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RFI - Attachment C - Slope Stability Assessment and Geotechnical report · MCU20/0026
SLOPE STABILITY ASSESSMENT AND GEOTECHNICAL REPORT PROJECT NO. 1-22543 JUNE, 2020 SANYA HOLDINGS PROPOSED RESIDENCE 40 PARK CRESCENT, SUNSHINE BEACH OFFICES IN BRISBANE AND GOLD COAST
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Soil Surveys Engineering Pty Limited
A.B.N. 70 054 043 631
www.soilsurveys.com.au
Ph +617 3369 6000
PO Box 317, Paddington, QLD, 4064, Australia
[email protected]
Brisbane
Level 2, 19 Finchley Street, Milton, QLD, 4064, Australia
Gold Coast
Unit 8, 140 Millaroo Drive, Helensvale, QLD, Australia
Document Information
Project Number 1-22543
Slope Stability Assessment and
Document Title - Proposed Residence
Geotechnical Report
Site Address 40 Park Crescent, Sunshine Beach
Client Sanya Holdings
File ref 1-22543, 2020-05-20, BR SSA VER 1
Author Glen Burkitt
Review
Approved for Issue
Ver No. Document Type Reviewed By
Name Date
1 Final Noel Perkins Glen Burkitt 15/06/2020
Distribution List
Ver No. Organisation Contact Email Date Sent
1 Sanya Holdings Asha Biswas [email protected] 15/06/2020
Shaun Lockyer
1 David Langley [email protected] 15/06/2020
Architects
Page 3
TABLE OF CONTENTS
1.0 INTRODUCTION .............................................................................................. 1
1.1 General .................................................................................................................. 1
1.2 Reason for this Report ......................................................................................... 1
1.3 Scope of Report .................................................................................................... 1
1.4 Controls and Procedures ..................................................................................... 1
1.5 Proposed Development ........................................................................................ 2
1.6 Provided Information ............................................................................................ 2
2.0 ASSESSMENT METHODOLOGY ................................................................... 3
2.1 Method ................................................................................................................... 3
2.2 Hazard & Risk Analysis ........................................................................................ 3
3.0 SITE ASSESSMENT AND DATA COLLECTION ............................................ 4
3.1 Field Investigation ................................................................................................ 4
3.2 Site Location ......................................................................................................... 4
3.3 Regional Geology ................................................................................................. 5
3.4 Published Stability Data ....................................................................................... 5
3.5 Site Factors ........................................................................................................... 5
3.6 Geotechnical Model .............................................................................................. 6
3.6.1 Subsurface Profile ........................................................................................................... 6
3.6.2 Groundwater and Seepage ............................................................................................. 7
3.6.3 Laboratory Testing .......................................................................................................... 7
3.7 Evidence of Possible Instability........................................................................... 7
4.0 RISK ASSESSMENT ....................................................................................... 8
4.1 General .................................................................................................................. 8
4.2 Identification of Potential Hazards on Site .......................................................... 8
4.3 Likelihood Estimation ........................................................................................... 8
4.3.1 Methods of Assessment .................................................................................................. 8
4.3.2 Evaluation of Likelihood Rating for Existing Overall Site ................................................ 8
4.3.3 Hazard A - Natural Shallow Landslide ............................................................................ 9
4.3.4 Hazard B - Proposed Cuts .............................................................................................. 9
4.3.5 Hazard C - Scour of Exposed Surfaces .......................................................................... 9
4.4 Consequence Analysis ....................................................................................... 10
4.4.1 General .......................................................................................................................... 10
4.4.2 Site Factors ................................................................................................................... 10
4.4.3 Consequence to Property ............................................................................................. 10
4.4.4 Consequence to Persons .............................................................................................. 11
4.5 Risk Determination and Evaluation - Property.................................................. 11
4.6 Risk Determination and Evaluation - Persons .................................................. 12
4.7 Risk Summary ..................................................................................................... 12
5.0 ENGINEERING STABILITY RECOMMENDATIONS..................................... 14
5.1 General Recommendations with Respect to Construction .............................. 14
5.2 Stormwater Drainage .......................................................................................... 14
5.3 Site Specific Recommendations ........................................................................ 15
5.3.1 General .......................................................................................................................... 15
5.3.2 Scour ............................................................................................................................. 15
5.3.3 Retaining Structures ...................................................................................................... 15
5.3.4 Continued Maintenance ................................................................................................ 15
6.0 ENGINEERING ASSESSMENT ..................................................................... 16
6.1 Civil Works .......................................................................................................... 16
6.1.1 Trafficability and Site Preparation ................................................................................. 16
6.1.2 Earthworks .................................................................................................................... 16
6.1.3 Batters ........................................................................................................................... 17
6.2 Building Foundations ......................................................................................... 17
6.2.1 General .......................................................................................................................... 17
6.2.2 Site Classification as per AS 2870-2011 ....................................................................... 17
6.2.3 Further Comments Regarding Foundations .................................................................. 18
6.2.4 High Level Footings ...................................................................................................... 18
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TABLE OF CONTENTS
6.2.5 Deep Foundations ......................................................................................................... 19
6.2.6 Slab on Ground ............................................................................................................. 21
6.2.7 Design Considerations .................................................................................................. 21
6.2.8 Site Management .......................................................................................................... 22
6.3 Basement Excavation and Retention Methods ................................................. 23
6.3.1 Introduction .................................................................................................................... 23
6.3.2 Geotechnical Model ...................................................................................................... 23
6.3.3 Boundary Conditions ..................................................................................................... 23
6.3.4 Possible Retention Options ........................................................................................... 23
6.4 Retaining Walls ................................................................................................... 24
6.4.1 General .......................................................................................................................... 24
6.4.2 Pressure Distribution ..................................................................................................... 24
7.0 CONSTRUCTION INSPECTIONS ................................................................. 27
8.0 REFERENCES ............................................................................................... 27
9.0 LIMITATIONS ................................................................................................ 27
Appendices
Appendix A
NOTES RELATING TO THIS REPORT
Appendix B
ASSESSMENT GUIDELINES
Appendix C
SITE INVESTIGATION MAPS
Appendix D
FRAMEWORK FOR LANDSLIDE RISK MANUAL
Appendix E
PHOTOGRAPHS
Appendix F
BOREHOLE AND DCP LOGS
Appendix G
LRM2007 – PRACTICE NOTE GUIDELINES – RISK MATRIX
Appendix H
DRAWINGS
Appendix I
HILLSIDE CONSTRUCTION GUIDELINES
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1.0 INTRODUCTION
1.1 General
This report presents the results of the geotechnical investigation carried out by Soil Surveys
Engineering Pty Limited on the 18th May and 20th May, 2020, for the Proposed Residence at 40
Park Crescent, Sunshine Beach.
The report was carried out following authorisation from Sanya Holdings on 13th May, 2020.
The objectives of this investigation were to assess subsurface conditions at the site in
accordance with the Scope of Services detailed in Section 1.3.
1.2 Reason for this Report
It is understood that given the site location and Noosa Shire Council regulations, a slope
stability assessment is required for the proposed development.
A provided Noosa Council Information Request (refer Section 1.6) also requires a “site specific
geotechnical report and site stability assessment.”
1.3 Scope of Report
The objective of this study was to undertake a geotechnical investigation and site mapping to
assess the existing geotechnical conditions to provide:-
Civil works recommendations
Trafficability
Earthworks recommendations
Batter recommendations
Foundation recommendations
Site classification according to AS 2870 (i.e. Reactivity)
Footing options and parameters
Retaining wall design parameters
Construction recommendations (where applicable)
Site management recommendations
Stability Assessment
1.4 Controls and Procedures
This report has been prepared in accordance with:-
National Disaster Mitigation Program (NDMP), Landslide Risk Management (LRM)
Guidelines, Practice Notes and Geoguidelines as published in the “Australian
Geomechanics Journal” Volume 42 No. 1 March 2007.
State Planning Policy July 2014-SPP.
The Noosa Plan, 2018, Part 7, Landslide Overlay.
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1.5 Proposed Development
It is understood that the proposed development is to consist of the construction of a new four
level residence, partially cut into and stepping down the natural slope of the site to the north
east.
Building loads have not been provided, however based on the provided drawings it is expected
that they would be generally consistent with domestic type structural loads.
Based on the provided drawings, earthworks are expected to consist of cuts of up to approx.
4.0m to 5.0m and retained filling of up to approx. 3.5m.
FIGURE 1 – PROPOSED RESIDENCE SECTION – MARCH, 2020
1.6 Provided Information
As part of the slope stability assessment, the following information has been provided to Soil
Surveys Engineering:-
Contour & Detail Survey by AJS Surveys, refer V0420-D1, dated 30th January, 2019.
Schematic Design drawings by Shaun Lockyer Architects Pty Ltd, refer drawing set
0309, Phase DA, 000.01 to 400.00, Rev. A, dated 30th March, 2020.
Information Request from Noosa Council, refer MCU20/0026, dated 1st May, 2020.
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2.0 ASSESSMENT METHODOLOGY
2.1 Method
The assessment of the stability of slopes at the site has utilised a Hazard and Risk Assessment
approach. In this method, the potential stability hazards on the site are assessed using certain
features/properties of the site (i.e. slope angle, ground water conditions, etc.). This method
provides a ranking for each of the identified hazards.
Using this hazard ranking and based on assumed consequences, an assessment of risk can
then be made using a risk matrix.
2.2 Hazard & Risk Analysis
The general processes in assessment and management of risks associated with landslides are
given in detail in the National Disaster Mitigation Program (NDMP), “Landslide Risk
Management (LRM) Guidelines”, Practice Notes and Geoguidelines as published in the
Australian Geomechanics Journal Volume 42 No. 1, March 2007. Figure D1 (Appendix D) is an
extract from the paper showing the typical processes in flow chart form.
As shown in Figure D1, hazard identification is an initial step in this process, and is the basis for
this study. Due to unknowns associated with development applications and their influence on
the surrounding environment, this study has been restricted to a qualitative evaluation of hazard
rating, with respect to natural slope instability. Landslide management areas were then derived
from these hazard/likelihood ratings and assessed consequences to evaluate the assessed risk.
The effects of earthquake on slope stability have not been included in this study.
The terminology adopted in this report has been designed to be consistent, as far as
practicable, with national standards including the Australian New Zealand Standard AS/NZS
ISO 31000:2009 “Risk Management – Principles and Guidelines” and the above NDMP LRM
Guidelines.
The methodology undertaken by Soil Surveys Engineering Pty Limited in assessing the Hazard
Management Areas was based on the following steps:-
A review of published information and aerial photographs to identify areas that have
potential to be affected by slope instability;
A review of regional features including topography, geology and geomorphology;
Identification of potential hazard zones based on topography, geology and
geomorphology;
Site inspection and review of field investigation data;
Development of a Risk Assessment map that can be used to assess the proposed
development.
This study has identified hazard ratings using a classification system consistent with the
procedures detailed in the paper entitled “A Method of Zoning Landslide Hazards”, prepared by
McGregor and Taylor. This method has been adopted on a wide range of projects and has
proven to be robust.
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3.0 SITE ASSESSMENT AND DATA COLLECTION
3.1 Field Investigation
Due to the existing structure on the site, the subsurface conditions were investigated by the
drilling and sampling one borehole at the front of the property (BH1) using our EVH1750 drilling
rig and undertaking two hand augers (HA, HA3) at the rear of the property where rig access
was not possible. In addition, dynamic cone penetrometer tests were carried out adjacent to
the borehole and hand auger as well as at one other location (DCP4).
A site inspection by a Senior Engineering Geologist, experienced in the assessment of slope
instability, was also carried out.
The description of the materials encountered in the field investigation is subjective, based on
the experience and judgement of the rig/site supervisor and some variations in the description,
from the actual material type may occur.
The soil classification descriptions, field and laboratory testing were carried out in general
accordance with the following Australian Standards:-
AS 1726 Geotechnical Site Investigations
AS 1289 Methods of Testing Soils for Engineering Purposes
Notes relating to this report, borehole records and a site plan showing the location of the
boreholes are included in the Appendices.
3.2 Site Location
The site is located at 40 Park Crescent, Sunshine Beach, also known as Lot 599 on RP48112
(refer Figure 2). Access to the front of the site is off Park Crescent and to the rear of the site via
a narrow easement off Webb Road from the north (refer Figure 3).
