Innovation FEATURE SECTION On good ground EQC is sponsoring innovative ground improvement techniques that will not only strengthen land in Christchurch to allow resilient rebuilding but may offer a solution for vulnerable land in other parts of the country. BY NICK HELM, FREELANCE WRITER, TENPOINT COMMUNICATIONS improvement methods that could be used to strengthen the upper few metres of land to reduce the liquefaction vulnerability,’ says Dr Hugh Cowan, General Manager Reinsurance, Research and Education at EQC. The work, which was completed in 2013, was led by Dr Sjoerd Van Ballegooy of the geotechnical engineering firm Tonkin & Taylor and supported by a number of research partners, sponsors and contractors from New Zealand and the United States. ‘The second stage, which is currently under way, is a land repair pilot programme, which involves installing those methods in real-life situations where people are repairing or rebuilding their homes.’ Four strategies identified Stone columns before blast trial. Simply reinstating the original ground level using fill had been ruled out in earlier ONE OF THE LASTING EFFECTS of the Strengthening liquefaction-prone land research, so EQC focused on trialling tech- Canterbur y e ar thqu ake s i s the land In response, EQC launched a programme to niques that could stiffen and densify the damage. The major events, particularly the investigate several unconventional ground upper soil and restore its strength and perfor- 22 February quake, caused tectonic subsid- remediation techniques in an effort to find mance, despite the lower overall ground level. ence and severe liquefaction in many parts practical, cost-effective and consentable ‘Most of the techniques were adapted from of eastern and central Christchurch. The methods to strengthen residential land that methods used on large-scale civil construc- subsidence meant that the non-liquefying is vulnerable to liquefaction tion projects and scaled down for residential soil layers had become much thinner, ‘The first stage of the Land Improvement properties in Christchurch,’ says Dr Cowan. increasing the vulnerability to liquefaction Programme involved testing in the residen- The first method, rapid impact compac- damage in future earthquakes. tial red zone to evaluate different ground tion, uses a falling weight to repeatedly Build 143 — August/September 2014 — 53 FEATURE SECTION Innovation compact the ground surface. This improves ‘The levels of shaking in the soil directly ground density down to several metres and beneath the T-Rex machine were strong pilot − 18 in Christchurch and 10 in Kaiapoi. is particularly suited for sandy soils in areas enough to liquefy the unimproved ground, ‘We are about to start installing a ground where there is adequate distance from whereas for most of the methods, greater improvement method on a number of prop- neighbouring buildings. This stage is coming to a close,’ he says. levels of shaking were required to trigger erties with repairable houses. This work Low-mobility grout involves injecting liquefaction. For some ground improvement uses horizontal soil mixing, which can be concrete bulbs into the ground to squeeze the methods, the T-Rex machine was unable to used to improve land under existing houses surrounding soil and increase the soil density. trigger liquefaction, even at the strongest without the need to jack the building up or levels of shaking,’ he says. temporarily remove it,’ he says. Short stone columns work on a similar principle by pushing gravel into the ground ‘Explosives were also used to induce large- ‘There will be several properties in this instead of concrete to improve ground scale liquefaction to test the performance part of the pilot, and the work will go on density and also stiffen the soil. The tech- of the ground improvements overlying a over the next few months.’ nique offers advantages on sandy sites, but thick layer of liquefied soil. The stiffened it also requires a clear site. and strengthened upper soil layers for most Will inform guidance document Fo r s i t e s w i t h v i b r a t i o n o r s p a c e of the ground improvement methods miti- Once the pilot is complete, the lessons constraints, soil cement mixing can be used. gated the occurrence of liquefaction ejecta learned from applying these methods Where the structure is still in place, hori- over the improved areas and also reduced will become publicly available. EQC is zontal soil mixing can be used. This creates the differential ground surface settlement working with MBIE to refine the ground horizontal cemented columns in the target compared to the natural unimproved soil.’ improvement method specification and layer that confines the soil and suppresses However, the trials showed low-mobility incorporate it into Guidance: Repairing and soil deformation during earthquake shaking. compaction grouting did not perform as rebuilding houses affected by the Canterbury expected and was unlikely to be successful earthquakes. Quakes recreated to test methods as a shallow-ground improvement method in ‘This means homeowners in Canterbury ‘Test sites were selected from a careful analysis a scaled-down residential application. This can apply these methods and have confi- of geotechnical information to ensure that technique was not used for the pilot. dence in the way their land, and the the engineering properties of the soil layers buildings on them, will perform in future closely matched the majority of soil conditions Moving on to real-world remediation that would be encountered in the rebuild ‘The first pilot programme focused on bare ‘It also means the methods will be avail- where ground improvements would be most land properties, where the house has been able throughout New Zealand if homeowners relevant,’ says Dr Cowan. demolished and needs to be rebuilt. We’re or developers want to build on land that has The effectiveness of the remediation was using stone columns, driven timber piles, in similar characteristics to the sandy and silty tested in two ways − controlled blasting and situ soil mixing and gravel raft construction soil areas in Christchurch.’ an earthquake simulator known as T-Rex. on these sites. There are 28 properties in the non- liquefiable crust non- liquefiable crust liquefiable soil earthquakes,’ says Dr Cowan. For more Visit www.eqc.govt.nz/canterbury. gravel liquefiable soil Rapid impact compaction 54 — Build 143 — August/September 2014 trench non- liquefiable crust liquefiable soil Stone columns Horizontal soil mixing