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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

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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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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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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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2

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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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 Geotechnical Stability Assessment Report. The slope instability hazard risk shall beassessed based on a quantitative Relative Frequency calculation in accordance with the methodpresented in the Landslip Study for the City of Gold Coast, prepared by Snowy MountainEngineering Corporation (SMEC), August 1999. An example of Relative Frequency calculationfollowing the SMEC method is given in Appendix A.

The results of Relative Frequency calculation shall be converted to a slope instability or landsliphazard rating following the correlation given in Appendix B.

The assessment must also examine the feasibility and suitability of the existing site for the

proposed development. If the development involves on-site effluent disposal system, the stabilityassessment shall consider potential saturation and softening of soils within the effluent disposalareas. Table 2.1 shows the extent of stability issues for different types of development applicationthat must be addressed in this slope stability assessment.


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Table 2.1: Extent of stability issues in slope stability assessmentType of development

applicationExtent of stability issues in the slope stability assessment

Material Change of Use(MCU)

This assessment shall examine the slope instability hazard risk of theexisting site, and also the feasibility and suitability of the site for theproposed development. The slope stability assessment must be made

based on site-specific geotechnical information including subsurfaceexploration, field and/or laboratory test results. If the report identifiesthe site with a Moderate or worse risk of slope instability, it musteither recommend appropriate and adequate stabilisation measuresthat will reduce the hazard rating of the site to Low or better, orpropose an alternate solution (e.g. for large allotments, buildingenvelops and effluent disposal areas may be restricted to Low orbetter hazard rating locations with a minimum 15m buffer frompotential slip areas).

If the application is for the purpose of building works, the geotechnicalreport shall include the stability assessment of all proposedexcavations for the construction of building foundations and/or

basement. The report must examine whether the excavation requiresany protective measures in order to contain the slope instability hazardrisk of the local excavation and also the overall site to Low or better.The stability assessment of the proposed excavations shall be madebased on detail calculations with due consideration to the existingground slope and geotechnical conditions of the site. The stabilityassessment shall refer to specific engineering drawings and cross-sections.

If the excavations are of temporary nature with/without anyshoring/retention system to facilitate the construction offoundations/basement, the stability assessment must ensure that theshort-term factor of safety of the excavation/retention system will be

greater than1.5.

If the excavations are of permanent type, the stability assessment shallensure that the long-term factor of safety of all excavationswith/without any permanent retaining structures/basement walls will begreater than 1.5.

If the development involves any permanent retaining structures, thestability of the retaining structures must be assessed to ensure that along-term factor of safety greater than 1.5 will be achieved.

The stability assessment shall also ensure that the proposedexcavations/earthworks will not adversely affect the stability and

integrity of the neighbouring properties, building foundations, buriedservices and infrastructures.

Reconfiguring a Lot(ROL)

This assessment shall examine the slope instability hazard risk foreach of the proposed subdivided lots, and also the feasibility andsuitability of each new lot for its intended use. The slope stabilityassessment must be made based on site-specific geotechnicalinformation including subsurface exploration, field and/or laboratorytest results. If the report identifies any proposed lot(s) with a Moderateor worse risk of slope instability, the report must either recommendappropriate and adequate stabilisation measures that will reduce thehazard rating of the lot(s) to Low or better, or propose an alternatesolution (e.g. for large allotments, building envelops and effluent

disposal areas may be restricted to Low or better hazard rating


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locations with a minimum 15m buffer from potential slip areas). Thisassessment must include a completed Subdivision LandslipEncumbrance Form, which is provided in Appendix C.

If the application expects any bulk earthworks including cut/fill batterswith or without any retaining structures, the report must assess thestability of all expected/preliminary earthworks including cut/fill batters

and/or retaining structures (if any) with reference to preliminary bulkearthworks engineering drawings and cross-sections. The stabilityassessment shall ensure that a long-term factor of safety greater than1.5 will be achieved.

Operational Works(OPW)

This assessment shall examine the slope instability hazard risk of thesite, and also the potential slope instability hazard risk associated withthe proposed operational works. The hazard risk must be assessedbased on detailed geotechnical site investigation including subsurfaceexploration, field and/or laboratory testing. If the report identifies thesite with a Moderate or worse hazard rating of slope instability, it mustrecommend appropriate and adequate stabilisation measures that willreduce the hazard rating of the site to Low or better.

The report shall include the results of detailed geotechnical siteinvestigation including subsurface exploration, field and/or laboratorytesting.

The report must include the stability assessment of all finalisedearthworks including cut/fill batters and/or retaining structures (if any)with reference to detailed and finalised bulk earthworks engineeringdrawings and cross-sections. This assessment shall include detailstability calculations of all batters and retaining structures (if any) withdue consideration to the existing ground slope and geotechnicalconditions of the site. The report must ensure that all cut/fill battersand/or retaining structures (if any) will achieve a long-term factor of

safety of greater than 1.5.

