Point load strength of clastic sedimentary rocks from the Semanggol Formation, Beris Dam, Kedah Darul Aman

Abstract

The Beris Dam is founded on a sequence of thick bedded conglomerates and pebbly to fine grained sandstones with minor mudstone mapped as the Semanggol Formation of Triassic age. The conglomerates have an average dry unit weight of 23.85 kN/m 3 , and apparent porosity of 9.8%, whilst the pebbly sandstones have corresponding values of 25.35 kN/m 3 , and 3.9%, respectively. Coarse, and medium, grained sandstones have similar apparent porosities of 3.8%, but dry unit weights of 25.33, and 25.36, kN/m 3. Fine grained sandstones, which are laminated, have an average dry unit weight of 25.81 kN/m 3 , and apparent porosity of 2.2%. Diametral tests on air dried core specimens (of 51.6 mm diameter) show them to fail in typical tensile splitting mode, except where inherent fracture or bedding planes are present. The fine grained sandstones split along laminae and yield a point load strength index [Is (50) ] of 2.44 MPa, whilst the relatively porous conglomerates yield an index of 5.16 MPa. Diametral tests on pebbly sandstones yield a strength index [Is (50) ] of 6.89 MPa, while those on coarse and medium grained sandstones yield indices of 8.64, and 8.83, MPa, respectively. Decreasing strength indices with increasing particle size are considered to result from the greater surface area of the coarser grains that allows preferential extension of failure crack traces along grain boundaries. It is concluded that the point load strength of clastic rocks from the Semanggol Formation is influenced by their texture as well as inherent discontinuity planes present.