EFFECT OF CYCLIC AND MONOTONIC SUSTAINED LOADING ON STRENGTH OF SAND.STONE

Abstract

The foundation rock of concrete dam is subjected to slow compressive loading of monotonic nature during its construction stage. Subsequently, the rock foundation undergoes sustained loading after construction of the dam and further subjected to cyclic loading during post construction period, 1.e., reservoir operation stage. It is therefore very essential to understand the deformation and strength behaviour of foundation rock under monotonic sustained and cyclic loading so as to assess the realistic rock engineering parameters for the design purpose. With this objective in mind, a large number of specimens of sand stone from Naina-Dovi area of lower Shiva/1k region (Himachal Pradesh) have been tested in the laboratory in order to know "Effect of Cyclic and Monotonic Sustained Loading on Strength of Sand-Stone". The effect of loading pattern maintaining uniform stress-rate on the deformational and strength characteristics/parameters viz. ultimate compressive strength, modulus of elasticity, failure pattern and creep have been studied on well cemented medium-grained micacious sand-stone specimens using a loading frame of capacity 20 tonnes fitted with load applying and measuring devices. This study comprises of preparation and testing of forty specimens of 38 mm diameter with length to diameter ratio 2.0 conforming to IS:9179-1979 and IS:9143-1979 besides detailed analysis of test data. Some physical properties of the rock samples were determined in order to check the uniformity of the specimens, which essentially included porosity, water content and specific-gravity. Besides this, mechanical properties such as dynamic modulus of elasticity, point load strength index on rock lumps were also determined to check the same. The analysis of test results indicate that unconrined uniaxial compressive strength and modulus of elasticity (tangent-modulus at 5014 qc, Compressive strength) increases significantly with sustained time period and number of cycles. It appears that the rock material gets consolidated gradually with time under the influence of sustained and cyclic loading. Consequently, the material becomes more compact and dense_ This results in increase in modulus of elasticity and failure strength.