This paper presents the results on the size effect of a tensile fracture in granite on the shear behavior determined by direct shear tests with a normal stress of 10MPa. Normal and shear stresses were unloaded at designated shear displacements, and the change in the surface topography was measured by using a non-contact surface profile measurement system with a laser profilometer, to determine the evolution of surface damage and aperture during shear. The standard deviation of the initial aperture of the sheared fracture significantly increases with both shear displacement and fracture size, resulting in an increase in the non-linearity of the closure curve with shear displacement. Furthermore, while the shear stiffness of the fracture in the initial stage decreases with fracture size, that in the residual stage increases with fracture size. In addition, it was shown that the damage zones are enlarged and localized with shear displacement and tend to form perpendicular to the shear displacement.
Over the past decade, considerable efforts have been made to better understand the mechanical behavior of rock fractures since it governs that of rock mass. Fractures in rock mass range in scale from a microfissure to a large fault system. Therefore, the size effect on the mechanical behavior of a fracture has to be considered when estimating the mean aperture and the hydraulic conductivity of a fracture under stress. Giwelli et al. (2009) have shown that the closure of a fracture at a given normal stress significantly increases with fracture size. This effect can be attributed to an increase in the standard deviation (SD) of the aperture with fracture size, since the SD of the aperture gives a measure for the matedness of the fracture. Accordingly, fracture size may affect the shear behavior of a fracture.