Yielding characteristics of unsaturated compacted soils

The elasto-plastic constitutive model for unsaturated soils postulates the existence of a loading collapse yield locus (LC) and a suction increase yield locus (SI) in the p:s stress plane. The characteristics of the LC yield locus have been reasonably well established in recent years but there is a much greater degree of uncertainty in relation to the SI yield locus. In the work reported in this paper a number of stress path tests in the p:s plane were conducted on samples of unsaturated compacted kaolin in a modified triaxial apparatus. Two series of tests were conducted on samples compacted to an identical initial condition so as to ensure a uniform soil structure prior to testing. Samples tested in Series 1 were taken through three stages: wetting at constant mean net stress, drying at constant mean net stress and a combination of isotropic compression and wetting. Samples tested in Series 2 were taken to pre-selected values of suction prior to isotropic compression. Data obtained from these tests indicate a possible form of single yield locus for unsaturated compacted soil. The data also indicate that the position of the yield locus, initially imposed by the compaction process will be influenced by the subsequent wetting process.