weathered bedrock (if encountered), though some surficial instability could still occur. These temporary
batters are expected to be achievable within the site geometry.
Some instability of temporary sand batters may occur at, or below, the level of any groundwater seepage
should it occur, especially after rain periods, and sand bagging may be required to stabilise the lower portion
of these batters.
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Stockpiles of construction materials, excavated materials, existing footings, etc. should be kept well clear of
the batter crests to avoid surcharging the slopes.
We expect that retaining walls will be constructed within the temporary batters, temporarily propped,
backfilled and permanently supported by the structure.
6.5 Retention Design Parameters
The following characteristic earth pressure coefficients and subsoil parameters may be adopted for the
design of retaining walls that will be propped by the structure and any landscape retaining walls:
For retaining walls that will be temporarily propped, backfilled and permanently restrained by the
structure, we recommend the use of a triangular lateral earth pressure distribution with an ‘at rest’ earth
pressure coefficient (ko) of 0.6 for the soil profile, assuming a horizontal backfill surface.
For landscape retaining walls, where we assume minor movements will be able to be tolerated, we
recommend the use of a triangular lateral earth pressure distribution with an ‘active’ earth pressure
coefficient (ka) of 0.4 for the soil profile, assuming a horizontal backfill surface.
A bulk unit weight of 20kN/m3 should be adopted for the retained profile.
Any surcharge affecting the walls (e.g. nearby footings, compaction stresses, sloping retained surfaces,
construction loads etc) should be allowed for in the design using the appropriate above earth pressure
coefficient.
Conventional retaining walls should be designed as drained and provision made for permanent and
effective drainage of the ground behind the walls. Subsurface drains should incorporate a non-woven
geotextile fabric, such as Bidim A34, to act as a filter against subsoil erosion. The subsoil drains should
discharge into the stormwater system.
Lateral restraint of retaining walls founded in the soil profile below bulk excavation level or adjacent
surface levels may be provided by the passive pressure of the soil below these levels. A ‘passive’ earth
pressure coefficient, Kp, of 3 may be adopted, provided a Factor of Safety of at least 2 is used in order to
reduce the high deflections that are associated with achieving a full passive case. Localised excavations
in front of the walls e.g. for buried services etc must also be taken into account when checking the
embedment depth design. We note that where the ground surface on the low side of the wall slopes
away from the wall, the lowest ground level within 5m of the wall should be adopted for assessing the
level at which passive resistance will be effective.
Friction between the base of a retaining walls and the underlying bedrock can be calculated using a
friction angle of 35°, provided the base is clean, rough and dry when the retaining wall footing is poured.
6.6 Footing Design
The ASI report has assessed the residual clays to have a high potential for shrink/swell reactivity with changes
in moisture content. However, considering the limited thickness of reactive residual clays and clay fill,
characteristic surface movements in the range for a Class S to Class M site are expected and are likely to be
differential across the proposed house. However, we expect trees to be removed as part of the work and so
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elevated characteristic surface shrink swell movements in the range of a Class M to H1 site could be expected
and would again be expected to be differential.
The upper residual sands are of variable relative density and very loose sands and soft to firm residual clays
are not considered to be suitable foundation materials due to the low allowable bearing capacity and
potential for high elastic settlements. The very loose sands are also not considered suitable for supporting
building loads as they are susceptible to additional settlements due to vibrations and infiltration of surface
water run-off.
For uniformity of support and for stability considerations, the footings supporting the proposed house should
be uniformly founded in bedrock which we expect to be encountered at depths between about 2.2m and
2.8m. Footings may be designed for an allowable bearing capacity of 600kPa, subject to inspection of a
representative number of footing excavations by a geotechnical engineer prior to placing the reinforcement
cages.
Locally over the deeper section of the excavation pad or strip footings will be suitable. However, over the
majority of the proposed building, we expect that bored piles will be suitable but will require rock sockets to
be formed and sacrificial liners will be required to support the upper sandy soils, where encountered. We
expect that powerful pile drilling plant with rock augers fitted with tungsten carbide teeth would be required
although the size of piling rig may be restricted by the limited site access and sloping nature of the site.
Further advice from the piling contractor should be sought in regard to the most suitable piling rig to form
the rock sockets particularly as medium (and possibly higher) strength bedrock may be encountered.
Due to the variability relative density and strength of the upper residual sand and clay profile, the potential
for differential movements and for uniformity of support, we recommend that the later southern extension
also be founded in bedrock. If it is preferred to found the southern extension in the residual sands or clays
then further geotechnical advice would need to be sought. However, we forewarn that the footings and floor
slabs would need to be designed with regard for the recommendations for a Class M or H1 site (due to the
removal of trees) provided in AS2870 and differential shrink swell movements would be expected across the
extension would be expected. A full height movement control joint would also need to be provided between
the extension and the proposed house, which will be founded in bedrock. An allowable bearing pressure of
100kPa may be adopted for design of footings founded in the residual clays of at least stiff strength or residual
sands of at least loose relative density but this would locally require piers to be founded in the order of 2.2m
depth (based on BH1). Differential reactive movements would also be expected and we reiterate that further
geotechnical advice must be sought if the later extension is to be founded in the residual soils.
The weathered bedrock will soften in the presence of water so the piles must be drilled, inspected and poured
without delay and preferably on the same day. All pile holes should be free from all loose or softened
materials prior to pouring. If water ponds in the base of the pile holes, the water should be removed and
then the footing excavation deepened to remove all loose and any water softened materials.
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6.7 Floor Slabs
We understand that a timber frame floor slab is proposed and will be entirely suspended, including the
proposed later extension although the car port will comprise a concrete slab on ground.
Whilst slab-on-grade construction is theoretically feasible for the car port floor slab, it will be difficult to
complete high quality earthworks on a small site such as this, as it would require removing and re-compacting
any existing poorly compacted fill. Also, on-grade floor slabs in contact with varying thickness of foundation
soils would result in differential deflections, and potential cracking of the car port floor slab. We recommend
that the car port floor slab also be fully suspended over the soil profile between footings founded in bedrock.
For floor slabs suspended over soil subgrade areas, unless sacrificial formwork is used (such as ‘Bondek’) the
subgrade preparation would comprise the removal of any topsoil and/or any soil containing organics,
completion of and re-profiling of site surface levels and the nominal tracking of ‘formwork fill’ to the required
subgrade level. We forewarn that even this nominal subgrade preparation will be difficult on this sloping site
and the use of sacrificial formwork is strongly recommended. However, void formers will be required if the
slabs are in contact with the reactive soils. Design of the void formers should be to accommodate maximum
shrink swell movements of about 30mm to 40mm below the slab and beams between the piles so that the
swell pressures from the clays are not transferred to the slab.
The car port on-grade floor slab, if preferred, should be provided with joints capable of resisting shear forces
but not bending moments by providing dowels or keys. The on-grade car port floor slab should be designed
as ‘floating’ i.e. separated from all footings and walls in order to permit relative movement.
The future on grade car port slab will be supporting traffic loads and should be provided with at least a
100mm thick subbase layer of unbound or equivalent good quality crushed rock (e.g. ‘Type 2’ material in
accordance with the latest revision of TMR Technical Specification MRTS05, compacted in a single layer using
a large smooth drum roller to at least 98% of Standard Maximum Dry Density (SMDD). A bond breaker should
be provided between the concrete base and the sub-base. Moisture conditioning to within 2% of Standard
Optimum Moisture Content (SOMC) should be provided during placement so as to reduce the potential for
material breakdown. The subbase will provide more uniform slab support and protect areas of prepared clay
subgrade from deterioration. The subbase will also reduce the “pumping” of “fines” at joints.
Surface drains should be provided along the perimeter of the car port floor slab, with inverts not less than
0.2m below design subgrade level. The drains should be excavated with a longitudinal fall to appropriate
discharge points so as to reduce the risk of water ponding. The car port subgrade should be graded to
promote water flow to the drains.
6.8 Earthworks
For on-grade construction of the car port floor slab, we reiterate that it will be difficult to complete high
quality earthworks over such small site areas. Even if the earthworks are completed in accordance with the
following recommendations, it is likely there will be a significant variation in subgrade conditions, resulting
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in some degree of differential deflection, and potential cracking of the floor slabs. If this cannot be accepted,
then the floor slabs should be suspended (and provided with void formers) as described in Section 6.7 above.
The following earthworks recommendations should be complemented by reference to AS3798-2007
“Guidelines on Earthworks for Commercial and Residential Developments”.
6.8.1 Subgrade Preparation
Prior to placement of engineered fill for treatment of poor subgrade areas and/or construction of on-grade
floor slabs, the soil subgrade should be prepared as follows:
Following removal of any existing paved surfaces, and stripping of topsoil and/or any root affected soils,
any remaining poorly compacted fill should be removed, as directed by the geotechnical engineer.
Proof roll the soil subgrade with a minimum 2 tonne deadweight smooth drum roller, using the static
(non-vibration) mode, following thorough moistening of the sand subgrade. Where access is restricted,
proof rolling is expected to require the use of a hand-held plate/sled compactor.
