Council archive · source document
Appendix G - Slope Stability Certification Report · MCU25/0022
ABN 21 613397934 CQA Laboratory P.O. Box 1225, Nambour Qld 4560
6 Mill Lane Nambour
Civil Quality Assurance Pty Ltd T/A CQA For all Geotechnical Engineering and Civil Testing
Ph 0429674334 Servicing Australasia
Slope Stability Assessment
193 Sankeys Road, Federal, Queensland
Client: Lachlan Robins
Testing Authority: Civil Quality Assurance Pty Ltd
Certification Authority: Lake Mac Geo Pty Ltd
Date: 17 July 2023
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Contents
1 INTRODUCTION .............................................................................................. 1
2 SITE DESCRIPTION AND PROPOSED WORKS .......................................... 2
3 GEOLOGY ......................................................................................................... 5
4 FIELDWORK..................................................................................................... 6
5 ENCOUNTERED GROUND CONDITIONS ................................................... 7
6 FIELD AND LABORATORY TESTING ......................................................... 8
7 ASSESSMENT OF STABILITY....................................................................... 9
8 GEOTECHNICAL CONSTRAINTS TO DEVELOPMENT ......................... 10
9 SLOPE RISK ASSESSMENT ......................................................................... 12
10 CONCLUSIONS AND RECOMENDATIONS .............................................. 13
APPENDIX A – LOCATION PLAN
APPENDIX B – BOREHOLE LOGS AND TEST RESULTS
APPENDIX C – SITE PHOTOGRAPHS
APPENDIX D – PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK
MANAGEMENT 2007
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1 INTRODUCTION
Civil Quality Assurance Pty Ltd (CQA) have been commissioned by Lachlan Robins to
undertake a slope stability assessment and to provide recommendations for a proposed
residential development at 193 Sankeys Road, Federal, Qld, 4568 (the Site).
The property is described as Lot 6 on RP216727.
The aim of the Report was to assess the geotechnical considerations of altering a slope to
provide a level pad for construction of a residence.
The report has been prepared by Phil Band of Lake Mac Geo RPEQ 23841.
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2 SITE DESCRIPTION AND PROPOSED WORKS
The Property is situated on the west side of Sankeys Road 1.5m north east of Federal CBD as
shown in Figure 1 below:
Figure 1 – Site Location
The site is currently a partially cleared and uncleared bushland. The Site is partially cleared
with mature and semi-mature trees.
Rural residential properties surround the site on all sides. Site photographs are presented as
Appendix C.
It is proposed to construct a single residence in the location indicated in Figure 1. The
proposed building is 8.5m by 16.5m, and elevated off the ground by up to 2.5m with a deck.
The existing slope is 21 and appears stable with no signs of creep, tension cracks, bulging or
other indicators of failure, nearby trees are vertical or near vertical.
As part of these works it is proposed to cut into the slope to provide a level building pad, this
will require creation of a slope at an angle of 80 to a height of approximately 2.6 high.
Figure 2 and 3 Illustrates the proposed house and slope. Long term the 80 slope will be
supported with a concrete block retaining wall.
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Figure 2 – Proposed Works – Cross Section
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\ EXISTING ACCESS ROAD WITHIN. EASEMENT -> ~ Dwelling / N / / —— Apain Gite aor Figure 3 — Proposed Works — Plan 193 Sankeys Rd Slope Stability Certification July 2023
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3 GEOLOGY
Reference to Queensland Geological Survey Mapping shown in Figure 4 below and taken
from https://georesglobe.information.qld.gov.au indicates the site is underlain by the Kin Kin
Beds comprising Strongly cleaved and commonly kinked mudstone with thin siltstone
laminae; minor volcanilithic sandstone of middle Triassic age.
.
Figure 4 - Geological Map
It is considered likely that the mudstone will have weathered to residual soils for up to 1m
depth. These generally comprise silty and/or sandy clays.
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4 FIELDWORK
The Fieldwork was undertaken by CQA and comprised:
• A Walkover Survey including:
o Assessment of soil and rock outcropping
o Surface drainage conditions
o Topography
o Four Hand Augured boreholes
o Two Dynamic Cone Penetrometer Tests (DCP)
o Erosion observations.
o Vegetation coverage/conditions.
o Three Permeameter tests.
Photographs from the site inspection are included as Appendix C.
