The influence of discontinuities in the rock mass
The influence of discontinuities on the excavation process is well illustrated by research done at the Norwegian Institute of Technology, Trondheim, Norway on the performance of tunnel boring machines cutting in strong rock. The larger rock excavating machines, like cutter suction dredgers, will excavate in a rock mass. Apart from the properties of the intact rock material, the properties of the rock mass are determined by fracture frequency and fracture orientation. If more than one or two sets of discontinuities are present, these tend to divide the rock mass into a system of blocks. On the basis of their geological origin, the types of discontinuities are distinguished. The effect of discontinuity spacing probably operates on two scales, one related to the penetration depth of each cutting tooth and one on the scale of the size of the cutterhead. The orientation of the discontinuities with respect to the cutting direction is known to have an important control on cutting performance.