FIGURE 2 - SITE LOCATION
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FIGURE 3 - SITE AERIAL VIEW (NEARMAP.COM, NOVEMBER, 2019)
3.3 Regional Geology
Based upon the 1:100,000 Gympie Special Geological Map (Department of Mines and Energy,
Queensland, 1999), the site is underlain by beach sand deposits of the Quaternary Age (1.8
Mya to present) described as Pleistocene dune sand (quartz sand) and Holocene beach ridge
and berm (quartz sand).
3.4 Published Stability Data
The searchable Geoscience Australia Landslide Database (2018) was reviewed. No recorded
landslides were located in the vicinity of the site (2km radius).
3.5 Site Factors
The principal site features were:-
The site is located on the eastern side of a low hill, with slopes falling relatively steeply
towards the east which roughly cross the north eastern end of the site diagonally (refer
Plate E1, Appendix E).
An existing partially two storey timber residence is located towards the south western
end of the site, elevated above the natural slope (refer Plate 1, Appendix E). The
columns for the structure appear to be founding on concrete piers.
A timber sleeper retaining wall exists along the western side of the residence supporting
the western sub ground floor level wall (refer Plate 2, Appendix E).
Park Crescent appears to be situated near the top of the natural slope with some filling
likely undertaken at the western side (upper part) of the site as part of the road
construction.
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The provided survey (refer Section 1.6) indicates a fall of approximately 11.5m across
the site with an RL of approx. 27.5m at the western boundary corner and approx. RL
16.0m at the eastern boundary corner (refer Plate E3, Appendix E).
A small concave type feature exists near the centre of the eastern boundary of the site,
extending into the site by approx. 15.0m.
An approx. 2.0m wide Council easement runs along the north western site boundary
from Park Cres to the easement at the rear of the site, understood to be for an
underground stormwater service.
Outside of the existing structure, the site is generally well landscaped and grassed with
several small to medium sized trees and shrubs (refer Plate E4, Appendix E).
The site plan (1-22543-01) in Appendix H, shows an overlay of the provided survey (refer
Section 1.6), as well as the proposed residence location and test locations.
3.6 Geotechnical Model
3.6.1 Subsurface Profile
Given access was only available for one borehole at the front of the lot and hand equipment
was used due to the steep slope to the rear of the lot, a combination of drilling and DCP tests
were used to provide an assessment of the competency of the subsurface profile. Therefore,
the subsurface profile adopted from the testing program is summarized below:-
Fill – Encountered in BH1 to 0.3m depth and in HA2 and HA3 to depths of 0.8m and
0.6m respectively, and assume din DCP4 to approx. 0.3m depth. It was described as
Silty SAND (SM) and SAND (SP), very loose to loose.
In the absence of documentation to confirm that the filling was placed and compacted in
accordance with AS 3798-2007, then the filling must be considered as being
‘uncontrolled’.
Sand – Encountered in BH1 to total depth of 7.5m, in both hand augers to refusal depths
of 1.8m (HA2) and 1.9m (HA3), and assumed beneath the fill in DCP4 to total depth.
Described and assumed (where required) as being generally very loose to medium
dense, increasing to dense at depths of 7.0m (BH1) and 3.6m (HA2).
A summary of the profiles encountered in the test locations is included in Table 1 with borehole
and hand auger logs and DCP results in Appendix F and the test locations shown on Drawing
No. 1-22543-01 in Appendix H.
TABLE 1 SUBSURFACE PROFILE SUMMARY
Fill - Very Hand Auger Consistency based on DCP Tests and
(3) Loose to assuming Full Sand Profile (m) DCP Total Total
Location RL (m) (4)
Loose Very Loose to Depth (m) Depth (m)
Medium Dense Dense
Sand (m) Loose
BH01 27.60 0.00-0.30 0.30-2.00 2.00-7.00 7.00-TD 7.50 7.50
5
HA02 18.20 0.00-0.80 0.80-2.20 2.20-3.60 3.60-TD 4.00 1.80
5
HA03 17.20 0.00-0.60 0.60-1.90 1.90-TD NE 4.00 1.90
DCP04 20.00 0.00-0.30 0.30-1.90 1.90-TD NE 4.00 NA
Notes:-
1. NE - Not Encountered; NA - Not Applicable; TD - Total Depth.
th
2. All depths below existing ground level (20 May, 2020).
3. RL's based on provided survey data.
4. DCP refusal (i.e. blows >20 per 100mm) was not reached.
5. Hand Auger refusal.
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3.6.2 Groundwater and Seepage
Groundwater seepage was not noted during the site investigation. However, seepage through
the upper granular material and along the fill/natural interface may be expected during and
following periods of rainfall.
3.6.3 Laboratory Testing
Due to the subsurface profile of sands intersected, no laboratory testing was carried out.
Samples were retained should any future testing be required.
3.7 Evidence of Possible Instability
Existing Land Slips
No obvious visual evidence of landslide instability was noted on the site at the time of the site
walkover and investigation.
A review of air photos of the site does not indicate any likely areas of medium to large scale
instability on the natural slopes. Any existing small areas of instability would not be able to be
detected using this method due to photo scale.
Surface Scour
Minimal evidence of surface scour was noted on the site, generally located within the exposed
sand surface beneath the existing residence. The majority of steeper areas were generally well
vegetated but overland flow paths could still produce continual scour paths during major rain
events.
Soil Creep
Slight leaning and/or bowed trunks of trees are generally considered to be indicative of soil
creep, with the affected trees being progressively tilted by the slow movement of soil down the
slope. The degree of tilting or bowing of these trees is considered to be a sign of relatively
minor movements, and is typical for trees growing on such a steep slope. Rapid or gross
movements of soil downslope would be likely to uproot or kill the trees.
The small to medium sized trees on the existing natural slopes showed minor (if any) signs of
curved or bowed trunks.
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4.0 RISK ASSESSMENT
4.1 General
For the purposes of this study, the assessment of Risk for the site has been based on risk to
property and risk to persons, where Risk is the product of Likelihood and Consequence.
This method is based on the method outlined in the “Practice Note Guidelines for Landslide
Risk Management 2007” (PNGLRM), produced by the Australian Geomechanics Society, 2007
and is based upon the Risk Matrix in Appendix G.
4.2 Identification of Potential Hazards on Site
Based on the site walkover, a review of the site information and experience on similar sites, the
following potential hazards have been identified for assessment at the location of the proposed
residence (subject site):-
A. Shallow landslides on the existing slopes – applies to overall site.
B. Proposed Cuts – applies to internal proposed cut areas for basement level.
C. Surface scour on the site during construction – applies to overall subject site.
4.3 Likelihood Estimation
4.3.1 Methods of Assessment
The site was assessed and zoned using the method as outlined in MacGregor and Taylor
(2001). The method has been adopted as a slope stability assessment tool and used in both the
Gold Coast and Redlands Shire regional stability assessments.
4.3.2 Evaluation of Likelihood Rating for Existing Overall Site
Once the relative frequency of slope instability for the site is calculated, MacGregor and Taylor
(2001) suggests a likelihood rating as outlined in Table 2 below.
This rating has then been compared to the Likelihood Descriptor from Appendix C of “Practice
Note Guidelines for Landslides Risk Management (PNGLRM) (2007c)”.
TABLE 2 LIKELIHOOD RATING
MacGregor and Taylor (2001) Landslide Risk Management (2007c) Appendix C
Adopted Indicative
Relative Frequency Likelihood Rating Likelihood Descriptor Value of Annual
Probability
-6
< 0.2 Very Low Barely Credible 10
-5 -4
0.2 to 0.6 Low Rare to Unlikely 10 to 10
-4 -3
0.6 to 2.0 Moderate Unlikely to Possible 10 to 10
-3 -2
2.0 to 6.0 High Possible to Likely 10 to 10
-1
> 6.0 Very High Almost Certain 10
Note: 1. McGregor & Taylor (2001).
2. Likelihood adopted based on Landslide Risk Management (2007c), Appendix C.
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4.3.3 Hazard A - Natural Shallow Landslide
The site was assessed using the parameters as described by MacGregor and Taylor (2001)
and the site features identified on the supplied information, observations made whilst on site
and where necessary worst case parameters.
Therefore, based on this assessment, natural shallow landslides on the site have been given
a likelihood hazard rating of Low. This rating translates to a Likelihood Descriptor of Rare to
Unlikely.
4.3.4 Hazard B - Proposed Cuts
Cuts are understood to be proposed within the site for the basement level floor. Given the
adjacent structures, this level is expected to be within approx. 1.0 to 1.5m from the southern
boundary.
Based upon previous similar types of structures in the area, we expect that all cuts will be
supported temporarily during construction by sheet pile walls then permanently by construction
of other retaining structures. To assess these cuts, the method outlined by MacGregor & Taylor
(2001) for the assessment of cuts has been adopted. No batters are expected to be adopted
for excavations.
The assessment has taken into account the proposed wall geometry (based on provided
drawings and site measurements), expected subsurface profile (from site investigation) and
proposed development (based on provided drawings).
Information in relation to the design parameters or structural elements of the wall is not
available at this stage. Therefore, the assessment has also assumed that all retaining walls
(both temporary and permanent) will be adequately engineer (RPEQ) designed and certified.
Assuming all cuts are adequately supported by retaining structures and that no evidence of
instability is noted in the cuts once completed, then a likelihood hazard rating of Low to
Moderate would apply to the temporary cuts. This translates to a Likelihood Descriptor of
Unlikely.
However, once suitable permanent retaining structures are in place, the hazard rating would
drop to Low which translates to a Likelihood Descriptor of Rare to Unlikely.
Note: The above assumes that all retaining structures on the site are designed by a
suitably qualified RPEQ and constructed in accordance with their design.
4.3.5 Hazard C - Scour of Exposed Surfaces
As noted above, the methods as adopted for Hazards A and B do not have a corresponding
method for the analysis of scour (Hazard C). The following comments will allow the hazard to be
assessed:-
The proposed development is likely to require the clearing and exposure of soil over
essentially the entire site (slope angles generally of 5º to 25º).
The exposed surfaces would most likely be fill and sandy soils.
Experience with these types of materials would suggest that the exposed material would
scour easily if left exposed and water is allowed to run over the slopes.
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Based on the above comments and observations on site, the likelihood rating for the scour of
exposed surfaces would be assessed as being Moderate to High which translates to a
Likelihood Descriptor of Unlikely to Likely.
However, if soils are protected from scour as would be expected to be required as part of the
development during and following construction, then the rating would reduce to Very Low or
Low which translates to a Likelihood Descriptor of Barely Credible to Rare.
4.4 Consequence Analysis
4.4.1 General
As part of the assessment of the risk of slope instability, the analysis of the consequence of any
failure must also be considered.
This analysis is based on the effect on the following for each type of hazard:-
i) Consequence to Property (proposed dwelling)
ii) Consequence to Persons within and around the proposed dwelling
4.4.2 Site Factors
The development consists of the construction of a new residence on the site. The following will
be used to assume consequence:-
A residence will be constructed with basement level formed by cuts into the existing
slope.
Based on the provided drawings, temporary cuts of up to approx. 4.0m to 5.0m will be
required for the basement (ground floor) near the middle of the site (including near
boundaries).
The cuts will be supported initially by temporary engineer designed and certified
retaining structures during construction and then with permanent retaining structures as
part of the residence.
4.4.3 Consequence to Property
For the purposes of this study, a semi-qualitative measure of consequence to property will be
adopted, as outlined in Appendix C of PNGLRM 2007c.
If slope instability were to occur, the aspect of property most at risk would be the proposed
residential structure located on the subject part of the site.
Based on Appendix A of PNGLRM 2007c the residential structures at risk would be classified as
Importance Level 2.
Considering the above information the consequence to property for each of the hazards is as
follows:-
Hazard A – Existing slopes – Minor to Medium
Hazard B – Proposed cuts – Medium as could affect several areas of the site or
adjacent sites during construction of the retaining walls.
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Hazard C – Initially Insignificant due to shallow soil profile but could be Medium if not
addressed in the long term.
4.4.4 Consequence to Persons
For the purposes of this study, the methodology as outlined in Section 7 of PNGLRM 2007, will
adopt the following assumptions with respect to persons on the lot. Different hazards have
been deemed to have varying probabilities based on their location on the site (Section 5.2):-
The affected study site will contain a proposed residential dwelling.
The probability of spatial impact (P(S:H)) - this has been based on the total width of the
site exposed to possible instability compared to the total width of the proposed structure
within that length. Assessed as 100%.
Proportion of time dwelling occupied - 50%.
Probability of not being able to escape - 20%.
