THE RELATIONSHIP BETWEEN THE SOIL-WATER CHARACTERISTIC CURVE AND THE UNSATURATED SHEAR-STRENGTH OF A COMPACTED GLACIAL TILL

Soils compacted at various ''initial'' water contents and to various d ensities should be considered as ''different'' soils from a soil mecha nics behavioral standpoint even though their mineralogy, plasticity, a nd texture are the same. The engineering behavioral change from one sp ecimen to another will vary due to differences in soil structure or ag gregation. The shear strength of an unsaturated soil and the soil-wate r characteristic curve are dependent on soil structure or the aggregat ion, which in turn is dependent on the ''initial'' water content and t he method of compaction. The laboratory preparation of specimens must, therefore, closely represent the physical conditions and the stress s tate conditions likely to occur in the field if a proper assessment of the shear strength parameters is to be achieved. This paper is primar ily concerned with the study of the relationship between the shear str ength of an unsaturated soil and its soil-water characteristic curve. Consolidated drained direct shear tests were conducted on statically c ompacted glacial till specimens, both under saturated and unsaturated conditions, representing three ''initial'' water contents and densitie s. The ''initial'' water contents and densities of the specimens were selected to represent the dry, optimum, and wet of optimum water conte nt conditions with reference to the compaction curve. Multistage, unsa turated, direct shear tests were conducted under three different net n ormal stresses with varying matric suction values for each case. The s oil-water characteristic curves were also developed on specimens with ''initial'' conditions similar to those used for the unsaturated shear strength tests. The shear strength variation with respect to matric s uction was found to be nonlinear for all the tests. The rate of increa se in the shear strength contribution due to matric suction, however, was found to be related to the rate of desaturation of the soil. The d esaturation characteristics are a function of the ''initial'' water co ntent of the compacted specimens. For any particular net normal stress and matric suction, specimens compacted wet of optimum water content offered more resistance to desaturation and exhibited a higher shear s trength when compared to specimens compacted at dry of optimum or at o ptimum water content conditions. Under similar ''initial'' conditions, the soil-water characteristic curve bears a close relationship to the unsaturated shear strength behavior of the soil.