If there is a Geotechnical Report associated with an earlier approvedMCU/ROL application, the applicant can alternatively amend thatreport to incorporate the stability assessment of all finalised earthworksincluding cut/fill batters and/or retaining structures (if any) withreference to detailed and finalised bulk earthworks engineeringdrawings and cross-sections.

The stability assessment shall also ensure that the proposedoperational works will not adversely affect the stability and integrity ofthe neighbouring properties, building foundations, buried services andinfrastructures.

2.2 Geotechnical Site Investigation

Section 2.1 earlier mentioned that the slope instability hazard risk must be assessed based on site-specific geotechnical information. Moreover, the stability assessment of all earthworks includingcut/fill batters and/or retaining structures associated with any development application shall also bemade based on geotechnical site investigation results. This section outlines the essential contentsof a geotechnical site investigation, which must be included in the Geotechnical StabilityAssessment Report.


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The geotechnical site investigation will provide a general description of the development site, sitegeology, methodology, site investigation techniques including subsurface exploration, field and/orlaboratory testing, test results, depth of water table, subsurface profile, recommended foundationtypes, depths and bearing capacities, predicted settlements, site constraints, construction issuesand any other site-specific geotechnical issues relevant to the development proposal. Thegeotechnical site investigation can be made by various techniques such as:

Test pits and open cuts for visual inspection of soil/rock (usually for less than 3m depth);

Drilling boreholes, conducting in-situ (field) blow count tests (e.g. Standard PenetrationTest or SPT), and collecting samples from different depths for visual inspection andlaboratory testing; and

Penetrating a special geotechnical probe (e.g. Cone Penetration Test or CPT, DilatometerTest or DMT etc.), measuring various responses to penetration and correlating theresponses to known geotechnical parameters.

Subsurface investigation by drilling boreholes or penetrating a probe can be as deep as 80m ormore. Drilling boreholes along with SPT is the most popular and widely used method forsubsurface geotechnical investigation mainly due its provision for deeper investigation, visualinspection of samples and well-established correlation of its blow count number N with shearstrength and other geotechnical design parameters. Recently, CPT tests, particularly electrical

friction CPT tests are getting increasingly popular among the designers and geotechnical engineersdue to its ability to provide almost continuous profile and improved correlation with variousgeotechnical design parameters.

Soil samples collected during site investigation are usually preserved and protected against anypossible disturbance or moisture changes and sent to the laboratory for various index tests, grain-size analysis and/or shear strength tests. The following laboratory tests are the usually conductedon the collected soil samples:

Liquid limit, plastic limit and plasticity index tests to classify fine-grained soils (clays andsilts) and other geotechnical correlations

Grain-size analysis (by sieve analysis for coarse-grained fraction of soils, by hydrometertest for fine-grained fraction of soils)

Unconfined compression test is one of the simple and quick tests that may be conducted onundisturbed samples in the laboratory in order to determine the undrained shear strength of fine-grained soils. This will provide the short-term shear strengths of cohesive soils.

In order to assess effective stress shear strength parameters (cand ), one may need to conductConsolidated Drained compression tests in the laboratory. These parameters will providerepresentative shear strengths of soils for the long-term conditions.

2.3 Geotechnical Certifications

In addition to the abovementioned slope stability assessment and geotechnical site investigationresults, the applicant shall provide a number of Geotechnical Certifications (see Table 2.2) froman experienced RPEQ for any development application on steep sloping grounds. Thesecertifications will provide assurance of geotechnical stability for the proposed development and willstrengthen the technical soundness of the development application. These certifications must beprepared following the standard pr-forma given in Appendix D and shall be included in theGeotechnical Report.

Table 2.2 below outlines the different types of Geotechnical Certifications required by Council forvarious types of development applications.


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Table 2.2: Geotechnical certifications for developments on steep sloping groundsType of application MCU ROL OPW

If the slope stability assessment determines the site/lot/building envelope with hazard riskof Low or better

Certification from a RPEQ stating that thesite/lot/building envelope is designated with a

slope instability hazard risk of Low or betterbased on site-specific geotechnical

information

If the slope stability assessment determines the site/lot/building envelope with hazard riskof Moderate or worse

Certification from a RPEQ stating that thesite/lot/building envelope will achieve a slope

instability hazard risk of Low or betterprovided the recommended stabilisationmeasures are implemented and that the

site/lot/building envelope will remain stable inthe long-term conditions (70 years minimum)

Certification from a RPEQ stating that the

post-developed site/lot/building envelope hasachieved a hazard risk of Low or better(include new hazard risk calculations)

*

If the application expects any bulk earthworks including cut/fill batters and/or retainingstructures

Certification from a RPEQ stating that allcut/fill batters and/or retaining structures willachieve a long-term factor of safety greater

than 1.5

Certification from a RPEQ stating that allearthworks including cut/fill batters and/or

retaining structures have been carried out inaccordance with the geotechnical

recommendations/report

*

* Certification shall be submitted immediately after completion of works


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3. DEVELOPMENTS WITHIN FLOOD PLAIN AREAS

For any development applications on sites within flood plain areas, one may expect that the subsoilmay contain a thick deposit of soft and compressible alluvium or marine clay. This thick deposit ofsoft clay, unless any pre-treatment/improvement is made, may consolidate over the time at thepost-developed stage, leading to a significant amount of settlement, which may cause cracks anddamages to various building elements, service utilities and infrastructures. In order to

prevent/minimise post-construction damage and associated costly remedial measures resultingfrom the consolidation of untreated soft subsoils, it is essential to conduct a geotechnical siteinvestigation including subsurface exploration and testing for any development proposal on such asite. If the subsoil contains a thick deposit of soft-clay, it is prudent and economic to improve thesite with one of the suitable soft ground improvement techniques prior to commencing the actualdevelopment works.