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5 ENCOUNTERED GROUND CONDITIONS
The two boreholes drilled at the shed location contained in Appendix B generally revealed the
following ground profile:
Borehole Silty CLAY Shale Stone DCP Refusal
Log of Visible 0.0-0.3m 0.3-1.5m+ -
Cutting
2 0.0-0.3m 0.3-0.5m+ 0.8
3 0.0-0.4m 0.4- 0.5m+ 0.8
4 0.0-0.5m 0.5-0.6m+ -
Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and some organics
present moist, soft to firm (CI-CH)
SHALE STONE, fractured, light red/brown, quartz present, moist, weak.
Groundwater was not encountered.
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6 FIELD AND LABORATORY TESTING
Field Permeameter testing was undertaken in three test pits on the site and all tests returned
values of 25mm/hr. A 150mm diameter permeameter embedded 600mm into the ground with
a 250mm depth of water was used.
A sample of the clay was tested for liquid limit and liner shrinkage and returned values of
38.5% and 14.5% respectively. 34,5% of the sample was retained on a 0.425mm sieve.
Report Sheets are included in Appendix B.
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7 ASSESSMENT OF STABILITY
Reference to Noosa Shire Council Planning, “Landslide Hazard Overlay Mapping” shows the
the lot is within Moderate and High Hazard Areas with he proposed residence location being
in a Moderate Hazard Area,
An extract from Council’s “Landslide Hazard Overlay Mapping” is shown in Figure 3 below.
Site
Figure 3 - Council Landslide Hazard Overlay Map
At the time of the inspection the existing 21 slope exhibited no evidence of failure, rotation
or sliding or cracking. No areas of groundwater seepage were observed. Surface erosion was
observed to be minimal.
The proposed Slope is to be modified provide a level building pad.
The existing slope will be cut back by approximately 8m at an angle of 80 to a maximum
height of 2.6m; any clay at the crest of the cutting will be cut back to 4h to 1v and will be
topsoiled and seeded to protect from erosion. Long term the slope will be
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8 GEOTECHNICAL CONSTRAINTS TO DEVELOPMENT
The following comments apply to construction on all the site.
Slope Stability
The site shows no signs of slope instability and will be cut into rock; however the new slope
will be considerably steeper than the existing and could be considered medium risk until the
retaining wall is constructed; once the wall has been constructed the slope can be considered
low risk; however, the development should not amend or alter other slopes without specific
geotechnical advice.
The following recommendations shall be adhered to:
• Any site earthworks carried out must comply with A.S. 3798, “Guidelines on
Earthworks for Commercial and Residential Development”.
• Any soil fill should be placed and compacted to “Level 1 Requirements” set out in
A.S.3798. to comply with Controlled Fill as stated in A.S.2870, “Residential Slabs
and Footings”.
• Excavate all of the clay above the rock in the platform area.
• Provide a drainage diversion to the crest of the slope to direct surface water away
from the crest of the cutting.
• Ensure that all of the site is level.
• Place fill which is at least CBR 15 or above with less than 15% passing the .075mm
sieve.
• Direct all water away from the site with surface drains.
• Dams, ponds, swimming pools, or any auxiliary services such as soakaways or septic
tanks plants are to be placed away from the site but not below it by at least 50m.
• Any fill material encountered shall be excavated and re-used if suitable and if
unsuitable either modified to be suitable, removed from site or used as landscape fill.
• Cut and fill areas may be battered at an angle of up to 1V to 3H or shall be retained
using engineer approved retaining methods. Steeper slopes may be achievable with
specific geotechnical advice. Prior to filling any topsoil, grass, roots or other organic
matter shall be removed from the construction area.
• Slopes steeper than 1V to 7H shall be benched to avoid introducing preferential
failure surfaces.
Surface Drainage
All structure roof drainage shall be collected and piped to approved legal points of stormwater
discharge.
The ground surrounding the structure shall be shaped so that surface runoff flows away from
all structures. Drainage should be provided to minimise variations in moisture content in soils
surrounding and below sites and prevent concentrated run‐off causing erosion.
During construction, measures will be required to prevent erosion, undercutting and over
saturating soil areas that may lead to slope instability. The basic principles of managing
stormwater on any development site are:
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• Control the flow volume
• Control the flow path
• Reduce run off velocity
Many techniques are available to achieve these objectives. The following list provides some
common methods for Stormwater Management.
• Vegetation Retention / Re‐Establishment: Plants provide a means of retarding
surface flow as well as acting as a natural filter. Where possible, natural vegetation
should be left undisturbed. Establishing vegetation cover in potential problem areas
(steep slopes, gullies etc.) will also assist in reducing erosion.
• Surface Drains / Grassed Swales: Well grassed diversion drains provided above
exposed surfaces redirect water around erosion prone areas. These should be used in
conjunction with level spreading devices to disperse fast moving, concentrated surface
flows.
