geotechnical stability assessment guidelines

Geotechnical Stability Assessment Guidelines

City Development Planning and Environment Directorate

March 2016

Geotechnical stability assessment guidelines

ii

Document #42081412 v2 Last updated 08/03/2016 i

Table of Contents 1. INTRODUCTION ............................................................................................................................................. 1

1.1 Developments with geotechnical stability issues ............................................................................................. 1

1.2 Geotechnical stability assessment criteria ...................................................................................................... 1

2. DEVELOPMENTS WITHIN LANDSLIDE HAZARD AREAS ........................................................................... 3

2.1 Landslide risk assessment .................................................................................................................... 3

2.2 Geotechnical site investigation .............................................................................................................. 5

2.3 Geotechnical certifications .................................................................................................................... 6

3. DEVELOPMENTS WITHIN FLOOD PLAIN AREAS ....................................................................................... 6

3.1 Ground improvement techniques .......................................................................................................... 6

3.1.1 Preloading ................................................................................................................................ 7

3.1.2 Preloading with vertical drains .................................................................................................. 7

3.1.3 Sand drains .............................................................................................................................. 9

3.1.4 Prefabricated vertical drains ..................................................................................................... 9

3.1.5 Stabilisation with lime/cement ................................................................................................ 11

4. DEVELOPMENTS INVOLVING DEEP EXCAVATIONS ............................................................................... 11

4.1 Deep excavation retention system ...................................................................................................... 11

4.2 Stability assessment of deep excavation ............................................................................................ 12

4.3 Geotechnical certifications .................................................................................................................. 13

5. DEVELOPMENTS INVOLVING BATTERS AND/OR RETAINING STRUCTURES ..................................... 13

5.1 Stability assessment of batters ........................................................................................................... 13

5.2 Stability assessment of retaining structures ........................................................................................ 14

5.3 Geotechnical certifications .................................................................................................................. 15

6. PRESENTATION OF THE REPORT ............................................................................................................ 15

7. REFERENCES .............................................................................................................................................. 15

8. APPENDICES ................................................................................................................................................ 16

Appendix A – Landslide susceptibility analysis form ..................................................................................... 16

Appendix B – Correlation between relative susceptibility and susceptibility rating ....................................... 17

Appendix C – Subdivision landslide encumbrance form ............................................................................... 18

Appendix D – Standard pro-forma for geotechnical certification ................................................................... 19


iiii

Document #42081412 v2 Last updated 08/03/2016 ii

Lists of Tables Table 1 – Criteria for requirement of a geotechnical stability assessment report ................................................... 1 Table 2 – Details of landslide risk issues for various development applications .................................................... 4 Table 3 – Geotechnical certifications for developments within landslide hazard areas .......................................... 6


iiiiii

Document #42081412 v2 Last updated 08/03/2016 iii

List of Figures Figure 1 – Flowchart for geotechnical stability assessment.................................................................................... 2 Figure 2 – Preloading for soft ground improvement ................................................................................................ 8 Figure 3 – Mandrel-driven pipes and continuous flight hollow auger methods ....................................................... 9 Figure 4 – Typical prefabricated vertical drain (PVD) ........................................................................................... 10 Figure 5 – Installation of prefabricated vertical drains .......................................................................................... 10 Figure 6 – Typical slope stability analysis using SLOPE/W .................................................................................. 14 Figure 7 – Typical retaining structure and lateral earth pressure distributions ..................................................... 14


1

Document #42081412 v2 Last updated 08/03/2016 Page 1 of 19

1. INTRODUCTION

This document has been developed to provide guidelines on geotechnical stability assessment and management issues associated with various types of development applications. These geotechnical issues should be assessed and submitted in support of any development applications to the Council of the City of Gold Coast (Council) for review and approval. The main purpose of this document is to provide a framework for informed decision-making process by Council regarding geotechnical stability issues associated with any development applications. This document will assist the applicants and their consultants in preparing a relevant geotechnical report in support of a development application and will also assist Council during the application assessment process, hence improves transparency and understanding between the two parties. The key objectives of this document are to: • provide clarity and transparency regarding geotechnical stability concerns, issues and requirements

by Council in relation to any development application assessment and approval process • provide guidelines for preparing and submitting a relevant Geotechnical Stability Assessment

Report, if required, in support of any development application • improve efficiency and consistency in the development application assessment process • support development applications, which are geotechnically stable, safe and sound.

This document has been prepared to provide detailed guidelines for addressing various geotechnical stability issues indicated in the Landslide Hazard Overlay Code and Change to Ground Level and Creation of New Waterways Code of the City Plan.

1.1. Developments with geotechnical stability issues

From a geotechnical point view, this document identifies four (4) types of developments involving geotechnical stability issues:

• Developments within landslide hazard areas • Developments within flood plain areas • Developments involving deep excavation • Developments involving batters and/or retaining structures.

1.2. Geotechnical stability assessment criteria

If any assessable development application falls within one of the following categories given in Table 1, the applicant should submit a Geotechnical Stability Assessment Report (Geotechnical Report) in order to proceed with the development application for assessment and approval by Council.

