Estimation by In - situ Investigations of Rock Property Changes Induced by Shaft Excavation in Tertiary Sedimentary Soft Rocks

摘要

A shaft with an inner diameter of 6 m has been excavated by blasting in Tertiary soft sedimentary rocks and we have estimated the changes of rock properties, such as deformation modulus, permeability and fracture frequency, of the rock around the shaft. We have performed four kinds of in-situ investigations; borehole jack test, cross-hole seismic velocity measurement, water injection test and borehole television survey. We have carried out these investigations before and after the shaft excavation in the same locations, with the same sets of equipment and through the same procedures for accurate estimation of rock property changes induced by the shaft excavation. The measurement locations have been selected in order to estimate the dependency of the excavation disturbance on the distance from the shaft wall and geological conditions such as lithology and faults.Within 0.5 to 1 m from the shaft wall, detectable property changes have been observed. In the rock zone, some kinds of deformation moduli have decreased. On the other hand, the number and apertures of fractures have increased and permeability has also increased by 2 approx 3 orders of magnitude in the rock zone. Blasting damage is supposed to be the main factor which has induced these rock property changes considering strength and stress condition of the rock. Effectiveness of each investigation method in the estimation of excavation disturbance is also discussed.