If soil softening occurs after rainfall, then the clay subgrade should be over-excavated to below the depth
of moisture softening and replaced with engineered fill. Conversely, if a clayey subgrade exhibits
shrinkage cracking, then the surface should be lightly watered and rolled until the shrinkage cracks are
no longer evident.
Proof rolling should be closely monitored by the geotechnical engineer to detect soft or unstable areas
which should be removed and replaced with engineered fill (as outlined below). Care should be taken
not to over-compact the clayey subgrade.
Care should also be taken when using vibrating equipment not to cause damage to any adjacent
structures. The vibrations should be qualitatively monitored by site personnel and if there is any cause
for concern then proof rolling should cease and further advice sought.
6.8.2 Drainage During Construction
The subgrade will comprise clay soils which may be found to be unstable if proper site drainage is not
implemented during construction. It is therefore important to provide good drainage in order to promote
run-off and reduce ponding. Earthworks platforms should be graded to maintain cross-falls during
construction. If the clays are exposed to periods of rainfall, softening may result and site trafficability will be
poor. If softening occurs, the subgrade should be over-excavated to below the depth of moisture softening.
The material removed should be replaced with engineered fill. Trafficability may be improved by the use of
a sacrificial surface layer of crushed demolition rubble.
6.8.3 Engineered Fill
For treatment of any poor subgrade areas and backfill to retaining walls, engineered fill should be used.
Engineered fill should be free from organic materials, other contaminants and deleterious substances and
have a maximum particle size not exceeding 40mm. The locally excavated clays may be used as engineered
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fill but would most likely require moisture conditioning which could result in delays. Also there may be a
shortfall in site won materials and imported fill would be required.
We recommend that engineered fill comprising well graded imported granular materials such as demolition
rubble would be suitable for treatment of poor subgrade areas and for backfilling retaining walls. The
imported engineered fill must be free of contaminants or deleterious substances and has a maximum particle
size not exceeding 40mm. Imported well graded granular fill should be compacted to at least 98% of SMDD
and within 2% of their SOMC. The compaction requirement may be reduced to 95% of SMDD in landscaped
areas, where settlement can be tolerated. Such fill should be compacted in horizontal layers as described
above. Care will be required to ensure excessive compaction stresses are not transferred to the retaining
walls.
Density tests should be carried out at the frequencies outlined in AS3798 (Table 8.1) for the volume of fill
involved. At least Level 2 testing of earthworks should be carried out in accordance with AS3798. Any areas
of insufficient compaction will require reworking.
As an alternative, single sized granular material (or ‘no fines’ gravel) may be used as backfill to the landscape
retaining walls and this would also act as the drainage behind the wall and would only require nominal
compaction (with no compaction testing). The drainage material should be wrapped in a non-woven
geotextile fabric (e.g. Bidim A34) to act as a filter against subsoil erosion. Further, retaining wall backfill
should be provided with a cap of at least 0.3m thickness of compacted crushed sandstone or clay at surface
level to reduce the likelihood of surface water surcharging the retaining wall.
6.9 Wastewater Treatment and Disposal
We note that the ASI “Site Soil Evaluation” report as recommended the following method of on-site effluent
disposal comprise an Advanced Secondary Treatment with an Advanced Enviro Septic System (AES)
Transpiration Bed.
For further details refer to the ASI “Site and Soil Evaluation” report.
6.10 Overall Effect of Development on Stability
We do not consider that the development will have any detrimental impact on the stability of the site. This
is on condition that the design and construction of the proposed development is completed in accordance
with the geotechnical advice presented in this report.
7 CONCLUSION
We consider that the site is suitable for the proposed development.
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The geotechnical advice presented in the preceding sections of this report are intended to achieve
“Acceptable” levels of risk of instability of the site and to control future risks. The above recommendations
address geotechnical issues only and other conditions may be required to address other aspects.
8 GENERAL COMMENTS
The recommendations presented in this report include specific issues to be addressed during the
construction phase of the project. As an example, special treatment of soft spots may be required as a result
of their discovery during proof-rolling, etc. In the event that any of the construction phase recommendations
presented in this report are not implemented, the general recommendations may become inapplicable and
JK Geotechnics accept no responsibility whatsoever for the performance of the structure where
recommendations are not implemented in full and properly tested, inspected and documented.
Occasionally, the subsurface conditions between and below the boreholes and DCP tests completed by ASI
may be found to be different (or may be interpreted to be different) from those expected. Variation can also
occur with groundwater conditions, especially after climatic changes. If such differences appear to exist, we
recommend that you immediately contact this office.
This report provides advice on geotechnical aspects for the proposed civil and structural design. As part of
the documentation stage of this project, Contract Documents and Specifications may be prepared based on
our report. However, there may be design features we are not aware of or have not commented on for a
variety of reasons. The designers should satisfy themselves that all the necessary advice has been obtained.
If required, we could be commissioned to review the geotechnical aspects of contract documents to confirm
the intent of our recommendations has been correctly implemented.
A waste classification is required for any soil and/or bedrock excavated from the site prior to offsite disposal.
Subject to the appropriate testing, material can be classified as Virgin Excavated Natural Material (VENM),
Excavated Natural Material (ENM), General Solid, Restricted Solid or Hazardous Waste. Analysis can take up
to seven to ten working days to complete, therefore, an adequate allowance should be included in the
construction program unless testing is completed prior to construction. If contamination is encountered,
then substantial further testing (and associated delays) could be expected. We strongly recommend that this
requirement is addressed prior to the commencement of excavation on site.
This report has been prepared for the particular project described and no responsibility is accepted for the
use of any part of this report in any other context or for any other purpose. If there is any change in the
proposed development described in this report then all recommendations should be reviewed. Copyright in
this report is the property of JK Geotechnics. We have used a degree of care, skill and diligence normally
exercised by consulting engineers in similar circumstances and locality. No other warranty expressed or
implied is made or intended. Subject to payment of all fees due for the investigation, the client alone shall
have a licence to use this report. The report shall not be reproduced except in full.
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Reference 1: Australian Geomechanics Society (2007c) ‘Practice Note Guidelines for Landslide Risk Management’,
Australian Geomechanics, Vol 42, No 1, March 2007, pp63-114.
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TABLE A
SUMMARY OF RISK ASSESSMENT TO PROPERTY
POTENTIAL LANDSLIDE Existing Conditions Following Construction and Implementation of Recommendations in Section 6 of the Report
HAZARD
1 2 3 1 2 3 4 5
Deep seated Near Surface Creep Instability of existing Deep seated Near Surface Creep Instability of existing Instability of Instability of new
instability of the steeper retaining walls instability of the steeper retaining walls temporary retaining walls
section of the site section of the site excavation batters
Assessed Likelihood Rare Almost Certain Unlikely Rare Almost Certain Unlikely Unlikely Unlikely
Assessed Consequence Minor Insignificant Minor Medium Insignificant Minor Minor Medium
Risk Very Low Low Low Low Low Low Low Low
Comments Instability affecting Slow moving only Localised section of Instability affecting Slow moving only Localised section of Localised instability. Localised instability.
the soil profile within impacting a localised retaining wall and the soil profile within impacting a localised retaining wall and Assumes walls are
Assumed
the site. steeper portion of small volume the site. steeper portion of small volume engineer designed.
recommended batter
the site. instability. the site. instability.
slopes are carried out
Assumes walls are Assumes walls are in accordance with
engineer designed. removed as part of the advice in our
the works and/or site report.
maintenance
measures
implemented.
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SITE
SOURCE: http://www.whereis.com/
SITE
WOONG
AR STR
E ET
DWG FILE: J:\6F GEOTECHNICAL JOBS\35000'S\35891\CAD\35891UR.DWG
ET
R STRE
HECTO
AERIAL IMAGE SOURCE: MAPS.AU.NEARMAP.COM Title:
SITE LOCATION PLAN
PLOT DATE: 28/03/2023 10:08:32 AM
Location:
20 WOONGAR STREET, BOREEN POINT, QLD
Report No: Figure No:
35891UR 1
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© JK GEOTECHNICS
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ROCK FACING
BUSHLAND
≈30°
MAX ≈30°
≈5°-10°
MAX ≈5°
≈5°
≈25°
≈40°
0.6m
OVERGROWN
APPROXIMATE OUTLINE OF
PROPOSED GROUND FLOOR LEVEL
FLAT
GRASS
≈2° FLAT
DWG FILE: J:\6F GEOTECHNICAL JOBS\35000'S\35891\CAD\35891UR.DWG
NOOSA COUNCIL
APPROVED PLAN
EXE23/0029 6 NOV 2023 WOONGAR STREET
PLOT DATE: 28/03/2023 10:10:04 AM
Title:
GEOTECHNICAL SITE PLAN
Location:
NOTE: 0 2 4 6 8 10 20 WOONGAR STREET, BOREEN POINT, QLD
REFER TO FIGURE 3 FOR GEOTECHNICAL MAPPING SYMBOLS.
SCALE 1:200 @A3 METRES Report No: Figure No:
35891UR 2
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© JK GEOTECHNICS
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DWG FILE: J:\6F GEOTECHNICAL JOBS\35000'S\35891\CAD\35891UR.DWG
Title:
GEOTECHNICAL MAPPING SYMBOLS
PLOT DATE: 28/03/2023 10:08:47 AM
Location:
20 WOONGAR STREET, BOREEN POINT, QLD
Report No: Figure No:
35891UR 3
This plan should be read in conjunction with the JK Geotechnics report.
