STRESS FRACTURING AROUND A DEEP-LEVEL BORED TUNNEL

Fracturing of the rock caused considerable difficulties during the boring of a deep-level tunnel in hard rock. A pilot bore was suggested as a possible means of reducing the effect of the fracturing, and this possibility was investigated theoretically by means of a three-dimensional elastic stress analysis. Failure criteria were applied to the results and it was found that fractures predicted by the use of a simple criterion of limiting tensile macrostrain criterion agreed with in situ observeration. It was concluded that fracture could not be avoided and that a pilot bore was likely to aggravate tunnelling conditions.