An Experimental Study of the Mechanics of Fracture Closure

Abstract

Fracture closure under uniaxial stress to 30 MPa was studied in the simulated fractures with controlled fracture surface roughness, obtained by lapping with # 240 and # 80 grit powders, within cylindrical cores. These cores were drilled from rock samples of Barre granite, metagabbro, granodiorite, metabasalt, altered pyroxene granulite, and dolomitic marble. The amount of fracture closure at a given normal stress depends largely on rock type as well as on fracture surface roughness. Most of the closure occurred at low stresses and the fracture became stiffer with increase in normal stress. At high normal stresses, the fracture stiffness, i.e, change in normal stress for unit displacement, was greatest in rocks containing hard, strong minerals. In rocks containing weak minerals, failure of surface irregularities or asperities, occurred causing the high-stress stiffness to be less than in strong rocks where asperities deformation was dominantly elastic. The amount of permanent closure due to asperity failure increased consistently from stronger to weaker rocks as well as from relatively smoother to rougher surfaces. Surface roughness has a proportionally greater effect on fracture closure for rocks containing strong minerals than those containing weak minerals.