The geotechnical site investigation results must be included in the Geotechnical StabilityAssessment Report. This section outlines Councils minimum requirements for geotechnicalstability assessments that shall be provided by the applicant for any development application withinflood plain areas.

3.1 Geotechnical Site Investigation

As mentioned earlier, all development applications on sites within flood plain areas shall submit aGeotechnical Stability Assessment Report including geotechnical site investigation results. Themain purpose of this geotechnical site investigation is to identify any potential thick deposit of softand compressible soil layers, which may cause significant amount of consolidation settlement atthe post-developed stage unless pre-treated/improved. If the report identifies such a layer of soft-soil at the site, it must recommend any appropriate and economic ground improvement techniques,depending on the geotechnical conditions, site constraints and type of development proposal. Thereport shall also contain other conventional geotechnical site investigation issues such as generaldescription of the development site, site geology, methodology, site investigations includingsubsurface exploration, field and/or laboratory testing, test results, subsurface profile,recommended foundation types, depths and bearing capacities, settlement issues, site constraints,

construction issues and any other site-specific geotechnical issues relevant to the developmentproposal.

The report shall also examine the feasibility and suitability of the existing site for the proposeddevelopment. Table 3.1 below provides details of various geotechnical stability issues for differenttypes of development application that must be addressed in this geotechnical siteinvestigation/report.


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Table 3.1: Extent of geotechnical site investigation for developments within flood plain areasType of development

applicationExtent of geotechnical issues in Geotechnical Site Investigation

Material Change of Use(MCU)

This investigation shall assess the presence of any soft andcompressible layer(s) of subsoil at the site, and feasibility andsuitability of the site for the proposed development. If the report

identifies any soft and compressible thick subsoil layer(s), it mustrecommend an appropriate and economic soft ground improvementtechnique for the relevant site.

The report shall clearly show the proposed ground improvement areasof the site in both plan and cross-sectional drawings, and providedetails of the proposed ground improvement technique. If a preloading(with/without any vertical drains) method is selected to improve the softsubsoils, the report shall provide estimated primary consolidationsettlement, time of consolidation and any secondary settlement due tocreep. For all other nominated ground improvement methods, thereport must provide predicted increase in shear strength, stiffnessand/or bearing capacity of the subsoils.

If the application is for the purpose of building works, this report shallinclude the stability assessment of all proposed excavations for theconstruction of building foundations and/or basement. The report mustexamine whether the excavations require any protective measures inorder to stabilise the excavation faces. The stability assessment of theproposed excavations shall be based on detail calculations with dueconsideration to the existing geotechnical conditions of the site. Thestability assessment shall refer to specific engineering drawings andcross-sections.

If the excavations are of temporary nature with/without anyshoring/retention system to facilitate the construction of

foundations/basement, the stability assessment must ensure that theshort-term factor of safety of the excavation/retention system will begreater than1.5.

If the excavations are of permanent type, the stability assessment shallensure that the long-term factor of safety of all excavationswith/without any permanent retaining structures/basement walls will begreater than 1.5.

If the development involves any permanent retaining structures, thestability of the retaining structures must also be assessed to ensurethat a long-term factor of safety greater than 1.5 will be achieved.

The stability assessment shall also ensure that the proposedexcavations/earthworks will not adversely affect the stability andintegrity of the neighbouring properties, building foundations, buriedservices and infrastructures.

Reconfiguring a Lot(ROL)

This investigation shall assess the presence of any soft andcompressible layer(s) of subsoil at the site, and assess the feasibilityand suitability of the site for the proposed development. If the reportidentifies any soft and thick subsoil layer(s), it must recommend anappropriate and economic soft ground improvement technique for therelevant site.

The report shall clearly show the proposed ground improvement areas

of the site in both plan and cross-sectional drawings, and provide


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details of the proposed ground improvement technique. If a preloading(with/without any vertical drains) method is selected to improve the softsubsoils, the report shall provide estimated primary consolidationsettlement, time of consolidation and any secondary settlement due tocreep. For all other nominated ground improvement methods, thereport must provide predicted increase in shear strength, stiffnessand/or bearing capacity of the subsoils.

If the application expects any bulk earthworks including cut/fill with orwithout any retaining structures, this report must also assess thestability of all expected/preliminary earthworks including cut/fill battersand/or retaining structures (if any) with reference to preliminary bulkearthworks engineering drawings and cross-sections. The stabilityassessment shall ensure that a long-term factor of safety greater than1.5 will be achieved.

