COMPLEXITIES OF ROCK FRACTURE AND ROCK FRICTION FROM DEFORMATION OF WESTERLY GRANITE

A series of rock friction experiments has been carried out to study th e complexities in rock fracture and rock friction. Intact Westerly gra nite samples were loaded to shear failure in a laboratory polyaxial lo ading apparatus. The resultant fractured samples were reloaded to caus e frictional sliding. Both polyaxial loading (sigma1 > sigma2 > sigma3 > 0) and equal confining condition (sigma1 > sigma2 = sigma3 > 0) wer e used. The deformation processes were monitored by macroscopic axial stress-strain, optical holography, and ultrasonic velocity measurement s. Intense localized deformation along the fracture occurred very earl y in the loading of fractured samples. Contacts on the fracture surfac es continuously broke during loading. No acoustic velocity anomaly was observed for the fractured sample, in contrast to a approximately 25% drop in the velocity before the failure of the corresponding intact s ample. The current study and previous research suggest that the deform ation localization is an important process in governing the instabilit y of rock friction. Instability analysis of rock friction needs to inc lude not only the deformation processes along the sliding surfaces, bu t also those adjacent to the fractures such as the localized deformati on along the fractures observed in the current study. The instability analysis of rock friction with rate- and state-dependent friction laws does not specifically include the deformation localization adjacent t o the faults and thus ignores an important class of instability as des cribed by RUDNICKI (1977). A dependence of frictional strengths on the stress components normal to the sliding and in the plane of the fract ure surface was observed. This dependence can be understood by conside ring the loading of the irregular fracture surface under polyaxial loa ding conditions. This observation requires the friction laws in the ma croscopic scale to be modified for those cases where the three princip al stresses (sigma1, sigma2, and sigma3) are significantly different.