Effects of Hysteresis on Shear Strength Envelopes from Constant Water Content and Consolidated Drained Triaxial Tests

Shear strength of unsaturated soil is commonly obtained from Consolidated Drained (CD) or Constant Water content (CW) triaxial tests. A series of CD and CW tests was carried out on statically compacted silt specimens. The results indicate that the effective angles of internal friction, φ', and the effective cohesions, c', of the compacted silt as obtained from both the CD and CW tests are identical. The φ^b angles from the CD and CW triaxial tests are essentially identical at matric suctions lower than the air-entry value. This could be attributed to the fact that the soil specimens remain saturated at matric suctions lower than the air-entry value in both the CD and CW tests. Similarly the φ^b angles from both tests are the same at matric suctions higher than the residual matric suction when the soil is at a relatively dry condition. However, the φ^b angles from the CD and CW tests are different at matric suctions between the air-entry value and the residual matric suction value. The difference is due to the lower failure envelope with respect to matric suction from the CW tests as compared to the failure envelope from the CD tests. This difference can be attributed to the hysteretic behavior of soil-water characteristic curve of the soil which will be explained in detail in the paper.