© JK GEOTECHNICS
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APPENDIX A
LANDSLIDE RISK
MANAGEMENT
TERMINOLOGY
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LANDSLIDE RISK MANAGEMENT
Definition of Terms and Landslide Risk
Risk Terminology Description
Acceptable Risk A risk for which, for the purposes of life or work, we are prepared to accept as it is with no regard to its
management. Society does not generally consider expenditure in further reducing such risks justifiable.
Annual Exceedance The estimated probability that an event of specified magnitude will be exceeded in any year.
Probability (AEP)
Consequence The outcomes or potential outcomes arising from the occurrence of a landslide expressed qualitatively
or quantitatively, in terms of loss, disadvantage or gain, damage, injury or loss of life.
Elements at Risk The population, buildings and engineering works, economic activities, public services utilities,
infrastructure and environmental features in the area potentially affected by landslides.
Frequency A measure of likelihood expressed as the number of occurrences of an event in a given time. See also
‘Likelihood’ and ‘Probability’.
Hazard A condition with the potential for causing an undesirable consequence (the landslide). The description
of landslide hazard should include the location, volume (or area), classification and velocity of the
potential landslides and any resultant detached material, and the likelihood of their occurrence within
a given period of time.
Individual Risk to Life The risk of fatality or injury to any identifiable (named) individual who lives within the zone impacted
by the landslide; or who follows a particular pattern of life that might subject him or her to the
consequences of the landslide.
Landslide Activity The stage of development of a landslide; pre failure when the slope is strained throughout but is
essentially intact; failure characterised by the formation of a continuous surface of rupture; post failure
which includes movement from just after failure to when it essentially stops; and reactivation when the
slope slides along one or several pre-existing surfaces of rupture. Reactivation may be occasional
(eg. seasonal) or continuous (in which case the slide is ‘active’).
Landslide Intensity A set of spatially distributed parameters related to the destructive power of a landslide. The parameters
may be described quantitatively or qualitatively and may include maximum movement velocity, total
displacement, differential displacement, depth of the moving mass, peak discharge per unit width, or
kinetic energy per unit area.
Landslide Risk The AGS Australian GeoGuide LR7 (AGS, 2007e) should be referred to for an explanation of Landslide
Risk.
Landslide The classification, and volume (or area) of landslides which exist or potentially may occur in an area or
Susceptibility may travel or retrogress onto it. Susceptibility may also include a description of the velocity and
intensity of the existing or potential landsliding.
Likelihood Used as a qualitative description of probability or frequency.
Probability A measure of the degree of certainty. This measure has a value between zero (impossibility) and 1.0
(certainty). It is an estimate of the likelihood of the magnitude of the uncertain quantity, or the
likelihood of the occurrence of the uncertain future event.
These are two main interpretations:
(i) Statistical – frequency or fraction – The outcome of a repetitive experiment of some kind like
flipping coins. It includes also the idea of population variability. Such a number is called an
‘objective’ or relative frequentist probability because it exists in the real world and is in principle
measurable by doing the experiment.
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Risk Terminology Description
Probability (ii) Subjective probability (degree of belief) – Quantified measure of belief, judgment, or confidence
(continued) in the likelihood of an outcome, obtained by considering all available information honestly, fairly,
and with a minimum of bias. Subjective probability is affected by the state of understanding of a
process, judgment regarding an evaluation,
or the quality and quantity of information. It may change over time as the state of knowledge
changes.
Qualitative Risk An analysis which uses word form, descriptive or numeric rating scales to describe the magnitude of
Analysis potential consequences and the likelihood that those consequences will occur.
Quantitative Risk An analysis based on numerical values of the probability, vulnerability and consequences and resulting
Analysis in a numerical value of the risk.
Risk A measure of the probability and severity of an adverse effect to health, property or the environment.
Risk is often estimated by the product of probability x consequences. However, a more general
interpretation of risk involves a comparison of the probability and consequences in a non-product form.
Risk Analysis The use of available information to estimate the risk to individual, population, property, or the
environment, from hazards. Risk analyses generally contain the following steps: scope definition,
hazard identification and risk estimation.
Risk Assessment The process of risk analysis and risk evaluation.
Risk Control or Risk The process of decision-making for managing risk and the implementation or enforcement of risk
Treatment mitigation measures and the re-evaluation of its effectiveness from time to time, using the results of
risk assessment as one input.
Risk Estimation The process used to produce a measure of the level of health, property or environmental risks being
analysed. Risk estimation contains the following steps: frequency analysis, consequence analysis and
their integration.
Risk Evaluation The stage at which values and judgments enter the decision process, explicitly or implicitly, by including
consideration of the importance of the estimated risks and the associated social, environmental and
economic consequences, in order to identify a range of alternatives for managing the risks.
Risk Management The complete process of risk assessment and risk control (or risk treatment).
Societal Risk The risk of multiple fatalities or injuries in society as a whole: one where society would have to carry
the burden of a landslide causing a number of deaths, injuries, financial, environmental and other
losses.
Susceptibility See ‘Landslide Susceptibility’.
Temporal Spatial The probability that the element at risk is in the area affected by the landsliding, at the time of the
Probability landslide.
Tolerable Risk A risk within a range that society can live with so as to secure certain net benefits. It is a range of risk
regarded as non-negligible and needing to be kept under review and reduced further if possible.
Vulnerability The degree of loss to a given element or set of elements within the area affected by the landslide
hazard. It is expressed on a scale of 0 (no loss) to 1 (total loss). For property, the loss will be the value
of the damage relative to the value of the property; for persons, it will be the probability that a
particular life (the element at risk) will be lost, given the person(s) is affected by the landslide.
NOTE: Reference should be made to Figure A1 which shows the inter-relationship of many of these terms and the
relevant portion of Landslide Risk Management.
Reference should also be made to the paper referenced below for Landslide Terminology and more detailed
discussion of the above terminology.
This appendix is an extract from PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT as presented in Australian
Geomechanics, Vol 42, No 1, March 2007, which discusses the matter more fully.
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FIGURE A1: Flowchart for Landslide Risk Management.
This figure is an extract from GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND RISK ZONING FOR LAND USE
PLANNING, as presented in Australian Geomechanics Vol 42, No 1, March 2007, which discusses the matter more fully.
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TABLE A1: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Indicative Notional Implied Indicative Landslide Recurrence Interval Description Descriptor Level
Value Boundary
10-1 10 years The event is expected to occur over the design life. ALMOST CERTAIN A
510-2 20 years
The event will probably occur under adverse conditions over the
10 -2 100 years LIKELY B
design life.
510 -3 200 years
The event could occur under adverse conditions over the design
10-3 1000 years POSSIBLE C
510 -4 life.
2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years UNLIKELY D
-5 design life.
510 20,000 years
-5 The event is conceivable but only under exceptional circumstances
10 100,000 years RARE E
-2
over the design life.
510 200,000 years
10-6 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
Indicative Notional Description Descriptor Level
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for stabilisation. Could
200% CATASTROPHIC 1
100% cause at least one adjacent property major consequence damage.
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant stabilisation works.
60% MAJOR 2
40% Could cause at least one adjacent property medium consequence damage.
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works. Could cause at
20% MEDIUM 3
10% least one adjacent property minor consequence damage.
5% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
1% Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a notional boundary of
0.5% INSIGNIFICANT 5
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.
Extract from PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT as presented in Australian Geomechanics, Vol 42, No 1, March 2007, which discusses the matter more fully.
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TABLE A1: LANDSLIDE RISK ASSESSMENT
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: INSIGNIFICANT
Approximate Annual 200% 60% 20% 5% 0.5%
Probability
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
B - LIKELY 10-2 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
E - RARE 10-5 M L L VL VL
F - BARELY CREDIBLE 10-6 L VL VL VL VL
Notes: (5) 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.
VL VERY LOW RISK Acceptable. Manage by normal slope maintenance procedures.
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.
Extract from PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT as presented in Australian Geomechanics, Vol 42, No 1, March 2007, which discusses the matter more fully.
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AUSTRALIAN GEOGUIDE LR2 (LANDSLIDES)
What is a Landslide?
Any movement of a mass of rock, debris, or earth, down a slope, constitutes a “landslide”. Landslides take many forms, some of
which are illustrated. More information can be obtained from Geoscience Australia, or by visiting its Australian landslide Database
at www.ga.gov.au/urban/factsheets/landslide.jsp. Aspects of the impact of landslides on buildings are dealt with in the book
“Guideline Document Landslide Hazards” published by the Australian Building Codes Board and referenced in the Building Code of
Australia. This document can be purchased over the internet at the Australian Building Codes Board’s website www.abcb.gov.au.