Operational Works(OPW)

This investigation shall assess the presence of any soft andcompressible thick layer of subsoil at the site, and feasibility andsuitability of the site for the proposed development. If the reportidentifies any soft and thick subsoil layer, it must recommend an

appropriate and economic soft ground improvement technique for therelevant site.

The report shall clearly show the proposed ground improvement areasof the site in both plan and cross-sectional drawings, and providedetails of the finalised ground improvement technique. If a preloading(with/without any vertical drains) method is selected to improve the softsubsoils of the site, the report shall provide estimated primaryconsolidation settlement, time of consolidation and any secondarysettlement due to creep. For all other nominated ground improvementmethods, the report must provide predicted increase in shear strength,stiffness and/or bearing capacity of the subsoils.

The report must also include the stability assessment of all finalisedearthworks including cut/fill batters and/or retaining structures (if any)with reference to specific earthworks engineering drawings and cross-sections. The report shall ensure that all finalised cut/fill batters and/orretaining structures (if any) will achieve a long-term factor of safetygreater than 1.5.

If there is a Geotechnical Report associated with an earlier approvedMCU/ROL application, the applicant can alternatively amend thatreport to incorporate the stability assessment of all finalised earthworksincluding cut/fill batters and/or retaining structures (if any) withreference to detailed and finalised bulk earthworks engineeringdrawings and cross-sections.

The stability assessment shall also ensure that the proposedoperational works will not adversely affect the stability and integrity ofthe neighbouring properties and infrastructures.


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3.2 Ground Improvement Techniques

There are a number of ground improvement techniques available in the literature and some of themare very effective and widely used by the practicing engineers and design consultants. This sectionprovides information and guidelines regarding some popular and widely used soft groundimprovement techniques.

3.2.1 Preloading

Preloading (or precompression) is one of the inexpensive and effective methods to improve a sitecontaining soft and compressible subsoils. Preloading is very effective on normal to lightlyoverconsolidated silts and clays. If the soft deposits are thick and do not intercepted by alternatingsand seams, the preloading alone may not be an effective method to consolidate the soft layer. Inthis case use of sand drains or prefabricated vertical drains (PVD) in addition to preloading wouldbe a better technique to improve the soft soils.

Usually, the preload surcharge would be greater than the estimated weight of the proposedstructure so that post-construction settlement becomes negligible and remains within tolerablelimits. The placement of preload surcharge would increase the total stresses as well as pore water

pressures on the soft deposits. Over the time, usually a couple of months to years depending onthe permeability and length of drainage paths, the excess pore water pressures will be dissipated,leading to increase in effective stresses and shear strengths of the subsoils.

In this technique, consolidation of soft subsoils generally takes a long time, which often may beunacceptable to many construction projects. In order to accelerate the consolidation process,preloading is often supplemented by vertical drains (sand drains or prefabricated vertical drains),as discussed below.

3.2.2 Preloading with Vertical Drains

In preloading, consolidation process usually takes a long time because the pore pressure

dissipates in one direction (vertical) only. For a thick, soft and saturated cohesive subsoil layer, thelength of this vertical drainage path can be substantially long, leading to longer consolidation time.By installing closely spaced vertical drains (e.g. sand drains or PVDs) through the soft deposits, thelength of drainage paths will be significantly reduced, as the pore pressure will now dissipatehorizontally towards the neighbouring vertical drains. Consequently, the consolidation time will besignificantly reduced compared to preloading alone. Figure 2 shows the typical preloading conceptand the benefits of preloading with vertical drains in terms of reduced consolidation time.


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(a)

(b) (c)

Time

Settlement

Without vertical drains

With vertical drains

Time saving with vertical drains

(d)

Fig.2: (a) Preloading concept, (b) preloading without vertical drains, (c) preloading with verticaldrains, and (d) time for consolidation settlement with and without vertical drains

Soft groundconsolidatesunder preloading

Surcharge Surcharge

Without drains With Vertical Drains


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The preloading technique with vertical drains, particularly with PVDs, is now widely used toconsolidate and improve ground containing soft and saturated cohesive soils. The followingsections give a brief overview of installation of sand drains and prefabricated vertical drains.

3.2.2.1 Sand Drains

Sand drains can be of diameters ranging from 150 to 750 mm. Sand drains can be installed by oneof the following methods:

Mandrel-driven pipes: A pipe is driven using a closed mandrel. Sand is poured in the pipewhich falls out the bottom cap as the pipe is withdrawn, forming the drain.

Driven pipes: A pipe is driven and the soil inside is then jetted, followed by the proceduresimilar to the Mandrel-drivel pipes above.

Rotary drill: A borehole is drilled using the rotary drill method, then the borehole is filled withsand to create the sand drain.

Continuous flight hollow auger: A borehole is drilled using continuous flight hollow augermethod, then the borehole is filled with sand.

Figure 3 illustrates Mandrel-driven pipes and Continuous flight hollow auger methods.