Temporal Spatial Probability (P(T:S)) - Proportion of time dwelling occupied multiplied by
the Probability of not being able to escape - 10%.
Vulnerability (V(D:T)), (Appendix F - PNGLRM 2007):- 70%
Note: the consequences to Persons have not been assessed for hazard C prior to construction
as it is not considered applicable.
4.5 Risk Determination and Evaluation - Property
Appendix C of “PNGLRM (2007)” outlines a method of assessing Risk to Property. The
calculation of likelihood is based on the estimate outlined in Sections 4.3.3 to 4.3.5 of this
report.
The consequence is based on the proposed development and the effects on the property of the
specific hazards in Section 4.4.
The risk is then assessed based on the Risk Matrix from LRMPNG, 2007. The assessment is
summarised in Table 3.
TABLE 3 SUMMARY OF RISK TO PROPERTY
Hazard Assessed Assessed Assessed Risk
Hazard Area of Site
Rating Likelihood Consequence to Property
A Overall Site Low Rare to Unlikely Minor to Medium Very Low to Low
Proposed
B 2 Low Rare to Unlikely Medium Low
internal cuts
Overall Site – Moderate to
Unlikely to Likely Insignificant Very Low to Low
initially High
C
Overall Site – Very Low to Barely Credible to
3 Medium Very Low to Low
long term . Low Rare
Note:-
1. Refer Appendix B of “Practice Note Guidelines for Landslides Risk Management (2007)”.
2. Assumes all cuts supported by engineer designed retaining walls.
3. Assumes it’s addressed by landscaping and control of drainage and overland flow.
A review of the table above indicates that provided the recommendations outlined in this
report are carried out, the risk of slope instability occurring on the site has been
assessed as no higher than Low.
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4.6 Risk Determination and Evaluation - Persons
Evaluation of the risk to life must be carried out on a quantitative basis to be consistent and for
comparison with acceptance criteria. To do this, the indicative probabilities given in Appendix C
of PNGLRM (2007) may be adopted as there are no other frequency/probability data. The
assessment results are shown in Table 4.
The analysis is based on formula 1 in Section 7 of PNGLRM (2007), i.e.:-
R(LoL) = P(H) × P(S:H) × P(T:S) × V(D:T)
Where R(LoL) = The risk for Loss of Life
P(H) = Annual probability of the Landslide - based on Section 4.3
The other factors are outlined in Section 4.4.4
The acceptance criteria (Table 5) adopted for this assessment is outlined in Section 8 of
PNGLRM (2007) which suggests the following Risk to Life as outlined in Table 4.
Based on the assessment below, the risk to life for the three hazards has been assessed
to be Acceptable.
TABLE 4 SUMMARY OF RISK TO LIFE CALCULATIONS
Assessed
Haz Area of Site P(H) P(S:H) P(T:S) V(D:T) R(LoL) Assessment
Likelihood
-5 -7
10 to 7x10 to
A Overall Site Rare to Unlikely -4 1.0 0.1 0.7 -6 Acceptable
10 7x10
-5 -7
Proposed 10 to 7x10 to
B 2 Rare to Unlikely -4 1.0 0.1 0.7 -6 Acceptable
internal cuts 10 7x10
-6 -8
Overall Site – Barely Credible 10 to 7x10 to
C 3 -5 1.0 0.1 0.7 -7 Acceptable
long term to Rare 10 7x10
Note:-
1. Refer Appendix B of “Practice Note Guidelines for Landslides Risk Management (2007)”.
2. Assumes all cuts supported by engineer designed retaining walls.
3. Assumes it’s addressed by landscaping and control of drainage and overland flow.
4.7 Risk Summary
A summary of the results of the risk assessment for the existing site condition as detailed in
Sections 4.6 and 4.7 are as follows:-
Risk to Property - This is outlined in Table 3 and provided the recommendations of this
report are followed, then the risk of any slope stability occurring on the site has been
assessed as Very Low to Low or Low.
Risk to Persons – This is outlined in Table 4 and based on that assessment, the risk to
persons from the identified hazards was assessed as Acceptable.
We also recommend that once a retention design has been completed, it be reviewed by
Soil Surveys Engineering to ensure compliance with this report and that no additional
stability issues have been created.
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TABLE 5 RISK CRITERIA
Situation Suggested Tolerable Loss of Life Suggested Acceptable Loss of
Risk limit for the person most at risk Life Risk limit for the person most
at risk
Existing Slope 10-4 / annum 10-5 / annum
Existing Development 10-4 / annum 10-5 / annum
New Constructed Slope 10-5 /annum 10-6 /annum
New Development 10-5 /annum 10-6 /annum
Existing Landslide 10-5 /annum 10-6 /annum
Notes:
“Existing Slopes” in this context are slopes that are not part of a recognizable landslide and have demonstrated non-failure
performance over at least several seasons or events of extended adverse weather, usually being a period of at least 10 to 20
years.
“Existing Development” includes existing structures, and slopes that have been modified by cut and fill, that are not located on or
part of a recognizable landslide and have demonstrated non-failure performance over at least several seasons or events of
extended adverse weather, usually being a period of at least 10 to 20 years.
“New Constructed Slope” includes any change to existing slopes by cut or fill or changes to existing slopes by new stabilisation
works (including replacement of existing retaining walls or replacement of existing stabilisation measures, such as rock bolts or
catch fences).
“New Development” includes any new structure or change to an existing slope or structure. Where changes to an existing
structure or slope result in any cut or fill of less than 1.0m vertical height from the toe to the crest and this change does not
increase the risk, then the Existing Slope / Existing Structure criterion may be adopted. Where changes to an existing structure
do not increase the building footprint or do not result in an overall change in footing loads, then the Existing Development
criterion may be adopted.
“Existing Landslides” have been considered likely to require remedial works and hence would become a New Constructed Slope
and require the lower risk. Even where remedial works are not required per se, it would be reasonable expectation of the public
for a known landslide to be assessed to the lower risk category as a matter of “public safety”
Acceptable risks are usually considered to be one order of magnitude lower than the Tolerable Risks.
It is important to distinguish between “acceptable risks” and “tolerable risks”.
Tolerable Risks are risks within a range that society can live with so as to secure certain benefits. It is a range of risk regarded as
non-negligible and needing to be kept under review and reduced further if practicable.
Acceptable Risks are risks which everyone affected is prepared to accept. Action to further reduce such risk is usually not
required unless reasonably practicable measures are available at low cost in terms of money, time and effort.
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5.0 ENGINEERING STABILITY RECOMMENDATIONS
5.1 General Recommendations with Respect to Construction
Continued long term stability of the site is subject to development of the site taking place in
accordance with the guidelines of this report and relevant Australian Standards and good
building practices.
Based on the field observations, published hazard mapping and observed subsurface
conditions, it is concluded that provided the proposed development is undertaken using
our recommendations, it would not adversely affect the slope stability conditions at this
site. This conclusion is qualified by the following provisions:-
The earthworks should be designed and constructed in accordance with sound and
proper engineering principles.
Likewise, the future construction should be planned designed and constructed in
accordance with sound and proper engineering principles, and more specifically in
accordance with good hillside construction practices (refer Appendix I).
All construction works to be carried out in accordance with the recommendations of this
report.
The access and drainage infrastructure must be properly and effectively maintained, to
ensure that all stormwater is intercepted and controlled. This requirement extends to all
owners and occupiers being aware that ongoing maintenance of the site drainage is
essential for continued site stability.
The following recommendations should be adopted:-
All engineering works should follow the appropriate codes i.e.:-
o Earthworks - AS 3798 "Guidelines on Earthworks for Commercial and
Residential Developments".
o Footings for buildings - AS 2870 'Residential Slabs and Footings'.
o Retaining structures - AS 4678 ‘Earth Retaining Structures’.
Vegetation - where possible, the prompt re-establishment of ground cover should be
undertaken to reduce the risk of surface scour during and following rainfall. Where not
possible, other forms of surface protection should be adopted.
5.2 Stormwater Drainage
Appropriate drainage provisions are essential in any development. Adequate subsoil and
surface drainage should also be incorporated in any retaining wall construction.
Preferably, where possible, roof water should be taken via piped conduits to the road drainage
system or to water storage tanks. Any overflow from water storage tanks should be piped to the
road drainage system. Any outlet for pool backwash will also be required to be discharged into
an adequately designed system and piped to the road drainage.
Other usual treatment options associated with Good Hillside Practice, as outlined in Appendix I
should also be adopted. Some of these are implicit or assumed by the analysis. For example,
provision of surface water drainage measures (such as a lined drain across the slope uphill
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from the dwelling) and properly engineered retaining walls. Such treatment options should be
clearly stated as part of the risk management requirements.
5.3 Site Specific Recommendations
5.3.1 General
The site risk assessment has assumed that several measures were in place including properly
designed and constructed retaining walls (where applicable), site drainage and scour protection.
5.3.2 Scour
It is essential that any permanent batters/slopes that result from the development (e.g. adjacent
to existing easements etc.) are suitably protected from erosion and scour by appropriate
drainage and the establishment of ground cover and shrubs, etc. Runoff should not be allowed
to discharge directly across the batters without suitable scour protection.
Top soiling and seeding or hydromulching on any cut areas steeper than 1:5 (11 degrees) is
suggested so that protection could be provided until such time as the grass cover is
established. The type of protection may also need to be discussed with the local council given
the adjacent dune environment.
5.3.3 Retaining Structures
At this stage, we understand that retaining structures will be utilised for several parts of the
proposed development. It is assumed and expected that any walls (regardless of type) would
be designed and constructed to meet relevant retaining wall codes and good engineering
practice as well as the recommendations from this report for this site.
5.3.4 Continued Maintenance
As with all developments, continued maintenance of the proposed site works is essential to
maintain the assessed risks outlined in this report.
Maintenance should include but not be limited to the following:-
Maintaining clear drainage paths for surface water flows.
Maintaining and promoting revegetation of exposed batter areas.
Regular maintenance checks to ensure any damage is repaired in a timely manner.
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6.0 ENGINEERING ASSESSMENT
6.1 Civil Works
6.1.1 Trafficability and Site Preparation
The fieldwork for this investigation was carried out by a ute mounted drilling rig (at street level)
and hand equipment (due to the steep slope which limited access onto the site for our
machinery). It should be noted that some problems may be anticipated during wet weather for
even light weight 4WD vehicles.
The soils on site are sensitive to repetitive vehicle loading and water (i.e. they will lose strength
through repetitive vehicle loading or if they become overly moist or wet). Further, seepage may
also result in a subsequent loss of strength. This may limit trafficability and create difficulties for
earthworks operations. This situation would be more pronounced if rainfall followed initial
clearing, stripping and grubbing.
It is recommended that after stripping, clearing and grubbing, the exposed surface in the
construction area be proof rolled (where appropriate) to assist in identifying weak areas and to
improve trafficability.
Maintaining adequate drainage conditions is essential. It should be ensured that runoff is
diverted away from the construction area to prevent ponding of water.
Potential trafficability problems with this site should not be underestimated. The site will
very quickly become untrafficable if appropriate seepage and drainage control
measures, along with construction practices appropriate for site conditions, are not
maintained.
The contractor should fully inform himself of the ground conditions on site prior to
commencement of earthworks. This requirement should be explicit in any earthworks
specifications or contract.
Working Platforms For Tracked Plant and Heavy Construction Vehicles
The scope of Soil Surveys Engineering’s study DOES NOT include the design of a working
platform for heavy construction vehicles or heavy tracked plant on or around the site.
Detailed design of a working platform should be carried out considering the operation of actual
machinery proposed to be used. This is particularly important when considering the use of
heavy piling rigs and heavy cranes - the piling/crane contractor should be consulted regarding
their requirements.
6.1.2 Earthworks
It is envisaged that the basement platform will be created with maximum cuts of up to approx.
4.0m to 5.0m with possible backfilling of up to approx. 2.5m required behind permanent
retaining structures.
Earthwork procedures should be carried out in a responsible manner in accordance with AS
3798-2007 'Guidelines on Earthworks for Commercial and Residential Developments'. It is
recommended that the earthworks contractor make themselves familiar with site conditions.
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Note: The surface silty sands will cause problems with trafficability/workability should
this material be wet/become wet prior to or during construction.
Given no structural filling is a currently proposed, no further specific earthworks
recommendation will be provided. The author can be contacted if further information is required.
6.1.3 Batters
It is understood that all cuts will be supported by temporary and later permanent retaining walls
with no requirements currently for batters on the site. Should batter angles be required, please
contact to author to discuss their use.
