The purpose of this study was to investigate the effect of consolidation under a principal stress difference on the strength response of a cohesive soil possessing two different types of initial structure. A laboratory sample of an illite clay was consolidated one-dimensionally from a slurry, and cylindrical specimens were cut with their axes parallel and perpendicular to the direction of the one-dimensional consolidation stress. Constant deformation rate undrained triaxial compression tests were conducted to determine the stress-strain-strength characteristics. Experimental results indicated that (1) the relative magnitudes of the axial and radial strains during consolidation depend on the initial soil structure, as well as the respective consolidation stresses; (2) the moisture content after consolidation decreased with an increase in the principal consolidation stress ratio for a constant value of the mean consolidation stress; and (3) the stress-strain-strength response was dependent on both the principal consolidation stress ratio and the initial structure. Greater strength's decreased pore pressures, and increased values of the pore pressure coefficient A at failure occurred for larger values of the principal consolidation stress ratio. In addition to its dependence on the initial soil structure, the ratio of the undrained shear strength to the major principal consolidation stress was greatest for isotropic consolidation conditions and decreased considerably for anisotropic consolidation; however, for both the maximum principal stress difference and the maximum principal effective stress ratio failure criteria, values computed by using both A_f for the isotropic case and A_f for the anisotropic case generally agree within about 10 per cent with the comparable experimental values.

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This paper contains the theoretical considerations on the basis of Coulomb-Navier's theory.The rock with parallel weak planes fails along the weak planes, before a new fracture plane cuts planes in the solid rock, if the angle between the planes and principal stress lies in some range in the two dimensional case. This range can be determined from the conditions for failure in solid rock and weak planes.The active Rankine's earth pressure of the rock with weak planes is not smaller than that of an intact solid one, and the passive Rankine's earth pressure of such rock is not larger than that of an intact solid one.In case of three dimensional development of parallel weak planes, failure along weak planes takes place before a new fracture plane arises in the solid rock, if direction cosines of the normal to the planes with principal axes lie in a certain range. The medium principal stress, in this case, plays a very important role in failure along weak planes and determining the dangerous range of the direction cosines.

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