Hydraulic isolation
The water level in the shaft is maintained below sea level by daily pumping of water. This reduces the amount of water that can flow through the shaft and contaminate the rock on the seaward side.
The first phase of decommissioning is hydraulic isolation, creating a containment barrier between the waste in the shaft and the groundwater that flows through fissures in the surrounding rock.
The selected method involves drilling approximately 400 boreholes in the rock around the shaft and injecting very fine grout under pressure to seal up the fissures in a 10-metre wide band of rock.
This phase of work also includes the construction of a raised working platform around the top of the shaft and infilling part of the tunnel.
The platform was completed in early 2006 and infilling of the tunnel was carried out later in 2006.
In Spring 2008, the team completed the grouting of the rock around the shaft.
The volume of water that needs to be pumped daily from the shaft to maintain its water level below that of sea-level has reduced from 15 cubic metres at the start of grouting two years ago to 1.3 cubic metres today.
Hydro testing is now being carried out to validate the performance of the curtain and identify any gaps in the curtain that need to be plugged.
Click here to see a presentation about shaft isolation.
Clearing the ground around the shaft to build the platform
Building the platform around the shaft in 2006
The working platform built around the top of the shaft
Computer-generated image of the grout curtain around the shaft
