NZ Geomechanics News

Guidance for the Specification of Ground Improvement

“Geotechnical earthquake engineering practice, Module 5A – Guidance for the Specification of Ground Improvement for Residential Properties in the Canterbury Region”, has recently been jointly published by the NZGS and Ministry of Business Innovation & Employment (MBIE), reports Neil Korte (Project Director, Tonkin & Taylor).

This is the first publication produced under a Memorandum of Understanding between NZGS and MBIE to support the development of a series of earthquake geotechnical engineering guidance documents.  Module 1 has now also been issued and a number of other guidance documents will be progressively released over the next 12 months.

Module 5A was prepared by a collaborative panel of geotechnical engineers and contractors from a wide variety of companies together with other key stakeholders involved in the Christchurch rebuild.  A draft of the document was issued for general industry comment prior to publication in November.

The intention of the document is to provide best practice guidance on what should be included in a contract technical specification for constructing ground improvement (for liquefaction mitigation purposes) for four of the most common ground improvement techniques being implemented in greater Christchurch as part of the Canterbury recovery.  It is intended that the guidance is used for small scale ground improvement works which are typically required for single residential sites (typically approximately 500 m2 in plan area).

The methods covered by the document have the same meaning and types as defined by the MBIE Guidance Document[1], i.e.

  • Densified Crust, Type G1a (excavate and re-compact) and Type G1d (reinforced crushed gravel raft).
  • Shallow Cement Stabilised Crust, Types G2a (ex-situ mixing) and G2b (stabilised in-situ mixing).
  • Stone Columns, Type G4 (deep stone columns) and Type G5a (shallow stone columns).
  • Driven Timber Poles, Type G5b.

The document includes a Preliminary & General specification (Section 1), a Testing specification (Section 2), a General Earthworks specification (Section 3), four technical specifications for the different ground improvement options covered and a section on Project Specific Requirements (Section 8) that might need to be included in a comprehensive set of contract documents.  It is intended that geotechnical practitioners will use various aspects of the document as appropriate for they particular application and need.  Large residential or commercial developments may have different or additional construction requirements not covered by this guidance document.

The ground improvement techniques in the guideline intend to support methods described in the MBIE Guidance Document1 for improving ground within Technical Category (TC) 3[2] sites in Canterbury. The ground improvement methods provide an integrated foundation solution consisting of both the improved ground and the enhanced surface foundation (constructed in accordance with TC2 foundation options as outlined in Section 5 of the MBIE guidance), to provide minimum acceptable performance for the supported superstructure.    The ground improvement methods provide a means to improve resilience against the effects of future liquefaction – such as reduced differential settlement across the residential building footprint at the ground surface.

The main objectives and benefits of producing the guidance document are to:

  1. Provide a guidance document that is available for use by individuals and organisation involved in designing and constructing ground improvement works.
  2. Lower overall industry costs to design ground improvement works as standard specifications are readily available.
  3. Lower overall construction costs as ground improvement techniques, specifications and materials are standardised.
  4. Standardise and improve consistency in ground improvement design and construction.
  5. A supporting document to the MBIE Guidance Document.

Module 5A also includes a set of Guidance Notes to provide background information to help users understand the principles behind certain clauses in the specification and to assist contract specification writers in the development of appropriate specifications, depending on the type of ground improvement chosen and the location of the site.

The document is freely available on both the NZGS and MBIE websites:

Neil Korte
Neil Korte

[1] “Repairing and rebuilding houses affected by the Canterbury earthquakes” (Chapter 15.3 update issued in April 2015)
[2] MBIE technical category (TC) as outlined in the MBIE Guidance Document

NZ Geomechanics News
Neil Korte
NZ Geomechanics News>Issue 91 - June 2016

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