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Ground Improvement Programme

The Canterbury Earthquake Sequence of 2010-2011 included four significant earthquakes that triggered widespread liquefaction and land damage varying in severity throughout the region.

Liquefaction occurs when soil below the groundwater level temporarily loses strength when shaken. This can cause the soil to liquefy, resulting in water, fine sand and silt ejecting to the surface. This places huge stress on buildings on top of liquefied ground.

The Ground Improvement Programme was an EQC-led research programme which informs more affordable and practical ways of making residential land less vulnerable to liquefaction.

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Collaborative research

The world-leading collaborative research was undertaken as part of EQC’s role of facilitating research and education that increases New Zealand’s natural disaster resilience.

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Co-ordinated by Tonkin + Taylor for EQC, the programme had contributions from many organisations and leading experts in liquefaction from New Zealand and overseas, including:

  • University of Canterbury
  • Ministry of Business, Innovation and Employment
  • Housing New Zealand
  • New Zealand Transport Agency
  • University of Texas at Austin, (USA)
  • Oregon State University (Corvallis, USA)
  • Brigham Young University (Provo, Utah, USA)
  • US National Science Foundation
  • US Network for Earthquake Engineering Simulation.

The Ground Improvement Programme has identified, developed and trialled affordable and practical shallow ground improvement methods which can be used to strengthen residential land. This research is useful to property owners and developers, engineers and builders, private insurers, local authorities and central government agencies. The findings of the research can be applied throughout New Zealand and globally.

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Ground Improvement Trials

The first workstream included a range of ground improvement methods tested in Canterbury to see if they could successfully be applied to residential ground improvement.

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The effectiveness of the methods was tested in two ways.

  • The earthquake simulator or T-Rex
  • Ground blasting

Geotechnical engineers, Tonkin + Taylor guided these trials with support from a global team including experts from the University of Canterbury, Cornell University, UC Berkley, University of Texas and other New Zealand engineering consultancies.

The trials component of the programme finished in December 2013.

You can findout more in our Residential Ground Improvement: Findings from trials to manage liquefaction vulnerability PDF (6.5 MB)

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Ground Improvement Pilot

In the second workstream EQC worked with a number of insurance companies including Tower, IAG, Lumley, Vero, Southern Response and AA to investigate how practical it was to use the trialled methods that were identified as promising in real-life situations. 

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The aim of the pilot was to understand the practicalities of implementing the methods by coordinating the various parties involved, such as property owners, EQC, insurance companies and local authorities. This work was tendered out on a competitive basis to determine the affordability of the methods and to assess local contractor capability.

In all, six different ground improvement solutions were constructed on 31 residential green zone properties and two residential red zone properties in Canterbury. Watch the videos below:

Horizontal Soil Mixing - Ground Improvement Trial

  • only method that can be installed directly underneath an existing home (i.e the site isn’t cleared)
  • takes a minimum of two weeks
  • requires specialist equipment and contractor knowledge

Ground Improvement: Stone columns (cleared section only)

  • takes approximately one week
  • requires stockpile area and good front road access
  • requires specialist equipment

Ground Improvement: Driven timber poles (cleared section only)

  • takes less than one week
  • does not require specialist equipment
  • requires stockpile area

 

Ground Improvement: Deep gravel raft (cleared section only)

  • takes less than one week
  • does not require specialist equipment
  • inclement weather can cause delays

 

Ground Improvement: Cement soil mixing (cleared section only)

  • takes approximately one week
  • requires specialist equipment
  • no stockpile required

 

Ground Improvement: Rotovated soil mixing (cleared section only)

  • takes approximately one week
  • does not require specialist equipment
  • requires stockpile area
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Related documents

A list of factsheets and findings from the programme of work.

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Questions and answers

A list of questions and answers on Ground Improvement Programme

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Public and homeowner related questions

Q. How much will these repairs cost?

For many sites MBIE endorsed ground improvement and TC2 foundation combinations will be more affordable than TC3 foundations. The cost vary depending on site specific requirements, a site specific assessment should be undertaken by an engineer to determine the most appropriate and cost effective solution for the property.

Q. Why is there only one method for properties with existing buildings – one that’s expensive and not suitable for all properties?

There are limited options available internationally to construct ground improvement beneath an existing building. Cement based permeation grouting, used in specialist commercial applications world-wide, was found to be unsuccessful in the Christchurch soils. One innovative method termed Horizontal Soil Mixed (HSM) beams was developed during the Science Trials that can reduce the damaging effects of liquefaction damage in a future earthquake. HSM beams require further technological investment to become practically applicable and affordable for widespread residential use.

Q. How aligned is GIP’s research with EQC’s land repair policies, such as Increased Liquefaction Vulnerability (ILV) and Increased Flooding Vulnerability (IFV)?

The ground improvement programme informed EQC’s understanding of technically viable and cost-effective repair solutions, but is otherwise independent of EQC’s insurance settlement policies.