Landslides vary in size. They can be small and localised or very large, sometimes extending for kilometres and involving millions of
tonnes of soil or rock. It is important to realise that even a 1 cubic metre boulder of soil, or rock, weighs at least 2 tonnes. If it falls,
or slides, it is large enough to kill a person, crush a car, or cause serious structural damage to a house. The material in a landslide
may travel downhill well beyond the point where the failure first occurred, leaving destruction in its wake. It may also leave an
unstable slope in the ground behind it, which has the potential to fall again, causing the landslide to extend (regress) uphill, or expand
sideways. For all these reasons, both “potential” and “actual” landslides must be taken very seriously. The present a real threat to
life and property and require proper management.
Identification of landslide risk is a complex task and must be undertaken by a geotechnical practitioner (GeoGuide LR1) with specialist
experience in slope stability assessment and slope stabilisation.
What Causes a Landslide?
Landslides occur as a result of local geological and groundwater conditions, but can be exacerbated by inappropriate development
(GeoGuide LR8), exceptional weather, earthquakes and other factors. Some slopes and cliffs never seem to change, but are actually
on the verge of failing. Others, often moderate slopes (Table 1), move continuously, but so slowly that it is not apparent to a casual
observer. In both cases, small changes in conditions can trigger a landslide with series consequences. Wetting up of the ground (which
may involve a rise in groundwater table) is the single most important cause of landslides (GeoGuide LR5). This is why they often
occur during, or soon after, heavy rain. Inappropriate development often results in small scale landslides which are very expensive
in human terms because of the proximity of housing and people.
Does a Landslide Affect You?
Any slope, cliff, cutting, or fill embankment may be a hazard which has the potential to impact on people, property, roads and
services. Some tell-tale signs that might indicate that a landslide is occurring are listed below:
Open cracks, or steps, along contours trees leaning down slope, or with exposed roots
Groundwater seepage, or springs debris/fallen rocks at the foot of a cliff
Bulging in the lower part of the slope tilted power poles, or fences
Hummocky ground cracked or distorted structures
These indications of instability may be seen on almost any slope and are not necessarily confined to the steeper ones (Table 1).
Advice should be sought from a geotechnical practitioner if any of them are observed. Landslides do not respect property boundaries.
As mentioned above they can “run-out” from above, “regress” from below, or expand sideways, so a landslide hazard affecting your
property may actually exist on someone else’s land.
Local councils are usually aware of slope instability problems within their jurisdiction and often have specific development and
maintenance requirements. Your local council is the first place to make enquiries if you are responsible for any sort of development
or own or occupy property on or near sloping land or a cliff.
TABLE 1 – Slope Descriptions
Slope Maximum
Appearance Angle Gradient Slope Characteristics
Gentle 0 - 10 1 on 6 Easy walking.
Moderate 10 - 18 1 on 3 Walkable. Can drive and manoeuvre a car on driveway.
Steep 18 - 27 1 on 2 Walkable with effort. Possible to drive straight up or down roughened
concrete driveway, but cannot practically manoeuvre a car.
Very Steep 27 - 45 1 on 1 Can only climb slope by clutching at vegetation, rocks, etc.
Extreme 45 - 64 1 on 0.5 Need rope access to climb slope.
Cliff 64 - 84 1 on 0.1 Appears vertical. Can abseil down.
Vertical or Overhang 84 - 90 Infinite Appears to overhang. Abseiler likely to lose contact with the face.
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Some typical landslides which could affect residential housing are illustrated below:
Rotational or circular slip failures (Figure 1) - can occur on moderate
to very steep soil and weathered rock slopes (Table 1). The sliding
surface of the moving mass tends to be deep seated. Tension cracks
may open at the top of the slope and bulging may occur at the toe.
The ground may move in discrete "steps" separated by long periods
without movement. More rapid movement may occur after heavy
rain. Figure 1
Translational slip failures (Figure 2) - tend to occur on moderate to
very steep slopes (Table 1) where soil, or weak rock, overlies stronger
strata. The sliding mass is often relatively shallow. It can move, or
deform slowly (creep) over long periods of time. Extensive linear
cracks and hummocks sometimes form along the contours. The
sliding mass may accelerate after heavy rain.
Figure 2
Wedge failures (Figure 3) - normally only occur on extreme slopes, or
cliffs (Table 1), where discontinuities in the rock are inclined steeply
downwards out of the face.
Rock falls (Figure 3) - tend to occur from cliffs and overhangs (Table
1).
Cliffs may remain, apparently unchanged, for hundreds of years.
Collections of boulders at the foot of a cliff may indicate that rock falls
are ongoing. Wedge failures and rock falls do not "creep". Familiarity
with a particular local situation can instil a false sense of security since
failure, when it occurs, is usually sudden and catastrophic.
Figure 3
Debris flows and mud slides (Figure 4) - may occur in the foothills of
ranges, where erosion has formed valleys which slope down to the
plains below. The valley bottoms are often lined with loose eroded
material (debris) which can "flow" if it becomes saturated during and
after heavy rain. Debris flows are likely to occur with little warning;
they travel a long way and often involve large volumes of soil. The
consequences can be devastating.
Figure 4
More information relevant to your particular situation may be found in other Australian GeoGuides:
GeoGuide LR1 - Introduction GeoGuide LR7 - Landslide Risk
GeoGuide LR3 - Soil Slopes GeoGuide LR8 - Hillside Construction
GeoGuide LR4 - Rock Slopes GeoGuide LR9 - Effluent & Surface Water Disposal
GeoGuide LR5 - Water & Drainage GeoGuide LR10 - Coastal Landslides
GeoGuide LR6 - Retaining Walls GeoGuide LR11 - Record Keeping
The Australian GeoGuides (LR series) are a set of publications intended for property owners; local councils; planning authorities; developers;
insurers; lawyers and, in fact, anyone who lives with, or has an interest in, a natural or engineered slope, a cutting, or an excavation. They
are intended to help you understand why slopes and retaining structures can be a hazard and what can be done with appropriate professional
advice and local council approval (if required) to remove, reduce, or minimise the risk they represent. The GeoGuides have been prepared
by the Australian Geomechanics Society, a specialist technical society within Engineers Australia, the national peak body for all engineering
disciplines in Australia, whose members are professional geotechnical engineers and engineering geologists with a particular interest in
ground engineering. The GeoGuides have been funded under the Australian governments’ National Disaster Mitigation Program.
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AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK)
Concept of Risk for a particular site you should expect to receive a report
prepared in accordance with current professional guidelines
Risk is a familiar term, but what does it really mean? It can be and in a form that is acceptable to your local council, or
defined as "a measure of the probability and severity of an planning authority.
adverse effect to health, property, or the environment." This
definition may seem a bit complicated. In relation to Risk to Property
landslides, geotechnical practitioners (see GeoGuide LR1) are
required to assess risk in terms of the likelihood that a Table 1 indicates the terms used to describe risk to property.
particular landslide will occur and the possible consequences. Each risk level depends on an assessment of how likely a
This is called landslide risk assessment. The consequences of landslide is to occur and its consequences in dollar terms.
a landslide are many and varied, but our concerns normally “Likelihood” is the chance of it happening in any one year, as
focus on loss of, or damage to, property and loss of life. indicated in Table 2. “Consequences” are related to the cost
of the repairs and temporary loss of use if the landslide occurs.
Landslide Risk Assessment These two factors are combined by the geotechnical
practitioner to determine the Qualitative Risk.
Some local councils in Australia are aware of the potential for
landslides within their jurisdiction and have responded by TABLE 2 – LIKELIHOOD
designating specific “landslide hazard zones". Development in Likelihood Annual Probability
these areas is normally covered by special regulations. If you Almost Certain 1:10
are contemplating building, or buying an existing house, Likely 1:100
particularly in a hilly area, or near cliffs, then go first for Possible 1:1,000
information to your local council. Unlikely 1:10,000
Rare 1:100,000
Landslide risk assessment must be undertaken by a Barely credible 1:1,000,000
geotechnical practitioner. It may involve visual inspection,
geological mapping, geotechnical investigation and The terms "unacceptable", "may be tolerable" etc. in Table 1
monitoring to identify: indicate how most people react to an assessed risk level.
However, some people will always be more prepared, or
potential landslides (there may be more than one that better able, to tolerate a higher risk level than others.
could impact on your site);
the likelihood that they will occur; Some local councils and planning authorities stipulate a
the damage that could result; maximum tolerable risk level of risk to property for
the cost of disruption and repairs; and developments within their jurisdictions. In these situations
the extent to which lives could be lost. the risk must be assessed by a geotechnical practitioner. If
stabilisation works are needed to meet the stipulated
Risk assessment is a predictive exercise, but since the ground requirements these will normally have to be carried out as
and the processes involved are complex, prediction tends to part of the development, or consent will be withheld.
lack precision. If you commission a landslide risk assessment
TABLE 1 – RISK TO PROPERTY
Qualitative Risk Significance - Geotechnical engineering requirements
Very high VH Unacceptable without treatment. Extensive detailed investigation and research, planning and
implementation of treatment options essential to reduce risk to Low. May be too expensive and not
practical. Work likely to cost more than the value of the property.
High H Unacceptable without treatment. Detailed investigation, planning and implementation of treatment
options required to reduce risk to acceptable level. Work would cost a substantial sum in relation to the
value of the property.
Moderate M May be tolerated in certain circumstances (subject to regulator's approval) but requires investigation,
planning and implementation of treatment options to reduce the risk to Low. Treatment options to
reduce to Low risk should be implemented as soon as possible.
