Compensation grouting is an active technique used to counteract relaxation of the ground resulting from tunnel excavation by injecting precise quantities of grout between the tunnel roof and the structures subject to settlement.
The key factors for success are; precise measurement of the quantities of grout injected, synchronisation of the grouting exercise and the advancement of the excavation, the grouting should react to the settlement, precise, real time monitoring of the grouting process and displacements of the soil and structures.
HOW IT WORKS
Compensation grouting works by injecting a grout mixture into the ground to either mitigate settlement or actively heave the strata above to compensate for ground movements resulting from tunnelling, shaft sinking and other invasive ground engineering.
Once the location and predicted settlements have been established, the design team will produce a grouting array – which is the planned layout of grouting pipes. Depending on the location, operations can either take place at ground level, or from shafts / tunnels beneath the ground. The grouting pipes are installed into drill holes at the predetermined locations identified by the design team on the planned layout.
As the below ground structure is constructed the buildings above are monitored for signs of movement. Once movement is detected grout can be injected via a grout port located within the grout pipe at precisely the right location to address the movement.
Capability: Boreholes can reach lengths of up to 150m, to a diameter of 180mm, depending upon ground conditions. Multiple injections (up to 12) at any one time, using computer piloted grout injection equipment, which in itself responds to real-time monitoring of the structures being protected.
Advantages: Perfect fit with the observational method, responding to real-time monitoring data. Process is non-invasive as can be carried out from shafts/tunnels below the structures.