High in situ stresses strongly influence the stability of the excavation face in deep tunnels. When hard rock fails under high acting stresses, it disintegrates and massive rock is transformed to blocky ground. Strong, instable rock blocks may appear at the tunnel face and this condition may prevent normal TBM operations When these instabilities appear, instead of showing a flat shape with clear marks of cutters, the tunnel face is irregular. This condition causes high frequency and great magnitude of variations on cutter force. As a consequence, high loading and heavy dynamic impacts of the cutters and the cutterhead against the hard rock blocks occur. Due to these effects abnormal cutter ring wear, such as pitted, chipped and flatted wear appears, and the cutter bearing failure is enhanced. Furthermore, extensive cutter-head vibration and the cutter loading process may lead to the instability of the rock blocks at the face. In return these falling blocks may destroy the cutters. To minimize the vibration of the cutterhead and the dynamic loading and impacts on cutters and cutterhead, the thrust force and the cutterhead rotational speed have to be decreased below the normal level. Finally, problems can be expected to the muck conveyor system which can be jammed or damaged by irregular rock fragments.
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