ABSTRACT
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);
1 INTRODUCTION
The instantaneous filling of the “annulus” created behind the segment lining at the end of the tail shield during TBM advance (Fig. 1) 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 [1, 2, 3]. The management and control of settlements induced by the construction of tunnels in urban areas is very important since they can damage nearby the buildings. Furthermore, the back-filling operation must [1, 4, 5, 6, 7, 8, 9, 12]:
• lock the segmental lining into the right position, avoiding movement owing to both segmental self-weight and the thrust forces and hoop stresses generated by the TBM;
• bear the loads transmitted by the TBM back-up weight;
• ensure a uniform, homogeneous and immediate contact between ground and lining;
• avoid puncture loads by ensuring the application of symmetrical and homogeneous loading along the lining;
• complement the waterproofing of the tunnel with the concrete lining and gasketry (i.e. if the lining has cracks due to wrong installation, back-fill grout should help mitigate any water inflow).