Fig. 3: Two methods of installing sand drains (after Bowles, J. E., Foundation Analysis and Design,4thedition, McGraw-Hill Inc, 1988)

3.2.2.2 Prefabricated Vertical Drains

In the recent times, prefabricated vertical drains (PVD) are more economic, popular and widelyused throughout the world to accelerate consolidation settlement of soft-clay deposits underpreloading. Many researchers indicate that PVDs are 5 to 8 times cheaper than sand drains. Aprefabricated vertical drain is a thin and flexible band drain consisting of a polymer or plastic

grooved core wrapped by a geotextile layer, as shown in Fig. 4.


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Fig. 4: Typical prefabricated Vertical Drain (PVD)

The geotextile cover acts as a filtering layer to reduce core clogging. The central grooved core acts

as the main drainage channel. The width of the PVD can vary from 100 to 300 mm with thethickness from 3 to 6 mm. The PVDs are available in roll and can be installed very fast (e.g. lessthan a minute for 1 PVD installation). Figure 5 shows typical installation of PVDs on a site usingspecial installation rigs.

Fig. 5: Installation of prefabricated vertical drains (PVD) on a site

3.2.3 Stabilisation with Lime/Cement

Lime and/or cement columns are installed by deep mixing methods (DMM) and they have beenused to stabilise soft clay in many parts of the world, particularly in Sweden, Finland, Norway andJapan. In this method, lime/cement columns are constructed by mechanically mixing lime and/or

cement with the soft clay subsoils at in-situ conditions. The chemical reaction of clay with lime


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and/or cement through the process of ion exchange, flocculation and pozzolanic reactions gives anincrease in shear strength and decrease in compressibility to the soft soils.

The lime/cement mixing method has been used to improve the shear strength of soft soils since theolden times. The use of cement columns, using cement powder, has been reported to besuccessful to improve soft soils in early 1980s. Since the mid-80s, lime and cement have beenincreasingly used as soil stabilising agents. The deep mixing method was originally developed toimprove soft grounds for the ports and harbour structures. The use of this method has now beenextended to the foundation of embankments, buildings and storage tanks. Further details oflime/cement stabilisation technique can be found in any ground improvement reference book suchas Soft Ground Improvement in Lowland and Other Environments by D.T. Bergado, L.R.Anderson, N. Miura, and A.S. Balasubramanium, ASCE Press (1996).


The applicant may need to provide a number of Geotechnical Certifications (see Table 3.2) froman experienced RPEQ for any development applications on sites within flood plain areas. Thesecertifications will provide assurance of geotechnical stability for the proposed development and will

strengthen the technical soundness of the development application. These certifications must beprepared following the standard pr-forma given in Appendix D and shall be included in theGeotechnical Report.

Table 3.2 below outlines the various Geotechnical Certifications that must be provided by theapplicant for various types of development applications.

Table 3.2: Geotechnical certifications for developments within flood plain areasType of application MCU ROL OPW

If the geotechnical report identifies any soft and thick deposit of subsoil layer(s)

Certification from a RPEQ specialising ingeotechnical engineering stating that the

site/lot will be geotechnically stable andsuitable for the proposed development

provided the recommended groundimprovement technique is implemented

If the application expects any bulk earthworks including cut/fill batters and/or retainingstructures

Certification from a RPEQ specialising ingeotechnical engineering stating that all

cut/fill batters and/or retaining structures willachieve long-term factor of safety greater

than 1.5

Certification from a RPEQ specialising ingeotechnical engineering stating that allcut/fill batters and/or retaining structures

have been carried out in accordance with thegeotechnical recommendations/report

*

* Certification shall be submitted immediately after completion of works


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4. DEVELOPMENTS INVOLVING DEEP EXCAVATIONS

For any development application such as high-rise building or multi-storied residential orcommercial complex that requires deep excavation for the construction of basement, foundationsor any other facilities, it must be supported by a geotechnical report demonstrating the stability ofthe proposed excavation including any temporary and/or permanent retention system/structures.The geotechnical report shall include subsoil information and ground water conditions of the site.The report must provide a detail stability assessment of the proposed excavation including any

temporary and/or permanent retaining structures. The assessment shall also provide the radius ofinfluence of drawdown cone due to dewatering (if any) and also the effects of drawdown on thesettlement and stability of the neighbouring buildings, properties, utility services and infrastructures.The stability assessment must be based on site-specific geotechnical information includingsubsurface exploration, field and/or laboratory test results. The stability assessment of deepexcavations shall be included in the Geotechnical Report.

This Section outlines the details of stability assessment for deep excavations associated with anydevelopment application.

4.1 Retaining Structures to Stabilise Deep Excavations

All deep excavations associated with any development application must be stable and adequately

protected against any potential sliding, rotational and base failure. The stability of deep excavationsis absolutely necessary to ensure safe construction within the excavated area, and also for thestability and integrity of the neighbouring buildings, properties, utility services and infrastructures.Unless the excavations are made with batters, which are very unusual, the stability of deepexcavations may be provided by retaining structures, either temporary or of permanent type, suchas:

Sheet piling (steel, concrete or wood)

Soldier piles and shotcrete panel walls

Drilled and cast-in place concrete piles (or piers)

Concrete poured in a cavity retained with slurry producing a slurry wall

Sheet piling can be of 3 types: cantilever sheet piling, anchored sheet piling and braced sheetpiling.

