Pore Pressures and Bearing Capacity of Layered Clays

The bearing capacity of layered clays can decrease temporarily after load application because of adverse changes of the pore-pressure distribution in the foundation soils resulting from water movement depending on pore-pressure dissipation. These pore-pressure changes are dependent on two soil properties: the permeability ratio and the compressibility-expansibility ratio. The case chosen is of an embankment founded on clays having different magnitudes of these properties, and the variations of the pore-pressure distributions with time are calculated. The resulting variations of bearing capacity with time of the clays are determined by means of stability calculations. It is concluded that if either the permeability ratio or the compressibility-expansibility ratio is small in magnitude (of the order of 3), then the bearing capacity of the clay can be expected to increase steadily after load application. If, however, both of these properties are large in magnitude, important temporary decreases of the bearing capacity of the clay can result. A review of published case records concerning pore-pressure measurements under loads on layered clays and bearing-capacity failures of layered clays shows that the field data are rather poor. Therefore, only tentative conclusions and comparisons can be drawn.