Low L Usually acceptable to regulators. Where treatment has been needed to reduce the risk to this level,
ongoing maintenance is required.
Very Low VL Acceptable. Manage by normal slope maintenance procedures.
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Risk to Life In NSW, the planning authorities consider that 1:1,000,000 is
the maximum tolerable risk for domestic housing built near
Most of us have some difficulty grappling with the concept of an obvious hazard, such as a chemical factory. Although not
risk and deciding whether, or not, we are prepared to accept specifically considered in the NSW guidelines there is little
it. However, without doing any sort of analysis, or difference between the hazard presented by a neighbouring
commissioning a report from an "expert", we all take risks factory and a landslide: both have the capacity to destroy life
every day. One of them is the risk of being killed in an and property and both are always present.
accident. This is worth thinking about, because it tells us a lot
about ourselves and can help to put an assessed risk into a TABLE 3 – RISK TO LIFE
meaningful context. By identifying activities that we either Risk (deaths per Activity/Event Leading to Death
are, or are not, prepared to engage in, we can get some participant per (NSW data unless noted)
indication of the maximum level of risk that we are prepared year)
to take. This knowledge can help us to decide whether we
really are able to accept a particular risk, or to tolerate a
1:1,000 Deep sea fishing (UK)
particular likelihood of loss, or damage, to our property
(Table 2). 1:1,000 to
Motor cycling, horse riding, ultra-
1:10,000
In Table 3, data from NSW for the years 1998 to 2002, and light flying (Canada)
other sources, is presented. A risk of 1 in 100,000 means that, Motor vehicle use
in any one year, 1 person is killed for every 100,000 people 1:23,000
undertaking that particular activity. The NSW data assumes
that the whole population undertakes the activity. That is, we 1:30,000 Fall
are all at risk of being killed in a fire, or of choking on our food,
but it is reasonable to assume that only people who go deep 1:70,000 Drowning
sea fishing run a risk of being killed while doing it.
1:180,000 Fire/burn
It can be seen that the risks of dying as a result of falling, using
a motor vehicle, or engaging in water-related activities 1:660,000 Choking on food
(including bathing) are all greater than 1:100,000 and yet few
people actively avoid situations where these risks are present. 1:1,000,000 Scheduled airlines (Canada)
Some people are averse to flying and yet it represents a lower
risk than choking to death on food. The data also indicate that, 1:2,300,000 Train travel
even when the risk of dying as a consequence of a particular
event is very small, it could still happen to any one of us today. 1:32,000,000 Lightning strike
If this were not so, there would be no risk at all and clearly
that is not the case.
More information relevant to your particular situation may be found in other Australian GeoGuides:
GeoGuide LR1 - Introduction GeoGuide LR7 - Landslide Risk
GeoGuide LR3 - Soil Slopes GeoGuide LR8 - Hillside Construction
GeoGuide LR4 - Rock Slopes GeoGuide LR9 - Effluent & Surface Water Disposal
GeoGuide LR5 - Water & Drainage GeoGuide LR10 - Coastal Landslides
GeoGuide LR6 - Retaining Walls GeoGuide LR11 - Record Keeping
The Australian GeoGuides (LR series) are a set of publications intended for property owners; local councils; planning authorities; developers;
insurers; lawyers and, in fact, anyone who lives with, or has an interest in, a natural or engineered slope, a cutting, or an excavation. They
are intended to help you understand why slopes and retaining structures can be a hazard and what can be done with appropriate professional
advice and local council approval (if required) to remove, reduce, or minimise the risk they represent. The GeoGuides have been prepared
by the Australian Geomechanics Society, a specialist technical society within Engineers Australia, the national peak body for all engineering
disciplines in Australia, whose members are professional geotechnical engineers and engineering geologists with a particular interest in
ground engineering. The GeoGuides have been funded under the Australian governments’ National Disaster Mitigation Program.
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REPORT EXPLANATION NOTES
INTRODUCTION
These notes have been provided to amplify the geotechnical report Cohesive soils are classified on the basis of strength (consistency)
in regard to classification methods, field procedures and certain either by use of a hand penetrometer, vane shear, laboratory testing
matters relating to the Comments and Recommendations section. and/or tactile engineering examination. The strength terms are
Not all notes are necessarily relevant to all reports. defined as follows.
The ground is a product of continuing natural and man-made Unconfined Indicative Undrained
processes and therefore exhibits a variety of characteristics and Compressive Shear Strength (kPa)
properties which vary from place to place and can change with time. Classification Strength (kPa)
Geotechnical engineering involves gathering and assimilating limited
facts about these characteristics and properties in order to Very Soft (VS) 25 12
understand or predict the behaviour of the ground on a particular Soft (S) > 25 and 50 > 12 and 25
site under certain conditions. This report may contain such facts Firm (F) > 50 and 100 > 25 and 50
obtained by inspection, excavation, probing, sampling, testing or Stiff (St) > 100 and 200 > 50 and 100
other means of investigation. If so, they are directly relevant only to
the ground at the place where and time when the investigation was Very Stiff (VSt) > 200 and 400 > 100 and 200
carried out. Hard (Hd) > 400 > 200
Friable (Fr) Strength not attainable – soil crumbles
DESCRIPTION AND CLASSIFICATION METHODS Rock types are classified by their geological names, together with
The methods of description and classification of soils and rocks used descriptive terms regarding weathering, strength, defects, etc.
in this report are based on Australian Standard 1726:2017 Where relevant, further information regarding rock classification is
‘Geotechnical Site Investigations’. In general, descriptions cover the given in the text of the report. In the Sydney Basin, ‘shale’ is used to
following properties – soil or rock type, colour, structure, strength or describe fissile mudstone, with a weakness parallel to bedding. Rocks
density, and inclusions. Identification and classification of soil and with alternating inter-laminations of different grain size
rock involves judgement and the Company infers accuracy only to (eg. siltstone/claystone and siltstone/fine grained sandstone) is
the extent that is common in current geotechnical practice. referred to as ‘laminite’.
Soil types are described according to the predominating particle size
SAMPLING
and behaviour as set out in the attached soil classification table
qualified by the grading of other particles present (eg. sandy clay) as Sampling is carried out during drilling or from other excavations to
set out below: allow engineering examination (and laboratory testing where
required) of the soil or rock.
Soil Classification Particle Size
Disturbed samples taken during drilling provide information on
Clay < 0.002mm plasticity, grain size, colour, moisture content, minor constituents
Silt 0.002 to 0.075mm and, depending upon the degree of disturbance, some information
Sand 0.075 to 2.36mm on strength and structure. Bulk samples are similar but of greater
Gravel 2.36 to 63mm volume required for some test procedures.
Cobbles 63 to 200mm Undisturbed samples are taken by pushing a thin-walled sample tube,
Boulders > 200mm usually 50mm diameter (known as a U50), into the soil and
withdrawing it with a sample of the soil contained in a relatively
Non-cohesive soils are classified on the basis of relative density, undisturbed state. Such samples yield information on structure and
generally from the results of Standard Penetration Test (SPT) as strength, and are necessary for laboratory determination of shrink-
below: swell behaviour, strength and compressibility. Undisturbed sampling
is generally effective only in cohesive soils.
SPT ‘N’ Value Details of the type and method of sampling used are given on the
Relative Density (blows/300mm) attached logs.
Very loose (VL) <4
Loose (L) 4 to 10
Medium dense (MD) 10 to 30
Dense (D) 30 to 50
Very Dense (VD) > 50
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INVESTIGATION METHODS Mud Stabilised Drilling: Either Wash Boring or Continuous Core
Drilling can use drilling mud as a circulating fluid to stabilise the
The following is a brief summary of investigation methods currently
borehole. The term ‘mud’ encompasses a range of products ranging
adopted by the Company and some comments on their use and
from bentonite to polymers. The mud tends to mask the cuttings and
application. All methods except test pits, hand auger drilling and
reliable identification is only possible from intermittent intact
portable Dynamic Cone Penetrometers require the use of a
sampling (eg. from SPT and U50 samples) or from rock coring, etc.
mechanical rig which is commonly mounted on a truck chassis or
track base.
Continuous Core Drilling: A continuous core sample is obtained
using a diamond tipped core barrel. Provided full core recovery is
Test Pits: These are normally excavated with a backhoe or a tracked
achieved (which is not always possible in very low strength rocks and
excavator, allowing close examination of the insitu soils and ‘weaker’
granular soils), this technique provides a very reliable (but relatively
bedrock if it is safe to descend into the pit. The depth of penetration
expensive) method of investigation. In rocks, NMLC or HQ triple tube
is limited to about 3m for a backhoe and up to 6m for a large
core barrels, which give a core of about 50mm and 61mm diameter,
excavator. Limitations of test pits are the problems associated with
respectively, is usually used with water flush. The length of core
disturbance and difficulty of reinstatement and the consequent
recovered is compared to the length drilled and any length not
effects on close-by structures. Care must be taken if construction is
recovered is shown as NO CORE. The location of NO CORE recovery
to be carried out near test pit locations to either properly recompact
is determined on site by the supervising engineer; where the location
the backfill during construction or to design and construct the
is uncertain, the loss is placed at the bottom of the drill run.
structure so as not to be adversely affected by poorly compacted
backfill at the test pit location.
