The uniqueness of the flow liquefaction line in the submarine bi-directional simple shear condition

Although the undrained shear strength has been widely studied for evaluating the factor of safety in slope, the effects of the initial shear stress induced by slope inclination are not fully understood. Some studies begin considering the conditions where the secondary shear direction along the direction of slope inclination, namely unidirectional shears. In the submarine environment, however, two directions are required to properly evaluate the true multidirectional shear strength because of the random direction of the wave and seismic load. This paper reports the bi-directional shear behavior of the Leighton Buzzard sand (Fraction B) from a specifically designed experimental program. The stress-strain relation, pore water pressure development, and flow liquefaction line are discussed. The test results show that the stress ratio at the peak of the stress-strain curve, called the gradient of flow liquefaction line, is unique for given vertical consolidation stress regardless of the direction and amount of initial shear stress. Though the sand behavior varies between different shear methods (bi-directional and unidirectional shear), the stress ratio at the initiation of instability is almost identical. Therefore, the total shear strength for submarine slopes under bi-directional shear conditions can be predicted through the constant gradient value of flow liquefaction line.