This preventive technique is an essential complementary expertise in tunnel construction.
Soletanche Bachy uses compensation grouting to limit surface settlement during tunnel construction.
The excavation of underground tunnels is always accompanied by surface settlements with follow a Gaussian bell curve profile and may result in significant levels of damage.
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:
- the calibration of quantities injected,
- the perfect synchronisation between grouting and earthworks: grouting must always precede settlement,
- the use of an accurate real-time control of grout placement, and the monitoring of ground and structural deformations.
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:
- It is important to locate a suitable ground layer for compensation grouting – the method is ineffective in soft clays, because the compensatory effect is not sustainable
- Structures built on piles are, by their nature, more difficult to protect using compensation grouting.
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.