Experimental study of the shear strength of soil from the Heifangtai Platform of the Loess Plateau of China

Abstract

Loess landslides induced by irrigation are common geological disasters in the Loess Plateau, China. Long-term watering decreases the matric suction and soil strength of loess, thus causing the frequent occurrence of landslides. Therefore, studying the strength characteristics of unsaturated loess is of great significance for knowing how such landslides occur and, a step further, preventing them. Undisturbed loess samples in Heifangtai Platform were collected to measure their soil water characteristic curves (SWCC) and a series of direct shear tests and triaxial shear tests were carried out with unsaturated loess. Moreover, the strength characteristics of the loess were discussed. The results showed that in the direct shear test, the cohesion (c) increased linearly with increasing matric suction whereas the suction internal friction angle (φb) was approximately constant. In the triaxial shear test, c increased nonlinearly with increasing matric suction. The value of φb was a constant when matric suction was 0–100 kPa, but decreased as matric suction increased within the range of 100–200 kPa. The effective stress parameter (χ) lay in the range from 0.5 to 0.8. When the matric suction was small, the shear strength calculated by Lu’s suction theory was almost the same as that obtained by Fredlund’s theory. With the increase of matric suction, the shear strength calculated by Fredlund’s theory gradually outperformed that of Lu’s suction theory. The shear strength calculated by the curve of moisture desorption was slightly larger than that of moisture absorption. The variation trends of strength parameters with matric suction under the two tests were similar. The effective internal friction angles obtained by the triaxial shear test are slightly larger than those by the direct shear test. The suction internal friction angle (φb) was approximately a constant and smaller than φ´. The φb obtained by the direct shear test was slightly larger than that obtained by the triaxial test. Lu’s theory can be used to evaluate the stability of unsaturated soils on some loess slopes with high soil moisture content.