EFFECTS OF DEGREE OF SATURATION ON DYNAMIC SOIL PROPERTIES

Abstract

Failure of soil subjected to dynamic loading is greater. Therefore, it is important to study the response of soil under dynamic loading. Thus the dynamic soil property is an important parameters to be studied. Dynamic soil property is affected by testing methods, method of sample preparation, relative density, confining pressure, degree of saturation of sample, loading frequency, soil type, fines percentage, mineral content etc. There are various lab tests available for determination of these properties and can be broadly classified as low and high strain testing system. Resonant Column Apparatus (RCA) is used for low strain tests (10-4 – 10-1 %) and Cyclic Triaxial System (CTS) for high strain tests (10-2 - 101 %). Various field tests are also available but the lab tests have an advantage that the various conditions can be simulated and different material can be tested that can be further used for ground improvement where ever required. Present study was carried out on samples of size 50 mm diameter and 100 mm height. The tests are done on three relative densities (40% , 60% & 80%) and three confining pressures (100 kPa, 200 kPa and 300 kPa). The tests were conducted using RCA. The effect of confining pressure, relative density and degree of saturation on dynamic soil property are examined. Soil properties that influence wave propagation are damping ratio, stiffness, Poisson’s ratio and density but among these stiffness and damping ratio are of utmost importance. It is being observed that the shear modulus of sample increases with increasing confining pressure and relative density. Thus decreases with saturation, the damping ratio is observed to decrease with increase in relative density and confining pressure while increases from unsaturated to saturated sample. It is also observed that as the stiffness increases, the strain induced is smaller. The damping ratio obtained in flexure mode of vibration is more than that in torsional mode of vibration. There is significant differences in behaviour of unsaturated, partially and fully saturated condition as the result of influence of capillarity as the saturation increase there is generation of pore pressure that affect effective stresses on the sample. Various degree of saturation simulates the condition of soil when the water was full there and the later stages where due to evaporation or withdrawal of water from the ground, the water decreases and the degree of saturation reduces. It is observed that the decrement in Shear and Modulus of Elasticity is greater up to 50% degree of saturation but after this saturation there is minor difference between 75% and 100% degree of saturation. There is not much effect of degree of saturation on the damping ratio