Abstract: In order to study the deformation characteristics of the undisturbed loess considering the change of the intermediate principal stress under high confining pressure conditions, equal b and equal p (p is the spherical stress) shear tests with controlled consolidation confining pressure and medium principal b values were conducted on undisturbed loess using a true triaxial instrument of Xi’an University of Technology. The relationships between the principal strains under high confining pressure were given, and the influence law of the failure strain and different b-value stages was revealed under different high confining pressure conditions. The results show that under the condition of high confining pressure, 3 of all b-value stages is expansion deformation, b = 0.2 is the critical point of intermediate principal strain deformation; 0≤b＜0.2 is the intermediate principal strain expansion deformation, and 0.2＜b≤1.0 is the intermediate principal strain compression deformation. In the high confining pressure state, the influence of the b value on the strain is much greater than the influence of the confining pressure on the strain. The degree of influence of b-value increment on strain is defined as b-value sensitivity K(ε), and b-value sensitivities K(ε2), K(ε3) and K(εs) all present an increasing trend when b-value lies between 0 and 0.7, are less affected by change in b-value between 0.7 and 0.9, and most affected by change in b-value between 0.9 and 1.0. K(εv) mostly falls in the range of 0 to 5, indicating that the failure volumetric strains in intermediate principal stress stages are less affected by the change of b-value.

Key words: undisturbed loess, true triaxial test, coefficient of intermediate principal stress, strain