6.2 Building Foundations
6.2.1 General
Given the subsurface profile encountered, the expected loads and likely excavation depths, a
high level strip and pad footing system may be possible to be adopted for the proposed
basement section (only) at the south western end of the development, provided all footings are
founded into at least medium dense sand and the presence of loose sand is taken into account.
Deep foundations would be required outside of the basement cut area to reach at least the
medium dense sands (i.e. up to approx. 2.5m below existing down slope surface levels) and
could also be adopted as an alternative foundation system across the entire site and/or for
retention systems.
6.2.2 Site Classification as per AS 2870-2011
General
A site classification, in accordance with AS 2870 'Residential Slabs and Footings' (it is
recommended that the reader satisfy themselves that the use of AS 2870 is applicable for the
proposed design) has been carried out with the following information:-
Subsurface profile:
Possible minor fill
Natural sand soils
Proposed earthworks:
Cuts of up to approx. 4.0m to 5.0m with backfilling up to approx. 3.5m.
Soil Moisture Model:
A zone of moisture variation of 1.8m
A crack zone of 0.9m
pF = 1.2
Site Classification
According to AS 2870-2011, the site has been designated Class 'P' due to:-
The potential presence of uncontrolled fill in excess of 0.4m in depth as defined in AS
2870-2011 Clause 2.5.3;
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The site being potentially subject to other factors resulting in foundation movement
beyond the reactive soil movements resulting from moisture changes due to the normal
site conditions described in AS 2870-2011 Clause 1.3.2 (i.e. Clause 1.3.3 abnormal
moisture conditions).
The site being potentially subject to landslip.
6.2.3 Further Comments Regarding Foundations
Foundation Design
As noted above this site has been classified as ‘P’, however, if the site was classified based on
reactivity alone, under normal moisture conditions, a ground surface movement (ys) of <20mm
(maximum) could be adopted, which relates to a Class ‘A/S’ (sand-rock/slightly reactive) site
(considering the proposed development, existing subsurface profile as per boreholes records,
and taking into account other relevant geotechnical data held by Soil Surveys Engineering).
It should also be noted that the methodology adopted for determination of the site classification
assumes the structure has performance requirements similar to domestic type construction.
Development of Abnormal Soil Conditions
It is recommended that design works for the proposed development take into account the long
term effect of any design elements on the subsurface moisture conditions under and around the
structure.
These design works should consider aspects of the design that may contribute to the
development of abnormal moisture conditions i.e.:-
Site drainage
Gardens and landscaping
Future planting of trees and shrubs
Services and repair of leaks
The designer is directed to AS 2870-2011 Appendix B and Section 8.2.7 of this report which
outlines measures that should be considered.
6.2.4 High Level Footings
Where high level footings are adopted, our recommendation is that all perimeter footings,
footings under isolated columns and strip footings below internal load bearing walls be founded
at least 300mm below platform level and a minimum of 300mm into the natural sands.
Table 6 outlines minimum allowable bearing capacities for high level foundations on the site.
TABLE 6 ALLOWABLE BEARING CAPACITIES FOR HIGH LEVEL FOOTINGS
Allowable Bearing Capacity (kPa)
Material
Strip Footing Pad Footing
Fill Uncontrolled NR NR
Sand Loose NR NR
Compacted to 70% density index 100 125
Medium Dense 100 125
Note: NR - Not Recommended
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The base of all pad and strip footings founding in sand should be individually re-compacted
using a vibrating plate compactor prior to placement of reinforcing steel and concrete as soils
may be loosened during the excavation process by machinery action.
Should any loose areas be encountered in the footing excavations, the density of these areas
must be improved. Dynamic cone penetrometer (DCP) tests should be carried out to confirm
the adequacy of the founding soils. Note that it may be necessary to use formwork to prevent
trench side collapse in the sand profile.
It should be noted that any relevant stability recommendations contained in Section 5.0
will also apply to high level foundations. Also should excavations exceed 0.5m in depth
they may need to be assessed for stability on a case by case basis.
Where necessary, footings deeper than 300mm may be made up with mass concrete poured to
the underside of the footings, or alternatively, footings may be constructed over mass concrete
filled, backhoe excavated pedestals.
6.2.5 Deep Foundations
General
A deep pile system may also be utilised for the proposed structure and for retention systems on
the site. The choice of suitable pile types should be made from non-vibration piling methods.
Push piles, continuous flight auger (CFA), bored piles or screw piles could be considered as
suitable pile types, although the construction of the bored piers may be problematic in loose
sand, where collapse of the pile hole is likely. It is recommended specialist piling contractors be
consulted to make their own assessment of pile types considering subsurface conditions and
pile load requirements. Access onto the site for piling machinery must also be considered.
Depending on the load requirements, and the piling contractors design, it is recommended that
the piles are founded in material which is at least medium dense, and at a minimum depth of
3m below existing ground surface.
Compressional Capacity
It is recommended that the deep foundation system on this project be designed in accordance
with AS 2159-2009 'Piling - Design and Installation'. This code uses the limit state design
method.
In the limit state design method the following must be taken into account:-
Considering limit state analysis (AS 2159-2009), the design geotechnical strength Rd,g is
calculated by multiplying the ultimate geotechnical strength Rd,ug by the geotechnical
strength reduction factor Øg, i.e.
Rdg = Rd,ug g
Ultimate strength - the design of a single pile or a pile group must be such that both the
design geotechnical strength (Rd,g) and the structural strength (Rd,s) are greater than or
equal to the Design action effect (Ed) i.e.
Rd,g ≥ Ed and Rd,s ≥ Ed
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Serviceability - Single piles and pile groups shall be designed for serviceability by
controlling or limiting pile movements.
Durability - This is outlined in Section 6 of AS 2159-2009 and will not be discussed any
further here.
Any other factors that need to be considered i.e. stability, scour, fatigue, cyclic loading or
seismic actions - As we are not aware that any of these factors will affect the pile design
they will not be considered any further.
The design geotechnical strength (Rd,g) can be calculated as the design ultimate geotechnical
strength (Rd,ug) multiplied by the geotechnical strength reduction factor (g). Uplift and lateral
capacities of piles are not considered, as these are both wholly dependent on the specific pile
type selected.
For design of piles in cohesionless soils, the following equations may be used. However, piling
contractors should be requested to provide their own calculations based on experience and
relevant pile load tests:-
Rug = Q b + Qs
Where: Qb = Ab Pd (Nq - 1)
And: Qs = Ks Pd' tan d As
Rg* = Rug x g
Where: Rug = ultimate geotechnical strength of pile
Qb = ultimate bearing capacity of pile base
Qs = ultimate capacity of pile skin friction
Rg* = allowable capacity of piles
g = geotechnical strength reduction factor, adopt 0.5
but refer also AS2159-2009
Nq = bearing capacity factor
Pd = effective overburden pressure
Pd' = effective overburden pressure at mid-layer height
Ab = area of pile base
As = circumferential area of pile
Ks = earth pressure coefficient
d = angle of wall friction
For this site and expected type of construction, the ultimate geotechnical strength pile
parameters for the material encountered on site are:-
Base Bearing
Dense Sand = 900kPa
Skin Friction
Loose Sand = Ignore
Medium Dense Sand = 10kPa
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Dense Sand = 20kPa
Lateral Capacity
Broms (1966) has related the values of Ks and d to effective angle of shearing resistance of
cohesionless soils and various pile materials, as shown in Table 7.
TABLE 7 VALUES OF KS
d Ks
Pile Material
Degrees Low Relative Density High Relative Density
Steel 20 0.5 1.0
Concrete ¾ 1.0 2.0
It should be noted that the calculated ultimate resistance of a given pile should also take into
account the installation effects of that particular type, i.e. whether displacement or non-
displacement pile performance can be expected.
6.2.6 Slab on Ground
General
Field and laboratory test results indicate that the existing natural soils are suitable for a
conventional slab on natural ground construction. In areas of any fill, localised pedestals would
be required to suspend the slab.
For constructability of the proposed slab, it is generally preferred to have the same type of slab
across the proposed development site.
Sub-Base
It is suggested that a layer of sub-base be provided under all concrete slabs. The sub-base
material should be compacted to a density not less than 95% of maximum dry density in
accordance with AS 1289 5.2.1 (Modified compaction). The sub-base is provided under a
concrete slab to:-
Provide a stable 'working platform' on which to operate construction equipment;
Facilitate the provision of a uniform bearing surface under the slab.
Sub-base material should be of good quality conforming to requirements (minimum) of Material
Type 2.4 as specified in the Queensland Department of Transport, Standard Specification MRS
11.05, Unbound Pavements (December, 1993).
6.2.7 Design Considerations
Inspections
It is recommended that inspections be undertaken by an experienced geotechnical engineer
from Soil Surveys Engineering following footing excavations to confirm the adequacy of the
founding soils. Inspections should be carried out prior to placement of reinforcing steel and
ordering of concrete.
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Underground Services
Where footings are located adjacent to underground services, the footings should extend to
base a minimum of 200mm below the trench base level for a distance of 1.0m out from the
trench. Beyond 1.0m the footings should be taken a minimum of 200mm below an imaginary
line drawn up at 450 from the trench base level. Figure 4 refers. These requirements do not
override minimum footing levels.
Service Trench 1
or 1
Retaining Wall 200mm
Found footings
1.0m Below these lines
200mm
FIGURE 4
6.2.8 Site Management
It is important that proper site management methods be observed for the existing soil conditions
by both the builder at the time of the construction and the owner throughout the life of the
proposed development.
Particular reference to this is set out in AS 2870-2011. It should be noted that where proper site
management, particularly with respect to change in moisture conditions, is not followed, the
foundation recommendations contained in this report could be considered void.
The following are some specific comments with respect to site management and the reader is
also directed to CSIRO pamphlet, 10-91, ‘to Home Owners on Foundation Maintenance and
Footing Performance’ for further information:-
It is important that the site be well drained. The ground around the building should
slope away at 1 in 20 for 2 metres and then fall to the stormwater system to prevent
ponding of water adjacent to the building.
Roof downpipes and garden taps should not be allowed to saturate founding soils.
Footings should be placed with minimal delay after excavation to avoid desiccation
or wetting of the founding soils. If footings cannot be poured on the same day as
excavation, a blinding layer of 50mm thickness is recommended. Piers should be
poured immediately following excavation.
Do not let the slab subgrade "dry out" prior to casting.
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6.3 Basement Excavation and Retention Methods
6.3.1 Introduction
The design and construction method adopted for basement retention depends upon a number
of factors. These include:-
Strength and type of subsurface.
Boundary conditions i.e. location and type (i.e. movement sensitivity) of nearby building,
need for underpinning, etc.
Final wall profile and need for waterproofing.
6.3.2 Geotechnical Model
Based on the supplied contour plan, the existing ground elevation varies from approximately RL
27.0m at the western corner of the site down to approximately RL 16.0m at the eastern corner
of the site. Assuming a basement (ground floor) excavation level of approx. RL 19.0m, the
overall cuts on the site will vary from 0m up to approximately 5.0m (i.e. basement excavation is
in from the boundary lines on the high side of the site).
Drilling, DCP testing carried out and a site inspection indicate a consistent subsurface profile
with very loose to loose sands to approx. 2.5m depth, transitioning to medium dense to dense
sands extending to the base of the proposed cuts.
6.3.3 Boundary Conditions
Boundary conditions for each face consist of:-
South Western Face – Park Crescent with underground services (council services,
water, comms, power).
North Eastern Face – Adjoining council easement.
North Western Face – Approx. 2m wide service easement then neighbouring two storey
residential property approx. on the easement boundary.
South Eastern Face – Neighbouring three storey residential property, approx. 2.0m off
the boundary line, with block retaining walls along parts of the boundary line.
The effect of these boundary conditions must be taken into account in the design of a suitable
retention option for the south western, north western and south eastern boundary lines.
6.3.4 Possible Retention Options
The following options could be considered for temporary basement retention along the three
site boundaries noted above:-
1) Batter slope to stable batter angle - This method is only applicable where sufficient
space is available. Batters could be utilised where space allows within the site,
however given the adjoining properties on all sides and predominantly loose sand
profile, this would not be possible or recommended.
2) Structural Solution - This method includes the construction of retention systems
(temporary or permanent) along the boundaries to support the material on the
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neighbouring properties and road. Given that the excavation is expected to be up to
5m deep overall, a braced retaining wall will be required. This retaining wall should be
installed prior to excavation to ensure stability of the sand profile.