Table 1 – Criteria for requirement of a geotechnical stability assessment report

Type of development Is a geotechnical stability assessment report required?

Developments within landslide hazard areas: • if the site is partially or completely identified on Landslide

Hazard Overlay Map of the City Plan

Yes

Developments within flood plain areas:

• if the site is located within the area identified on Flood Overlay Map of the City Plan

Yes

Developments involving deep excavation that is greater than 3 metres in depth.

Yes

Developments involving:

• cut/fill batters greater than 3 metres in height/depth Yes • retaining structures greater than 3 metres in height Yes


iiii


No

Yes

Yes

Yes No

No

Yes

No

Figure 1 shows a flowchart for various geotechnical stability assessments that should be carried out and include in a Geotechnical Report.

Figure 1: Flowchart for geotechnical stability assessment

Development application

Does the site partially or completely identified on

Landslide Hazard Overlay Map?

Assess risk of landslide using site-specific geotechnical information

Does the risk assessment determine the site/lot/building envelope with a landslide risk rating of moderate or worse?

Provide risk mitigation measures to reduce landslide risk rating to low or better and certify that the site/lot/building envelope will achieve a landslide risk rating of low or better subject to compliance with the risk mitigation measures

Provide certification confirming that the site/lot/building envelope has been assessed with a landslide risk rating of low or better

Does the site contain any soft subsoil layer(s)?

Provide details of recommended soft ground improvement techniques

Does the development expect any deep excavation,

cut/fill batters or retaining structures?

Provide stability assessments for deep excavation, cut/fill batters and/or retaining structures, and certify the stability of deep excavation, cut/fill batters and/or retaining structures

Geotechnical report is not warranted


iiiiii


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

2. DEVELOPMENTS WITHIN LANDSLIDE HAZARD AREAS

For any proposed development on land within landslide hazard areas, as identified on Landslide Hazard Overlay Map of the City Plan, there is a risk of landslide which must be assessed by a qualified expert and submitted to Council for review and approval. The level of landslide risk depends on a number of factors including ground slope angle and shape, strength of geomaterials and its distribution within the subsurface, depth of groundwater table, potential for surface run-off concentration, orientation of rock mass defects, etc. The applicant needs to assess the risk of landslide which may adversely affect the subject site, adjoining properties and the proposed development.

According to Table 1 if the proposed development site is partially or completely identified on Landslide Hazard Overlay Map, a Geotechnical Report is required by Council. In this case, the Geotechnical Report should include a landslide risk assessment for the site in relation to the proposed development.

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

The following section outlines the details of landslide risk assessment, associated geotechnical site investigation and geotechnical certifications that are required by Council for any proposed development within landslide hazard areas.

2.1. Landslide risk assessment

The landslide risk assessment for the proposed development site should be conducted by a Registered Professional Engineer of Queensland (RPEQ) specialising in geotechnical engineering, particularly experienced in landslide risk assessment and management issues. The landslide risk assessment should be carried out using site-specific geotechnical information, site slope, surface features, historical landslide information, groundwater table and any other relevant information of the site. The landslide risk assessment results should be included in the Geotechnical Report. The landslide risk assessment should be carried out generally in accordance with the methodology presented in the document: Landslide Susceptibility Assessment Report for the City of the Gold Coast, prepared by SMEC AUST PTY LTD, dated August 2011.

According to the above referenced report, for any proposed development or re-development on any site/lot mapped with landslide hazard, a landslide relative susceptibility analysis should be carried out first using the Landslide Susceptibility Analysis Form attached in Appendix A. The calculated relative susceptibility should then be correlated to susceptibility rating using the table given in Appendix B.

If the result of the landslide susceptibility rating analysis is ‘Low’ or ‘Very Low’, then a further risk assessment of the proposed development impacting any adjoining buildings/properties should be conducted. Finally, the report should include a certification from a RPEQ specialising in geotechnical engineering confirming that the proposed development site/lot has been assessed with a landslide risk rating of ‘Low’ or better and that the proposed development will not cause any adverse impact on any adjoining buildings, properties and infrastructures.

In case the result of the landslide susceptibility rating analysis is ‘moderate’ or higher, a detailed landslide risk assessment following the Australian Geomechanics Society (AGS) ‘Landslide Risk Management Guideline 2007’ should be carried out in order to determine whether the risk to life and property is acceptable. In this regard a ‘Low’ or ‘Very Low’ risk is acceptable to Council. If the result of the landslide risk assessment following the AGS 2007 method is still ‘Moderate’ or higher, a detailed risk mitigation measures and engineering recommendations to reduce the landslide risk to an acceptable level should be included with the report. Finally, the report should include a certification from a RPEQ specialising in geotechnical engineering confirming that the proposed development site/lot has been assessed with a landslide risk rating of ‘Low’ or better and that the proposed development will not cause any adverse impact on any adjoining buildings, properties and infrastructures, subject to compliance with the risk mitigation measures and engineering recommendations (if any) of the report.

