Residual shear strength of shales from the Gemas Formation based on ring shear tests

Fresh, dark grey to black, shales with minor sandstones of the Triassic Gemas Formation outcrop at the slope cut between Km 81.30 and 81.05 (southbound) of the North-South Expressway near Ayer Hitam in Johore State. The beds strike 165o with eastward dips of 35o to 43o and have thickness of between 0.2 and 1.5 m. Over-lying the fresh bedrock along an irregular boundary is a bleached zone, some 14 to 18 m thick, comprising light grey to white, in situ oxidized shales and sandstones. Three samples were collected from a thick shale bed; fresh samples A and B at 18 m and 15 m depth, respectively, and a bleached shale sample C at 11 m depth. All samples were air dried, finely ground, and then passed through a wire mesh sieve with 180 μm aperture sieve. The remoulded samples were tested with the Bromhead ring shear apparatus, employing the pre-shearing test procedure with multi-stage loading. Plots of shear stress versus cumulative horizontal shear displacement under low normal stresses (<150 kPa) yield failure envelopes with residual friction angles of 27.4o, 27.4o and 27.9o for samples A, B, and C, respectively. Under moderate to large normal stresses (150-350 kPa), plots of shear stress versus linear displacement sometimes slope upward due to increased friction as a result of sample extrusion and settlement of the top platen. It is concluded that the pre-shearing test procedure with multi-stage loading is best suited for low to moderate, effective normal stresses (<250 kPa). Based on the test results, the residual friction angle for shales from the Gemas Formation is between 27o and 28o.