Abstract: Water-sensitivity and structural property of loess which have been widely studied are the two marked characteristics compared with the other soils. But most test researches are always conducted by using soil samples taken only in the vertical direction, which means only the vertical mechanical behaviour is taken into account. It is absolutely both reasonable and correct to study the vertical mechanical properties of loess mainly loaded vertically by upper foundations, but for those constructions with complicated stress state at different parts such as loess slopes, tunnels and/or underground structures, the anisotropy of loess might be more significant. The undisturbed Q4 loess in Lanzhou is sampled both from vertical and horizontal directions; then triaxial shear tests are conducted to investigate the difference of shear strength and deformation parameters under different confining pressures. The shear strength parameters obtained from triaxial test are also compared with those from direct shear test. The test results show that the anisotropy in mechanical behaviours of Q4 loess in Lanzhou is remarkable. The anisotropy in shear strength is mainly externalized by cohesion but the friction is nearly the same. Both the vertical and horizontal deformations can be characterized by conversion of brittle failure to plastic failure while the confining pressures are increasing. The difference in deformation modulus of loess between vertical and horizontal directions is quite obvious, i.e. the deformation modulus in vertical directions is averagely twice than that in horizontal directions. The test results are explained by using the structural strength and corresponding forming mechanism of loess. The anisotropy in mechanical behaviours of loess can be considered as another type of structural property of loess, which should be paid more attention in order to get more safe and reasonable parameters.

Key words: loess, anisotropy, structural property, shear strength, deformation modulus