Back-filling grout with two component mix in EPB tunneling to minimize the surface settlements:Rome Metro - Line C case history

The instantaneous filling of the annulus created behind the segment lining at the end of the tail shield during TBM advance is an operation of paramount importance. Its main goal is to minimize displacements around the tunnel and induced surface settlements due to lost volume at the tail shield. For correctly achieving these goals, the simultaneous back-filling system and the injected material should satisfy technical, operational and performance characteristics. The two-component injection system for back-filling while excavating with shielded TBMs is progressively replacing the traditional use of cementitious mortars. In the paper different systems of back-filling grout and in particular the two-component system are analyzed and discussed together with a description of the Rome Metro (Line C) case history where this type of back-filling was widely applied. For correctly achieving the above goals, the simultaneous back-filling system and the injected material should satisfy the following technical, operational and performance characteristics: • the back-filling should ideally be instantaneous in order to avoid the presence of voids in the “annulus” while advancing with the TBM. For this reason, back-filling is typically carried out through pipes inserted inside the TBM tail skin, uniformly distributed around the tail skin (Fig. 1); • the “annulus” must be regularly and completely filled so that the lining is fully linked to the surrounding ground (the system becomes monolithic); • the reliability of the system must be guaranteed in terms of transportability of the mix. The grout must be designed to avoid choking off the injection pipes, segregating in pumps and bleeding, in conjunction with the time the grout is being transported and distance from batching to injection; • the injected material has to gel very quickly after injection (which is carried out progressively with generation of the “annulus”) but without choking the injection pipes and nozzles (especially the ones for the accelerator admixture). The injection must always be carried out until either achieving the maximum pressure that is a function of the TBM face pressure or the theoretical volume);