Standard Penetration Tests: Standard Penetration Tests (SPT) are
used mainly in non-cohesive soils, but can also be used in cohesive
Hand Auger Drilling: A borehole of 50mm to 100mm diameter is
soils, as a means of indicating density or strength and also of
advanced by manually operated equipment. Refusal of the hand
obtaining a relatively undisturbed sample. The test procedure is
auger can occur on a variety of materials such as obstructions within
described in Australian Standard 1289.6.3.1–2004 (R2016) ‘Methods
any fill, tree roots, hard clay, gravel or ironstone, cobbles and
of Testing Soils for Engineering Purposes, Soil Strength and
boulders, and does not necessarily indicate rock level.
Consolidation Tests – Determination of the Penetration Resistance of
a Soil – Standard Penetration Test (SPT)’.
Continuous Spiral Flight Augers: The borehole is advanced using
75mm to 115mm diameter continuous spiral flight augers, which are The test is carried out in a borehole by driving a 50mm diameter split
withdrawn at intervals to allow sampling and insitu testing. This is a sample tube with a tapered shoe, under the impact of a 63.5kg
relatively economical means of drilling in clays and in sands above hammer with a free fall of 760mm. It is normal for the tube to be
the water table. Samples are returned to the surface by the flights or driven in three successive 150mm increments and the ‘N’ value is
may be collected after withdrawal of the auger flights, but they can taken as the number of blows for the last 300mm. In dense sands,
be very disturbed and layers may become mixed. Information from very hard clays or weak rock, the full 450mm penetration may not be
the auger sampling (as distinct from specific sampling by SPTs or practicable and the test is discontinued.
undisturbed samples) is of limited reliability due to mixing or
The test results are reported in the following form:
softening of samples by groundwater, or uncertainties as to the
original depth of the samples. Augering below the groundwater table In the case where full penetration is obtained with successive
is of even lesser reliability than augering above the water table. blow counts for each 150mm of, say, 4, 6 and 7 blows, as
Rock Augering: Use can be made of a Tungsten Carbide (TC) bit for N = 13
auger drilling into rock to indicate rock quality and continuity by 4, 6, 7
variation in drilling resistance and from examination of recovered In a case where the test is discontinued short of full penetration,
rock cuttings. This method of investigation is quick and relatively say after 15 blows for the first 150mm and 30 blows for the next
inexpensive but provides only an indication of the likely rock strength 40mm, as
and predicted values may be in error by a strength order. Where rock N > 30
strengths may have a significant impact on construction feasibility or 15, 30/40mm
costs, then further investigation by means of cored boreholes may
be warranted. The results of the test can be related empirically to the engineering
properties of the soil.
Wash Boring: The borehole is usually advanced by a rotary bit, with A modification to the SPT is where the same driving system is used
water being pumped down the drill rods and returned up the with a solid 60 tipped steel cone of the same diameter as the SPT
annulus, carrying the drill cuttings. Only major changes in hollow sampler. The solid cone can be continuously driven for some
stratification can be assessed from the cuttings, together with some distance in soft clays or loose sands, or may be used where damage
information from “feel” and rate of penetration. would otherwise occur to the SPT. The results of this Solid Cone
Penetration Test (SCPT) are shown as ‘Nc’ on the borehole logs,
together with the number of blows per 150mm penetration.
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Cone Penetrometer Testing (CPT) and Interpretation: There are limitations when using the CPT in that it may not penetrate
The cone penetrometer is sometimes referred to as a Dutch Cone. obstructions within any fill, thick layers of hard clay and very dense
The test is described in Australian Standard 1289.6.5.1–1999 (R2013) sand, gravel and weathered bedrock. Normally a ‘dummy’ cone is
‘Methods of Testing Soils for Engineering Purposes, Soil Strength and pushed through fill to protect the equipment. No information is
Consolidation Tests – Determination of the Static Cone Penetration recorded by the ‘dummy’ probe.
Resistance of a Soil – Field Test using a Mechanical and Electrical
Cone or Friction-Cone Penetrometer’. Flat Dilatometer Test: The flat dilatometer (DMT), also known as the
Marchetti Dilometer comprises a stainless steel blade having a flat,
In the tests, a 35mm or 44mm diameter rod with a conical tip is
circular steel membrane mounted flush on one side.
pushed continuously into the soil, the reaction being provided by a
specially designed truck or rig which is fitted with a hydraulic ram The blade is connected to a control unit at ground surface by a
system. Measurements are made of the end bearing resistance on pneumatic-electrical tube running through the insertion rods. A gas
the cone and the frictional resistance on a separate 134mm or tank, connected to the control unit by a pneumatic cable, supplies
165mm long sleeve, immediately behind the cone. Transducers in the gas pressure required to expand the membrane. The control unit
the tip of the assembly are electrically connected by wires passing is equipped with a pressure regulator, pressure gauges, an audio-
through the centre of the push rods to an amplifier and recorder unit visual signal and vent valves.
mounted on the control truck. The CPT does not provide soil sample
The blade is advanced into the ground using our CPT rig or one of our
recovery.
drilling rigs, and can be driven into the ground using an SPT hammer.
As penetration occurs (at a rate of approximately 20mm per second), As soon as the blade is in place, the membrane is inflated, and the
the information is output as incremental digital records every 10mm. pressure required to lift the membrane (approximately 0.1mm) is
The results given in this report have been plotted from the digital recorded. The pressure then required to lift the centre of the
data. membrane by an additional 1mm is recorded. The membrane is then
deflated before pushing to the next depth increment, usually
The information provided on the charts comprise:
200mm down. The pressure readings are corrected for membrane
Cone resistance – the actual end bearing force divided by the stiffness.
cross sectional area of the cone – expressed in MPa. There are
two scales presented for the cone resistance. The lower scale The DMT is used to measure material index (ID), horizontal stress
has a range of 0 to 5MPa and the main scale has a range of 0 to index (KD), and dilatometer modulus (ED). Using established
50MPa. For cone resistance values less than 5MPa, the plot will correlations, the DMT results can also be used to assess the ‘at rest’
appear on both scales. earth pressure coefficient (Ko), over-consolidation ratio (OCR),
Sleeve friction – the frictional force on the sleeve divided by the undrained shear strength (Cu), friction angle (), coefficient of
surface area – expressed in kPa. consolidation (Ch), coefficient of permeability (Kh), unit weight (),
and vertical drained constrained modulus (M).
Friction ratio – the ratio of sleeve friction to cone resistance,
expressed as a percentage. The seismic dilatometer (SDMT) is the combination of the DMT with
an add-on seismic module for the measurement of shear wave
The ratios of the sleeve resistance to cone resistance will vary
velocity (Vs). Using established correlations, the SDMT results can
with the type of soil encountered, with higher relative friction in
also be used to assess the small strain modulus (Go).
clays than in sands. Friction ratios of 1% to 2% are commonly
encountered in sands and occasionally very soft clays, rising to
Portable Dynamic Cone Penetrometers: Portable Dynamic Cone
4% to 10% in stiff clays and peats. Soil descriptions based on
Penetrometer (DCP) tests are carried out by driving a 16mm
cone resistance and friction ratios are only inferred and must not
diameter rod with a 20mm diameter cone end with a 9kg hammer
be considered as exact.
dropping 510mm. The test is described in Australian Standard
Correlations between CPT and SPT values can be developed for both 1289.6.3.2–1997 (R2013) ‘Methods of Testing Soils for Engineering
sands and clays but may be site specific. Purposes, Soil Strength and Consolidation Tests – Determination of
Interpretation of CPT values can be made to empirically derive the Penetration Resistance of a Soil – 9kg Dynamic Cone
modulus or compressibility values to allow calculation of foundation Penetrometer Test’.
settlements. The results are used to assess the relative compaction of fill, the
Stratification can be inferred from the cone and friction traces and relative density of granular soils, and the strength of cohesive soils.
from experience and information from nearby boreholes etc. Where Using established correlations, the DCP test results can also be used
shown, this information is presented for general guidance, but must to assess California Bearing Ratio (CBR).
be regarded as interpretive. The test method provides a continuous Refusal of the DCP can occur on a variety of materials such as
profile of engineering properties but, where precise information on obstructions within any fill, tree roots, hard clay, gravel or ironstone,
soil classification is required, direct drilling and sampling may be cobbles and boulders, and does not necessarily indicate rock level.
preferable.
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Vane Shear Test: The vane shear test is used to measure the GROUNDWATER
undrained shear strength (Cu) of typically very soft to firm fine
Where groundwater levels are measured in boreholes, there are
grained cohesive soils. The vane shear is normally performed in the
several potential problems:
bottom of a borehole, but can be completed from surface level, the
bottom and sides of test pits, and on recovered undisturbed tube Although groundwater may be present, in low permeability soils
samples (when using a hand vane). it may enter the hole slowly or perhaps not at all during the time
it is left open.
