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Engineering a change

New Zealand’s Royal Commission report into the earthquakes that devastated Christchurch recommended greater use of cone penetration testing. Carlton Hall reports on why this approach is being taken and how it is being delivered on site.

Pyne_Gould_building

The Pyne Gould building in Christchurch after the February 2011 earthquake

In the 16 months between September 2010 and December 2011, New Zealand’s South Island city of Christchurch was hit by four devastating happening in the middle of the day on 22 February 2011. The 6.2 magnitude earthquake was shallow, at around 5km below the surface, with its epicentre in the Port Hills area which is 10km south east of Christchurch’s Central Business District (CBD).

The quake caused the catastrophic collapse of two large offices - the Canterbury Television building and the Pyne Gould Corporation building, killing 133 people. In all, 185 people died, many more were injured and large areas were affected by the liquefaction and lateral spreading of weak near surface alluvial soils. Hundreds of buildings have since been demolished and, with large sections of infrastructure also damaged, it is understood that rebuild costs are estimated between NZ$20bn and NZ$30bn (£10bn - £15bn).

The Canterbury Earthquakes Royal Commission was set up in 2011 to investigate how buildings performed during the earthquakes and to assess the adequacy of best practice for design, construction and maintenance of buildings according to seismic risk. The commission also made a number of recommendations on the measures that need to be taken to minimise building failures in the event of future earthquakes.

The first part of the Commission’s report was published in August 2012, a section of which made specific recommendations relating to geotechnical aspects.

These recommendations highlighted the need to establish detailed ground models that will properly support the design of buildings and infrastructure to withstand future earthquakes.

New_Zealand_working_2_011

One of the main recommendations was that “greater use should be made of in situ testing of soil properties by the cone penetrometer test (CPT), standard penetration test (SPT) or other appropriate methods”.

It is unsurprising that CPT has been identified as a key investigation technique. Developments in CPT applications and ever-more powerful data management systems over the last decade have resulted in it becoming a quick and versatile way of acquiring a range of ground data which, with skilful interpretation, can make a strong contribution to the definition of ground models.

Ground conditions in Christchurch consist of 20-25m of generally weak and poorly consolidated gravels, sands, silts and peat overlying the very thick dense gravels. The water table is shallow and often appears within 1.5m of the ground surface in the CBD and at approximately 5m depth further west.

The combination of weak soils, coupled with a high water table, means the regional ground is at high risk of liquefaction or excessive softening during a significant seismic event. As such, the commission recommended that buildings constructed in areas at risk should be founded on deep piles or shallow foundations after ground improvement has been carried out.

While CPT has been identified as an important investigation technique, the depth of penetration can be curtailed by the near surface Christchurch geology which contains pockets of dense gravel overlying weaker materials below.

It was with this in mind that Lankelma decided to transfer one of its 20t 6x6 wheeled CPT trucks from the Middle East to New Zealand in early 2012.

The truck is equipped with standard piezocone instrumentation plus a range of supplementary tools and provides a substantial reaction force which, when combined with a cased push rod support system and skilful operators, has proved a good match for the challenging ground.

Since May 2012, Lankelma has been working with New Zealand-based KGA Geotechnical Investigations and has completed more than 90 CPT investigations to date. The projects range across several sectors of the recovery process including bridges and piped services for infrastructure, commercial buildings, public buildings and residential developments.

The work programmes range from two weeks of CPT on a large expanse of new build to two to three hours work on a single residential property.

Most clients need an electronic dataset of measurements for use with ground modelling, parameter determinations and liquefaction assessments. Over this time, Lankelma has acquired good experience of the variations within regional ground conditions and the operators have developed a feel for what apparatus ‘works’ in what ground.

The type of schemes have included forensic investigations on behalf of insurers, liquefaction studies and depth mapping of the regional primary bearing strata known as the Riccarton Gravel, mapping and quantification of peat. Other work has involved piezometer standpipe installations for groundwater monitoring and ground model and parameter determinations for structural foundation assessment and general land development.

While piezocone is being deployed effectively in Christchurch and much data is being generated, there are other techniques available that can also help improve confidence in assessing liquefaction potential and other CPT data.

For example, the MOSTAPCPT sampling technique involves obtaining 1.2m long discrete samples down to about 15m. The samples can be used to provide laboratory sub samples for the testing of fines content and plasticity resulting in an enhanced soil behaviour assessment.

Seismic CPT provides shear wave velocity data which is used along with unit weight to generate soil stiffness values. Additionally, the data enables an alternative route to the assessment liquefaction potential. The technique gathers shear wave velocity data which is then processed to provide soil stiffness. This data can be used to enhance soil classification and behaviour.

While the benefits of CPT are clear, one issue that often arises, in Christchurch and elsewhere, is the need to maintain a high level of field quality control. This is critical to reduce the potential for error in the raw field data, which can then be carried through to analysis. For a start, maintaining instrumentation and ensuring it is fit for purpose is key. Rigorous field practice requires checks of cone wear to be undertaken and, when a cone is subject to excessive wear, it has to be taken out of use, serviced, re-calibrated.

Another issue Lankelma has come across in Christchurch is the potential for erroneous data when a cone exits from a dense layer into a weak layer as the standard 20mm/ sec push rate of CPT is too fast for the cone instrumentation to adjust. Pausing testing for five minutes enables the instrumentation to recover and maintain data quality (see diagram). Without a pause, data quality has been shown to drop, reducing confidence.

The Royal Commission has recommended that there should be greater cooperation and dialogue between geotechnical and structural engineers - and it is clear that the engineering community is working hard in this respect.

One example is the keenness of New Zealand’s geotechnical community to engage with Lankelma since it arrived in the country and raise the bar on CPT data confidence, which has been very encouraging.

  • Carlton Hall is engineering director for Lankelma

Canterbury Earthquake Royal Commission’s geotechnical recommendations

- A thorough and detailed geotechnical investigation of each building site, leading to development of a full site model, should be recognised as a key requirement for achieving good foundation performance.

- There should be greater focus on geotechnical investigations to reduce the risk of unsatisfactory foundation performance. The Department of Building and Housing should lead the development of guidelines to ensure a more uniform standard for future investigations, and as an aid to engineers and owners.

- Geotechnical site reports and foundation design details should be kept on each property file by the territorial authority and made available for neighbouring site assessments by geotechnical engineers.

- The Christchurch City Council should develop and maintain a publicly available database of information about the sub-surface conditions in the Christchurch CBD, building on the information provided in the Tonkin and Taylor report. Other territorial authorities should consider developing and maintaining similar databases of their own.

- Greater use should be made of in situ testing of soil properties by the CPT, SPT or other appropriate methods.

- The Department of Building and Housing should work with the New Zealand Geotechnical Society to update the existing guidelines for assessing liquefaction hazard to include new information and draw on the experience from the Christchurch earthquakes.

- Further research should be conducted into the performance of building foundations in the Christchurch CBD, including sub-surface investigations as necessary, to better inform future practice.

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