FAILURE MODES AND STRENGTH ANISOTROPY OF GRANITE AND GRANODIORITE GNEISSES FROM OKENE SOUTHWESTERN BASEMENT COMPLEX OF NIGERIA

Abstract

Gneiss, as an anisotropic rock, is considered to constitute a serious challenge when encountered in underground works and thus the proper evaluation of its failure modes relative to its strength properties becomes necessary to prevent major flaws in project design and execution. In an attempt to correlate the strength properties of granite gneiss and granodiorite gneiss with their associated failure modes, a total of 20 core samples, 10 each for granite gneiss and granodiorite gneiss was obtained from Okene for uniaxial compressive strength (UCS), Brazilian tensile strength (BTS) tests and determination of failure mode. Shear failure typical of rocks with low to intermediate confining pressure was observed in granite gneiss (GRN) at UCS values between 57.8 Mpa – 142.2 Mpa. Whereas axial splitting typical of low strength rocks was observed in granodiorite gneiss (GRn) for UCS values ranging between 57.6 Mpa – 88.1 Mpa. Under tensile loading the central failure was commonly observed for both gneisses. But more unlilkely low tensile strength was observed in granodiorite gneiss regardless of the central failure mode. However higher tensile strength (4.68 Mpa – 12.45 Mpa) was observed in granite gneiss (5.06 Mpa – 7.68 Mpa)