The vane comprises four rectangular blades arranged in the form of
A localised perched water table may lead to an erroneous
a cross on the end of a thin rod, which is coupled to the bottom of a
indication of the true water table.
drill rod string when used in a borehole. The size of the vane is
dependent on the strength of the fine grained cohesive soils; that is, Water table levels will vary from time to time with seasons or
larger vanes are normally used for very low strength soils. For recent weather changes and may not be the same at the time of
borehole testing, the size of the vane can be limited by the size of the construction.
casing that is used. The use of water or mud as a drilling fluid will mask any
For testing inside a borehole, a device is used at the top of the casing, groundwater inflow. Water has to be blown out of the hole and
which suspends the vane and rods so that they do not sink under self- drilling mud must be washed out of the hole or ‘reverted’
weight into the ‘soft’ soils beyond the depth at which the test is to chemically if reliable water observations are to be made.
be carried out. A calibrated torque head is used to rotate the rods More reliable measurements can be made by installing standpipes
and vane and to measure the resistance of the vane to rotation. which are read after the groundwater level has stabilised at intervals
With the vane in position, torque is applied to cause rotation of ranging from several days to perhaps weeks for low permeability
the vane at a constant rate. A rate of 6° per minute is the soils. Piezometers, sealed in a particular stratum, may be advisable
common rotation rate. Rotation is continued until the soil is in low permeability soils or where there may be interference from
sheared and the maximum torque has been recorded. This value perched water tables or surface water.
is then used to calculate the undrained shear strength. The vane
is then rotated rapidly a number of times and the operation FILL
repeated until a constant torque reading is obtained. This torque The presence of fill materials can often be determined only by the
value is used to calculate the remoulded shear strength. Where inclusion of foreign objects (eg. bricks, steel, etc) or by distinctly
appropriate, friction on the vane rods is measured and taken into unusual colour, texture or fabric. Identification of the extent of fill
account in the shear strength calculation. materials will also depend on investigation methods and frequency.
Where natural soils similar to those at the site are used for fill, it may
LOGS be difficult with limited testing and sampling to reliably assess the
The borehole or test pit logs presented herein are an engineering extent of the fill.
and/or geological interpretation of the subsurface conditions, and The presence of fill materials is usually regarded with caution as the
their reliability will depend to some extent on the frequency of possible variation in density, strength and material type is much
sampling and the method of drilling or excavation. Ideally, greater than with natural soil deposits. Consequently, there is an
continuous undisturbed sampling or core drilling will enable the increased risk of adverse engineering characteristics or behaviour. If
most reliable assessment, but is not always practicable or possible to the volume and quality of fill is of importance to a project, then
justify on economic grounds. In any case, the boreholes or test pits frequent test pit excavations are preferable to boreholes.
represent only a very small sample of the total subsurface conditions.
The terms and symbols used in preparation of the logs are defined in LABORATORY TESTING
the following pages. Laboratory testing is normally carried out in accordance with
Interpretation of the information shown on the logs, and its Australian Standard 1289 ‘Methods of Testing Soils for Engineering
application to design and construction, should therefore take into Purposes’ or appropriate NSW Government Roads & Maritime
account the spacing of boreholes or test pits, the method of drilling Services (RMS) test methods. Details of the test procedure used are
or excavation, the frequency of sampling and testing and the given on the individual report forms.
possibility of other than ‘straight line’ variations between the
boreholes or test pits. Subsurface conditions between boreholes or ENGINEERING REPORTS
test pits may vary significantly from conditions encountered at the Engineering reports are prepared by qualified personnel and are
borehole or test pit locations. based on the information obtained and on current engineering
standards of interpretation and analysis. Where the report has been
prepared for a specific design proposal (eg. a three storey building)
the information and interpretation may not be relevant if the design
proposal is changed (eg. to a twenty storey building). If this happens,
the Company will be pleased to review the report and the sufficiency
of the investigation work.
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Reasonable care is taken with the report as it relates to be pleased to assist in this regard and/or to make additional report
interpretation of subsurface conditions, discussion of geotechnical copies available for contract purposes at a nominal charge.
aspects and recommendations or suggestions for design and
Copyright in all documents (such as drawings, borehole or test pit
construction. However, the Company cannot always anticipate or
logs, reports and specifications) provided by the Company shall
assume responsibility for:
remain the property of Jeffery and Katauskas Pty Ltd. Subject to the
Unexpected variations in ground conditions – the potential for payment of all fees due, the Client alone shall have a licence to use
this will be partially dependent on borehole spacing and the documents provided for the sole purpose of completing the
sampling frequency as well as investigation technique. project to which they relate. Licence to use the documents may be
Changes in policy or interpretation of policy by statutory revoked without notice if the Client is in breach of any obligation to
authorities. make a payment to us.
The actions of persons or contractors responding to commercial
pressures. REVIEW OF DESIGN
Details of the development that the Company could not Where major civil or structural developments are proposed or where
reasonably be expected to anticipate. only a limited investigation has been completed or where the
geotechnical conditions/constraints are quite complex, it is prudent
If these occur, the Company will be pleased to assist with to have a joint design review which involves an experienced
investigation or advice to resolve any problems occurring. geotechnical engineer/engineering geologist.
SITE ANOMALIES SITE INSPECTION
In the event that conditions encountered on site during construction The Company will always be pleased to provide engineering
appear to vary from those which were expected from the inspection services for geotechnical aspects of work to which this
information contained in the report, the Company requests that it report is related.
immediately be notified. Most problems are much more readily
resolved when conditions are exposed rather than at some later Requirements could range from:
stage, well after the event. i) a site visit to confirm that conditions exposed are no worse than
those interpreted, to
REPRODUCTION OF INFORMATION FOR CONTRACTUAL ii) a visit to assist the contractor or other site personnel in
PURPOSES identifying various soil/rock types and appropriate footing or
Where information obtained from this investigation is provided for pile founding depths, or
tendering purposes, it is recommended that all information, iii) full time engineering presence on site.
including the written report and discussion, be made available. In
circumstances where the discussion or comments section is not
relevant to the contractual situation, it may be appropriate to
prepare a specially edited document. The Company would
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SYMBOL LEGENDS
SOIL ROCK
OTHER MATERIALS
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CLASSIFICATION OF COARSE AND FINE GRAINED SOILS
Group
Major Divisions Symbol Typical Names Field Classification of Sand and Gravel Laboratory Classification Laboratory Classification Criteria
GRAVEL (more A well graded coarse grained soil is one for which the coefficient of uniformity
GW Gravel and gravel-sand mixtures, Wide range in grain size and substantial amounts of all intermediate sizes, not ≤ 5% fines Cu > 4
Cu > 4 and the coefficient of curvature 1 < Cc < 3. Otherwise, the soil is poorly
Coarse grainedsoil (more than 65%of soil excludingoversize fraction is
than half little or no fines enough fines to bind coarse grains, no dry strength 1 < Cc < 3
of coarse graded. These coefficients are given by:
fraction is larger GP Gravel and gravel-sand mixtures, Predominantly one size or range of sizes with some intermediate sizes missing, ≤ 5% fines Fails to comply
little or no fines, uniform gravels not enough fines to bind coarse grains, no dry strength with above ( )
than 2.36mm = and =
GM Gravel-silt mixtures and gravel- ‘Dirty’ materials with excess of non-plastic fines, zero to medium dry strength ≥ 12% fines, fines Fines behave as
sand-silt mixtures are silty silt Where D10, D30 and D60 are those grain sizes for which 10%, 30% and 60% of
greater than 0.075mm)
the soil grains, respectively, are smaller.
GC Gravel-clay mixtures and gravel- ‘Dirty’ materials with excess of plastic fines, medium to high dry strength ≥ 12% fines, fines Fines behave as
sand-clay mixtures are clayey clay
SAND (more SW Sand and gravel-sand mixtures, Wide range in grain size and substantial amounts of all intermediate sizes, not ≤ 5% fines Cu > 6 NOTES:
than half little or no fines enough fines to bind coarse grains, no dry strength 1 < Cc < 3 1 For a coarse grained soil with a fines content between 5% and 12%,
of coarse the soil is given a dual classification comprising the two group symbols
fraction SP Sand and gravel-sand mixtures, Predominantly one size or range of sizes with some intermediate sizes missing, ≤ 5% fines Fails to comply
separated by a dash; for example, for a poorly graded gravel with
is smaller than little or no fines not enough fines to bind coarse grains, no dry strength with above between 5% and 12% silt fines, the classification is GP-GM.
2.36mm) SM Sand-silt mixtures ‘Dirty’ materials with excess of non-plastic fines, zero to medium dry strength ≥ 12% fines, fines 2 Where the grading is determined from laboratory tests, it is defined by
are silty coefficients of curvature (Cc) and uniformity (Cu) derived from the
N/A particle size distribution curve.
SC Sand-clay mixtures ‘Dirty’ materials with excess of plastic fines, medium to high dry strength ≥ 12% fines, fines 3 Clay soils with liquid limits > 35% and ≤ 50% may be classified as being
are clayey of medium plasticity.
4 The U line on the Modified Casagrande Chart is an approximate upper
bound for most natural soils.
