The stiffness degradation and strength reduction of clays due to cyclic loading are investigated with both the cyclic and static triaxial tests. Two procedures to predict the deformation modulus of the clay subjected to cyclic loading are presented. One requires the static and cyclic test results and the other requires the static test results and the plasticity index.
　Following remarks are obtained:
1) The shear strength reduction of clays was not observed, when the double amplitiude of axial strain caused by the cyclic loading was less than 2%. This value represents the threshold strain in the strength reduction of clays.
2) The static shear strength of clays which experienced the double amplitude of axial strain of 10% decreases up to 0.7 times the initial static shear strength. In such a case, as a large strain is induced by external forces, the reduction should be considered in the design of clay foundations.
3) The deformation modulus degradation was 3 times larger than the strength reduction. The effects of the cyclic loading appear more notably in the deformation modulus than the strength reduction. The deformation of foundations due to cyclic loading should be considered appropriately in design procedures.
4) Two kinds of procedure to predict the deformation modulus degradation are proposed based on the laboratory test results. It may be thought that the proposed methods have appropriate accuracy, since the average value of the prediction coincides with the measured one.
5) The procedure using the static test results and plasticity index is not so appropriate for evaluating the deformation modulus of the overconsolidated clay.