Influence of Joint Orientation on the Behavior of Dam Foundation Resting on Jointed Rock Mass Under Earthquake Loading Condition

1. Introduction

With the dam being a lifeline structure and serves the purpose of irrigation, hydro-electric power generation, flood control, domestic and industrial water supply, etc., which are important for human existence. In India, there are over 5100 large dams which have been serving such purposes over the years. Therefore, the foundation stability is the key factor influencing the safety of gravity dams and should be considered as the integral part for safe and economic design. The safety of the dam foundation system depends on the stress-strain properties of rock mass constituting the foundation. It is very rare to encounter intact rock foundation in actual field conditions. The foundation of the dam is very complex and less informative beforehand. It mainly contains joints, faults, shear seams etc. The presence of such weak layers, joints, faults, and fissures can affect the safety of the structure in various ways. The weak layers may lead to concentration of stresses in the dam, transmission of uplift pressures through pervious layers thereby causing instability of the dam. The foundation investigation is carried out to bring in light all the weaknesses so that these are duly treated to make safe design of dams. Usually gravity dams experience large loads due to body weight and water loads. As a result, high stresses are generated in the foundation. These stresses become critical when earthquake motion comes into play. India being a seismically active country possesses more than 1040 active faults which resulted some devastating earthquakes like 1988 Bihar, 1991 Uttarkashi, 1999 Chamoli, 2001 Bhuj, 2004 Sumatra, 2005 Kashmir and 2011 Sikkim over the years. A number of instances of dam failure due to earthquake are reported in the literatures such as Augusta dam failure in South Carolina earthquake (1886), concrete dam disaster in Koyna earthquake (1967) in India and Pacoima dam failure in San Fernando earthquake (1971) in California. Concrete gravity dams may get uprooted during earthquake events when blind faults lying below the dam body turn active. So, the dynamic response of gravity dams during earthquake have always attracted the attention of researchers.