EXPERIMENTAL STUDY OF FRACTURE DEVELOPMENT IN MULTILAYERS OF CONTRASTING STRENGTH AND DUCTILITY

The effect of mean ductility, interlayer thickness, and magnitude of shortening on fracture development in bedded rock was investigated by shortening multilayer cylinders (5 cm dia.) 4 to 14% normal to layering in a triaxial apparatus. Multilayers were constructed by stacking two 1.4-cm thick layers of Berea sandstone (relatively strong and brittle) with interlayers of Indiana limestone (relatively weak and ductile). Thickness of the interlayer between the sandstone was 30%, 100%, or 150% the thickness of the sandstone layer. Mean ductility was varied by shortening at confining pressures (Pc) of 5, 25, 50, and 100 MPa. Sandstone layers fracture at all conditions. Fractures have preferred orientation symmetric to the cylinder axis, and display systematic spacing. At the lowest Pc and mean ductility, fractures in the sandstone are dominantly opening mode (joints) and oriented at high angles to layer boundaries. At greater Pc and mean ductility, fractures are dominantly shear mode (faults) and display conjugate geometry. Average dihedral angle of the conjugates increases from 16 to 67 degrees with increase in mean ductility. Maximum fracture density in the sandstone occurs at intermediate mean ductility and maximum interlayer thickness. Fractures propagate from the sandstone into the limestone and may link across the limestone interlayer as shortening is increased. Linkage is enhanced with decreasing mean ductility and interlayer thickness, and increasing shortening. At high mean ductility, fractures are confined to the sandstone layers. Limestone deforms by faulting and compactive cataclastic flow at low and high mean ductility, respectively. Faults in limestone are more variable in orientation and display larger dihedral angles than in the sandstone. Fracture mode and orientation are consistent with Mohr-Coulomb failure, and a spatially heterogeneous stress state where the most tensile stress occurs in the sandstone. Fracture modes and orientations are consistent with a continuous transition from joints to faults with an increase in the least compressive principal stress. Fracture spacing depends on layer and interlayer thickness, mean ductility and ductility contrast, and magnitude of shortening.