Analysis of the pure mode I crack propagation (KII = 0) from tips of compressible joints in rock is improved by a minute computation of internal stresses in the joints, which takes a compressibility of joint and a dilatational overall sliding on joint surface into consideration. The Stress Compensation-Displacement Discontinuity Method (SC-DDM) is successfully applied to evaluate not only the internal stresses but also the stress intensity factors (KI, KII) at tips of propagating open cracks. It is firstly confirmed that the open crack propagates, describing a smooth curve, to the direction of the maximum compressive pressure applied at infinity. Subsequently the critical applied pressure to realize the critical state of KI = KIC is presented as a function of the crack elongation, and it is clarified, as a significant effect of interactive crack propagation, that the critical pressure has a maximum, and that the maximum critical pressure decreases with decreasing the joint spacing. Thus it is discussed that the quasi-brittle crack propagation in rock consists of a toughening process and a subsequent softening process, and that the maximum critical pressure appears at the transition from the toughening to the softening. Moreover, it is discussed the present improved method is a promising scheme available for the positive analysis of the critical pressure-crack elongation curve and the prediction of the strength characteristics of rock, including the delayed fracture, the effect of confining pressure and so on.
