The narrow winding TeKoa Road, in North Canterbury, is supported over a short 30m section above the Mandamus River by an aging four-metre high gabion wall. The gabions are subject to regular poundings by the Mandamus River when it floods, and the steel wire had begun to corrode, causing some of the gabions to split and open up. GeoStabilization International® was commissioned by the Hurunui District Council to provide a design/build solution for protecting the road. Various options were considered, including excavating and rebuilding the gabion wall. However, with the road only 4m wide and constrained by a vertical cliff on the upslope side, the Client was eager to maintain vehicular traffic access.
Our engineers proposed a simple soil nail and mesh solution. Using high-tensile steel wire mesh and our proprietary low carbon self-drilling steel hollow bar soil nails, the system will contain the existing failing gabion baskets and basket’s fill material. The soil nails, installed on a 2m x 2m diamond offset pattern, were drilled to varying depths up to 8m to provide sufficient load capacity for the failing gabion wall’s likely loads. To minimise traffic disruption during construction, the Client provided access to the base of the wall using excess material from a river training project further upstream. This access allowed our technicians to use our 16-ton SuperNailer located in the river bed. Full traffic access continued throughout the project. The soil nails were installed using grout flush (self-drilling nails) as the road embankment fill material was subject to collapse. Using grout flush ensured the integrity of the hole.
Since global slope failure was not an issue at this site, our engineers Geobrugg’s high tensile TECCO® mesh for the facing. The solution provided the Client with a long term, low-cost solution with minimal disruption to the road users.
The site was classified a “Class D –Deep or Soft soil site” as defined in the New Zealand Standard NZS 1170.5:2004, with soils comprising Hinuera Sands and Silts up to 10m thick overlying Puketoka Clays. The upper sands and silts were deemed liquefiable and the new bridge structure needed suitable ground improvement measures incorporated into its foundation design.