Soil Instruments Ltd has recently completed the instrumentation scheme associated with the 25,000m3 St.Helier Storm Water Cavern in Jersey.

During the summer season, St.Helier’s population significantly increases due to tourism, putting a strain on its sewer and waste water systems. This problem is further compounded when rainfall is heavy. The result is untreated waste discharged into the sea and polluting the very beaches on which the island tourist trade relies.

The cavern, one of the most extensively instrumented in Europe, together with 1.1 km of tunnels, is designed to relieve the combined sewer/storm water system during these times of high loading. The cavern is located under approximately 60m of granite which, on the surface, forms part of Fort Regent. More than 80,000 tonnes of rock has been excavated to produce the 70m long, 17.6m wide holding tank, the height of which varies from 15m to 27m.

Soil Instruments were selected by the contractor, Balfour Beatty, to supply, install and commission the instrumentation for the project, in view of their extensive experience of similar schemes in underground environments.

The comprehensive monitoring programme was designed by Geo-Engineering to monitor safety of the works and long term stability of the cavern throughout its operating lifetime. One of the key issues in the selection of the instruments was that the cavern had to be excavated using drill and blast techniques and many of the instruments were to be installed within 1m of the blast face. Vibrating Wire Transducers in the form of Multiple Rod Extensometers, Piezometers, Load Cells and Strain Gauges were selected in the knowledge that they would be robust and accurate within this aggressive environment. All the sensors installed for long term monitoring are linked via a network of 3.6 km of cabling to a 192 channel Data Logger located in the control room 30m above the cavern.

Prior to commencing the tunnelling operations, Soil Instruments also installed extensive ground level instruments along the proposed tunnel routes, including Vibrating Wire Piezometers, Standpipes and Magnetic Extensometers, all of which have been monitored manually during the course of the contract.