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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. [ABSTRACT FROM AUTHOR]

Flow liquefaction and flow slide of soils are two fundamental concerns in current geotechnical engineering. Previous studies mainly focused on level ground conditions where soils were consolidated under isotropic or K 0 conditions. However, in geotechnical engineering, sloping conditions are more unfavorable for geostructure because of the inherent initial shear stress induced by the self-weight of slope soils. In this study, a series of monotonic and cyclic bidirectional tests are conducted on Leighton Buzzard sand (Fraction B) to investigate the effects of initial shear stress on soil behavior. This study employs an energy-based method to interpret the test results by establishing a correlation between energy dissipation and soil behavior. The monotonic tests show that the energy dissipation for initiating strain-softening behavior is highly dependent on the level of initial shear stress but independent of the initial shear stress direction. The cyclic tests show that both the magnitude and direction of initial shear stress can affect the liquefaction resistance and cumulative energy. Through the energy-based method, this research provides new insights into evaluating soil strength in sloping ground conditions. • Employing an energy-based method to interpret the soil behavior in sloping ground conditions. • The required energy for triggering the flow failure decreases with the increase of initial shear stress. • The dissipated energy at the peak stress is independent of the initial stress orientation. • Specimens show an obvious degeneration in the liquefaction resistance due to the presence of the initial shear stress. • The relationship between dissipated energy and pore water pressure is independent of the initial shear stress orientation. [ABSTRACT FROM AUTHOR]

• Employing an energy-based method to interpret the post-liquefaction behavior of submarine soil in sloping ground conditions. • This paper refines the equations proposed in former studies to estimate the liquefaction energy in the initial and post-liquefaction stages. Introducing a new variable α, thereby the equation can be extended to submarine-inclined terrains. • The peak shear strength increases with initial shear stress, but the energy required to initiate the strain-softening state reduces with initial shear stress. • Both virgin and post-liquefaction specimens show an obvious degeneration in the liquefaction resistance due to the presence of the initial shear stress. However, the energy required for particle rearrangement is improved under initial shear stress. • The relationship between dissipated energy and pore water pressure is closely related to the initial shear stress level and stress histories. For generating the same pore pressure, post-liquefaction samples require more energy than virgin samples. In offshore earthquake regions, granular soils are prone to liquefaction and flow failure, resulting in imponderable damage to adjacent structures. When the foundation is constructed on inclined terrains, initial static shear stress induced by the self-weight of slope soils can significantly affect the soil behavior and liquefaction potential. This paper discusses the effects of initial shear stress on the liquefaction and post-liquefaction behavior through an energy-based approach. A series of monotonic and cyclic undrained simple shear tests are performed under various initial shear stress levels. The cumulative dissipated energy is used to evaluate the effects of initial shear stress on typical soil behaviors, including the onset of instability in monotonic tests and the liquefaction in cyclic tests. In monotonic tests, the cumulative energy required for triggering instability decreases with the initial shear stress level at the post-liquefaction stage. Moreover, at the same relative density, a strain-softening response on virgin samples can be transformed into a strain-hardening type at the post-liquefaction stage. In cyclic tests, a unique correlation is found between cumulative energy and pore water pressure and it is closely related to the initial shear stress level. Based on the test results, two relationships are proposed to estimate the cumulative energy required for liquefaction or flow-type failure at the initial liquefaction and post-liquefaction stages. [ABSTRACT FROM AUTHOR]

In submarine inclined terrains, granular soils are subjected to considerable initial shear stress, which significantly affects their cyclic behavior and liquefaction potential. When the external cyclic load is applied in varying directions relative to the dip direction, the prediction of liquefaction resistance may be complicated using conventional stress-based methods. This paper presents the effects of initial static shear stress in submarine slopes through an energy-based approach. Oval-shaped and linear shear stress paths are employed to replicate the stress condition commonly encountered in offshore environments. The test results indicate that the correlation between cumulative energy and pore water pressure is independent of the orientation of initial shear stress. On the other hand, increasing the initial shear stress level within a certain range will increase the dissipated energy required for reaching a given pore water pressure. At the end of this paper, a relationship is given for estimating the liquefaction energy considering the effects of initial shear stress. • Employing an energy-based method to interpret the submarine soil behavior in sloping ground conditions. • This paper introduces two new variables α and θ, thereby the energy equation can be extended to submarine-inclined terrains. • Specimens show an obvious degeneration in the liquefaction resistance due to the presence of the initial shear stress. • The relationship between dissipated energy and pore pressure is determined by the initial shear stress level and is independent of its orientation. [ABSTRACT FROM AUTHOR]