Mechanical properties of carbonaceous rocks and their shear strength at interface with mortar

Carbonaceous rock is of easy disintegration, making it hard to prepare standard samples for assessing its compressive strength. The point load test was conducted on both fresh and weathered carbonaceous rocks to investigate the variation of compressive strength under different conditions. The shear strength of carbonaceous rock-mortar interface was measured and the corresponding stress–strain behaviors were systematically analyzed. The test results revealed that the point load test was one of the effective methods for evaluating compressive strength of carbonaceous rocks. The calculated compressive strength R c presented obvious variability for fresh and weathered rocks, while their statistical distributions followed a normal pattern. The expected value of R c can be selected as an evaluation indicator, and their values for fresh and weathered cases were respectively 127.65 MPa and 50.06 MPa. Two days of water soaking led to a reduction of R c exceeding 50%, and the softening coefficient K R decreased to 0.38 with soaking time extended to one month. An exponentially decreasing trend between K R and soaking time was proposed to capture the experimental data. The weathered rock collected from the slope surface was classified into a highly weathered one due to its weathering coefficient of 0.392. The stress-displacement curves of rock-mortar interfaces can be roughly divided into: elastic, elastoplastic, softening, and stable parts. The great variability of cohesion and friction angle was consistent with the anisotropy and the disintegration of tested carbonaceous rock.