4.2 Stability of Deep Excavations

The following are the possible modes of failure for a deep excavation supported by a retainingstructure. Stability and factor of safety against these possible failure modes must be assessed.

Flexural failure of the retaining structural element

The retaining structural elements may fail due to the flexural stresses caused by the maximumbending moment exceeding the yield strength of the material.

Tensile failure of anchor rod or tendon (for anchored sheet pile)

The anchor rod or tendon may fail in tension if the tensile stress exceeds the yield strength ofthe rod or tendon.

Pull out failure of anchor (for anchored sheet pile)

The anchor may fail if the tensile force in the anchor exceeds its pull out resistance.

Compressive failure of bracing elements (for braced sheet pile)

The bracing element may fail in compression if the compressive stress exceeds the yieldstrength of the bracing material.


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Rotational failure (for cantilever sheet pile)

The retaining structure may become unstable or fail in rotation if the resisting moment causedby the passive resistance within the embedment depth becomes less than the driving momentcaused by active lateral earth pressure of the retained soils.

System failure

The whole retaining system may fail in a particular slip circle. In this case, a slip circle stabilityanalysis shall be carried out where the trial slip circles will pass through the toe of the retainingstructure or sheet pile.

Base failure (by upward movement of excavation base)

The pressure loss due to excavation may result in a base instability, where the soil flowbeneath the sheeting into the excavation, producing a rise in the base elevation commonlytermed as heave. This can be analysed using Mohrs circle or bearing capacity theory.

The stability assessment shall demonstrate that the proposed shoring/retention system forsupporting the basement excavation will be stable enough with a factor of safety greater than 1.5

against failure.

The applicant shall also assess the suitability of the proposed basement excavation methodologyand examine whether the basement excavation support system requires any ground anchoring intoany adjacent properties or road reserve. If ground anchoring is proposed to penetrate into anyadjacent private properties, the details of the ground anchoring including a plan showing the extentof the affected adjacent property and a signed letter from the owner of the affected propertyconsenting to carrying out of those works must be included in the report. If ground anchoring isproposed to penetrate into any adjacent road reserve, the applicant shall obtain a groundanchoring approval from Council in accordance with the requirements of Ground Anchor InterimPolicy, July 2006 prior to the commencement of any excavation on site.

The applicant shall assess the overall potential adverse effects on the stability and integrity of the

neighbouring properties/structures in terms of their total predicted vertical and lateral movementsdue to the proposed basement excavation and dewatering.

Moreover, the applicant must submit a site-monitoring plan to ensure the stability of theneighbouring structures both during construction and at post-construction period of at least 3months. The monitoring plan shall include plans and cross-sectional drawings showing thelocations and parameters to be monitored, frequency of monitoring, threshold value of anyparameter that will trigger immediate cessation of all site works in order to maintain the stability andintegrity of the neighbouring properties/structures.


All development applications involving deep excavations must submit a geotechnical certification

from a RPEQ specialised in geotechnical engineering. The certification must be prepared followingthe standard pr-forma given in Appendix D and shall be included in the Geotechnical Report. Thecertification shall state that:

The proposed excavation and the temporary supporting structures are geotechnicallystable with a factor of safety greater than 1.5 against rotational failure, system failure andbase failure.

The permanent retaining structures (if any) associated with the excavation aregeotechnically stable with a long-term factor of safety greater than 1.5.

The proposed excavation and dewatering (if any) will not cause any adverse effects on thestability and integrity of the neighbouring buildings, properties, utility services andinfrastructures.


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5. DEVELOPMENTS INVOLVING BATTERS AND/OR RETAINING STRUCTURES

All development applications involving cut/fill batters and/or retaining structures must be assessedfor their stability against potential sliding, rotational or slip circle failures. The stability assessmentof all cut/fill batters and/or retaining structures shall be included in the Geotechnical StabilityAssessment Report.

This Section outlines the details of stability assessment for all batters and retaining structuresassociated with any development application.

5.1 Stability of Batters

The stability assessment of all proposed cut/fill batters shall be carried out according toconventional slip circle failure analysis. In this type of analysis, a number of potential slip circleswith varying centres and radius are assumed, and the factor of safety for each of the assumed slipcircles is computed. The minimum factor of safety amongst those slip circles is considered to bethe factor of safety for that selected batter. The accuracy of stability assessment in this methoddepends on the number of slip circles analysed.