Field Classification of Laboratory
Silt and Clay Classification
Group
Major Divisions Symbol Typical Names Dry Strength Dilatancy Toughness % < 0.075mm Modified Casagrande Chart for Classifying Silts and Clays
according to their Behaviour
SILT and CLAY ML Inorganic silt and very fine sand, rock flour, silty or None to low Slow to rapid Low Below A line
ine grained soils (more than 35%of soil excluding
(low to medium clayey fine sand or silt with low plasticity
plasticity)
oversize fraction is less than 0.075mm)
CL, CI Inorganic clay of low to medium plasticity, gravelly Medium to high None to slow Medium Above A line
clay, sandy clay
OL Organic silt Low to medium Slow Low Below A line
SILT and CLAY MH Inorganic silt Low to medium None to slow Low to medium Below A line
(high plasticity)
CH Inorganic clay of high plasticity High to very high None High Above A line
OH Organic clay of medium to high plasticity, organic Medium to high None to very slow Low to medium Below A line
silt
Highly organic soil Pt Peat, highly organic soil – – – –
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LOG SYMBOLS
Log Column Symbol Definition
Groundwater Record Standing water level. Time delay following completion of drilling/excavation may be shown.
Extent of borehole/test pit collapse shortly after drilling/excavation.
C
Groundwater seepage into borehole or test pit noted during drilling or excavation.
Samples ES Sample taken over depth indicated, for environmental analysis.
U50 Undisturbed 50mm diameter tube sample taken over depth indicated.
DB Bulk disturbed sample taken over depth indicated.
DS Small disturbed bag sample taken over depth indicated.
ASB Soil sample taken over depth indicated, for asbestos analysis.
ASS Soil sample taken over depth indicated, for acid sulfate soil analysis.
SAL Soil sample taken over depth indicated, for salinity analysis.
Field Tests N = 17 Standard Penetration Test (SPT) performed between depths indicated by lines. Individual
4, 7, 10 figures show blows per 150mm penetration. ‘Refusal’ refers to apparent hammer refusal within
the corresponding 150mm depth increment.
Nc = 5 Solid Cone Penetration Test (SCPT) performed between depths indicated by lines. Individual
7 figures show blows per 150mm penetration for 60 solid cone driven by SPT hammer. ‘R’ refers
to apparent hammer refusal within the corresponding 150mm depth increment.
3R
VNS = 25 Vane shear reading in kPa of undrained shear strength.
PID = 100 Photoionisation detector reading in ppm (soil sample headspace test).
Moisture Condition w > PL Moisture content estimated to be greater than plastic limit.
(Fine Grained Soils) w PL Moisture content estimated to be approximately equal to plastic limit.
w < PL Moisture content estimated to be less than plastic limit.
w LL Moisture content estimated to be near liquid limit.
w > LL Moisture content estimated to be wet of liquid limit.
(Coarse Grained Soils) D DRY – runs freely through fingers.
M MOIST – does not run freely but no free water visible on soil surface.
W WET – free water visible on soil surface.
Strength (Consistency) VS VERY SOFT – unconfined compressive strength 25kPa.
Cohesive Soils S SOFT – unconfined compressive strength > 25kPa and 50kPa.
F FIRM – unconfined compressive strength > 50kPa and 100kPa.
St STIFF – unconfined compressive strength > 100kPa and 200kPa.
VSt VERY STIFF – unconfined compressive strength > 200kPa and 400kPa.
Hd HARD – unconfined compressive strength > 400kPa.
Fr FRIABLE – strength not attainable, soil crumbles.
( ) Bracketed symbol indicates estimated consistency based on tactile examination or other
assessment.
Density Index/ Density Index (ID) SPT ‘N’ Value Range
Relative Density Range (%) (Blows/300mm)
(Cohesionless Soils) VL VERY LOOSE 15 0–4
L LOOSE > 15 and 35 4 – 10
MD MEDIUM DENSE > 35 and 65 10 – 30
D DENSE > 65 and 85 30 – 50
VD VERY DENSE > 85 > 50
( ) Bracketed symbol indicates estimated density based on ease of drilling or other assessment.
Hand Penetrometer 300 Measures reading in kPa of unconfined compressive strength. Numbers indicate individual
Readings 250 test results on representative undisturbed material unless noted otherwise.
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Log Column Symbol Definition
Remarks ‘V’ bit Hardened steel ‘V’ shaped bit.
‘TC’ bit Twin pronged tungsten carbide bit.
Penetration of auger string in mm under static load of rig applied by drill head hydraulics
T60 without rotation of augers.
Soil Origin The geological origin of the soil can generally be described as:
RESIDUAL – soil formed directly from insitu weathering of the underlying rock.
No visible structure or fabric of the parent rock.
EXTREMELY – soil formed directly from insitu weathering of the underlying rock.
WEATHERED Material is of soil strength but retains the structure and/or fabric of the
parent rock.
ALLUVIAL – soil deposited by creeks and rivers.
ESTUARINE – soil deposited in coastal estuaries, including sediments caused by
inflowing creeks and rivers, and tidal currents.
MARINE – soil deposited in a marine environment.
AEOLIAN – soil carried and deposited by wind.
COLLUVIAL – soil and rock debris transported downslope by gravity, with or without
the assistance of flowing water. Colluvium is usually a thick deposit
formed from a landslide. The description ‘slopewash’ is used for thinner
surficial deposits.
LITTORAL – beach deposited soil.
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Classification of Material Weathering
Term Abbreviation Definition
Material is weathered to such an extent that it has soil properties. Mass
Residual Soil RS structure and material texture and fabric of original rock are no longer visible,
but the soil has not been significantly transported.
Material is weathered to such an extent that it has soil properties. Mass
Extremely Weathered XW
structure and material texture and fabric of original rock are still visible.
The whole of the rock material is discoloured, usually by iron staining or
bleaching to the extent that the colour of the original rock is not recognisable.
Highly Weathered HW Rock strength is significantly changed by weathering. Some primary minerals
Distinctly have weathered to clay minerals. Porosity may be increased by leaching, or
Weathered DW may be decreased due to deposition of weathering products in pores.
(Note 1)
The whole of the rock material is discoloured, usually by iron staining or
Moderately Weathered MW bleaching to the extent that the colour of the original rock is not recognisable,
but shows little or no change of strength from fresh rock.
Rock is partially discoloured with staining or bleaching along joints but shows
Slightly Weathered SW
little or no change of strength from fresh rock.
Fresh FR Rock shows no sign of decomposition of individual minerals or colour changes.
NOTE 1: The term ‘Distinctly Weathered’ is used where it is not practicable to distinguish between ‘Highly Weathered’ and ‘Moderately Weathered’ rock.
‘Distinctly Weathered’ is defined as follows: ‘Rock strength usually changed by weathering. The rock may be highly discoloured, usually by iron staining.
Porosity may be increased by leaching, or may be decreased due to deposition of weathering products in pores’. There is some change in rock strength.
Rock Material Strength Classification
Guide to Strength
Uniaxial Point Load
Compressive Strength Index
Term Abbreviation Strength (MPa) Is(50) (MPa) Field Assessment
Very Low VL 0.6 to 2 0.03 to 0.1 Material crumbles under firm blows with sharp end of pick;
Strength can be peeled with knife; too hard to cut a triaxial sample by
hand. Pieces up to 30mm thick can be broken by finger
pressure.
Low Strength L 2 to 6 0.1 to 0.3 Easily scored with a knife; indentations 1mm to 3mm show
in the specimen with firm blows of the pick point; has dull
sound under hammer. A piece of core 150mm long by 50mm
diameter may be broken by hand. Sharp edges of core may
be friable and break during handling.
Medium M 6 to 20 0.3 to 1 Scored with a knife; a piece of core 150mm long by 50mm
Strength diameter can be broken by hand with difficulty.
High Strength H 20 to 60 1 to 3 A piece of core 150mm long by 50mm diameter cannot be
broken by hand but can be broken by a pick with a single
firm blow; rock rings under hammer.
Very High VH 60 to 200 3 to 10 Hand specimen breaks with pick after more than one blow;
Strength rock rings under hammer.
Extremely EH > 200 > 10 Specimen requires many blows with geological pick to break
High Strength through intact material; rock rings under hammer.
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Abbreviations Used in Defect Description
Symbol
Cored Borehole Log Column Abbreviation Description
Point Load Strength Index 0.6 Axial point load strength index test result (MPa)
x 0.6 Diametral point load strength index test result (MPa)
Defect Details – Type Be Parting – bedding or cleavage
CS Clay seam
Cr Crushed/sheared seam or zone
J Joint
Jh Healed joint
Ji Incipient joint
XWS Extremely weathered seam
– Orientation Degrees Defect orientation is measured relative to normal to the core axis
(ie. relative to the horizontal for a vertical borehole)
– Shape P Planar
C Curved
Un Undulating
St Stepped
Ir Irregular
– Roughness Vr Very rough
R Rough
S Smooth
Po Polished
Sl Slickensided
– Infill Material Ca Calcite
Cb Carbonaceous
Clay Clay
Fe Iron
Qz Quartz
Py Pyrite
– Coatings Cn Clean
Sn Stained – no visible coating, surface is discoloured
Vn Veneer – visible, too thin to measure, may be patchy
Ct Coating 1mm thick
Filled Coating > 1mm thick
– Thickness mm.t Defect thickness measured in millimetres
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