Review of the TBM performance in blocky rocks with potential face stability issues

In blocky/jointed rock masses, the combined effects of the rock mass structure and the in-situ state of stress may lead to severe instabilities of the excavation face which may become "blocky". Large breakout and rock block detachment from the tunnel face may occur with consequent paramount effects on the normal TBM operations. These effects generally comprise the abnormal wear of the cutters and cutterhead, due to the heavy dynamic impacts against the hard rock blocks at the face, as well as frequent damages to the muck transportation facilities due to the presence of blocks in the muck. Therefore, an increase of the maintenance requirements and TBM downtimes is generally registered when the face presents an irregular/blocky structure. This work focuses on the review of the TBM performance at the Lotschberg Base Tunnel, where substantial occurrence of face instability phenomena was observed. More in detail, the main effects that blocky rock conditions may have on the TBM performance, in terms of machine penetration rate, utilization time and total advance rate, are underlined. Finally, a new prediction model suitable for blocky rock conditions with potential face stability issues is presented.