The report should generally examine feasibility and suitability of the proposed development with regard to landslide risk issues for the site. If the proposed development involves on-site effluent disposal system, the risk assessment should consider potential saturation and softening of the soils within the effluent disposal areas and their impacts on the long-term stability of the site.


ivi v


Table 2 describes the details of landslide risk issues for different types of development applications that should be addressed in a geotechnical stability assessment report.

Table 2 – Details of landslide risk issues for various development applications

Material Change of Use (MCU) For any MCU development application on any site/lot identified on the Landslide Hazard Overlay Map, the application needs to be supported by a landslide risk assessment report (Geotechnical Report). The report should assess the risk of landslide on the subject site as well as any risk of landslide on any upslope external properties which may impact the proposed development. In case any risk of landslide on any upslope external properties impacting the proposed development is identified, the report should provide suitable risk mitigation measures including appropriate buffer to protect the proposed development. If the proposed development is on a portion of a large allotment, the landslide risk assessment may be limited to the proposed development footprint only. In this case the landslide risk assessment for the proposed building envelope and effluent disposal area of the site may be sufficient, rather than for the entire allotment. The risk assessment should take into account availability of a suitable driveway access to the proposed building envelope. In case the proposed development is associated with any significant excavation/cutting and/or filling on the site, the risk assessment should take into account the proposed bulk earthworks and finished levels, and determine the overall risk of landslide including the proposed bulk earthworks. The assessment report should provide any restrictions on any earthworks including excavation/cutting and filling in order to achieve and maintain acceptable risk of landslide in the long-term conditions. The landslide risk assessment should be carried out in accordance with the procedure described in Section 2.1 of this document. The assessment report should confirm that the risk of landslide on the subject site/lot adversely impacting the proposed development and adjoining properties/structures and the risk of landslide on any upslope external properties impacting the proposed development is ‘Low’ or ‘Very Low’.

Reconfiguring of a Lot (ROL) For any ROL development application on any site/lot identified on the Landslide Hazard Overlay Map, the application needs to be supported by a landslide risk assessment report (Geotechnical Report). The report should assess the risk of landslide for each of the proposed lots. In case the proposed lots are large allotments, the landslide risk assessment for each proposed lot may be limited to the nominated building envelope and effluent disposal area only, rather than for the entire allotment. The risk assessment should take into account availability of a suitable driveway access to the proposed building envelope of the lot. The risk assessment report should identify any clear exclusion zone (if any) which is deemed not suitable for any future development due to unacceptable risk to life and/or property. In this case the report should recommend a suitable buffer zone outside the exclusion zone. In case the proposed development is associated with any significant excavation/cutting and/or filling on the site/lots, the risk assessment should take into account the proposed bulk earthworks and finished levels, and determine the risk of landslide for each of the proposed lots at their proposed finished levels. The landslide risk assessment should be carried out in accordance with the procedure described in Section 2.1 of this document. The assessment report should confirm that the risk of landslide adversely affecting each of the proposed lots (or their nominated building envelopes and effluent disposal areas) is ‘Low’ or ‘Very Low’. The report should include a completed and signed Subdivision Landslide Encumbrance Form (attached in Appendix C) for each of the proposed lots.


5


Operational Works (OPW) For any OPW development application (for change to ground level or civil works) on any site/lot identified on the Landslide Hazard Overlay Map, the application needs to be supported by a landslide risk assessment report (Geotechnical Report). The report should assess the overall risk of landslide on the subject site/lot taking into account the proposed bulk earthworks, excavation/cutting, filling, retaining walls and the proposed finished level. If the proposed development is on a portion of a large allotment, the landslide risk assessment may be limited to the proposed development footprint only, rather than for the entire site/lot. The assessment report should provide any restrictions on any earthworks including excavation/cutting and filling in order to achieve and maintain acceptable risk of landslide in the long-term conditions. The landslide risk assessment should be carried out in accordance with the procedure described in Section 2.1 of this document. The assessment report should confirm that the risk of landslide on the subject site/lot after completion of the proposed works is ‘low’ or ‘very low’ and that the proposed works will not cause any adverse impact on any adjoining properties/structures.

2.2. Geotechnical site investigation

As stated earlier in Section 2.1, the landslide risk should be assessed based on site-specific geotechnical information. Furthermore, the stability assessment of all proposed bulk earthworks including cut/fill batters and/or retaining structures should also be carried out using site-specific geotechnical investigation results. This section outlines the essential contents of a geotechnical site investigation, which should be included in the Geotechnical Stability Assessment Report.

The geotechnical site investigation will provide a general description of the development site, site geology, methodology, site investigation techniques employed during subsurface exploration, field and/or laboratory testing, test results, depth of watertable, subsurface profile, recommended foundation types, depths and indicative bearing capacities, anticipated settlements, any site constraints, construction issues and any other site-specific geotechnical issues relevant to the proposed development. The geotechnical site investigation may be carried out employing various techniques such as:

• test pits and open cuts for visual inspection of soil/rock (usually not more than three (3) to four (4) metres in depth)

• drilling boreholes, conducting in-situ (field) tests, (for example, standard penetration test (SPT)), and collecting undisturbed or disturbed samples from different depths for visual inspection and laboratory testing

• penetrating a special geotechnical probe, (for example, cone penetration test (CPT), dilatometer test (DMT), etc.) and recording various responses to penetration. The penetration response data are very useful in correlating with various geotechnical design parameters.

