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    Working for our future today

    Geotechnical StabilityAssessment Guidelines

    JUNE 2007: Version 1.0

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    Geotechnical Stability Assessment Guidelines

    TABLE OF CONTENTS

    Page1. INTRODUCTION 1

    1.1 Developments with Geotechnical Stability Issues 1

    1.2 Geotechnical Stability Assessment Criteria 12. DEVELOPMENTS ON SLOPING GROUNDS 3

    2.1 Slope Stability Assessment 3

    2.2 Geotechnical Site Investigation 5

    2.3 Geotechnical Certifications 63. DEVELOPMENTS ON FLOOD PLAIN AREAS 8

    3.1 Geotechnical Site Investigation 8

    3.2 Ground Improvement Techniques 11

    3.3 Geotechnical Certifications 154. DEVELOPMENTS INVOLVING DEEP EXCAVATIONS 16

    4.1 Retaining Structures to Stabilise Deep Excavations 16

    4.2 Stability of Deep Excavations 16

    4.3 Geotechnical Certifications 17

    5. DEVELOPMENTS INVOLVING BATTERS AND/OR RETAINING

    STRUCTURES18

    5.1 Stability of Batters 18

    5.2 Stability of Retaining Structures 19

    5.3 Geotechnical Certifications 19

    6. PRESENTATION OF THE REPORT 207. REFERENCES 208. APPENDICES 21 APPENDIX A 22

    APPENDIX B 24APPENDIX C 25APPENDIX D 26

    i

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    Geotechnical Stability Assessment Guidelines

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    LIST OF TABLES

    Table 1.1 Criteria for the requirement of a Geotechnical Stability AssessmentReport

    1

    Table 2.1 Extent of stability issues in slope stability assessment 4

    Table 2.2 Geotechnical certifications for developments on steep slopinggrounds

    7

    Table 3.1 Extent of geotechnical site investigation for developments within floodplain areas

    9

    Table 3.2 Geotechnical certifications for developments within flood plain areas 15

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    Geotechnical Stability Assessment Guidelines

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    LIST OF FIGURES

    Figure 1 Flowchart of geotechnical stability assessments 2

    Figure 2 (a) Preloading concept, (b) preloading without vertical drains, (c)preloading with vertical drains, and (d) time for consolidation

    settlement with and without vertical drains

    12

    Figure 3 Two methods of installing sand drains (after Bowles, J. E.,Foundation Analysis and Design, 4

    thedition, McGraw-Hill Inc, 1988)

    13

    Figure 4 Typical prefabricated Vertical Drain (PVD) 14

    Figure 5 Installation of prefabricated vertical drains (PVD) on a site 14

    Figure 6 Typical slope stability analysis using SLOPE/W 18

    Figure 7 Typical retaining structure and the lateral earth pressure distributions 19

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    Geotechnical Stability Assessment Guidelines

    1

    1. INTRODUCTION

    This document contains various geotechnical stability aspects and issues associated with differenttypes of development application. These geotechnical stability issues shall be assessed andsubmitted by the applicant to Gold Coast City Council (Council) for review and approval. The mainpurpose of developing this document is to provide a framework for informed decision-making by theCouncil regarding geotechnical stability issues associated with any development application. This

    document will accelerate both the application preparation process by the applicant and applicationassessment process by the Council, hence will improve transparency and understanding betweenthe two parties. The key objectives of this document are to:

    Provide clarity and transparency regarding geotechnical stability concerns, issues andrequirements by the Council for the assessment and approval of any developmentapplication;

    Provide guidelines for preparation and submission of a geotechnical stability assessmentreport (if required) for supporting any development application;

    Improve efficiency and consistency in the development application assessment process;and

    Allow development permits, which are geotechnically stable, safe and sound.

    This document has been prepared to provide detailed guidelines for addressing variousgeotechnical stability issues indicated in the constraint code Steep Slopes or Unstable Soils andspecific development code Changes to Ground Level and Creation of New Waterbodies of thedocument Our Living City Gold Coast Planning Scheme.