One critical and very important issue in the stability assessment of batters is the estimation ofrepresentative shear strengths for the constituting soil layers. In stability analysis of batters, theworst possible shear strengths of the soil layers during the design life of the batters shall be used,rather than using the existing shear strengths of the soil layers. In other words, the shear strengthsof soils in saturated conditions (in case there is a prolonged and heavy rainfall) with the highestestimated water table shall be used. The other potential worst-case scenario for the stabilityassessment of batters adjacent to any water body is the sudden drawdown of water table. Inassessing the stability of batters adjacent to any water body, the factor of safety for this particularcase of sudden drawdown must be taken into account.

The stability analysis of batters can be carried out manually, however, the use of a professionalsoftware such as SLOPE/W (by Geoslope: www.geo-slope.com) would be cost-effective with muchless computational efforts and time. Figure 6 shows an example of slope stability analysis using

SLOPE/W software.

1

2

1

2 3

4

5

6 7

8

9

10

11 12

1.479

1

2 3

4

5

6 7

8

9

10

11 12

Material #: 1 Unit Weight: 15 C: 5 Phi: 20 Model: MohrCoulombMaterial #: 2 Unit Weight: 18 C: 10 Phi: 25 Model: MohrCoulomb

Distance (m)

0 5 10 15 20 25 30 35 40

Elevation

(m)

0

4

8

12

16

Fig. 6: Typical slope stability analysis using SLOPE/W


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5.2 Stability of Retaining Structures

Geotechnical stability of all proposed retaining structures must be carried out against sliding,overturning and global slope instability. The retaining structures must also be stable againstbearing capacity failure (or excessive base settlements). Moreover, the retaining structure itselfmust be adequately designed against any potential structural failures such as flexural failure orshear failure.

Fig. 7: Typical retaining structure and the lateral earth pressure distributions

Figure 7 shows a typical retaining structure including lateral earth pressure distributions. Theretained soil behind the retaining structure will exert active lateral earth pressure if the retainingstructure allows some lateral movement; otherwise lateral earth pressure at rest (Ko condition)should be used during design and stability assessments. The soil in front of the wall will causepassive earth pressure, as shown in Fig. 7.

All development applications involving retaining structures must assess the geotechnical stabilityand factor of safety against the following:

a) Sliding caused by the active earth pressure and resistance by passive earth pressure andfrictional force at the base the retaining structure;

b) Overturning about the toe (point O in Fig. 7) as a result of driving moment caused by theactive earth pressure and resisting moment caused by the passive earth pressure, the self-weight of the retaining structure and weight of the retained soils behind the structure; and

c) Global slope instability considering a large slip circle passing through the underneath of theretaining structure and the retained soils.

The stability assessment shall ensure that all retaining structures will achieve a factor of safety

(FOS) > 1.5 against sliding, overturning and global slope instability.


All development applications involving batters and/or retaining structures must include acertification from a RPEQ experienced in geotechnical engineering stating that all cut/fill battersand/or retaining structures will achieve a long-term factor of safety greater than 1.5 and that theproposed cut/fill batters and/or retaining structures will not cause any adverse effects on thestability and integrity of the neighbouring buildings, properties, utility services and infrastructures.The certification must be prepared following the standard pr-forma given in Appendix D and shallbe included in the Geotechnical Report.

6. PRESENTATION OF THE REPORT

Pa = Active earth pressure

Pp = Passive earthressure O


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The geotechnical stability assessment report must be written in such a way that it is regarded as aself-contained document, which does not require the reader to refer to any other documentsincluding Council files, maps, drawings, previous application or submitted reports. If in any case, itdoes require referring to any other documents (as mentioned earlier), it must include a copy ofthose documents as attachments. The report shall include but not necessarily limited to thefollowing:

a) A covering page showing the report reference number, revision, title of the report, theproperty address, Real Property Description (Lot and Plan numbers), Councils referencenumber (if available), authors name and date;

b) The body of the report including the context within which the report was commissioned, thepurpose of the report, geotechnical site investigation results, slope stability assessments (ifany), ground improvement techniques (if any), stability of deep excavations (if any), andstability assessments of batters and/or retaining structures (if any);

c) Quantitative Relative Frequency calculations (if any) as explained in section 2.1 of thisdocument;

d) Subdivision Landslip Encumbrance Form, if required according to Table 2.1 of thisdocument;

e) Geotechnical Certifications, as required according to this document; and

f) Any maps, plans, drawings, cross-sections referred in this report.

7. REFERENCES

Gold Coast City Council (2005), Our Living City Gold Coast Planning Scheme, Ver. 1.1

Gold Coast City Council (2005), Land Development Guidelines

Queensland Government (2003), Mitigating the Adverse Impacts of Flood, Bushfire andLandslide, State Planning Policy, June 2003

Gold Coast City Council, Guidelines for Control of Slope Instability within the City of Gold Coast

SMEC (1999), Landslip Study for the City of Gold Coast, final report, prepared by SnowyMountain Engineering Corporation (SMEC), August 1999

MacGregor, P. and Taylor, B. (2001), A method of Zoning Landslide Hazard, AustralianGeomechanics, September 2001, pp. 55-61

Australian Geomechanics Society (2000), Landslide Risk Management Concepts andGuidelines prepared by Sub-Committee on Landslide Risk Management, Australian

Geomechanics, March 2000, pp. 51-92.


