Undisturbed clay samples were taken from alluvial deposits at seven different urban coastal locations in Japan. The samples were then triaxially tested in both undisturbed and remoulded states. The cyclic data for normally consolidated samples was used to define the cyclic strength of the clays as an intrinsic function of the plasticity index 4. This was modified to take into account the initial vertical yield stress ratio and degree of aging. A further series of tests were carried out on samples which were anisotropically consolidated by being subjected to an initial drained shear stress. The cyclic strength of the anisotropically consolidated clays at low initial drained shear stress ratios increased with increasing 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 against the Initial drained shear stress ratio.
The shortage of land around the large conurbations has resulted in numerous coastal engineering schemes including coastal defences, break waters, harbour and reclaimed land developments which are often located on soft marine clay deposits. Thus a knowledge of the properties of these soils particularly under wave and earthquake loading conditions is essential. The major concern in seismic design has usually been related to the problems of liquefaction of sands while clays have always been considered stable materials under earthquake conditions. However there are recorded instances of earthquake initiated failures in clay deposits. For example in the 1964 Alaska earthquake, a major 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, embankments founded on clay deposits underwent slope failures. Although relatively clear guidelines exist on the liquefaction behaviour of sands, a similar knowledge base for clays under cyclic loading needs to be developed, particularly for in-situ undisturbed clays.
