Annulus grouting (also called back grouting or tail-skin grouting) is the grouting around the tunnel annulus – the backfilling of the gap between the tunnel segments and the ground. This grouting is done systematically, which means it is always done as part of a cyclic procedure of the TBM operation: Thrust/excavation > segment erection > annulus grouting.
Why is there a gap between the segments and the ground?
Whoever has used a saw would know that the saw teeth are slightly towards pointing, creating an overcut, to reduce the ‘grip’ from the timber. The exact overcut is also required for the TBM operation, reducing the ‘grip’ of the ground on the TBM. Also, the tunnel alignment is never straight, and even if it is straight, the TBM will need to make corrections to its alignment from time to time. Therefore the TBM needs to be able to negotiate required bends, which requires certain amount of overcut – imagine a long bus making a turn around a tight corner.
For these reasons the very front of the TBM is largest in diameter and the shield slightly tapers down, to create the required overcut. When the overcut is first made, it will create a gap in between the tunnel segments and the ground.
What is annulus grouting for?
It has the following functions:
Ensuring contact between the tunnel segments and the surrounding ground. The excavated ground profile is inevitably jagged to certain extent. Grouting ensures the gap between the ground and the tunnel segments is stuffed and compacted.
Ensuring an even distribution of ground pressure on the tunnel lining, avoiding sharp concentrated loads that induce localised stresses
‘Locking in’ the assembled ring, minimising the ring displacement while the TBM rams thrust against the previously installed ring, and also enabling the segments to take the loads from TBM backups
Filling the gap, preventing ground moving into the gap, thereby minimising the ground settlement as a result of tunnelling
Creating a water cut-off barrier around the tunnel, reducing water ingress even if the gaskets fail to meet the water tightness requirement.
What is annulus grouting made of?
Grouting can be two types by material:
‘active’ grouting – containing cement, which hydrates and hardens, just like concrete
‘inert’ grouting – containing no cement, and mostly hydraulic lime
There can also be ‘in-betweens’ which can be called ‘semi-active’ grouting. The exact type of grout should be selected based on the specific tunnelling considerations, such as settlement, waterproofing and ground conditions.