Subsurface investigation by drilling boreholes or penetrating a probe can be as deep as 80 metres or higher. Drilling boreholes along with SPT test is the most popular and widely used method for subsurface geotechnical investigation mainly due its provision for deeper investigation, visual inspection of samples and well-established correlation of the blow count number N with shear strength and other geotechnical design parameters. Recently, CPT tests, particularly electrical friction CPT tests are getting increasingly popular among the designers and geotechnical engineers due to its ability to provide almost continuous profile and improved correlation with various geotechnical design parameters.

Soil samples collected during the site investigation are usually preserved and protected against any possible 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 usually conducted on the collected soil samples:

• liquid limit, plastic limit and plasticity index tests to classify fine-grained soils (clays and silts) and for other geotechnical correlations

• grain-size analysis (by sieve analysis for coarse-grained fraction of soils, by hydrometer test for fine-grained fraction of soils).

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


vi vi


In order to assess the effective stress shear strength parameters (c′ and φ′), one may need to conduct consolidated drained compression tests in the laboratory. These parameters will provide representative shear strengths of soils for the long-term conditions.

2.3. Geotechnical certifications

In addition to the abovementioned landslide risk assessment, the applicant should provide a number of Geotechnical Certifications (refer to Table 3) from a Registered Professional Engineer of Queensland (RPEQ) specialising in geotechnical engineering for any proposed development within landslide hazard areas. These certifications will provide assurance of geotechnical stability for the proposed development site and will also provide a summary of the complex landslide risk assessment process. These certifications should be prepared using the standard pro-forma given in Appendix D and should be included with the Geotechnical Report.

Table 3 describes the relevant Geotechnical Certifications required by Council for various types of development applications within the landslide hazard areas.

Table 3 – Geotechnical certifications for developments within landslide hazard areas

If the landslide risk assessment determines the site/lot/building envelope with a landslide risk rating of ‘Low’ or better

Certification from a RPEQ specialising in geotechnical engineering confirming that the proposed development is appropriate for the sloping nature of the site, the risk of landslide on the subject site/lot (or each of the proposed lots – for Subdivisions) adversely affecting the proposed development and adjoining properties/ structures and the risk of landslide on any upslope external properties impacting the proposed development is ‘Low’ or better.

MCU ROL OPW

If the landslide risk assessment determines the site/lot/building envelope with a landslide risk rating of

‘Moderate’ or worse Certification from a RPEQ specialising in geotechnical engineering confirming that the proposed development is appropriate for the sloping nature of the site, the risk of landslide on the subject site/lot (or each of the proposed lots – for Subdivisions) adversely affecting the proposed development and adjoining properties/ structures and the risk of landslide on any upslope external properties impacting the proposed development is ‘Low’ or better, subject to compliance with the risk mitigation measures and engineering recommendations of the report.

MCU ROL OPW

3. DEVELOPMENTS WITHIN FLOOD PLAIN AREAS

For any development applications on sites within flood plain areas, one may expect that the subsoil may contain a thick deposit of soft and compressible alluvium or marine clays. This thick deposit of soft clay, unless any pre-treatment/improvement is made, may consolidate over time at the post-developed stage, leading to large consolidation settlements, which may cause cracks and/or damages to various building elements, service utilities and infrastructures. In order to prevent/minimise post-construction damage and associated costly remedial measures resulting from the consolidation of the untreated soft subsoils, it is essential to conduct a geotechnical site investigation including subsurface exploration and testing for any development proposal on such sites. If the subsoil contains a thick deposit of soft clay, it is prudent and economic to improve the strength of the soft clays by one of the suitable soft ground improvement techniques prior to commencement of the actual development works.

In case, the geotechnical site investigation report identifies thick and soft clays underneath any development site and the proposed development requires significant filling (to elevate the site above flood level or for any other reasons), the applicant should demonstrate how the soft clays will be stabilised in order to minimise post-construction settlements.

3.1. Ground improvement techniques

There are a number of soft ground improvement techniques available in the literature and some of them are very effective and widely used by the practicing engineers and design consultants. This section provides brief information and guideline regarding some popular and widely used soft ground improvement techniques.


vii vii


3.1.1. Preloading

Preloading (or pre-compression) is one of the inexpensive and effective methods to improve a site containing soft and compressible clays. Preloading is very effective on normal to lightly over-consolidated silts and clays. If the soft deposits are thick and not intercepted by alternating sand seams, the preloading alone may not be an effective method to consolidate the soft clay layer. In this case, the use of sand drains or prefabricated vertical drains (PVD’s) in addition to preloading would be a better technique to improve the soft subsoils.