• Level Spreaders: A level spreader typically consists of a coarse gravel filled trench
running parallel to the natural surface contours. This redistributes surface water and
allows concentrated flows to spread over a large area and decreasing the velocity.
Surface Erosion
Soils lost from land surfaces as a result of surface water run off not only removes valuable
topsoil from the area, but also result in siltation of downstream waterways when the sediments
settle. Loss of soil is usually the result of leaving bare surfaces exposed to the elements, or
from concentrated surface flows scouring the natural surface. Temporary measures can be
taken to control and contain sediment removal during construction such as:
• Covering Stockpiles: Stockpiles of soil can be covered with plastic sheeting and re‐
spread when construction is completed.
• Sediment Control Fencing: Hay bales or Hessian sacks supported between pickets
and placed across the paths of surface water can catch sediments for future re‐
spreading. Specialised sediment fencing is available for broad scale hillside scouring
protection.
• Sediment Basin: A suitably sized earth basin may be constructed during the
development to capture turbid stormwater runoff, where the sediment is allowed to
settle before the cleaner water is pumped to a legal point of discharge. The sediment
should be regularly emptied during drier periods.
In general, it is recommended that all development on hillside areas should follow Good
Hillside Construction Practice recommended in the information sheets presented in Australian
Geomechanics Society “Practice Note Guidelines for Landslide Risk Management”,
Australian Geomechanics Vol 42 No. 1 March 2007. An extract is attached as Appendix E.
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9 SLOPE RISK ASSESSMENT
“Risk is defined in the Australian Geomechanics Society publications as a measure of the
probability and severity of an adverse effect to health, property or environment. Risk is often
estimated by the product of numerical probability and consequences; however, a more general
interpretation involves a comparison of the probability and consequences in a non‐product
form. To carry out a full risk assessment of the site would require a full understanding of the
type of structure proposed and the associated earthworks along with the likely consequences
should failure occur.
The site was assessed in terms of “Landslip Risk Management Concepts and Guidelines”
published by The Australian Geomechanics Society.
RISK LEVEL IMPLICATIONS
RISK LEVEL EXAMPLE IMPLICATIONS
VH VERY HIGH RISK Extensive detailed investigation and research, planning and
implementation of treatment options essential to reduce risk
to acceptable levels; may be too expensive and not practical
H HIGH RISK Detailed investigation, planning and implementation of
treatment options required to reduce risk to acceptable levels
M MODERATE RISK Tolerable, provided treatment plan is implemented to maintain or
reduce risks. May be accepted. May require
investigation and planning of treatment options.
L LOW RISK Usually accepted. Treatment requirements and responsibility
to be defined to maintain or reduce risk.
VL VERY LOW RISK Acceptable. Manage by normal slope maintenance
procedures.
It is our opinion, based on Appendix C of Australian Geomechanics Society, “Practice Note
Guidelines for Landslide Risk Management 2007”, that if development is managed in
accordance with the recommendations in this report and the geotechnical development
constraints presented are strictly followed, the site can be developed with a MODERATE
RISK of damage to persons or property prior to construction of the retaining wall and a LOW
RISK of damage to persons property once the retaining wall has been constructed.
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10 CONCLUSIONS AND RECOMENDATIONS
Site stability as assessed on the basis of visual inspection and sub‐surface investigation. No
evidence of previous movement, seepage or soil creep was observed in the area.
We consider the site can be developed with a MODERATE RISK of damage to persons or
property prior to construction of the retaining wall and a LOW RISK of damage to persons
property once the retaining wall has been constructed in accordance with “Landslip Risk
Management Concepts and Guidelines” published by The Australian Geomechanics Society
and recommended building and construction methods as set out in this report.
Weekly visual inspections of the slope are recommended during construction with additional
inspections after heavy rain. If any signs of movement i.e. cracking, bulging, spalling etc are
observed CQA shall be informed immediately and all work in the area suspended until a
geotechnical inspection can be carried out.
An inspection and removal of any loose rocks shall be undertaken prior to any workers
accessing the toe of the slope for construction of the retaining wall or waste collection area.
All construction should be in accordance with good hillside practice as illustrated in Figure J1
of Australian Geomechanics Society Sub‐Committee recommendations for Landslide Risk
Management attached as Appendix D.
Report Prepared by:
Phil Band BEng MSc MIEAust CPEng NER AGS RPEQ 23841
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LIMITATIONS
Civil Quality Assurance (CQA) has prepared this report for the express and exclusive use
by the client. The report should not be used or relied upon for other projects or purposes on
the same or and other sites or by a third party. Any persons or party so relying upon this
report beyond the exclusive use and purpose stated above, and without the express written
consent of CQA, does so entirely at their own risk and without recourse to CQA for any
loss or damage. In preparing this report CQA has necessarily relied upon information
provided by the client and their associated agents.
