Apparent cohesion of unsaturated soils as correlated with suction

A series of shearing tests were conducted on loosely compacted specimens using a modified triaxial apparatus which incorporates the measurement of suction. The test results of three different materials revealed that the components of shearing resistance arising from net stress (difference between total stress and air pressure) and suction are independent from each other. Therefore, it was noted that the strength of unsaturated soils can be expressed in the form of Mohr-Coulomb criterion with respect to net stress in which the total cohesion consisting of the effective cohesion and the apparent cohesion attributed to suction. It was observed that the apparent cohesion generally increases with suction at a decreasing rate and reaches the maximum value when suction becomes high enough. By introducing an assumption that the maximum apparent cohesion appears when suction approaches infinity, a hyperbolic function of suction having an initial slope of tan phi' was derived for the apparent cohesion to characterize the non-linearity between apparent cohesion and suction. The validity of hyperbolic function was examined, and values of the maximum apparent cohesion for eight different materials were summarized.