Characteristics of pore water pressure and strength of undisturbed saturated marine clay under complex stress conditions

The apparatus for static and dynamic universal triaxial and torsional shear soil tests is employed to perform stress-controlled coupling vertical and torsional shear tests under different three-dimensional initial anisotropic consolidation stress conditions. Through experimental tests the effects of the initial orientation angle of the major principal stress, the ratio of the two stress components and the cyclic stress level on the pore water pressure and the strength of the saturated marine clay are examined. The initial orientation angle of the major principal stress has a significant influence on the characteristics of pore water pressure: as the angle increases, both the residual pore water pressure and the amplitude of fluctuated pore water pressure increase. Meanwhile, the relations between the normalized residual pore water pressure ratio and the cyclic number ratio with different initial orientation angles of the major principal stress are established. Under the conditions of keeping the area bounded by the elliptical stress path unchanged, there exists a certain critical value for the ratio of the axial and torsional shear stresses. At this critical value, the residual pore water pressure is the lowest. Both the fluctuated pore water pressure and the residual pore water pressure increase with increasing cyclic stress ratio. However, there is great difference between the high and the low stress levels. Furthermore, the critical cyclic stress ratio is determined according to the time history curves of pore water pressure under different cyclic stress ratios. The initial orientation angle of the major principal stress has a significant influence on the dynamic strength: as the angle increases, the dynamic strength decreases. The dynamic strength curve indicates that there is basically a linear relationship between the cyclic stress ratio and the logarithm of the cyclic number required at failure.