Q. How will this research be adopted by the insurance industry?

The GIP has shown that almost all TC3 property in Canterbury has an applicable cleared site ground improvement solution. The MBIE guidance specifies which foundation systems and ground improvement methods are appropriate to be used on various soil profiles. A geotechnical engineer is consulted to determine the appropriateness of any method for a particular property.

Q. The report says these repair methods will reduce a building’s foundation costs. By how much?

MBIE endorsed ground improvement solutions enable the use of lighter TC2 foundations as an alternative to robust TC3 surface structure foundations. TC2 foundations are more affordable and when combined with ground improvement allow the use of a wider range of cladding and roofing systems. Use of ground improvement improves the performance of the land, reducing the likely need for re-levelling in a future earthquake.
The cost varies depending on site specific requirements, a geotechnical site specific assessment should be undertaken by an engineer to determine the most appropriate and cost effective solution for the property.

Q. My property does not qualify for ILV land damage but had liquefaction. Do you recommend I use one of these repair methods anyway?

The GIP has shown that almost all TC3 property in Canterbury has an applicable cleared site ground improvement solution. The MBIE guidance specifies which foundation systems and ground improvement methods are appropriate to be used on various soil profiles. A geotechnical engineer must be consulted to determine the appropriateness of any method.

Q. How much has the Ground Improvement Programme cost EQC?

The programme budget is $8 million, which has been funded by EQC, the Ministry of Business, Innovation and Employment, Housing New Zealand and the National Science Foundation in the United States. Canterbury Earthquake Recovery Authority and local authorities also provided input.

Q. What did the research involve?

The research involved a science trial, which tested how a range of repair methods performed during simulations of earthquakes. Based on the findings of the science trial, well performing repair methods were then piloted on residential homes in TC3 areas of Christchurch and Waimakariri District to assess real-world application. The pilots were carried out in collaboration with private insurers and enabled interested parties to better understand the design, consenting and construction requirements of using the methods.

Q. What techniques were identified?

Five methods emerged from the trials as viable and potentially affordable for dealing with land vulnerable to liquefaction in a residential setting. Four methods apply to cleared sites:
• Stone columns
• Driven timber poles
• Reinforced gravel raft
• Reinforced soil-cement raft
Horizontal Soil Mixed Beams is a method that may be able to be applied under certain conditions to properties where there is still a house on site.
More information can be found about these techniques on the EQC website at: http://www.eqc.govt.nz/canterbury-earthquakes/land-claims/land-improvement-programme/ground-improvement-pilot

Q. Why has EQC funded the trials when it may not use the findings?

The research was carried out as part of EQC’s role of funding research that will increase New Zealand’s resilience to the effects of natural disasters. The results of the research are potentially extremely useful to a number of parties interested in building on land prone to liquefaction in New Zealand and internationally. These include including property owners, private insurance companies and local authorities. Through this research, the pool of contractors available to carry out these works has also been increased.

Engineer and building industry related questions

Q. Have other countries shown any interest in these types of remedies?

New Zealand has a long history of working collaboratively with the U.S.A in geotechnical earthquake engineering science; much of this work has been facilitated by the Earthquake Commission.
In undertaking this ground improvement research EQC was able to convene the involvement of some of the world’s leading experts on liquefaction which included the University of Canterbury, the Ministry of Business, Innovation and Employment, Housing New Zealand, the New Zealand Transport Agency, the University of Texas at Austin, (USA), Oregon State University (Corvallis, USA), Brigham Young University (Provo, Utah, USA), the US National Science Foundation, and the US Network for Earthquake Engineering Simulation.

Q. Will these repair methods match with recommended TC3 foundation designs?

All methods in the MBIE guidance are compliant with the New Zealand Building Code. For many sites MBIE endorsed ground improvement and TC2 foundation combinations will be more affordable than TC3 foundations. Although the two systems provide similar levels of building performance, ground improvement with a TC2 foundation also improves the performance of the land, reducing the likely need for re-levelling in a future earthquake. Ground improvement also allows for the use of a wider range of cladding and roofing systems.

Q. These methods seem to favour bare land only. Why is this?

There are limited options available internationally to construct ground improvement beneath an existing building. Cement based permeation grouting, used in specialist applications world-wide, was found to be unsuccessful in the fine grained soils susceptible to liquefaction which underlie many parts of Christchurch. As a result, an innovative method was developed during the Science Trials, which was termed Horizontal Soil Mixed (HSM) beams. This method is not widely applicable due to various construction constraints and its high cost, with few experienced contractors able to undertake the work.

Q. How rigorous has the peer review and testing of these methods been?

Between April 2013 and April 2015 the ground improvement science trial data, assessment and conclusions have been peer reviewed by leading national and international experts in earthquake engineering and liquefaction mitigation.

Q. The report says you worked with the insurance industry on the trials, what feedback have you had from them?

All participants found the collaborative approach to planning and engagement very positive and beneficial for the rebuilding of more resilient homes.

Q. Is there any continuation with research on this?

EQC and the US National Science Foundation are currently funding some follow up studies to further improve teste methods for predicting soil behaviour in areas susceptible to liquefaction.

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