Clay Strength Under Earthquake Loading Conditions

A procedure for determining the combinations of sustained stress and pulsating stress that will cause failure of a given soil is described and such data is presented for three soil types. The relationship depends on the nature of the loading conditions (one-directional or two-directional), the soil type, the principal stress ratio during consolidation, the frequency and duration of the pulsating stresses, the number of stress pulses, and the form of the stress pulse. Data is also presented on the strains resulting from different stress combinations. For typical field and earthquake conditions, it is shown that the relationship between total stress (sustained plus pulsating) and total strain for some soils will not differ greatly from the stress versus strain relationship for the soils determined by conventional undrained test procedures. Finally, it appears that in sensitive clay, a series of vibrations or stress pulses that will not in themselves cause failure, may induce an increase in pore water pressure and initiate creep movement that will lead to failure some time after the vibrations have ceased.