Several types of retaining walls could be employed including steel sheet piles,
contiguous CFA piles or diaphragm wall. Each of these have relative advantages and
disadvantages (technical and economic) which are best evaluated at the detailed
design/tender stage.
Once the preferred retention option for the site boundaries has been selected, SSE should be
contacted to provide further design detail (if required).
6.4 Retaining Walls
6.4.1 General
It is understood that retaining walls will be required for parts of the site. The lateral earth
pressure distribution that affects the retaining walls on the site will depend upon the following
parameters:-
In-situ and backfill material properties
Design water regime at the rear of the wall
Wall and cut geometry
Surcharges affecting the wall
Wall type
The structural bracing of the wall and the timing of backfilling behind the wall
6.4.2 Pressure Distribution
The following situations should be considered:-
For cantilever walls, which allow some movement at the top, (i.e. at least 0.005H in
clays) the active case (Ka) applies with a triangular distribution in both the short and
long term situations.
For cantilever walls which cannot tolerate this movement, the at rest case (Ko) applies
with a triangular distribution in both the short and long term situations.
For structurally braced walls the wall design should be checked for both a trapezoidal
(clay soils - short term conditions) and triangular distribution with Ko values (long term
conditions).
The pressure distributions as referred to above are shown in Figures 5 and 6. The parameters
selected for use in the figures are dependent on the preconstruction geometry of the face being
retained. It is expected that without battering, the earth parameters will control.
The lateral pressure distributions shown in Figures 5 and 6 include hydrostatic pressure and
show typical pressure distributions due to surcharge loadings. It is recommended that where
the retaining walls are expected to be surcharged (e.g. by footings, traffic loads, sloping ground
surface, etc.) Soil Surveys Engineering should be contacted to provide a recommended lateral
earth pressure distribution.
SOIL SURVEYS
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TRIANGULAR DISTRIBUTION
Lateral Earth Pressure Lateral Earth Pressure due to Surcharges
H K(H-hw)
hw
KH+hww(1-K)
K = appropriate earth pressure coefficient
H = Vertical height of the wall UDL Point Load or
Hw = Height of design water table Strip Footing
= Soil bulk density (kN/m3)
w= water bulk density (kN/m3)
FIGURE 5
As a general guide (using an elastic model) line loads located 2.5H away from the wall (where
H is the wall height) results in lateral forces of less than 10% of the line load. Parameters
(unfactored) for assessment of lateral earth pressures are outlined in Table 8.
Any backfill placed behind the wall should be loose granular material. The backfill should not
be heavily compacted since research has shown that compaction can raise the earth pressure
to above the 'at rest' pressure.
Adequate surface and subsoil drainage should be provided for all retaining walls on the site.
Cut-off / interceptor drains should also be provided around the high side of the wall to ensure
stormwater runoff from the area above the wall is suitably diverted.
The placement of a filter fabric between the retained soil and the drainage material (e.g.
granular backfill) for protection against silting of the drainage material is recommended. The
outlets to subsoil pipe drains must be located beyond the ends of the walls and connected to a
proper drainage system. It is suggested the pipes be wrapped in filter fabric to minimise silting.
SOIL SURVEYS
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TABLE 8 RETAINING WALL DESIGN PARAMETERS (UNFACTORED)
Earth Pressure Coefficient Vertical
Material Density Wall
Long Term
3
(kN/m ) Drained Ø (o)
Ka Ko Kp
Back Fill 17 0.27 0.43 NR 35
2
Gravel 19 0.33 0.50 3.0 30
Loose Sand 17 0.33 0.50 3.0 30
Medium Dense Sand 18 0.29 0.46 3.4 33
Dense Sand 20 0.27 0.43 3.0 35
Note: 1) NC – Not considered, NR Not recommended.
2) Infers granular free draining material placed behind the retaining wall as a drainage layer.
TRAPEZOIDAL DISTRIBUTION
Lateral Earth Pressure Lateral Earth Pressure due to Surcharges
0.65KaH
hw
hw
Earth Pressure Water UDL Point Load or
Strip Footing
FIGURE 6
In weather exposed locations, to reduce infiltration by surface runoff, the surface of the backfill
should be sealed. This can be achieved by either compacting a material of low permeability i.e.
clay or concrete, etc. with a slope towards an open drain.
Due to possible long term problems with blocking of gravel filters and drains and short term
storm conditions that could flood the fill behind retaining walls, it is recommended that all
retaining walls be designed for some water pressure distribution. For ultimate limit state the
suggested water pressure distribution for retaining walls on this site would be half height water
pressure.
During installation of any retaining walls, the in-situ soils should be battered back to minimise
fall-in and subsequent disruption of works. Temporary batter angles should be requested from
Soil Surveys should any battering on site be proposed. Suitable precautions to satisfy Health &
Safety requirements must also be adhered to.
SOIL SURVEYS
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7.0 CONSTRUCTION INSPECTIONS
It is recommended that all footing excavations be inspected by Soil Surveys Engineering Pty.
Limited. or a duly qualified Geotechnical Engineer.
Should subsurface conditions other than those described in this report be encountered, Soil
Surveys Engineering Pty. Limited. should be consulted immediately and appropriate
modifications developed and implemented if necessary.
8.0 REFERENCES
Australian Geomechanics Society (2000) Landslide Risk Management Concepts and
Guidelines, Australian Geomechanics, Vol 35 No.1 March 2000, pp49-92
Australian Geomechanics Society (2007) Landslide Risk Management Guidelines and
commentary, Australian Geomechanics, Vol 42 No. 1 March 2007, pp1-182
Geoscience Australia web site - www.ga.gov.au
McGregor P. and Taylor B. (2001) A Method of Zoning Landslide Hazard, Australian
Geomechanic Journal Volume 36, No. 3 September 2001.
Walker, BF (2002) An example of Semi Quantitation Landslide Basic Management for an
individual Lot in Northern Sydney, Australian Geomechanics, Vol 37 No.2 May 2002.
9.0 LIMITATIONS
We have prepared this report for the use of Sanya Holdings, for design purposes in
accordance with generally accepted geotechnical engineering practices. No other warranty,
expressed or implied, is made as to the professional advice included in this report. This report
has not been prepared for use by parties other than Sanya Holdings; it may not contain
sufficient information for purposes of other parties or for other uses. Please note that any third
party relying on the information contained in this report for any purpose whatsoever does so
entirely at its own risk, and any duty of care to that third party is excluded.
Any interpretation or recommendation given by Soil Surveys Engineering shall be understood to
be based on judgement and experience and not on greater knowledge of the facts than the
reported investigations would imply. The interpretation and recommendations are therefore
opinions provided for our Client’s sole use in accordance with the specific brief. As such they do
not necessarily address all aspects of ground behaviour on the subject site. Information
provided by others has been taken in good faith, but no liability can be accepted for information
provided by others.
Your attention is drawn to ‘Appendix A’, ‘Notes Relating to this Report’. Interpretation of factual
data given in this report is based on judgement, not a greater knowledge of facts other than
those reported.
Interpretation of the information shown on the logs, and its application to design and
construction, should therefore take into account the spacing and depth of boreholes, the
SOIL SURVEYS
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Project No. 1-22543 Page | 28 June, 2020 Ref: 1-22543, 2020-05-20, BR SSA VER 1 Sanya Holdings - Slope Stability Assessment and Geotechnical Report - Proposed Residence, 40 Park Crescent, Sunshine Beach method of drilling, the frequency of sampling and testing and the possibility of other than “straight line” variations between the boreholes. Subsurface conditions between and below boreholes may vary significantly from conditions encountered at the borehole locations. Please note, if following detailed design, the founding depth of any footings or piers/piles is within 3B (B = footing width/pier diameter) above the termination depth of the boreholes/pits/CPTU or if any excavations extend below the borehole/pit/CPTU termination levels, then Soil Surveys Engineering should be contacted immediately. If the above were to occur then the geotechnical data in this report should be considered preliminary only; additional investigation is likely to be required. In the event that conditions encountered on site during construction appear to vary from those expected from the information contained in the report, the Company strongly recommends that it immediately be notified. Most problems are more readily resolved when conditions are exposed than at some later stage, after the event. Should Soil Surveys Engineering not be notified or if this notification is delayed, then Soil Surveys can not be held responsible for the effect that any variation has on any aspect of the development. Soil Surveys Engineering consider that a documentation review service (during the design phase and prior to construction) to verify that the intent of geotechnical recommendations is properly reflected in the design, along with construction inspections, forms a very important component of the geotechnical engineering design service/process. The geotechnical review ensures geotechnical risks to our Client and their project are minimised at the design and tender stage of the project. Further, with Soil Surveys Engineering being commissioned to carry out geotechnical construction inspections, an opportunity at the time of construction to confirm any assumptions made in the preparation of the report and allow the effect of any normally occurring variation in ground conditions to be assessed with respect to construction becomes available. The above statements are not intended to reduce the level of responsibility accepted by Soil Surveys Engineering in accordance with our commission, but rather to ensure that all parties who may rely on this report are aware of the responsibilities each assumes in doing so and the risks they accept should they decline to have Soil Surveys Engineering carry out a geotechnical documentation review and geotechnical construction inspections. It is highly recommended that the Client avail themselves of these review and inspection services; our standard rates will apply. Ee - 2 G. BURKITT _ N.T. PERKINS (RPEQ 7527) ENGINEERING GEOLOGIST PRINCIPAL ENGINEERING GEOLOGIST For and on behalf of SOIL SURVEYS ENGINEERING PTY LIMITED SOIL SURVEYS
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APPENDICES
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APPENDIX A
NOTES RELATING TO THIS REPORT
Page 35
NOTES RELATING TO THIS REPORT
September, 2019
INTRODUCTION determination of shear strength, volume change potential
and compressibility. Undisturbed sampling is generally
These notes are provided by Soil Surveys Engineering Pty
effective only in cohesive soils.
Limited (the Company) to complement the geotechnical
report in regard to classification methods and field Details of the type and method of sampling used are given
procedures. Not all notes are necessarily relevant to all on the attached logs.
reports.
SAMPLE STORAGE – SOIL, ROCK AND WATER
The ground is a product of continuing natural and man-made
SAMPLES
processes and therefore exhibits a variety of characteristics
and properties which vary from place to place and can Soil samples (not subject to testing) are not stored beyond a
change with time. Geotechnical engineering involves period of 90 days of taking or receiving said soil sample.
gathering and assimilating limited information about these Rock core (not subject to testing) is not stored beyond a
characteristics and properties in order to understand or period of six months of taking or receiving said rock core.
predict the behaviour of the ground on a particular site under
Should any party require that soil samples (not subject to
certain conditions. This report may contain such information
testing) be stored beyond 90 days, or rock core (not subject
obtained by inspection, excavation, probing, sampling,
to testing) be stored beyond six months, please contact Soil
testing or other means of investigation. If so, they are directly
Surveys Engineering.
relevant only to the ground at the place where and at the
time when the investigation was carried out. Water samples (not subject to testing) are not stored beyond
a period of seven days of taking or receiving water samples.
DESCRIPTION AND CLASSIFICATION METHODS
TEST LOCATIONS
Soils - The methods of description and classification of soils
and rocks used in this report are based on Australian Test locations (e.g. boreholes, CPT’s, test pits etc.) were
Standard 1726-2017 (Geotechnical Site Investigations), based on available access at the time of testing. Test
where appropriate. In general, descriptions cover the locations may have been shifted if access was not suitable.
following properties - soil or rock type, colour, structure,
Unless noted otherwise, accuracy of test locations are to the
strength or density, and inclusions. Identification and
accuracy of hand held GPS equipment.
classification of soil and rock involves judgement and the
Company infers accuracy only to the extent that is common INVESTIGATION METHODS
in current geotechnical practice.
The following is a brief summary of investigation methods
Soil types are described according to the dominant particle currently adopted by the Company and some comments on
size and behaviour as set out in AS 1726-2017. their use and application.
Cohesive soils are classified on the basis of strength Test Pits - These are normally excavated with a backhoe or
(consistency) either by use of hand penetrometer, shear a tracked excavator, allowing close examination of the insitu
vane, laboratory testing or engineering examination. The soils if it is safe to descend into the pit. The depth of
strength terms are defined in AS 1726-2017 Table 11. penetration is limited to approximately 3.0m for a backhoe
and up to 6.0m for an excavator. Limitations of test pits are
Non-cohesive soils are classified on the basis of relative
the problems associated with disturbance and difficulty of
density usually based on insitu testing or engineering
reinstatement and the consequent effects on close-by
examination (see AS 1726-2017 Table 12).
structures. Care must be taken if construction is to be carried
Rocks - Rock types are classified by their geological names out near test pit locations to either properly recompact the
(AS 1726-2017 Tables 15 to 18), together with descriptive backfill during construction or to design and construct the
terms regarding weathering (AS 1726-2017 Table 20), structure so as not to be adversely affected by poorly
strength (AS 1726-2017 Table 19), defects (AS 1726-2017 compacted backfill at the test pit location.
