The principle of compensation grouting is to avoid soil decompression as a result of earthworks by delivering precise and calibrated quantities of grout between the tunnel and the structures to be protected. The key factors are:
Compensation grouting can be used wherever there is a risk that a tunnel worksite could cause a sensitive structure to deform. It is relevant for all tunnelling techniques: tunnel boring machines (TBM), traditional tunnel, NATM, etc.
It has the following limitations from the geotechnical point of view:
Compensation grouting can perfectly be used with the Observational Method. Commonly, compensation grouting plays a major role within a remediation programme to provide protection to existing structures in case something occurs as the tunnel boring machine is progressing.
It is perfectly adapted to tunnel construction
It limits the settlement of neighbouring structures
It is a “surgical” treatment, used only when and where necessary
Compensation grouting drilling can be either sub-vertical or sub-horizontal. On surface, motion sensors are installed on the structures to be protected.
An initial phase of pre-conditioning injections precedes tunnel boring to firm up the soil around the injection drillings. The compensation grouting process must be perfectly monitored and controlled, as it involves small quantities of grout at low flow rates and pressures.
The second phase is carried out during the tunnel excavation using either traditional methods or mechanised methods, such as tunnel boring machines or micro-tunnel boring machines. Additional compensation grouting is delivered as soon as any excessive movement has been detected on the surface structures.
Once the tunnel has been built, a final consolidation phase compensates for any further settlement of the soil, which is particularly important in clay soils.
Soletanche Bachy carried out the first compensation grouting operation with real-time settlement monitoring on the Jubilee Line underground project in London (UK) back in 1994.