Laboratory study of four basalts

From the proposed Khargihill-Pinjal Tunnel alignment, four variants of basalt, i.e., Fragmentary Porphyritic Basalt, Amygdular Basalt, Massive Porphyritic Basalt, and Massive Basalt, have been investigated in the laboratory. The assessments include: uniaxial compressive strength in both, dry and saturated, states, indirect tensile strength, and shear strength parameters, along with compression and shear wave velocities in both, dry and saturated, states. In both, dry and saturated, states, the uniaxial compressive strength data of the four variants overlaps. But, still, for each variant, uniaxial compressive strength has been separately recommended. However, in case of shear strength parameters, the triaxial test data of all the four variants is combined, because the triaxial test data for each of the variant does not lend to realistic drawing of the classical Mohr-Coulomb failure envelop. Whereas, the collated triaxial data, in the ‘axial stress at failure versus confining pressure’ plot, clearly suggests that the lower-bound and upper-bound curves, encompassing most of the data, need to be drawn. And, tangents to these bounds can be treated as traditional strength envelops obtained as per Mohr-Coulomb failure criterion. These tangents (treated as strength envelops), help determine the probable range for the shear strength parameters. The data of waves’ velocities helps in understanding the scatter in the test-data of uniaxial compressive strength and triaxial, because after evaluation of waves’ velocities, the same samples are used for these tests. The assessment of engineering parameters is complemented by the evaluation of water-related and identification properties, which exhibit marginal variation between different variants.