Usually, the preload surcharge is greater than the estimated weight of the proposed structures so that post-construction settlement becomes negligible and remains within tolerable limits. The placement of preload surcharge would increase the total stresses as well as pore water pressures in the soft deposits. Over time, usually after a couple of months to years, depending on the permeability and length of the 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 may be unacceptable to many construction projects. In order to accelerate the consolidation process, preloading is often supplemented by vertical drains (sand drains or PVD’s), as discussed below.

3.1.2. Preloading with vertical drains

In preloading, consolidation process usually takes a long time because the pore water pressure dissipates in one direction – vertical only. For a thick, soft and saturated cohesive subsoil layer, the length of this vertical drainage path can be substantially long, leading to longer consolidation time. By installing closely spaced vertical drains (for example, sand drains or PVD’s) through the soft deposits, the length of drainage paths will be significantly reduced as the pore pressure will now dissipate horizontally towards the adjacent vertical drains. Consequently, the consolidation time will be significantly reduced compared to preloading without vertical drains.

Figure 2 shows the typical preloading concept and the benefits of preloading with vertical drains in terms of reduced consolidation time.


viii viii


Figure 2 – Preloading for soft ground improvement (a) Preloading concept (b) Preloading without vertical drains (c) Preloading with vertical drains (d) Time for consolidation settlement with and without vertical drains

(a)

(b)

(c)

Time

Settl

emen

t

Without vertical drains

With vertical drains

Time saving with vertical drains

(d)

The preloading technique with vertical drains, particularly with PVD’s, is now widely used to consolidate and improve grounds containing soft and saturated cohesive soils. The following sections provide a brief overview regarding sand drains, PVD and lime/cement stabilisation.

Surcharge Surcharge

Without drains With Vertical Drains

Soft ground consolidates under preloading


ixi x


3.1.3. Sand drains

Sand drains typically range from 150 to 750 millimetres in diameter. Sand drains are usually installed by one of the following methods: • mandrel-driven pipes

A pipe is driven using a closed mandrel. Sand is poured in the pipe which 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 procedure similar to the mandrel-drivel pipes;

• rotary drill A borehole is drilled using the rotary drill method and then the borehole is filled with sand to create the sand drain;

• continuous flight hollow auger A borehole is drilled using continuous flight hollow auger method and then the borehole is filled with sand.

Figure 3 refers to two (2) methods of installing sand drains (after Bowles, J. E., Foundation Analysis and Design, 4th Edition, McGraw-Hill Incorporated 1988).

Figure 3 – Mandrel-driven pipes and continuous flight hollow auger methods

3.1.4. Prefabricated vertical drains

In recent times, prefabricated vertical drains (PVD’s) are becoming more economic, popular and are widely used throughout the world to accelerate consolidation of soft clays under preloading. Many researchers indicate that PVD’s are five (5) to eight (8) times cheaper than sand drains. A PVD is a thin and flexible band drain consisting of a polymer or plastic grooved core wrapped by a geotextile layer, as shown in Figure 4.


xx


Figure 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 typically varies from 100 to 300 millimetres with the thickness from three (3) to six (6) millimetres. The PVD’s are available in rolls and can be installed very fast (for example, less than a minute for one PVD installation).

Figure 5 shows typical installation of PVD’s on a site using special installation rigs.

Figure 5 – Installation of prefabricated vertical drains


xi xi


3.1.5. Stabilisation with lime/cement

Lime and/or cement columns are installed by deep mixing methods (DMM) and they have been used in stabilising soft clays in many parts of the world, particularly in Sweden, Finland, Norway and Japan. 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 and/or cement through the process of ion exchange, flocculation and pozzolanic reactions gives an increase 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 the olden times. The use of cement columns, using cement powder, has been reported to be successful in improving soft soils in early 1980’s. Lime and cement have been increasingly used as soil stabilising agents since the mid 1980’s. The DMM was originally developed to improve soft grounds for the ports and harbour structures. The use of this method has now been extended to the foundation of embankments, buildings and storage tanks. Further details of lime/cement stabilisation technique can be found in any ground improvement reference book such as Soft Ground Improvement in Lowland and Other Environments by D.T. Bergado, L.R. Anderson, N. Miura, and A.S. Balasubramanium, ASCE Press (1996).

4. DEVELOPMENTS INVOLVING DEEP EXCAVATIONS

For any development application such as high-rise building or multi-storied residential or commercial complex that requires deep excavation for the construction of in-ground basement car park or any other facilities, it should be supported by a Geotechnical Report demonstrating the stability of the proposed excavation including any temporary and/or permanent retention system/structures. The Geotechnical Report should include subsoil information and groundwater conditions of the site. The report should provide a detail stability assessment (including calculations) for the proposed deep excavation demonstrating that it will achieve a factor of safety of at least 1.5 against geotechnical failure, and that the proposed basement excavation/construction will not cause any adverse impact on the stability and integrity of the adjoining properties/structures. The assessment should also provide the effects of any dewatering on potential settlements and lateral movements of the adjoining properties/structures. The stability assessment should be based on site-specific geotechnical information and the proposed/selected excavation retention system. The basement excavation stability assessment shall be carried out using specialised geotechnical software such as WALLAP, PLAXIS etc.