The results provided in the report are indicative of the sub-surface conditions on the site
only at the specific sampling and/or test locations and only to the depths investigated and
at the time the investigation/work was carried out. Sub-surface conditions can change
abruptly due to variable geological processes and the result of human influences. Such
changes may occur after CQA has completed field testing.
CQA advice is based upon the conditions encountered during the investigation. The
accuracy of the advice provided by CQA in this report may be affected by undetected
variations in ground conditions across the site between and beyond the sampling and or
testing locations. The advice may also be limited by budget constraints imposed by others
or by site accessibility. Tests and sampling results relate only to the items tested or
sampled. Results applied to the samples received/tested and any abnormalities may
contribute to the report being compromised.
The assessment of atypical safety hazards arising from this this advice is restricted to the
geotechnical components set out in the report and based on known project conditions and
stated design advice and assumptions. While some recommendations for safety controls
may be provided, detailed safety in design assessment is outside the current scope of this
report and requires additional project data and assessment.
This report must be read in conjunction with all of the attached and should be kept in its
entirety without separation of individual pages or sections. CQA can not be held
responsible for interpretations or conclusions made by others unless they are supported by
the express statement, interpretation, outcome or conclusion stated in the report.
The report or sections from this report, shall not be used as part of a specification for a
project, without review and agreement of CQA. This is because this report has been written
as advice and opinion rather than detailed instructions for construction.
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Appendix A — Location Plan 193 Sankeys Rd Slope Stability Certification July 2023
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Waterway ~ Stream Order X
———~ Riparian Buffer Area
SITE LAYOUT
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Scale 1:100 ot AS
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PROPOSED RESIDENCE
SITE PLAN 2022026
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@_indecom Solutions Pty Lid
Page 19
Appendix B – Borehole Logs and Test
Results
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Civil Quality Assurance T/A _ ABN 21 613397934 Civil Quality Assurance Pty Ltd PH 0429 674334 Project:__ 193 Sankeys Rd Federal Client: __ Lachlan Robins P.O. Box 1225, Nambour Qld 4560 For all Civil Testing Servicing South East Queensland Requested By:__L.Robins Date Tested:__30-6-23 Report No.:__ A191 Drilling Method: Log of cutting Bore Hole No.:1 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and some organics present, moist, soft to firm (CI-CH) 300 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 1500 Hole Terminated Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full 6 Mill Lane Nambour CQA Laboratory ABN 21 61339 7934 Form R26 July 2021 Approved Person: C. Templeton Date: 10-7-23
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Civil Quality Assurance T/A _ ABN 21 613397934 Civil Quality Assurance Pty Ltd PH 0429 674334 Project:__193 Sankeys Rd Federal Client: __ Lachlan Robins P.O. Box 1225, Nambour Qld 4560 For all Civil Testing Servicing South East Queensland Requested By:__L.Robins Date Tested:__30-6-23 Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:2 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, 300 500 moist, soft to firm (CI-CH) SHALE STONE, fractured, light red/brown, quartz present, moist, weak Hole Terminated in shale stone ‘Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full 6 Mill Lane Nambour CQA Laboratory ABN 21 61339 7934 Form R26 July 2021 jes Approved Person: ec of C. Templeton Date: 10-7-23
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Civil Quality Assurance T/A _ ABN 21 613397934 P.O. Box 1225, Nambour Qld 4560 Civil Quality Assurance Pty Ltd For all Civil Testing PH 0429 674334 Servicing South East Queensland Project:__193 Sankeys Rd Requested By:__L.Robins Federal Date Tested:__30-6-23 Client: __ Lachlan Robins Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:3 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, moist, soft to firm (CI-CH) 400 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 500 Hole Terminated in shale stone Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full > FT Approved Person: Gurl 6 Mill Lane Nambour C. Templeton CQA Laboratory ABN 21 61339 7934 Date: 10-7-23 Form R26 July 2021
