Geostabilization New Zealand Ltd (GeoStabilization NZ) was engaged by Connell Contractors following issues with traditional sheet pile and dewatering techniques for the drainage pipe installation as part of the replacement Hamilton Western Interceptor. This project forms part of the local council’s $2B, 10-year resilience and future growth program to improve the city’s infrastructure.
High water table and fluid silt materials were causing excessive settlement of the surrounding ground during piping installation, and with the Crawford Street section running alongside the existing State Highway, a stabilisation technique needed to be adopted to facilitate safe and stable drainage operations.
GeoStabilization NZ was able to offer an in-situ mass stabilization solution to consolidate the loose and saturated silts and prevent localised ground collapse for this project.
GeoStabilization NZ’s skilled operators were able to mix around existing services, including close proximity to a 33kVA power cable with a Utility Service Company observer in place throughout the entire task within 2m of the high voltage power cable.
Environmental controls include the use of a reduced dug trench to contain wet grout soil mixing and prevent migration to the surrounding areas. A stabilised soil block allows safe and secure excavation for drainage installation, with 1.5-metres stabilised walls and trench base to prevent groundwater ingress.
Jet grout columns were also used on the project to form a shroud around an existing manhole, with more targeted grouting aimed at stabilising the gravel surrounding the manhole. Jet grouting can provide discrete isolated columns whereas in-situ mass stabilization is a block treatment.
2,300 cu meters of soils were blended with cement grout binders to a depth of 5m below ground level in a work. Binders were prepared as a wet grout and blended into the ground in accordance with strict Quality Controls.
By providing a stabilised trench, the main contractor was able to excavate and install drainage pipes in dry conditions and without the risk of ground settlement as a consequence of dewatering.
An as-built 3D model was used to ensure 100% soil stabilisation. Daily wet grab samples were UCS tested at 7 & 28 days to verify the soil strengths were greater than 1 MPa. Cells were individually referenced and data recorded including date installed, volume of grout, and depth of treatment.
Following a series of sub-tropical rainstorm events, Kawakawa and the wider Auckland region suffered extensive landslip issues. Many roads in the region developed both underslips and overslips, severely affecting the flow of traffic.