Alteration of the hydraulic conductivity of rock by tunnel excavation

Theory as well as small- and large-scale experiments indicate that tunnel excavations in rock result in an increased axial permeability close to the tunnels. This is due to stress relief as well as to blasting, of which the former is generally assumed to be most important, at least when smooth blasting is applied. The Buffer Mass Test in Stripa offered a possibility of quantifying the increase in axial permeability and it was concluded from this experiment that the increase may have been as high as 1000–10,000 times in a 0.5–1.0 m wide zone adjacent to the excavation, assuming that the value 10−10 m/sec derived from the preceding, large-scale “Macropermeability Test” is representative of the hydraulic conductivity of the undisturbed rock. If this figure is correct, disturbance by blasting would be the major cause of the increased perviousness.