Elastic–Perfectly Plastic Behavior of Lower Siwalik Sandstones

Establishing shear strength parameters, cohesion and friction angle of rock mass, is an essential task in the design of dam foundations, tunnels, shafts, nuclear waste repositories, caverns for oil and gas storage, etc. In rock engineering practices, in-situ direct shear test is performed in the drifts, and depending upon on the requirements, these tests can also be conducted in trenches, which requires girders, beams, and loading arrangements to provide the normal load, and for applying shear force, a concrete reaction pad is need to be casted. In the present paper, the findings of in-situ shear tests conducted in open trench to determine the shear strength parameters of rock over rock (RR) and concrete over rock (CR) interfaces in sandstone are discussed. These sandstones belong to lower Siwalik group of rocks, forming the youngest mountain belt, and are extended from East to West foothills of the Himalayas. The rock is classified as fine to medium grained poorly to moderately cemented gray silty sandstone and is highly to moderately jointed, with bands of mudstone (1–10 m thick). The permeability of rock mass is ranging from 10–4 to 10–5 cm/s. The rock mass falls in Class-IV, poor category as per RMR classification. In-situ shear tests were conducted in two open trenches in lower Siwalik sandstone. An innovative design was made at site to arrange the ISMB girders in such a way that the dead load was equally transferred on three testing blocks in single loading of gunny bags, meeting the load requirements from top. Five blocks each of RR and CR were tested at different normal loads. The plot of shear stress versus shear displacement and the photographs of the overturned blocks clearly indicate the rock mass has shown progressive shearing and elastic–perfectly plastic case, as the shear strength parameters in peak and residual are same.