THE INTERACTION OF 2 CLOSELY SPACED CRACKS - A ROCK MODEL STUDY

The influence of preexisting fractures on fracture development and the geometric patterns of crack growth have been investigated experimentally. Rectangular specimens of Berea sandstone with two parallel thin cuts were compressed along their longest dimension at a confining pressure of 60 MPa, a pore pressure of 20 MPa, an average displacement rate of 2x10(4) cm/s, and room temperature. The precuts are inclined 35-degrees to the loading direction. They are en echelon with each other, each extending only part way from the specimen boundary. When the sense of shear (left-or right-lateral), overlap (left or right stepping), and the amount of separation are changed, significant difference of fracture interaction occurs. An extensional fracture zone forms between left-stepping precuts subjected to left-lateral shear (or right-stepping precuts under right-lateral shear). Secondary fractures tend to develop from one precut and toward the other precut. The sequence of the fracture development is as follows: alpha-fractures which are about 60-degrees to the precuts, beta-fractures which are about 30-degrees to the precuts, and gamma-fractures for large overlap pattern which are about 70-degrees to the precuts but opposite in direction. The fracture zone is confined by precuts and beta-fractures. Therefore, the larger the overlap, the larger the fracture zone that develops. For large overlap patterns, a set of alpha-fractures forms nearly parallel and equally spaced within the fracture zone. In cross sections, fractures are normal faults with surface depression. Microscopic study shows that microfractures are highly concentrated in the area immediately adjacent to the macrofractures and at angles less than 30-degrees to the macrofracture. Grains away from macrofractures remain unfractured. For right-stepping precuts subject to left-lateral shear (or left-stepping precuts under right-lateral shear), a compression zone forms between the precuts. The zeta-fractures propagate at precut tips and diverge away. The eta-fractures develop within the compression zone and are bounded by precuts and zeta-fractures. The eta-fractures develop within the compression zone and are bounded by precuts and zeta-fractures. The eta-fractures have reverse offsets in cross section.