Critical State Behavior of a Compacted Silt Specimen

Shear strength of unsaturated soil is commonly obtained from a consolidated drained (CD) triaxial test. However in many field situations, fill materials are compacted where the excess pore-air pressure developed during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. This condition can be simulated in a constant water content (CW) triaxial test. The critical states of an unsaturated soil obtained from the CW and the CD triaxial tests have not been extensively investigated and compared. A series of CW and CD triaxial tests was carried out on a statically compacted silt. The results from this study show that the critical state lines at different matric suctions on the (q-p) plane were parallel with a slope of 1.28 for both the CW and CD triaxial tests, indicating the unique relationship between deviator stress and mean net stress. The results also indicate the unique relationship between the specific volume, v, and mean net stress, p, on the (v-p) plane for both the CW and CD triaxial tests. The slope of the critical state lines on the (v-p) plane for both the CW and CD triaxial tests decreased with the increase in matric suction.