Table 22), etc.
Hand Auger Drilling - A borehole of 50mm to 100mm
SAMPLING diameter is advanced by manually operated equipment.
Refusal of the augers can occur on a variety of materials
Sampling is carried out during drilling or from other
such as hard clay, gravel or rock fragments and does not
excavations to allow engineering examination (and
necessarily indicate rock level.
laboratory testing where required) of the soil or rock.
Continuous Spiral Flight Augers - The borehole is
Disturbed samples taken during drilling provide information
advanced using 75mm to 300 mm diameter continuous spiral
on plasticity, grain size, colour, moisture content, minor
flight augers, which are withdrawn at intervals to allow
constituents and, depending upon sample disturbance,
sampling or insitu testing. This is a relatively economical
(information on strength and structure).
means of drilling in clays and in sands above the water table.
Undisturbed samples are taken by pushing a thin walled Samples are returned to the surface by the flights or may be
sample tube, usually 50mm diameter (U50), into the soil and collected after withdrawal of the augers. Information from
withdrawing it with a sample of the soil contained in a the drilling (as distinct from specific sampling) is of relatively
relatively undisturbed state. Such samples yield information lower reliability due to remoulding, inclusion of cuttings from
on structure and strength, and are necessary for laboratory above or softening of samples by groundwater, or
Page 36
NOTES RELATING TO THIS REPORT
September, 2019
uncertainties as to the original depth of the samples. In a case where the test is discontinued short of full
Augering below the groundwater table has a lower reliability penetration, say after 15 blows for the first 150 mm and 30
than augering above the water table. Various drill bits are blows for the next 40 mm, the record shows:
attached to the base of the augers during the drilling. The
15, 30/40mm
depth of refusal of the different bit types can provide
information as to the strength of the material encountered. The results of the test can be related empirically to the
Generally the ‘TC’ bit (a tungsten carbide tipped screw type engineering properties of the soil.
bit) is used.
Occasionally, the drop hammer is used to drive 50mm
Wash Boring - The borehole is usually advanced by a rotary diameter thin walled sample tubes (U50) in clays. In such
bit with water or fluid pumped down the hollow drill rods and circumstances, it is noted on the borehole logs.
returned up in the space between the rods and the soil or
A modification to the SPT test is where the same driving
casing, carrying the drill cuttings. Only major changes in
system is used with a solid 600 tipped steel cone of the
stratification can be determined from the cuttings, together
same diameter as the SPT hollow sampler. The solid cone
with some information from "feel" and rate of penetration.
can be continuously driven for some distance in soft clays or
More accurate information on soil strata is gained by regular
loose sands, or may be used where damage would
testing and sampling using the Standard Penetration Test
otherwise occur to the SPT. The results of this Solid SPT are
(SPT) and undisturbed thin walled tube samples (U50).
shown as "Nc" on the borehole logs, together with the
Mud Stabilized Drilling - Either Wash Boring or Continuous number of blows per 150 mm penetration.
Core Drilling can use drilling mud as a circulating fluid to
Cone Penetration Tests - Test Method - Cone Penetration
stabilize the borehole. The term "mud" encompasses a range
Tests (CPT) are carried out in accordance with AS 1289 Test
of products ranging from bentonite to polymers such as
6.5.1-1999, using an electrical friction-cone penetrometer.
Revert or Biogel. The mud tends to mask the cuttings and
reliable identification is only possible from regular intact The test essentially comprises the measurement of
sampling (e.g. from SPT and U50 samples) or from rock resistance to penetration of a cone of 35.7 mm diameter
coring, etc. pushed into the soil at a rate of 10-20 mm per second by
hydraulic force. The resistance to penetration is recorded in
Continuous Core Drilling - A continuous core sample is
terms of pressure on the end area of the cone (cone
obtained using a diamond or tungsten carbide tipped core
resistance, qc, in MPa) and friction on the side of the 135 mm
barrel. Provided full core recovery is achieved (which is not
long sleeve immediately above the top of the cone (friction
always possible in very weak rocks and granular soils), this
resistance, fs, in kPa). These forces are measured by
technique provides a very reliable method of investigation.
electrical transducers (strain gauges) within the cone device.
In rocks, NMLC coring (nominal 52 mm diameter) is usually
The ratio between friction resistance and cone resistance is
used with water flush. The length of core recovered is
also calculated as a percentage, i.e.-
compared to the length drilled and any length not recovered
𝑭𝒓𝒊𝒄𝒕𝒊𝒐𝒏 𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆, 𝒇𝒔 (𝒌𝑷𝒂) 𝒙 𝟏𝟎𝟎
is shown as CORE LOSS. The location of losses is Friction Ratio (FR) =
𝒄𝒐𝒏𝒆 𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆, 𝒒𝒄 (𝒌𝑷𝒂)
determined on site by the supervisor. If the location of the
loss is uncertain, it is placed at the top end of the run, when The friction ratio, FR, is generally low in sands (less than 1%
the core is placed in a storage tray and recorded on the log. or 2%) and generally higher in clays (say 3% or more). The
Standard Penetration Tests - Standard Penetration Tests interpretation of sandy clays, clayey sands and material with
(SPT) are used mainly in non-cohesive soils, but can also be a high silt content is more difficult, but intermediate values
used in cohesive soils, as a means of indicating density or (between 1% and 3%) would be expected. Highly organic
strength. The test procedure is described in Australian clays and peats generally have a friction ratio in excess of
Standard 1289, "Methods of Testing Soils for Engineering 5%.
Purposes" - Test 6.3.1. Static cone data is recorded in the field on disc for later
The test is carried out in a borehole by driving a 50mm presentation using computer aided drafting.
diameter split sample tube with a tapered shoe, under the The equipment can be operated from any conventional drill
impact of a 63 kg hammer with a free fall of 760 mm. It is rig. A total applied load in the range of 4 to 10 tonnes is
normal for the tube to be driven in three successive 150 mm required for practical purposes, although lighter loads may
increments and the 'N' value is taken as the number of blows be used. The cone penetrometers are available with various
for the last 300 mm, the upper 150 mm being neglected due capacities of cone resistance ranging up to 100 MPa for
to possible disturbance from the drilling method. In dense general purpose investigations, while a range of 0 to 10 MPa
sands, very hard clays or weak rock, the full 450 mm can be used where more sensitive investigations of soft clay
penetration may not be practicable and the test is are required.
discontinued at a reduced penetration. The cone resistance value provides a continuous measure of
In the case where full penetration is obtained with successive soil strength or density, and together with the friction ratio,
blow counts for each 150 mm of, say 4, 6 and 7 blows, the provide very useful indications of the presence of narrow
record shows, bands of geotechnically significant layers such as thin, soft
clay layers or lenses of sand which might otherwise be
4, 6, 7 N = 13 missed using conventional drilling methods.
Page 37
NOTES RELATING TO THIS REPORT
September, 2019
The lithology of the encountered soils is interpreted from Although groundwater may be present in lower
static cone data and is generally presented on the static permeability soils, it may enter the hole slowly or
cone log sheets. perhaps not at all during the time the hole is open.
It is important to note that the lithology is interpreted A localized perched water table may lead to an
information and is based on research by Schmertmann erroneous indication of the true water table.
(1970), Sanglerat (1972), Robinson and Campinalli (1986), Water table levels will vary from time to time with
modified to suit local conditions as indicated by borehole seasons or recent weather changes and may not be the
information and laboratory testing. same at the time of construction.
As soils generally change gradually it is sometimes difficult to The use of water or mud as a drilling fluid will mask any
accurately describe depths of strata changes, although groundwater inflow. Water has to be bailed out of the
greater accuracy is obtained with the static cone compared bore and mud must be washed out of the hole or
with conventional drilling. In addition, friction ratios decrease "reverted" if water observations are to be made.
in accuracy with low cone resistance values, and in
More reliable measurements can be made by use of
desiccated soils. As a result, some overlap and minor
standpipes which are read after stabilizing at periods ranging
discrepancies may exist between static cone and nearby
from several days to perhaps weeks for low permeability
borehole information.
soils. Piezometers, sealed in a particular stratum, may be
Portable Dynamic Cone Penetrometers - Portable advisable in low permeability soils or where there may be
Dynamic Cone Penetrometer (DCP) tests are carried out by interference from perched water tables or surface water.
driving a rod into the ground with a falling weight hammer
and measuring the blows for successive 100mm increments
FILL
of penetration. The presence of fill materials can often be determined only
by the inclusion of foreign objects (e.g. bricks, steel, etc.) or
The DCP comprises a Cone of 20 mm diameter with 30
by distinctly unusual colour, texture or fabric. Identification of
degree taper attached to steel rods of smaller section.
the extent of fill materials will also depend on investigation
The cone end is driven with a 9 kg hammer falling 510 mm methods and frequency. Where natural soils similar to those
(AS 1289 Test 6.3.2). The test was developed initially for at the site are used for fill, it may be difficult with limited
pavement subgrade investigations, and empirical testing and sampling to reliably determine the extent of the
correlations of the test results with California Bearing Ratio fill.
have been published by various Road Authorities. The
The presence of fill materials is usually regarded with caution
Company has developed their own correlations with
as the possible variation in density, strength and material
Standard Penetration tests and Density Index tests in sands.
type is much greater than with natural soil deposits.
LOGS Consequently, there is an increased risk of adverse
engineering characteristics or behaviour. If the volume and
The borehole or test pit logs presented herein are an
quality of fill is important to a project, then frequent test pit
engineering and/or geological interpretation of the
excavations are preferable to boreholes.
subsurface conditions, and their reliability will depend to
some extent on the frequency of sampling and the method of LABORATORY TESTING
drilling or excavation. Ideally, continuous undisturbed
Laboratory testing is normally carried out in accordance with
sampling or core drilling will enable the most reliable
Australian Standard 1289 "Methods of Testing Soil for
assessment but is not always practicable or possible to
Engineering Purposes". Details of the test procedure used
justify on economic grounds. In any case, the boreholes or
are given on the individual report forms and the attached
test pits represent only a very small sample of the total
explanatory notes summarize important aspects of the
subsurface conditions.
Laboratory Test Procedures adopted.
The attached explanatory notes define the terms and
ENGINEERING REPORTS
symbols used in preparation of the logs.
Engineering reports are prepared by qualified personnel and
Interpretation of the information shown on the logs, and its
are based on the information obtained and on current
application to design and construction, should therefore take
engineering standards of interpretation and analysis. The
into account the spacing of boreholes or test pits, the method
information provided in Soil Surveys Engineering reports is
of drilling or excavation, the frequency of sampling and
opinion and interpretation and not factual. The
testing and the possibility of other than "straight line"
client/contractor increases their risk by not retaining the
variations between the boreholes or test pits. Subsurface
person who authored the geotechnical report, to carry out
conditions between boreholes or test pits may vary
site inspection and review (overseeing role) during
significantly from conditions encountered at the borehole or
construction, to confirm opinion and interpretation expressed
test pit locations.
in the report is accurate. Where the report has been
GROUNDWATER prepared for a specific design proposal the information and
interpretation may not be relevant if the design proposal is
Where groundwater levels are measured in boreholes, there
changed. If this happens, the Company will be pleased to
are several potential problems.
Page 38
NOTES RELATING TO THIS REPORT
September, 2019
review the report and the sufficiency of the investigation complex, it is prudent to have a joint design review which
work. involves a senior geotechnical engineer. We would be happy
to assist in this regard as an extension of our investigation
Every care is taken with the report as it relates to
commission. Construction drawings should be reviewed by
interpretation of subsurface conditions, discussion of
Soil Surveys Engineering, with sufficient time to allow
geotechnical aspects and recommendations or suggestions
changes if required, prior to inspections. Otherwise Soil
for design and construction. Since the test sites in any
Surveys Engineering reserves the right to refuse to carry out
exploration represent a very small proportion of the total site
inspections.
and since the exploration only identifies actual ground
conditions at the test sites, even under the best SITE INSPECTION
circumstances actual conditions may vary from those
The Company will always be pleased to provide engineering
inferred to exist. No responsibility is taken for:-
inspection services for geotechnical aspects of work to which
Unexpected variations in ground and/or groundwater this report is related.
conditions.
i. Site visits during construction to confirm reported
Changes in policy or interpretation of policy by statutory ground conditions
authorities. ii. Site visits to assist the contractor or other site
personnel in identifying various soil/rock types such
The actions of other persons.
as appropriate footing or pier founding depths, the
Any work where the company is not given the stability of a filled or excavated slope; or
opportunity to supervise the construction using the iii. Full-time engineering presence on site.
