REBOUND CHARACTERISTICS IN CLAYS

Abstract

Upon unloading clay undergoes expansion or swelling which is explained partly due to a (i) physical mechanism, in which negative pore water pressure is a criteria for swelling and partly due to a (ii) mechanical swelling in which the load when applied to the clay particles, the grains are compressed itself and most important is that platy clay particles get deformed (bent) under load resulting in decrease in volume and when load is released, the elastic rebound of grain itself take place, the bent particles become straight again due to elastic recovery which causes swelling (Seed et al.). Another swelling mechanism is known as physico-chemical mechanism, which occurs on the basis of double layer theory (Guoy-Chapman) on imbibition of water and is* independent of loading. This mechanism is somehow related to physical mechanism. Terzaghi's theory of swelling/consolidation is based on pore water pressure dissipation and gradual transfer of stresses from pore water to soil skeleton. On the other hand swelling due to elastic recovery on unloading has not been considered so far. In the present investigation, consolidation including rebound tests have been carried out on four different type of clays such as Dhanauri clay, Kaolin, Black Cotton Soil and bentonite. The experimental time factors have been compared with Terzaghi's theoretical time factors. The consolidation tests have also been conducted on rubber specimen to simulate the elastic rebound of the soil grains. An attempt has been made to modify Terzaghi's theory of swelling considering the elastic recovery of the soil grains in addition to the pore water pressure dissipation. A critical assessment of assumptions in Terzaghi's theory of swelling have been carried out. The tests for swelling and swelling pressures have also been performed in order to examine its relation-ships with the index properties and rebound characteristics. xiv 2 It has been observed that Terzaghi's assumptions of linear relationships between. pressure decrements with coefficient of expansibility, coefficient of volume expansibility, coefficient of permeability are in fact non-linear. Again as unloading progresses, the non-linear relationship becomes more promi-nent. Terzaghi's theoretical time factors do not exactly coincide with the experimental time factors but vary to some extend. A generalised equation has been developed bringing out the importance of elastic recovery in clay particles in addition to pore water dissipation theory. It is observed that the dimensionless time factor governing the diffu-sion process is exactly analogous to Terzaghi's time factor whereas time factor associated with structural rebound depends on the time-dependent elasticity of the clay particles. Although many equations are available correlating compression index with index properties and initial conditions, no such definite, equations are available for swelling index. An attempt has also been made to find out empirical equations for swelling index correlating it with liquid limit, % of clay fraction and activity. AbOut 175 experimental results have been considered for establishing the equations. *-3F-