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Civil Quality Assurance T/A ABN 21 613397934 P.O. Box 1225, Nambour Qld 4560 Civil Quality Assurance Pty Ltd For all Civil Testing PH 0429 674334 Servicing South East Queensland Project:__193 Sankeys Rd Requested By:__L.Robins Federal Date Tested:__30-6-23 Client: __Lachlan Robins Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:4 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, moist, soft to firm (CI-CH) 500 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 600 Hole Terminated in shale stone Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full : Approved Person: Ce / 6 Mill Lane Nambour C. Templeton CQA Laboratory ABN 21 61339 7934 Date: 10-7-23 Form R26 July 2021
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ABN 21 613397934 CQA Laboratory P.O. Box 1225, Nambour QlId 4560 6 Mill Lane Nambour Civil Quality Assurance Pty itd T/A For all Geotechnical Engineering and Civil Testing Ph 0429674334 Servicing South East Queensland Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full Test Report CQA Laboratory Accredited for compliance with ISO/IEC 17025 - testing Project:- 193 Sankeys Rd Requested By: L. Robins Federal Date Tested: 30-6-23 Client: Lachlan Robins Report No.: A191 Laboratory Test Procedure: AS1289 3.6.1 soil classification tests-determination of the particle size distribution of a soil-standard method of analysis by sieving, AS1289 3.9.2 Soil classification tests- determination of the cone liquid limit of soil-one point method, AS1289 3.4.1 Soil classification tests-determination of the linear shrinkage of a soil-standard method. Sample Method: AS1289.1.2.1 (6) hand augured bore hole sample method Date Sampled: 30-6-23 Sample/ Material Description: BH2 200-500 Material Source: see attached Bore hole Nominal Size(mm): <0.425mm Liquid Limit (%): 38.5 Linear Shrinkage (%): 14.5 Percentage of material retained on 0.425mm sieve (%): 34.5 Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full NATA Accredited facility No.: 20084 —_ COA Laboratory Signatory, C_./ Ca ABN 21 613397934 6 Mill Lane Nambour C. Templeton Form R1 July 2021 Date Reported: 10-7-23
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ABN 21 613397934 CQA Laboratory P.O. Box 1225, Nambour Qld 4560 6 Mill Lane Nambour Civil Quality Assurance Pty Ltd T/A For all Geotechnical Engineering and Civil Testing Ph 0429674334 Servicing Queensland Test Report CQA Laboratory Accredited for compliance with ISO/IEC 17025 - testin: Dynamic Cone Penetrometer AS1289 6.3.2 Project:- 193 Sankeys Rd Requested By:L. Robins Federal Date Tested: 30-6-23 Client: Lachlan Robins Report No.: A191 Laboratory Test Procedure: AS1289 6.3.2 Reduced ground level at test site(mm): 00 Moisture Condition: moist Sample/ Material Description: See attached Water Table(mm): na Depth below surface at commencement of Penetration(mm): 0.0 Depth Location of test (mm) Blows/100mm Location | Pl at Approx. P2 at Approx. Approx. Approx. Approx. KPA KPA KPA KPA KPA BH2 equivalent BH3 equivalent equivalent equivalent equivalent 00 00 00 100 3 75 3 75 200 3 75 3 75 300 4 100 3 75 400 5 125 4 100 500 8 200 6 150 600 12 300 10 250 700 12 300 10 250 800 12 300 12 300 900 a = 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full. KPa have been developed in = CQA Laboratory ABN 21 613397934 Approved Person: Cof 6 Mill Lane Nambour C. Templeton Form R5 July 2021 Date Reported: 10-7-23
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ABN 21 613397934 Ph. 0429674334 Project:_193 Sankeys Rd Federal Client: _Lachlan Robins P.O. Box 1225, Nambour Qld 4560 For all Civil Testing Servicing South East Queensland TEST REPORT Requested By:_L. Robins Date Tested: _30-6-23 Report No. A191 Field Test Procedure: AS/NZ 1547:2000 Sample method: Hand Auger Bore Hole Hole Number: TP1 TP2 TP3 Radius of all holes(mm): 100 100 100 Depth of Holes(mm): 600 600 600 Impermeable Layer: n/a n/a n/a Depth of water in Hole(mm): 250 250 250 Site Vegetation: Heavily treed with small shrubs Soil Moisture: moist moist moist What is the water table: n/a n/a n/a Soil Classification see Bhl Permeameter readings(mm/Hr): 25 25 aa Permeameter Diameter: 150mm 6 Mill Lane Nambour Form 4A September 2016 Signatory: C. / — C. Templeton & Date: 10-7-23
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Appendix C — Site Photographs 193 Sankeys Rd Slope Stability Certification July 2023
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Photo 1: Photo 2: Photo 3:
. .
Photo 4: Photo 5: Photo 6:
SITE PHOTOGRAPHS
Date: 17 July 2023 LACHLAN ROBINS
193 Sankeys Road, Federal
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Appendix D - Practice Note Guidelines
for Landslide Risk Management 2007
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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
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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
Appendix G - Slope Stability Certification Report.pdfRelated documentApplication case desk ↗
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