plan change 67 - geotechnical

Plan Change 67 - Geotechnical Karen HartelURS New Zealand Ltd.

Plan Change 67 Highfield Park - Geotechnical2

Purpose

•Review of the “Preliminary Geotechnical Assessment Report, Highfield Park” prepared by Golder Associates Ltd. for Highfield Park Ltd, dated September 2011;

- Assess whether the findings of the report are accurate, adequateand reasonable in the context of the proposed plan change;

- Comment on the implications and consequences of the proposed development from a geotechnical perspective.

Plan Change 67 Highfield Park - Geotechnical

Overview of Evidence

•Field Investigations

•Preliminary geotechnical model

•Liquefaction susceptibility assessment

•Ground improvement for liquefaction mitigation

•Proposed site grading

•Factors to consider relative to additional investigations

•Submitter’s concerns

•Summary

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Guidance Documents

•Department of Building and Housing (DBH) “Revised guidance on repairing and rebuilding houses affected by the Canterbury earthquake sequence”, November 2011

referred to as: DBH 2011 guidance

•DBH “Interim guidance for repairing and rebuilding foundations in Technical Category 3”, April 2012

referred to as: DBH 2012 guidance

•Ministry of Business, Innovation and Employment (Ministry) “Guidelines for the investigation and assessment of subdivisions on the flat in Canterbury”, September 2012

referred to as: Ministry 2012 guidance

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Field Investigations

Minimum requirements for assessing liquefaction for land development in Canterbury at the Plan Change stage (Reference: Ministry 2012 guidance)

- Minimum depth of exploration = 15 m

- Minimum density = 0.2 to 0.5 exploration locations per ha

Therefore, minimum density required for the Highfield Park site:

- 50 to 130 exploration locations

Existing Investigations meeting minimum requirements:

- 26 CPTs

- 2 BH

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Field Investigations

Comparison with Plan Change guidance:

•Existing investigations adequate to demonstrate that subsurface conditions across the site are susceptible to liquefaction

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Preliminary Geotechnical Model

The site geologic setting has been adequately and accurately described;

Existing subsurface data confirms the general geologic model;

Stratigraphy in the upper 15 m of the soil profile is characteristically variable;

? Characterisation of soil profile in upper 15m:

• Based on interpretation of CPT data and shallow test pits;

• Soil behaviour type index provides a general indication of material type;

• Sample recovery (visual logging and laboratory tests) required for positive soil classification;

• Distinction between silt and clay;

• Potential presence of peat;

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Preliminary Geotechnical Model

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CPT20


Liquefaction Susceptibility Assessment

Preliminary liquefaction assessment suggests areas of TC2 and TC3 will predominate;

? Preliminary liquefaction assessment based on analysis of CPT data. Analysis relies in part on the soil behaviour type index to determine liquefaction susceptibility

• Clays: not liquefaction susceptible

• Silts: potentially liquefaction susceptible

• South of Preston’s Road soils modelled as clay

Comparative analysis at CPT20

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Liquefaction Susceptibility Assessment

Vertical ground deformation based on comparison of LiDAR survey data

Reference: https://canterburyrecovery.projectorbit.com June 2012

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Ground improvement for Liquefaction Mitigation

Some form of ground improvement will be required to mitigate the effects of liquefaction;

Many techniques are available - feasibility depends on:- Soil type- Fines content- Plasticity- Relative density- Depth

Additional site investigation data will be required to determine the distribution and nature of the required ground improvement;

? Proposed ground improvement strategies are presented based on preliminary liquefaction assessment.

- general strategy presented based on assumed non-liquefiable crust

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Ground Improvement for Liquefaction Mitigation

Applicability of Guidance documents

•DBH 2012 – “Interim guidance for repairing and rebuilding foundations in Technical Category 3”

C5 New Foundations in TC3

C5.1 Foundation types and selection considerations

This section covers foundations for new houses as well as situations where foundations are completely rebuilt for existing houses in TC3

•DBH 2011 – “Revised guidance on repairing and rebuilding houses affected by the Canterbury earthquake sequence”

Part A

Chapter 5 New Foundations

5.1 GENERAL

This section covers both foundations for new houses and situations where foundations are completely rebuilt for existing houses in the Green Zone on the flat.

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Ground Improvement for Liquefaction Mitigation

New foundations - TC3

Deep piles

Site ground improvements (shallow and deep)

Surface structures with shallow foundations

-Foundation design depends on:

• Magnitude and distribution of vertical settlement

• Magnitude of lateral spreading

• Type of construction

Deep treatments (and/or deep foundations) may be required

New foundations – TC2 Various options available

-Foundation design depends on:

• Bearing capacity of underlying soils

• Type of construction

Peat and soft clay - specific design may be required

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Proposed Site Grading

•Up to 3 m fill proposed south of Preston’s Road;

•Consolidation settlement of underlying soft clays and peat;

•Longer-term post-construction settlement:

- Overlying structures

- Roads

- Utilities

- Adjacent structures

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Factors to Consider Relative to Additional Investigations

•Boreholes to minimum depth where CPTs are infeasible;

•Boreholes collocated with CPTs

- Correlation of CPT interpreted soil behaviour type with visual logging and lab testing of samples from boreholes

- Determine extent and consistency of clay, silt and organic soils

•Additional investigation and liquefaction assessment to delineate land classification zones (TC2, TC3);

•Define engineering characteristics

- Liquefaction

- Consolidation

- Bearing capacity

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Submitter’s Concerns

•Submission 62 raises concerns relative to geologic suitability and comments on geotechnical issues:

- Broad land classification

- Liquefaction susceptibility of site soils

- Lack of field investigation data

- Potential presence of peat

- Necessity for widespread mitigation

- Design groundwater elevation

- Potential impact of soil improvement on adjacent untreated properties

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Summary

•Highfield Park is geotechnically viable for residential subdivision under the current design criteria for residential developments on liquefaction susceptible soils, given appropriate engineering preparatory works.

•Further geotechnical investigation and analysis is required to:- refine the ground model;- delineate the extent of potentially liquefaction susceptible soils;- determine broad land classification;- determine technical feasibility of ground improvement methodologies;- determine technical feasibility of proposed areal fill on the southern portion

of the site;- address bearing capacity considerations

•Technically feasible engineering solutions exist that can be applied to the site to mitigate the geotechnical issues presented by the challenging subsurface conditions. Additional information is required to determine the feasibility of specific engineering solutions.

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