Fatigue Analysis of Insitu Rock Mass for Dam Foundation

As many rocks are non-elastic, characterization of rock mass deformability by elastic constants alone is not sufficient. Elasticity refers to the property of reversibility of deformation in response to load. Many fresh, hard rocks are elastic when considered as laboratory specimens. But on the field scale, where rock can be expected to contain fissures, bedding planes, contacts and zones of altered rock and clays with plastic properties, most rocks do not exhibit perfect elasticity. The extent of irrecoverability of strain in response to load cycles may be as important for the design as the slope of the load/deformation curve. Repeated cycles of loading unloading in response to cyclic operation of the reservoir would produce the series of loops (hysteresis). This could open joints in the rock or simply lower the compressive strength. Some sites have been considered unacceptable for concrete dams because large hysteresis even though the modulus of elasticity of the rock itself was considered reasonable. The insitu fatigue test has been conducted at the dam foundation of the Jobat Irrigation Project (M.P.) to stimulate the filling and emptying of reservoir. This paper deals with the study of fatigue behaviour of rock foundation. Due to repeated loading/unloading cycles, the elastic modulus increases and the deformation modulus decreases, indicating reducing ductility of rock, which may affect the designed life of damfoundation.