The following section provides brief information and guideline regarding deep excavation, typical retention system and stability requirements.

4.1. Deep excavation retention system

All deep excavations associated with any proposed development must be stable and adequately protected against any potential sliding, rotational and base failure. The stability of deep excavation is absolutely necessary to ensure safe construction within the excavated area and also to maintain the stability and integrity of the adjoining buildings, properties, utility services and infrastructures. Unless the excavations are made with safe batters, which are often impossible due to very limited setback for the proposed in-ground basement from the property boundaries in most high density urban areas, the stability of deep excavations may be provided by structural retention system, either temporary or of permanent type, such as:

• sheet piling (steel, concrete or timber) • soldier piles and shotcrete panel walls • drilled and cast-in place concrete piles (contiguous piles, secant piles) • diaphragm walls.

The structural system for supporting the excavation faces can be of three (3) types:

• cantilever • anchored • braced.


xii xii


4.2. Stability assessment of deep excavation

The following are the possible modes of failure for a deep excavation supported by a structural retention system. Stability and factor of safety against these possible failure modes should be assessed.

• Flexural failure of the retaining structural element The retaining structural elements may fail due to the flexural stresses caused by the maximum bending moment exceeding the yield strength of the material.

• Tensile failure of anchor rod or tendon (for anchored retention system) The anchor rod or tendon may fail in tension if the tensile stress exceeds the yield strength of the rod or tendon.

• Pull out failure of anchor (for anchored retention system) The anchor may fail if the tensile force in the anchor exceeds its pull out resistance.

• Compressive failure of bracing elements (for braced retention system) The bracing elements may fail in compression if the compressive stress exceeds the yield strength of the bracing material.

• Rotational failure (for cantilever retaining walls) The retaining wall may become unstable or fail in rotation if the resisting moment caused by the passive resistance within the embedment depth becomes less than the driving moment caused 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 stability analysis should be carried out where the trial slip circles will pass through the toe of the retaining structure or sheet pile.

• Base failure (by upward movement of excavation base) The pressure loss due to the excavation may result in a base instability, where the soil flow beneath the sheeting into the excavation, producing a rise in the base elevation commonly termed as ‘heave’. This can be analysed using ‘Mohr’s circle’ or ‘bearing capacity theory’.

The stability assessment should demonstrate that the proposed shoring/retention system for supporting the basement excavation will be stable enough with a factor of safety not less than 1.5 against failure.

The applicant should also assess suitability of the proposed basement excavation methodology and examine whether the basement excavation support system requires any ground anchoring into any adjacent properties or road reserve. If ground anchoring is proposed to penetrate into any adjacent Council maintained road reserve, the applicant needs to obtain a separate permit from Council to interfere with a road – temporary ground anchors (Subordinate of Local Law 11.1, section 5). Please refer to: http://www.goldcoast.qld.gov.au/documents/fa/fm673-permit-to-interfere.pdf.

The applicant should note the following in case ground anchoring is required into any adjacent private property or State controlled road/reserve:

• the installation of any ground anchors into any adjacent private property will require consent of the relevant property owner(s) and is not assessed or approved by Council

• the installation of any ground anchors into any adjacent State controlled road/reserve will require a Road Corridor Permit from the Department of Transport and Main Roads, and is not assessed or approved by Council.

In addition to the abovementioned stability assessment, the applicant should provide a site-monitoring plan for the entire construction period and a post-construction period of at least three months in order to ensure no adverse impact on the stability and integrity of the adjacent properties/structures. The monitoring plan should include plans and cross-sectional drawings showing the locations and parameters to be monitored, frequency of monitoring, threshold value of any parameter that will trigger immediate cessation of all site works in order to maintain the stability and integrity of the adjoining properties/structures. The applicant also needs to provide a contingency plan in case any instability of the adjacent properties/structures arises or is detected during the construction period.

http://www.goldcoast.qld.gov.au/documents/fa/fm673-permit-to-interfere.pdf


xiii xiii



All development applications involving deep excavations should be supported by a Geotechnical Certification from a RPEQ specialising in geotechnical engineering. The certification should be prepared using the standard pro-forma given in Appendix D and should be included with the Geotechnical Report.

The certification should confirm that:

• the proposed basement excavation and associated batters or supporting structures will achieve a factor of safety greater than or equal to 1.5 against failure

• the proposed basement excavation/construction including any dewatering will not cause any adverse impact on the stability and integrity of the adjacent buildings, properties, utility services and infrastructures.

5. DEVELOPMENTS INVOLVING BATTERS AND/OR RETAINING STRUCTURES

The proposed development often requires significant bulk earthworks including cut/fill batters and/or retaining structures to achieve the desired finished levels. The geotechnical stability of the proposed cut/fill batters and/or retaining structures should be assessed against potential sliding, rotational and slip circle failure. The stability assessment of the proposed cut/fill batters and/or retaining structures should be included with the Geotechnical Stability Assessment Report.

This section provides a brief guideline and Council requirements regarding stability assessment of cut/fill batters and retaining structures associated with any proposed development.

