Undrained Settlement of Plastic and Organic Clays

For most foundation loading situations, initial settlements due to undrained shear deformations are small. The common practice of computing consolidation settlements using the conventional 1-dimensional model therefore generally yields total settlement estimates of sufficient accuracy for design purposes. Initial settlements can become a predominant design consideration with highly plastic or organic foundation soils, or both, however—especially when loaded to low stability factors of safety, as illustrated by three case histories. Moreover, with slow field rates of consolidation, the occurrence of large initial settlements may also be followed by excessive undrained creep deformations. A semi-empirical method is presented for predicting the magnitude of initial settlement and the likelihood of excessive creep movements. Ways of reducing detrimental undrained shear deformations in design practice are explored.