Abstract: In order to study the strength and deformation characteristics of Q3 sand loess from Heifangtai, Yongjing, Gansu province, using the true triaxial apparatus developed by Xi'an University of Technology, true triaxial tests are carried out under conditions of different confining pressures (50, 100, 200 kPa), ratios of intermediate principal stress (b =0, 0.25 and 0.5) and water contents (5%, 10% and 15%). The test results illustrate that: (1) The shear failure modes of Q3 sand loess are displayed by lateral bulge failure and monolete shear failure under the true triaxial conditions; and there are other modes for few soil samples including taper failure, double-slit shear failure and T-shape shear failure. (2) The deviatoric stress difference of Q3 sandy loess increases with the ratio of intermediate principal stress when the confining pressure and moisture content are the same, while the ratio of intermediate principal stress (b) is different. Meanwhile, the stress-strain relationship curve shows hardening and obvious shear dilatancy phenomenon. However, the ratio of intermediate principal stress effects the relationship between generalized shear stress and generalized shear strain; the relation curve increases accordingly with the increase of b. With the increase of b, the strength decreases; and the relation curve between the generalized shear stress and the average spheric stress ratio (p/q) becomes flat gradually, and decreases accordingly. (3) With the increasing of water content, the Q3 sandy loess transfers to semisolid state from initial solid state. The cohesions with different ratios of intermediate principal stress all declines remarkably; but the internal friction angle increases a little.

Key words: Q3 sand loess, true triaxial test, stress-strain relationship, intermediate principal stress, failure mode