5.1. Stability assessment of batters

The stability assessment of all proposed cut/fill batters should be carried out following a conventional slip circle failure analysis method. In this type of analysis, a number of potential slip circles are assumed, and the factor of safety for each of the assumed slip circles is calculated. The minimum factor of safety amongst those assumed slip circles is considered to be the factor of safety for that designed batter. The accuracy of the stability assessment depends on the number of slip circles analysed and the calculation method followed.

One very important issue in the stability assessment of batters is the estimation of representative shear strengths for the constituting soil layers. In stability analysis of batters, the worst possible shear strengths of the soil layers during the design life of the batters should be used, rather than using the existing shear strengths of the soil layers. In other words, the shear strengths of soils in saturated conditions, in case there is a prolonged and heavy rainfall, with the highest estimated watertable should be used. Another potential worst case scenario for the stability assessment of batters adjacent to any water body is sudden drawdown of watertable. In this instance, the factor of safety for the sudden drawdown case should be calculated, rather than for the high water level condition.

The stability analysis of batters may be carried out manually, however, the use of professional softwares, such as SLOPE/W by Geoslope: www.geo-slope.com would be cost effective with much less computational efforts and time.

Figure 6 shows an example of slope stability analysis using SLOPE/W.

http://www.geo-slope.com/


xi vxi v


Figure 6 – Typical slope stability analysis using SLOPE/W

Material number 1: Unit weight: 15 C: 5 Phi: 20 Model: MohrCoulomb Material number 2: Unit weight: 18 C: 10 Phi: 25 Model: MohrCoulomb

5.2. Stability assessment of retaining structures

Geotechnical stability of all proposed retaining structures should be carried out against sliding, overturning and global slope instability through the geomaterials. The proposed retaining structures should also be checked against bearing capacity failure or excessive base settlements. Furthermore, the retaining structure itself must be adequately designed against any potential structural failure such as flexural failure or shear failure.

Figure 7 shows a typical retaining structure including lateral earth pressure distributions. The retained soil behind the retaining structure will exert active lateral earth pressure if the retaining structure allows lateral movement. Otherwise, lateral earth pressure at rest ‘Ko condition’ should be used during the design and stability assessments. The soil in front of the wall will provide passive earth pressure (resistance).

Figure 7 – Typical retaining structure and lateral earth pressure distributions


xvxv


For the proposed retaining structures the applicant should assess factor of safety against the following:

• sliding caused by the active earth pressure and resistance by passive earth pressure and frictional force at the base the retaining structure

• overturning about the toe (point O in Figure 7) as a result of the driving moment caused by the active 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

• global slope instability considering a number of large slip circles passing through the underneath of the retaining structure and the retained soils.

The stability assessment should ensure that all retaining structures will achieve a factor of safety (FOS) greater than or equal to 1.5 against sliding, overturning and global slope instability.


All development applications involving batters and/or retaining structures should include a certification from a RPEQ specialising in geotechnical engineering confirming that all cut/fill batters and/or retaining structures associated with the proposed development have been adequately designed to achieve a long-term factor of safety greater than or equal to 1.5 against geotechnical failure. The certification should be prepared using the standard pro-forma given in Appendix D and should be included with the Geotechnical Report.

6. PRESENTATION OF THE REPORT

The Geotechnical Stability Assessment Report should be written in such a way that it is regarded as a self-contained document, which does not require the reader to refer to any other documents including Council files, maps, drawings, previous applications or other reports (if any). If it does require referring to any other documents, it should include a copy of those documents as attachments.

The report should include, but not necessarily limited to, the following:

• a cover page with a title of the report, revision number, property address, real property description (lot and plan numbers), report reference number, author’s name and date

• the body of the report including the context within which the report was commissioned, the purpose of the report, geotechnical site investigation results, landslide risk assessment results, slope stability assessment results for cut/fill batters and/or retaining walls, soft ground improvement techniques and stability assessment of deep excavations (if any)

• any maps, plans, drawings, cross-sections referred to in the report • borehole records, laboratory and field test results • landslide susceptibility rating calculations • slope stability calculations for batters and retaining walls • basement excavation stability calculations • geotechnical certifications.

7. REFERENCES

City of Gold Coast: Gold Coast City Plan;

SMEC (2011): Landslide Susceptibility Assessment Report for the City of the Gold Coast, August 2011;

Australian Geomechanics Society (2007): Practice Note Guideline for Landslide Risk Management 2007, Journal of the Australian Geomechanics Society, Vol. 42, No. 1, March 2007.


xvi xvi


8. APPENDICES

Appendix A – Landslide susceptibility analysis form (extracted from the document: Landslide Susceptibility Assessment Report for the City of the Gold Coast, SMEC, August 2011)

LANDSLIDE SUSCEPTIBILITY ANALYSIS Analysis No. Location: Site No. Site Name: 1 Natural Surface Slope 9 Material in cutting

