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.

 

 

Applications for this technique

 

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.

What are the advantages of this technique?

Implementation

 

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.

Did you know?

 

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.