Companies designs/recommendations.
In the vast majority of cases it is advantageous to the
If differences occur, the Company will be pleased to assist principal for the geotechnical engineer who wrote the
with investigation or advice to resolve any problems investigation report to be involved in the construction stage
occurring. of the project.
SITE ANOMALIES The geotechnical engineer cannot take responsibility for
variations in encountered conditions, where he is not given
In the event that conditions encountered on site during the opportunity to review plans for the proposed
construction appear to vary from those expected from the development with sufficient time to allow review and make
information contained in the report, the Company requests changes to the proposed development if required, and where
that it immediately be notified. Most problems are more he is not given the opportunity to inspect the site and
readily resolved when conditions are exposed than at some oversee construction methods with regard to site conditions
later stage, well after the event. with sufficient time to observe all relevant site conditions and
Extreme events including but not limited to the results of operations.
climate change, e.g. flood levels above previously identified RESPONSIBLE USE OF GEOTECHNICAL
levels, beach scour or erosion beyond normal expectations
(as identified by local authorities) extreme rainfall events, INFORMATION
war, espionage, sabotage may result in different conditions Recommendations in our report are for design purposes only
between time of investigation and time of construction. and provided on the basis that inspections are carried out to
allow finalisation of opinions and recommendations
REPRODUCTION OF INFORMATION FOR
contained in our report.
CONTRACTUAL PURPOSES
The geotechnical investigation consisting of field and
Attention is drawn to the document “Guidelines for the laboratory testing has been carried out to indicate typical
Provision of Geotechnical Information in Construction conditions by indicating conditions and parameters at the
Contracts (1987)”, published by the Institution of Engineers, specific locations of boreholes/test pits. Subsurface
Australia. Where information obtained from this investigation conditions are indicated at these locations only and the
is provided for tendering purposes, it is recommended that all inference of conditions between or away from these locations
information, including the written report and discussion, be (interpolation and extrapolation) involves a certain degree of
made available. In circumstances, where the discussion or risk. Persons inferring such conditions or carrying out such
comments section is not relevant to the contractual situation, inferences should do so with a degree of caution and
it may be appropriate to prepare a specially edited conservatism which is commensurate with the consequences
document. The Company would be pleased to assist in this of the risk of error.
regard and/or to make additional report copies available for
contract purposes at a nominal charge. Estimates of volumes based on our findings require
interpolation and extrapolation between test locations and as
REVIEW OF DESIGN such may be significantly different from actual volumes.
Where major civil or structural developments are propose or
where only a limited investigation has been completed or
where the geotechnical conditions/ constraints are quite
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APPENDIX B
ASSESSMENT GUIDELINES
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B. ASSESSMENT GUIDELINES
B.1 State Planning Policy (July, 2014)
B.1.1 General
In 2003 the Queensland State Government adopted under the Integrated Planning Act 1997
(IPA) the State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and
Landslide” and an associated guideline.
In 2014, the Queensland State Government reissued all the State Planning Policies under a
single document (State Planning Policy July 2014 - SPP). Landslide has now been
addressed under two sections of the document these being:-
Part D – The State Interests and Plan Making policies - Planning for
Safety and Resilience to Hazards.
Making or amending a planning scheme and designating land for community
infrastructure
The planning scheme is to appropriately integrate the state interest by:
For all natural hazards:
3) Identifying natural hazard areas for flood, bushfire, landslide and coastal hazards
based on a fit for purpose natural hazard study, and
4) Including provisions that seek to achieve an acceptable or tolerable level of risk,
based on a fit for purpose risk assessment consistent with AS/NZS ISO 31000:2009
Risk Management, and
5) Including provisions that require development to:
a) Avoid natural hazard areas or mitigate the risks of the natural hazard to an
acceptable or tolerable level, and
b) Support, and not unduly burden, disaster management response or recovery
capacity and capabilities, and
c) Directly, indirectly and cumulatively avoid an increase in the severity of the
natural hazard and the potential for damage on the site or to other properties, and
d) Maintain or enhance natural processes and the protective function of landforms
and vegetation that can mitigate risks associated with the natural hazard, and
6) Facilitating the location and design of community infrastructure to maintain the
required level of functionality during and immediately after a natural hazard event.
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Part E - Interim Development Assessment Requirements
These requirements apply to development applications as follows:
A development application for a material change of use, reconfiguring a lot or operational
works on land within:
7) A flood hazard area, or
8) A bushfire hazard area, or
9) A landslide hazard area, or
10) A coastal hazard area.
The development application is to be assessed against the following
requirements:
For all natural hazards:
Development:
11) Avoids natural hazard areas or mitigates the risks of the natural hazard to an
acceptable or tolerable level, and
12) Supports, and does not unduly burden, disaster management response or recovery
capacity and capabilities, and
13) Directly, indirectly and cumulatively avoids an increase in the severity of the natural
hazard and the potential for damage on the site or to other properties, and
14) Avoids risks to public safety and the environment from the location of hazardous
materials and the release of these materials as a result of a natural hazard, and
15) Maintains or enhances natural processes and the protective function of landforms
and vegetation that can mitigate risks associated with the natural hazard.
The SPP defines the following:-
Landslide hazard area means an area that is:
a) Identified by a local government in its planning scheme as a landslide hazard
area, on the basis of a fit for purpose landslide study of the local government’s
area; or
b) If the local government has not identified landslide hazard areas in its planning
scheme in accordance with paragraph (a) above; an area of land with a slope
greater than, or equal to 15 percent.
Natural hazard means a naturally occurring situation or condition, such as a flood,
bushfire, landslide or coastal hazard, including erosion-prone areas and storm-tide
inundation areas, with the potential for loss or harm to the community, property or
environment.
Natural hazard area means a flood hazard area, a bushfire hazard area, a landslide
hazard area or a coastal hazard area.
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Sanya Holdings – Slope Stability Assessment and Geotechnical Report - Proposed Residence, 40 Park Crescent, Sunshine Beach
In accordance with the SPP, “risks to the community for proposed developments in identified
Natural Hazard Management Areas should be adequately considered during design and
assessment of a proposed development. Where risks are unacceptable they should be
suitably minimised and/or controlled”.
B.1.2 Application to Site
The applicability of State Planning Policy 2014 (SPP) to the development site was assessed
using the requirements as outlined in Section 3.1.1 above:-
Is the development proposal in a local government defined Landslide Hazard Area -
Yes - refer The Noosa Plan, November, 2017, Landslide Overlay Code.
On the basis of the above, the SPP is applicable to the development site.
B.2 The Noosa Plan, 2017
B.2.1 Landslide Hazards
The Noosa Shire Council’s Noosa Plan, 2017 addresses slope instability and landslide
hazards within its Eastern Beaches Locality Plan (Part 7) and Natural Hazards Map for the
Eastern Beaches.
B.2.2 Applicability to Development Site
Figure C1 (Appendix C) shows the relevant output from the New Noosa Plan 2019 Online
Overlay Map (not yet in force) indicating that the entire site is located within the High Hazard
Area (refer Figure C1).
Therefore the Noosa Plan Landslide Hazard Code (2017) will apply to the site.
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APPENDIX C
SITE INVESTIGATION MAPS
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FIGURE C1 - PORTION OF LANDSLIDE OVERLAY FROM NEW NOOSA PLAN 2019
MAP
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APPENDIX D
FRAMEWORK FOR LANDSLIDE RISK
MANUAL
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FIGURE D1 LRM FLOWCHART
Extract from “Landslide Risk Management Concepts and Guidelines”, Australian
Geomechanics Society Journal, Volume 42 No. 1, March, 2007.
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APPENDIX E
PHOTOGRAPHS
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Sanya Holdings – Slope Stability Assessment and Geotechnical Report - Proposed Residence, 40 Park Crescent, Sunshine Beach
PLATE E1 – SITE LOOKING NORTH EAST ALONG SOUTH EAST BOUNDARY LINE
PLATE E2 – EXISTING WALLS AT WESTERN END OF SITESUPPORTING
DRIVEWAYAND BOUNDARY LINE, LOOKING WEST
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PLATE E3 – SITE LOOKING WEST UP SLOPE BENEATH RESIDENCE
PLATE E4 – SITE LOOKING WEST UP SLOPE FROM EASTERN BOUNDARY CORNER
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APPENDIX F
BOREHOLE AND DCP LOGS
Page 51
Soil Surveys Engineering Pty. Limited BOREHOLE RECORD SHEET
Specialists in Applied Geotechnics
PO Box 317, Paddington, 4064
Location Number: BH 01
+61 7 3369 6000
[email protected]
Project Number: 1-22543
www.soilsurveys.com.au Project Name: Proposed Residence
Location: 40 Park Crescent, Sunshine Beach
Easting: 511076 Northing: 7079137 RL: 27.6 m Client: Sanya Holdings Pty Ltd
Logger: RH Operator: RH Machine: EVH1750 Date: 20/05/2020 Page: 1 OF 2
Drilling Method DCP Test
Graphic (blows/100mm) Samples and
Depth Description
Casing
NMLC
Remarks
WB
RR
0 6 12 18 24 30
TC
FILL Silty SAND (SM) Loose to very loose, fine to coarse
grained, dark brown, trace of fine to medium sized gravel,
with organics, moist.
0.30
NATURAL SAND (SP) Very loose, fine to medium grained,
dark grey, trace of silt fines, moist. D
0.5
0.90
1.0 SAND (SP) Very loose, fine to medium grained, light grey
mottled light brown, moist.
D
1.5
2.0
2.10
SAND (SP) Loose, fine to medium grained, dark grey, moist.
SOIL SURVEYS 2.00 LIB 2017-07-13.GLB Log SOIL_SURVEY_AUGER_LOG2 1-22543_GINT.GPJ <<DrawingFile>> 15/06/2020 16:21 10.0.000 Developed by Datgel
2.5
2.70
SAND (SP) Medium dense, fine to medium grained, dark D
grey, moist.
3.0
3.5
4.0
4.5
4.70
SAND (SP) Loose to medium dense, fine to medium
grained, light grey, moist.
5.0
Weathering Grades Samples
Comments: RS - Residual Soil
XW - Extremely weathered U50
1. Groundwater not encountered during augering.
HW - Highly weathered
DW - Distinctly weathered
2. RL estimated from provided survey. MW - Moderately weathered
SW - Slightly weathered
FR - Fresh
SPT
Rock Strength Disturbed
RS - Residual Soil
VL - Very low Sample
L - Low
M - Medium Bulk Approved: GB
H - High
Sample
Water First Noted Water Steady Level VH - Very high
EH - Extremely high Date: 15/06/2020
Page 52
Soil Surveys Engineering Pty. Limited BOREHOLE RECORD SHEET
Specialists in Applied Geotechnics
PO Box 317, Paddington, 4064
Location Number: BH 01
+61 7 3369 6000
[email protected]
Project Number: 1-22543
www.soilsurveys.com.au Project Name: Proposed Residence
Location: 40 Park Crescent, Sunshine Beach
Easting: 511076 Northing: 7079137 RL: 27.6 m Client: Sanya Holdings Pty Ltd
Logger: RH Operator: RH Machine: EVH1750 Date: 20/05/2020 Page: 2 OF 2
Drilling Method DCP Test
Graphic (blows/100mm) Samples and
Depth Description
Casing
NMLC
Remarks
WB
RR
0 6 12 18 24 30
TC
SAND (SP) Loose to medium dense, fine to medium
grained, light grey, moist. (continued)
5.5 5.50
SAND (SP) Medium dense, fine to medium grained, light
grey, moist.
D
6.0
6.5
7.0 7.00
SAND (SP) Dense, fine to medium grained, light grey, moist.