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APPENDICES


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APPENDIX A:

EXAMPLE OF RELATIVE FREQUENCY CALCULATION FOR ASSESSING

SLOPE INSTABILITY HAZARD RISK

Appendix A contains one example of quantitative landslip frequency analysis following the methodpresented in the Landslip Study for the City of Gold Coast, prepared by Snowy MountainEngineering Corporation (SMEC), August 1999.


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LNADSLIDE FREQUENCY ANALYSIS Analysis No.: V-A1

GEOLOGY: Volcanics LOCATION: A1 GEOMORPHOLOGY: THICK RESIDUAL SOIL

NATURAL SHALLOW LANDSLIDES

1 Basic Frequency 0.004 6 Concentration of Surface Water

2 Slope Angle Area Level Factor

Ridge L 0.7

Area Level Factor Crest M 0.8

1 Less than 5 degrees L 0.1 Upper slope M 0.9

Between 5 and 15 degrees M 0.5 Mid slope H 1.2

Between 15 and 30 degrees M 0.8 1 Lower slope H 1.5

Between 30 and 45 degrees H 1.2More than 45 degrees M 0.8 7 Evidence of Ground Water

3 Slope Shape Area Level Factor

None apparent L 0.7

Area Level Factor 1 Minor moistness M 0.9

1 Crest or ridge L 0.7 Generally wet H 1.5

Planar M 0.9 Surface springs VH 3

Convex M 0.9

Concave H 1.5 8 Evidence of Instability

4 Area Geology Area Level Factor

1 No sign of instability L 0.8

Area Level Factor Minor irregularity VH 2

1 Volcanic rock H 1.1 Major irregularity VH 5

Sedimentary rock M 1 Active instability VH 10

Low grade metamorphic rock M 1

High grade metamorphic rock L 0.9

Granitic rock M 1 Summary

Factor

5 Material Strength 2 Slope Angle 0.1

3 Slope Shape 0.7Area Level Factor 4 Area Geology 1.1

Rock at surface VL 0.1 5 Material Strength 0.9

Residual soil < 1 m deep L 0.5 6 Concentration of Surface Water 1.5

1 Residual soil 1-3 m deep M 0.9 7 Evidence of Ground Water 0.9

Residual soil > 3 m deep H 1.5 8 Evidence of Instability 0.8

Colluvial soil < 1 m deep H 1.5 9 Relative Frequency (2x3x4x5x6x7x8) 0.07

Colluvial soil 1-3 m deep VH 2

Colluvial soil > 3 m deep VH 4 Area Frequency (1 x 9) 0.0003

Note: The numerical factors allocated to these site features are based on judgement and experience


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APPENDIX B:

(Excerpt from Section 3.6 of the SMEC Report: Gold Coast City Council: LandslipStudy for the City of Gold Coast , 1999)

CORRELATION BETWEEN RELATIVE FREQUENCY AND HAZARD RATING

Relative Frequency Hazard Rating

< 0.2 Very Low

0.2 0.6 Low0.6 2.0 Moderate

2.0 6.0 High

> 6 Very High

To ensure consistency certain rules were adopted. For example if there is evidence ofactive slope instability the Hazard Rating must be at least High.


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APPENDIX C:

SUBDIVISION LANDSLIP ENCUMBRANCE FORM

ADDRESS:

ESTATE NAME AND STAGE

SUBDIVISION FILE REFERENCE:

PARENT PARCEL OF LAND

Lot No. Registered No. Encumbrance

PROPOSED SUBDIVIDED ALLOTMENTSFinal Hazard RatingLotNo.

RegisteredPlan No.

RelativeFrequency For Lot For Building

PadFor Effluent

Disposal Area

Please use the following abbreviations to re-categorise the SMEC soil instability hazardrating.

Encumbrances shall be defined in accordance with the following abbreviations:

VH = Very High hazard of soil instabilityH = High hazard of soil instabilityM = Moderate hazard of soil instabilityL = Low hazard of soil instabilityVL = Very Low hazard of soil instability

Geotechnical Engineering Consultancy:

Signature:

Name:

RPEQ No:

Date:


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APPENDIX D:

Standard Pro-forma forGeotechnical Certification

Gold Coast City Council file reference: .

PROPOSED WORKS AT(LOCATION)________________________________________________

FOR (proposeddevelopment)______________________________________________________

I,, RPEQ No. ., of..(Consulting Engineers), beingduly authorised on this behalf, do certify that

..

I am aware that the Gold Coast City Council will rely upon this certificate and anyassociated geotechnical reports, maps, graphs, tables, attachments etc. produced as aconsequence of commissioning this development proposal.

Signed: _____________________________

Designation: __________________________

Certified this: _________________________ day of _________ 20___.


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Gold Coast City Council

Implementation & Assessment Branch

Planning Environment & Transport Directorate

PO Box 5042 Gold Coast MC Qld 9729 Australia

Email:[email protected]

Web:goldcoastcity.com.au

GCC

C3357

Slope Gold Coast

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