The Island of Jersey’s major wastewater treatment facility at Bellozanne was constructed in 1959, and with a growing population and future environmental standards needing to be met, the State of Jersey made the decision to renew the facility. With limited land that could be readily developed, and with the existing facility needing to be kept operational throughout the construction period, a phased approach to building the new works was required.

The chosen solution was to remove a hillside adjacent to the existing plant, involving excavation of 70,000m3 of rock, to create the required space. This would allow the new facility to be fully constructed prior to decommissioning and demolishing the existing one. CAN were contracted to provide a design and build solution to stabilise the steep rock faces created by removing this material.

The CAN design solution was for the hillside to be cut down in a series of “benches” cut into the rock to allow four 22t excavator mounted drill rigs and a Casagrande C6 tracked drill rig access to drill rock bolts into the newly formed rock faces. The faces exposed by each bench excavation were stabilised with 2700 individual rock bolts ranging from 6m to 15m in length, with a combined total length of 30,250 lin m. Rock netting was installed, secured to the rock bolts, with Deltax netting installed on slopes <40° and high tensile Tecco netting installed on steeper sections. This was carried out on each bench prior to further excavation, with the rock bolts pre-loaded prior to the excavation continuing.

Over 2130lin m of drilled drainage was installed in the lower rock faces to allow excess water flow out into newly constructed drainage channels. During the course of the works areas of loose rock were encountered, requiring an alternative design solution. In these areas a sprayed concrete facing was installed, reinforced with steel mesh and 2m deep dowels, to provide further support to these areas.

Duration:   10 months

Client:         Brenwal Ltd

Location:    St Helier, Jersey