The variation of strength parameters of unsaturated soil with suction and its engineering application

Two strength parameters (i.e., internal friction angle and cohesion) are prerequisites for determining the shear strength of soils. To clarify the variation patterns of these two strength parameters with suction for unsaturated soils, the failure stresses from 225 unsaturated soil triaxial shear tests and 102 unsaturated soil direct shear tests were analyzed. It was found that both the cohesion and internal friction angle of various unsaturated soils change with suction. Based on this, a new method ( CΦΜ ) for describing the shear strength of unsaturated soils was proposed, which can simultaneously consider the influence of suction on both the cohesion and internal friction angle of unsaturated soils. In contrast, the Effective Stress Method (ESM) and the method suggested by Fredlund (FCM) attribute the influence of suction solely to frictional effects and cohesive effects, respectively, which are one-sided and unreasonable. The cohesion of all soil types increases with increasing suction, while the internal friction angle exhibits various patterns of change with suction, which can be summarized into eight types. For each type, mathematical expressions describing the variation of internal friction angle and cohesion with suction were established, each containing only 2-3 parameters for ease of application. Theoretical expressions for calculating earth pressure, critical edge pressure, ultimate bearing capacity, and slope stability safety factors of unsaturated soils using ESM, FCM, and CΦΜ respectively were proposed, providing a scientific basis for engineering applications. For a steep slope terrace of intact Q3 loess at Lanzhou University of Technology, the earth pressure, critical edge pressure, ultimate bearing capacity, and slope stability safety factor at the site were calculated using ESM, FCM, and CΦΜ , respectively. It was found that at high suction levels (where the corresponding effective stress parameter is small), the calculation results using ESM contradicted common sense, while the results using CΦΜ were in good agreement with measured data. This also explains the mechanical mechanism behind the stable existence of very high loess columns and loess walls in loess regions.