A simple precaution for preventing of the lining failures in the tunnelling by the shielded TBMs through squeezing ground
The use of shield around the TBM allows the machine to pass through weak grounds and adverse geological conditions. However, using the shielded machine limits access to the walls for observation of ground conditions and presence of shield makes the machine susceptible to entrapment or seizure in weak rocks under high stresses which results in high convergence. On the other hand, failure of the segments occurs just after the shield passes through the ground and when the segmental lining are subjected to contact with the backfilling and ground. This is due to high pressures that transform from shield to the segmental linings. Therefore for coping with such difficult ground and preventing of failures, some points must be considered when we intend to design a shielded TBM for utilizing in squeezing ground. The main points are as follows: -In the shielded tunnelling through ground with high squeezing level, the closure of overcut between shield and ground occurs right after a few distance to the face. Therefore, in contrary to the tunnelling in non-squeezing ground, uplifting phenomenon does not occur here because of closing all of the gaps. This causes to high contact pressures between ground and shield in the invert of the tunnel in comparison to its crown. (Note that the overcut is non-uniform around shield, maximum value in the crown and minimum in the invert). -With advancing of the tunnel, shield or shields passes through the ground and installing of segmental lining is commenced. In the most of TBM types, installing of the segmental linings is performed into the shield. However, since the radial distance at circumference between shield and segmental ring is uniform or nearly uniform, then stress redistribution occur according to the previous operating stage so that the ground pressure transformed to the precast segments in the invert part of tunnel is very higher than other points. -Due to higher ground pressure in the invert part, after the shield passes through the segmental ring, and with advancing of the tunnel, the segmental rings in the invert parts of tunnel are subjected to higher ground pressure in comparison to other points. It leads to a non-uniform stress redistribution around segmental rings. The non-uniform stress redistribution around segmental lining firstly cause to create tensile cracks on the segments located at the crown of the tunnel. After that, failure of segments is started in the invert due to high ground pressures. In this study, a simple precaution is proposed when applied in design stage of the shielded TBMs can be useful for avoiding of failures of the segments during tunnelling. The method includes choosing an appropriate non-uniform radial gap between the shield and the segmental ring at the installation step of the linings. Sensitivity analyses on the different gaps between the shield and the segmental rings were performed by developing a fully 3D numerical simulation of the shielded TBMs. By evaluating of the uniformity of the ground pressure and the size of plastic zone around the segmental lining, the appropriate gap was selected for the studied machine to keep lining safe during tunnelling. The method can be applied for any type and sizes of the shielded TBMs at the design stage of the machine.