True triaxial mechanical properties of unsaturated loess in foundation pit engineering

Abstract

Loess is a typical structural unsaturated soil. Because the matric suction of unsaturated loess changes with water migration, the properties of unsaturated soil clearly change. Therefore, the matric suction is considered the main factor affecting the properties of unsaturated loess. Generally, soils are subjected to three principal stresses in practical engineering. To investigate the properties of unsaturated natural loess under complex stress states, a suction-controlled true triaxial apparatus with a rigid-flexible boundary was applied to test unsaturated loess under true triaxial loading. Four sets of suction-controlled tests, including isotropic consolidation followed by monotonic shearing, were carried out, and the matric suction was controlled in the true triaxial tests. The compression yield characteristics of loess were investigated under different matric suctions. The influence of the matric suction and intermediate principal stress parameter were analysed based on the test results. Consolidated drained tests of loess with constant matric suctions were performed under a constant net mean stress and deviatoric stress with different intermediate principal stress parameters. Stress-strain curves and failure envelopes of loess were also presented. The stress-strain-strength properties of unsaturated loess are related to the matric suction and intermediate principal stress parameter under true triaxial stress conditions. The earth pressure of the Q3 loess stratum of the loess foundation pit was analysed based on a strength criterion of unsaturated loess with matric suction.