Coalescence of offset rock joints under biaxial loading

摘要

Crack coalescence in rock masses was studied by performing a series of biaxial compresion tests on specimens made of rock-like material. Specimens of size 63.5 X 27.9 X 20.3 cm, made of 72 percent silica sand, 16 percent cement (Type I) and 12 percent water by weight were tested. The joint inclination angle was maintained at 45 deg, while the offset angle i.e. angle between the plane of the joint and the line that connects the two inner tips of the joints, was changed from 0 deg to 90 deg with an increment of 15 deg. Three levels of lateral stress were used; 0.35 MPa, 0.7 MPa and 1.5 MPa on each sample. HP data acquisition system was used to record the data for each sample. In each sample, four LVDTs were fixed to measure the axial and lateral displacement along the sample. The failure mechanisms were monitored by eye inspection and a magnifier to detect crack initiation and propagation. For each test, the failure surfaces were investigated to determine the characteristics of each surface. Wing cracks initiated at the tip of the joint for the low confining stress applied, while at higher confining stresses wing cracks also initiated at the middle of the joint. Secondary cracks initiated at the tip of the joint due to shear stress, Three modes of failure took place due to coalescence of the secondary and wing cracks. The bridge inclination was the main variable that controlled the mode of failure. For bridge inclination of 0 deg, the coalescence occured due to shear failure and for bridge inclination of 90 deg the coalescence occurred due to tensile failure while for the other bridge inclinations coalescence occured due to mixed tensile and shear failure.