Use Of Deep Soil Mixing for Excavation Retention And Groundwater Control

Several buildings at St George’s Hospital were damaged as a result of the 2010/2011 Canterbury earthquake sequence. Four new buildings are being constructed, which when completed, must remain operational following a similar seismic event.

Arup provided design services to Hiway Geotechnical, for the use of Deep Soil Mix (DSM) columns below the buildings to mitigate the effects of liquefaction and provide support and groundwater cutoff for a 4m deep basement excavation. With a groundwater table within 1.0m of the ground surface and a ground profile consisting of loose sands which were susceptible to instability and piping, the DSM columns provide an alternative to typical sheet pile solutions.

Based on previous research by Arup, the design also made use of ground improvement effects in the soils between the DSM columns. On-site trials and testing verified the ground improvement, enabling cost savings compared with traditional DSM column layouts and other ground improvement solutions. DSM columns also provided advantages over sheet pile which have installation issues and don’t provide the same level of versatility.

The adoption of DSM enabled several design issues to be addressed with one construction technique, providing construction cost and programme time savings.

This paper presents the main geotechnical challenges for the site, describes how various elements of the DSM columns were designed to address these issues and summarises site observations and performance during construction including observations of wall movement.