    1.1 Developments with Geotechnical Stability Issues

    From geotechnical point view, this document identifies the following 4 types of developmentsinvolving geotechnical stability issues and assessment:

    a) Developments on steep sloping groundsb) Developments within flood plain areasc) Developments involving deep excavationd) Developments involving batters and/or retaining structures

    1.2 Geotechnical Stability Assessment Criteria

    If any development application falls within one of the following categories given in Table 1.1, theapplicant must submit a Geotechnical Stability Assessment Report (Geotechnical Report) inorder to proceed with the development application for assessment and approval by the Council.

    Table 1.1: Criteria for the requirement of a Geotechnical Stability Assessment ReportTypes of Development Submission of a Geotechnical

    Stability Assessment ReportDevelopments on steep sloping grounds:

    IfOverlay Map OM16 Areas of Unstable Soils andAreas of Potential Land Slip Hazardshows the site is on

    land within hazard rating of Moderate/High/Very Highor

    the site contains land with slope more than 25%

    Developments within flood plain areas:If the site is within the designated area shown in the

    Overlay Map OM17 Natural Hazard (Flood)Management Area

    Developments involving deep excavation(>2.0m depth)

    Developments involving cut/fill batters (>1.0m depth/height)and/or retaining structures (>1.0m height)

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    Geotechnical Stability Assessment Guidelines

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    Figure 1 shows a flowchart for various geotechnical stability assessments that must be carried outand include them in the Geotechnical Report.

    Fig.1: Flowchart of geotechnical stability assessments

    Development application

    Does theOverlay Map OM16shows the site is within hazard

    rating of Moderate/High/Very Highor the site contains land with slope

    more than 25%?

    Assess slope instability hazardrisk based on site-specificgeotechnical information

    Does the assessmentdetermine the site/lot/buildingenvelope with hazard risk of

    Moderate or worse?

    Provide recommendations on stabilisation measuresto reduce hazard risk to Low or better, and certify

    that the site/lot/building envelope will achievehazard risk of Low or better and will remain stable

    in the long-term conditions (70 years minimum)

    Does the site contain softand thick subsoil layer(s)?

    Provide details of proposed soft groundimprovement technique including certification

    Does the development expectany deep excavation, cut/fill

    batters or retaining structures?

    Provide stability assessment of deep excavation, cut/fillbatters and/or retaining structures and certify the stability of

    all deep excavation, cut/fill batters and/or retainingstructures

    No

    No

    No

    No

    Yes

    Yes

    Yes

    Yes

    Provide certification statingthat the site/lot/building

    envelope is designated withhazard risk of Low or better

    Geotechnical report is notwarranted

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    Geotechnical Stability Assessment Guidelines

    The following sections describe in detail the extent of geotechnical stability issues, assessmentsand certifications that may need to be included in the Geotechnical Report.

    2. DEVELOPMENTS ON STEEP SLOPING GROUNDS

    For any development application on sites with steep sloping ground in the hilly topography, there isa risk of slope instability or landslip that must be analysed, assessed and submitted to the Councilfor review and approval. The degree of slope instability or landslip hazard risk depends on anumber of factors including ground slope angle and shape, strength of geomaterials and itsdistribution in the subsurface, depth of ground water table, potential for surface runoffconcentration, orientation of rock mass defects etc. The applicant shall assess the potential risk ofslope instability or landslip hazard risk for the proposed development site at its existing state aswell as at the post-developed state.

    If the proposed development involves with or is expected to be involved with bulk earthworksincluding cut/fill with or without any retaining structures, the applicant must also assess the stabilityof all cut/fill batters and retaining structures, as detailed in Section 5 of this document.

    Table 1.1 earlier states that if Overlay Map OM16 Areas of Unstable Soils and Areas ofPotential Land Slip Hazardshows the site is on land within a hazard rating of Moderate/High/VeryHigh or the site contains land with slope exceeding 25%, a Geotechnical Stability AssessmentReport shall be submitted. In this case, the Geotechnical Report shall include a slope stabilityassessment of the site in relation to the proposed development.

    This section outlines the details of slope stability assessment, any associated geotechnical siteinvestigations and geotechnical certifications required for any developments on steep slopinggrounds.

    2.1 Slope Stability Assessment

    The slope stability assessment for the proposed development site shall be prepared by aRegistered Professional Engineer of Queensland (RPEQ) specialising in geotechnical engineering,particularly in slope instability hazard risk assessment. This stability assessment must be includedin the