Shear behaviors of ‘real’ natural un-matching joints of granite with equivalent joint roughness coefficients

Abstract

Contact planes of a natural unfilled rock joint often portray dissimilar roughness characteristics particularly at shallow depth. Some researchers investigated shear strength behaviors of un-matching joints (i.e. joints with dissimilar surface morphologies) through laboratory experiments using mimicked discontinuities. This study aimed to explore shear behaviors of ‘real’ natural un-matching joints (with similar averaged joint roughness coefficients (JRCs)) in the Singhbhum Granite (India) based on laboratory investigations. Direct shear tests were performed on a total of 18 encapsulated jointed samples, split into three groups, under (a) three different low normal stresses (≤0.70MPa) with increasing values; (b) three different high normal stresses (within a range of 0.98–1.99MPa) with increasing values; and (c) one particular intermediate normal stress (≈ 0.99MPa) for five times, where direction of shearing was kept same as the dip direction of the joints in situ. Shear behaviors of the investigated joints portray diverse patterns. Moreover, most of the obtained behavioral patterns look different from the typical shear stress-shear displacement graph in the literature that are based on direct shear test results in case of matching rock discontinuities. Influence of surface morphology on shear behavior, shear stress fluctuations with displacement, variation of JRC values with the increase in normal stresses and observations on peak friction angle were critically analyzed. Effect of cycles of shearing on shear strength was also examined. Plausible reasons behind the uniqueness of the results were explained.