SOIL SURVEYS 2.00 LIB 2017-07-13.GLB Log SOIL_SURVEY_AUGER_LOG2 1-22543_GINT.GPJ <<DrawingFile>> 15/06/2020 16:21 10.0.000 Developed by Datgel
7.5 7.50
BOREHOLE BH 01 TERMINATED AT 7.50 m
8.0
8.5
9.0
9.5
10.0
Weathering Grades Samples
Comments: RS - Residual Soil
XW - Extremely weathered U50
1. Groundwater not encountered during augering.
HW - Highly weathered
DW - Distinctly weathered
2. RL estimated from provided survey. MW - Moderately weathered
SW - Slightly weathered
FR - Fresh
SPT
Rock Strength Disturbed
RS - Residual Soil
VL - Very low Sample
L - Low
M - Medium Bulk Approved: GB
H - High
Sample
Water First Noted Water Steady Level VH - Very high
EH - Extremely high Date: 15/06/2020
Page 53
Soil Surveys Engineering Pty. Limited HAND AUGER RECORD SHEET
Specialists in Applied Geotechnics
PO Box 317, Paddington, 4064
Location Number: HA 02
+61 7 3369 6000
[email protected]
Project Number: 1-22543
www.soilsurveys.com.au Project Name: Proposed Residence
Location: 40 Park Crescent, Sunshine Beach
Easting: 511096 Northing: 7079159 RL: 18.2 m Client: Sanya Holdings Pty Ltd
Logger: RH Operator: RH Machine: Hand Auger Date: 20/05/2020 Page: 1 OF 1
Drilling Method DCP Test
Graphic (blows/100mm) Samples and
Depth Description
Casing
NMLC
Remarks
WB
RR
0 6 12 18 24 30
TC
FILL SAND (SP) Very loose, fine to medium grained, dark
brown mottled light brown, with organics, moist.
0.20
FILL SAND (SP) Very loose, fine to medium grained, dark
brown mottled light brown, trace of organics, moist.
0.5 D
0.80
NATURAL Silty SAND (SM) Very loose, fine to medium
grained, dark grey, trace of organics, moist.
1.0
1.10
SAND (SP) Very loose, fine to medium grained, light brown,
moist.
1.40
1.5 SAND (SP) Very loose to loose, fine to medium grained,
mottled light grey light brown, moist.
1.80
HAND AUGER HA 02 TERMINATED AT 1.80 m
2.0
SOIL SURVEYS 2.00 LIB 2017-07-13.GLB Log SOIL_SURVEY_AUGER_LOG2 1-22543_GINT.GPJ <<DrawingFile>> 15/06/2020 16:21 10.0.000 Developed by Datgel
2.5
3.0
3.5
4.0
4.5
5.0
Weathering Grades Samples
Comments: RS - Residual Soil
XW - Extremely weathered U50
1. Groundwater not encountered during augering.
HW - Highly weathered
DW - Distinctly weathered
2. Hand auger refusal at 1.80m. MW - Moderately weathered
SW - Slightly weathered SPT
3. RL estimated from provided survey. FR - Fresh
Rock Strength Disturbed
RS - Residual Soil
VL - Very low Sample
L - Low
M - Medium Bulk Approved: GB
H - High
Sample
Water First Noted Water Steady Level VH - Very high
EH - Extremely high Date: 15/06/2020
Page 54
Soil Surveys Engineering Pty. Limited HAND AUGER RECORD SHEET
Specialists in Applied Geotechnics
PO Box 317, Paddington, 4064
Location Number: HA 03
+61 7 3369 6000
[email protected]
Project Number: 1-22543
www.soilsurveys.com.au Project Name: Proposed Residence
Location: 40 Park Crescent, Sunshine Beach
Easting: 511096 Northing: 7079150 RL: 17.2 m Client: Sanya Holdings Pty Ltd
Logger: RH Operator: RH Machine: Hand Auger Date: 20/05/2020 Page: 1 OF 1
Drilling Method DCP Test
Graphic (blows/100mm) Samples and
Depth Description
Casing
NMLC
Remarks
WB
RR
0 6 12 18 24 30
TC
FILL SAND (SP) Very loose, fine to medium grained, dark
0.14 brown mottled light brown, with organics, moist.
FILL SAND (SP) Very loose, fine to medium grained, dark
brown mottled light brown, trace of organics, moist.
0.5
0.60
NATURAL Silty SAND (SM) Very loose, fine to medium
grained, dark grey mottled light grey, trace of organics,
moist. D
1.0
1.30
SAND (SP) Loose to medium dense, fine to medium
grained, light grey mottled light brown, moist.
1.5
1.90
2.0
HAND AUGER HA 03 TERMINATED AT 1.90 m
SOIL SURVEYS 2.00 LIB 2017-07-13.GLB Log SOIL_SURVEY_AUGER_LOG2 1-22543_GINT.GPJ <<DrawingFile>> 15/06/2020 16:21 10.0.000 Developed by Datgel
2.5
3.0
3.5
4.0
4.5
5.0
Weathering Grades Samples
Comments: RS - Residual Soil
XW - Extremely weathered U50
1. Groundwater not encountered during augering.
HW - Highly weathered
DW - Distinctly weathered
2. Hand auger refusal at 1.90m. MW - Moderately weathered
SW - Slightly weathered SPT
3. RL estimated from provided survey. FR - Fresh
Rock Strength Disturbed
RS - Residual Soil
VL - Very low Sample
L - Low
M - Medium Bulk Approved: GB
H - High
Sample
Water First Noted Water Steady Level VH - Very high
EH - Extremely high Date: 15/06/2020
Page 55
Soil Surveys Engineering Pty. Limited DYNAMIC CONE PENETROMETER TESTING
Specialists in Applied Geotechnics
Project Number: 1-22543
PO Box 317, Paddington, 4064
+61 7 3369 6000 Project Name: Proposed Residence
[email protected]
www.soilsurveys.com.au Location: 40 Park Crescent, Sunshine Beach
Client: Sanya Holdings Pty Ltd
DCP 04
Easting: 511088 Northing: 7079143 RL 20.0
DCP Test
(blows/100mm)
0.0 0 6 12 18 24 30
0.5
1.0
1.5
2.0
Depth (m)
2.5
3.0
3.5
4.0
4.5
5.0
Notes
1. Operator : RH
2. Date: 20/05/2020
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APPENDIX G
LRM2007 – PRACTICE NOTE
GUIDELINES – RISK MATRIX
Page 57
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
-4
D - UNLIKELY 10 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
92 Australian Geomechanics Vol 42 No 1 March 2007
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APPENDIX H
DRAWINGS
Page 59
7079170
7079165
HA 02
7079160
7079155
LEGEND
HA 03 Approx. SSE Borehole Locations 2020
7079150
Approx. Lot Boundary
DCP 04
Approx. Proposed House Location
7079145
Image from Nearmap.com (07/11/2019)
7079140
BH 01
7079135
7079130
511060 511065 511070 511075 511080 511085 511090 511095 511100 511105 511110
0 10 20 30 40
SCALE DRAWING TITLE CLIENT
1:300
REFERENCE
V:\Milton\Milton 22001 - 23000\1-22401 - 1-22600\1-22543 Sunshine Beach\Graphics
SITE PLAN Sanya Holdings Pty Ltd
LOCATION
A4
DRW. NO
1-22543-01
DATE
03/06/20
CHECKED
NTP Proposed Residence 40 Park Crescent, Sunshine Beach
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APPENDIX I
HILLSIDE CONSTRUCTION
GUIDELINES
Page 61
AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE)
HILLSIDE CONSTRUCTION PRACTICE
Sensible development practices are required when building on hillsides, particularly if the hillside has more than a low
risk of instability (GeoGuide LR7). Only building techniques intended to maintain, or reduce, the overall level of landslide
risk should be considered. Examples of good hillside construction practice are illustrated below.
WHY ARE THESE PRACTICES GOOD?
Roadways and parking areas - are paved and incorporate kerbs which prevent water discharging straight into the
hillside (GeoGuide LR5).
Cuttings - are supported by retaining walls (GeoGuide LR6).
Retaining walls - are engineer designed to withstand the lateral earth pressures and surcharges expected, and include
drains to prevent water pressures developing in the backfill. Where the ground slopes steeply down towards the high
side of a retaining wall, the disturbing force (see GeoGuide LR6) can be two or more times that in level ground.
Retaining walls must be designed taking these forces into account.
Sewage - whether treated or not is either taken away in pipes or contained in properly founded tanks so it cannot soak
into the ground.
Surface water - from roofs and other hard surfaces is piped away to a suitable discharge point rather than being allowed
to infiltrate into the ground. Preferably, the discharge point will be in a natural creek where ground water exits, rather
than enters, the ground. Shallow, lined, drains on the surface can fulfil the same purpose (GeoGuide LR5).
Surface loads - are minimised. No fill embankments have been built. The house is a lightweight structure. Foundation
loads have been taken down below the level at which a landslide is likely to occur and, preferably, to rock. This sort of
construction is probably not applicable to soil slopes (GeoGuide LR3). If you are uncertain whether your site has rock
near the surface, or is essentially a soil slope, you should engage a geotechnical practitioner to find out.
Flexible structures - have been used because they can tolerate a certain amount of movement with minimal signs of
distress and maintain their functionality.
Vegetation clearance - on soil slopes has been kept to a reasonable minimum. Trees, and to a lesser extent smaller
vegetation, take large quantities of water out of the ground every day. This lowers the ground water table, which in turn
helps to maintain the stability of the slope. Large scale clearing can result in a rise in water table with a consequent
increase in the likelihood of a landslide (GeoGuide LR5). An exception may have to be made to this rule on steep rock
slopes where trees have little effect on the water table, but their roots pose a landslide hazard by dislodging boulders.
Possible effects of ignoring good construction practices are illustrated on page 2. Unfortunately, these poor construction
practices are not as unusual as you might think and are often chosen because, on the face of it, they will save the
developer, or owner, money. You should not lose sight of the fact that the cost and anguish associated with any one of
the disasters illustrated, is likely to more than wipe out any apparent savings at the outset.
ADOPT GOOD PRACTICE ON HILLSIDE SITES
174 Australian Geomechanics Vol 42 No 1 March 2007
Page 62
AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE)
WHY ARE THESE PRACTICES POOR?
Roadways and parking areas - are unsurfaced and lack proper table drains (gutters) causing surface water to pond and
soak into the ground.
Cut and fill - has been used to balance earthworks quantities and level the site leaving unstable cut faces and added
large surface loads to the ground. Failure to compact the fill properly has led to settlement, which will probably continue
for several years after completion. The house and pool have been built on the fill and have settled with it and cracked.
Leakage from the cracked pool and the applied surface loads from the fill have combined to cause landslides.
Retaining walls - have been avoided, to minimise cost, and hand placed rock walls used instead. Without applying
engineering design principles, the walls have failed to provide the required support to the ground and have failed,
creating a very dangerous situation.
A heavy, rigid, house - has been built on shallow, conventional, footings. Not only has the brickwork cracked because
of the resulting ground movements, but it has also become involved in a man-made landslide.
Soak-away drainage - has been used for sewage and surface water run-off from roofs and pavements. This water
soaks into the ground and raises the water table (GeoGuide LR5). Subsoil drains that run along the contours should be
avoided for the same reason. If felt necessary, subsoil drains should run steeply downhill in a chevron, or herring bone,
pattern. This may conflict with the requirements for effluent and surface water disposal (GeoGuide LR9) and if so, you
will need to seek professional advice.
Rock debris - from landslides higher up on the slope seems likely to pass through the site. Such locations are often
referred to by geotechnical practitioners as "debris flow paths". Rock is normally even denser than ordinary fill, so even
quite modest boulders are likely to weigh many tonnes and do a lot of damage once they start to roll. Boulders have
been known to travel hundreds of metres downhill leaving behind a trail of destruction.
Vegetation - has been completely cleared, leading to a possible rise in the water table and increased landslide risk
(GeoGuide LR5).
DON'T CUT CORNERS ON HILLSIDE SITES - OBTAIN ADVICE FROM A GEOTECHNICAL PRACTITIONER
More information relevant to your particular situation may be found in other Australian GeoGuides:
• GeoGuide LR1 - Introduction • GeoGuide LR6 - Retaining Walls
• GeoGuide LR2 - Landslides • GeoGuide LR7 - Landslide Risk
• GeoGuide LR3 - Landslides in Soil • GeoGuide LR9 - Effluent & Surface Water Disposal
• GeoGuide LR4 - Landslides in Rock GeoGuide LR10 - Coastal Landslides
• GeoGuide LR5 - Water & Drainage • 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.
Australian Geomechanics Vol 42 No 1 March 2007 175
Used in these records
RFI - Attachment C - Slope Stability Assessment and Geotechnical report.pdfRelated documentApplication case desk ↗
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