Site Level Factor Site Level Factor Less than 5 degrees L 0.1 High strength rock L 0.5 Between 5 and 15 degrees M 0.5 Medium strength rock L 1 Between 15 and 30 degrees M 0.8 Low strength rock M 1.2 Between 30 and 45 degrees H 1.2 Very low strength rock and soil H 1.5 More than 45 degrees M 0.8 Soil VH 2 2 Slope Shape 10 Cut slope support Site Level Factor Site Level Factor Crest or ridge L 0.7 Concrete wall L 0.5 Planar / Convex M 0.9 Crib wall M 0.9 Rough / Irregular H 1.2 Gabion wall M 1 Concave H 1.5 Rock wall H 1.5 3 Site geology Unsupported H 2 Site Level Factor Volcanic Extrusive rock H 1.1 11 Concentration of surface water Sedimentary rock M 1 Site Level Factor Low grade metamorphic rock M 1 Ridge L 0.7 High grade metamorphic rock L 0.9 Crest M 0.8 Volcanic Intrusive rock M 1 Upper slope M 0.9 4 Soils Mid slope H 1.2 Site Level Factor Lower slope H 1.5 Rock at surface VL 0.1 12 Wastewater Disposal Residual soil < 1m deep L 0.5 Site Level Factor Residual soil 1-3m deep M 0.9 Fully Sewered M 1 Residual soil > 3m deep H 1.5 Onsite disposal – Surface M 1.2 Colluvial soil < 1m deep H 1.5 Onsite disposal – Soak Pit/Trenches H 1.5 Colluvial soil 1-3m deep VH 2 Colluvial soil > 3m deep VH 4 13 Stormwater Disposal 5 Fill height Site Level Factor Site Level Factor All stormwater piped into road drainage L 0.7 None L 0.9 Rain water tank with overflows M 1 Less than 1m M 1.1 Stormwater discharge on site H 1.5 Between 1 and 3m M 1.3 Between 3 and 6m H 1.7 14 Evidence of instability More than 6m VH 2.5 Site Level Factor 6 Evidence of groundwater No sign of instability L 0.8 Site Level Factor Soil Creep H 1.2 None apparent L 0.7 Minor irregularity VH 2 Minor moistness M 0.9 Major irregularity VH 5 Generally wet H 1.5 Active instability VH 10 Surface springs VH 3 Summary 7 Cut height Factor Site Level Factor 1 Natural Surface Slope None L 0.9 2 Slope Shape Less than 1m M 1.1 3 Site Geology Between 1 and 3m M 1.3 4 Soils Between 3 and 6m H 1.7 5 Fill Height More than 6m VH 2.5 6 Evidence of Groundwater 7 Cut height 8 Slope of cut face 8 Slope of Cut Face Site Level Factor 9 Material in Cutting Less than 30 degrees L 0.5 10 Cut Slope Support Between 30 and 45 degrees M 1 11 Concentration of Surface Water Between 45 and 60 degrees H 1.5 12 Wastewater Disposal More than 60 degrees VH 3 13 Stormwater Disposal 14 Evidence of Instability Relative Susceptibility (1x2x3x4x5x6x7x8x9x10x11x12x13x14)


xvii xvii


Appendix B – Correlation between relative susceptibility and susceptibility rating (extracted from the document: Landslide Susceptibility Assessment Report for the City of the Gold Coast, SMEC, August 2011)

Relative Susceptibility Susceptibility Rating

Less than 0.2 Very Low

0.2 – 0.6 Low

0.6 – 2.0 Moderate

2.0 – 6.0 High

Greater than 6.0 Very High


xviii xviii


Appendix C – Subdivision landslide encumbrance form

Property details

Address

Estate name

Estate stage

Council reference

Parent parcel of land

Lot number

Registered plan number

Encumbrance

Please use the following abbreviations to re-categorise the SMEC landslide risk rating. Encumbrances shall be defined in accordance with the following abbreviations for landslide risk rating:

VH = Very high H = High M = Moderate L = Low VL = Very low

Proposed subdivided allotments

Proposed lot number Plan number Relative susceptibility Final landslide risk rating for:

Lot Building pad Effluent disposal area

Geotechnical engineer details

Consulting engineering company

Name of engineer

Registered Professional Engineer of Queensland (RPEQ) number

Signature

Date


xi xxi x


Appendix D – Standard pro-forma for geotechnical certification

Property details Lot number

Registered plan number

Address

Proposed works

Description

Proposed development

Description

Declaration

I, Registered Professional Engineer of Queensland (RPEQ) number

of (Consulting engineer’s firm)

being duly authorised on this behalf, do certify that:

I am aware that the City of Gold Coast will rely upon this certificate and any associated geotechnical reports, maps, graphs, tables, attachments, etc, produced as a consequence of commissioning this technical assessment.

Signature Designation

Certified this Day of Year


#42081412v2 Last updated 14/11/2014 Page 20 of 24

City of Gold Coast P 1300 GOLDCOAST (1300 465 326) F 07 5596 3653 E [email protected] W cityofgoldcoast.com.au Chief Executive Officer City of Gold Coast PO Box 5042 GCMC QLD 9729

geotechnical stability assessment guidelines

Documents