Undisturbed clay samples were taken from alluvial deposits at seven different urban coastal locations in Japan using a thin wall sampler. The samples were then triaxially tested in both undisturbed and remoulded states. The cyclic data for the normally consolidated samples was used empirically to define the cyclic strength of the clays as an intrinsic function of the plasticity index Ip. This was then modified taking into account the vertical yield stress ratio σvy'/σv0', degree of aging. The cyclic strength of anisotropically consolidated clays at low initial drained shear stress ratios increased with decreasing plasticity. This reversed as the drained shear stress ratio increased. It was possible to define a simple function for the cyclic strength normalised by the isotropic cyclic strength RISO against the initial drained shear stress ratio. This function was shown to be dependent on plasticity index Ip.
The shortage of land around the large conurbations in Japan has resulted in numerous coastal engineering schemes including coastal defences, break waters, harbour and reclaimed land developments. These are often located on soft marine clay deposits. Additionally many cities and urban areas are located on alluvial clay deposits. Thus a knowledge of the properties of these soils particularly under wave and earthquake loading conditions is essential. The major concern m seismic design has usually been related to the problems of liquefaction of sands while clays have always been considered as stable materials under earthquake conditions. However there are frequently recorded instances of earthquake initiated failures in clay deposits. For example in the 1964 Alaska earthquake, a major slope failure occured in quickday; in the 1985 Mexico earthquake, buildings founded on clay suffered severe damage (Mendoza,1985) and in the 1964 Niigata, 1978 Miyagi-ken Oki (Sasaki, et al. 1980) and the Nihonkai Chubu earthquakes
