Abstract: A series of consolidated undrained triaxial shear tests were conducted on Qingyang undisturbed loess samples with different confining pressure and matrix suction combinations. The test results reveal that 1) with increasing in suction or confining pressure, the shear strength and initial shear modulus are both increased for the undisturbed loess samples. 2) increasing in the matric suction will cause the increase of apparent cohesion c″=c′+(u_a-u_w) tanφ″, the matric suction in the range of s=0~100 kPa will lead to an obvious increase in the apparent cohesion, whereas s>100 kPa will lead to a slow increase in the apparent cohesion in the undisturbed loess samples. This also shows that internal frictional angle φ″ induced by the matrix suction is not a constant and it decreases nonlinearly as the matrix suction increases. 3) the shear interlocking effect among particles increases with the increase of matric suction. This will induce an increase of the effective friction angle. 4）the apparent cohesion c″ =2 kPa of undisturbed loess in saturated state with suction s=0, this shows that runoff in the field occurs frequently for Qingyang loess being saturated and gradually lost the matrix suction under the continuous rainfall conditions.

Key words: Undisturbed Loess, Matrix Suction, Consolidated Undrained Shear, Shear Strength, Strength Parameters