Influence of moisture content on shearing strength of unsaturated undisturbed quaternary system middle pleistocene

ZHONG Zu-liang(钟祖良)^{1, 2, 3}, LIU Yuan-xue(刘元雪)³, LIU Xin-rong(刘新荣)^{1, 2}, LI Xiao-yong(李小勇)¹, WANG Sui(王睢)¹

(1. School of Civil Engineering, Chongqing University, Chongqing 400045, China;
2. Key Laboratory of New Technology for Construction of Cities in Mountain Area of Ministry of Education
(Chongqing University), Chongqing 400045, China;
3. Department of Civil Engineering, Logistical Engineering University, Chongqing 401311, China)

Abstract:The unsaturated undisturbed quaternary system middle pleistocene loess, a typical unsaturated soil, often occurs in the implementation of western development strategy. To obtain the shearing strength characteristics of this unsaturated undisturbed loess, based on the analysis of mineral composition, the triaxial shear test of undisturbed quaternary system middle pleistocene loess under different moisture contents is conducted with the specialized triaxial instrument for unsaturated soil. The test results show that the mainly mineral composition of undisturbed quaternary system middle pleistocene loess is quartz and albite. Under the same confining pressure, the matric suction increases with the decrease of moisture content. The smaller the moisture content, the larger the matric suction; the higher the moisture content, the lower the matric suction. Under the same moisture content, the matric suction increases with the confining pressure and reaches a maximum when the confining pressure is 100 kPa, and then decreases with the increase of confining pressure. This phenomenon is closely related to the grain contact tightness of soil mass under high confining pressure. According to the triaxial test of loess, the sample of loess experiences 4 stages from loading to failure: 1) compaction stage; 2) compression stage; 3) microcrack developing stage; 4) shear failure stage. The test sample is of brittle failure (weak softening) under low moisture content and confining pressure. With the decrease of matric suction and the increase of consolidated confining pressure, the stress-strain curve changes from softening type to ideal plastic type. In the shearing strength parameters of unsaturated undisturbed loess, the influence of moisture content on internal friction angle is small, but that on cohesive force is obvious. Therefore, the shearing strength of unsaturated undisturbed loess is higher than that of saturated undisturbed loess and varies with the moisture content.

Key words:unsaturated undisturbed loess; matric suction; test; shearing strength