Full face shield machines are used diffusely for tunnel excavation. The process is made by a rotating cutter head fitted with disk cutters while the face stability is guaranteed by the pressure in the bulk chamber, behind the cutter head, obtained with a bentonite slurry (Slurry Shield and Hydroshield) or with conditioned soil (Earth Pressure Balance Shields). The tunnel is supported by a concrete segmental lining that is installed continuously during the advance of the machine. The excavation diameter is usually bigger than the external diameter of the lining due to the overcutting necessary to allow the advancing of the machine. Therefore, around the lining there is an open space, known as "anulus". It should be continuously filled during the machine's advance to minimize the surface settlements due to any over-excavation generated by the passage of the tunnel boring machine (TBM). Furthermore, the back-filling operation should:1) tock the segmental lining into position, avoiding movement owing to the segmental self-weight,2) ensure a uniform, homogeneous and immediate contact between the ground and the lining,3) improve the waterproofing of the tunnel (for example, in case of cracks of the concrete lining the presence of concrete cracks, back-fill grout should mitigate any water inflow).
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