A Case Study: Trench Stabilization Using Cutter Soil

The Wynyard Edge Alliance (Auckland Council, the Crown represented by Ministry of Business, Innovation and Employment, Downer, McConnell Dowell, Beca, Tonkin and Taylor) was formed to design and construct the infrastructure to support the 36th America’s Cup event in Auckland. In addition to the core infrastructure required for the event, a wider programme of works is underway on the waterfront. One of these projects is the extension of the Daldy Street stormwater pipeline – the Daldy Street Outfall Extension (DSOE) which is a Healthy Waters project (a department of Auckland Council). The DSOE scope of works was included as part of the WEA scope of works in 2019. The DSOE works involves the extension of the pipeline from its current outfall position underneath North Wharf, to a new outflow position at the end of Wynyard Point. To complete this, 510m of High-Density Polyethylene (HDPE) pipe is to be placed beneath Brigham Street.

Cutter Soil Mixing (CSM) was adopted as the preferred ground improvement methodology to stabilise the site to enable the trench excavation. This paper intends to report the quality and performance of the panels produced by the Cutter Soil Mix method, as well as to compare different sampling methods, including Wet Sampling, Double Tube Sampling and core sampling from boreholes. It will also discuss the interlocking observed between two adjacent panels and discussions around the practicality of the CSM method in similar site conditions.