The phenomenon of seabed liquefaction during ocean storm has been recognized, which may exert damaging influences to the coastal and offshore installations. To study problems associated with seafloor instability, a constitutive model for marine sand is formulated within the general framework of bounding surface plasticity. One feature of this model is the vanishing yield surface for stress reversal processes. The model is relatively simple, yet capable of simulating marine sand behaviour under various loading conditions either monotonic or cyclic, in particular, under rotational shear. The importance of principal stress rotation in practice is related to seabed loading conditions occurred under ocean wave propagation. A comparison of model predictions and test results is presented.


The classical concept of a yield surface implies a purely elastic stress range contrary to the reality for many soils. Some very important aspects of soil behaviour, mainly relating to the cyclic response such as strain accumulation for drained cyclic loading or pore water pressure build up for undrained cyclic loading, cannot be adequately described. Nevertheless, the load situations caused by earthquakes, vehicular traffic and sea waves are further complicated due to "rotational shear", or the cyclic rotation of principal stress directions (Ishihara, 1983). There are two distinguished loading conditions belonging to this class. The first one is realized experimentally in a true triaxial device. The stress path remains on the octahedral plane (I1 = constant) a circle centered at the origin whose radius is a measure of the octahedral shear stress tool, entailing a continuous change of principal stress value but with fixed principal stress directions. The second is performed in a hollow cylindrical torsional apparatus. It is a cyclic orientational loading that entails a rotation of principal stress directions with a fixed value of the maximum shear stress. Numerous experimental studies (e.g. Arthur et al., 1980) have shown the important effects of rotational shear on the response of soils.

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