While clayey soils subjected to cyclic loading may show a decrease or an increase in undrained shear strength during and immediately after the stress applications, their moduli have been observed to decrease considerably. Cyclic loads produced by earthquake or storm wave action may, even at relatively low strain level cause considerable shear deformations in soft clays which are under initial shear stresses, as in sloping delta and bay deposits.

Laboratory test data are presented which show the effects of cyclic strain amplitude, rate and number of applications on the strength and modulus of clays. The use of the laboratory data to the solution of typical field problems is illustrated with an example.

Laboratory shake table tests on soft clay banks appear to validate the analysis procedure. Moreover, the results of the application of the method to the field case appear to be reasonable. It is concluded that this simplified method is a practical and reasonable way to obtain estimates of the dynamic deformation of sloping soft clay deposits.


The expansion of industrial and commercial installations in highly populated near-water areas requires, in many cases, the use of land with poor subsurface conditions. This situation is illustrated in Figure 1, where fill is shown overlying a soft San Francisco Bay Mud deposit for subsequent use in a highway extension in the area.

Cyclic loading of the clay produced by water wave activity, or ground shaking induced by earthquakes in the region requires that the engineer search for appropriate answers to questions, such as:

  1. Is the site stable during and immediately after the cyclic loading?; or,

  2. What is the order of magnitude of the permanent deformations that the fill may sustain? Answers to both of these questions can be given at this time with a reasonable degree of confidence.

It is the purpose of this paper to review the experimental evidence that may be utilized to answer the first question, and to present a simplified method which provides an answer to the latter. The experimental validation of the method and its application to a practical case are also presented in the paper.


Castro and Christian (1) have recently reviewed the findings of various investigators on this subject. The authors concluded that when a saturated undrained soil is subjected to cyclic loading followed by undrained compressive loading to failure, the static shear strength is equal to or very close to the static undrained shear strength for samples that failed without initial cyclic loading. For example, tests on San Francisco Bay Mud (2) indicated that for cyclic strains less than one half of the strain to failure in static tests, the loss of strength after cyclic loading was less than 10 per cent of the original peak strength.

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