A correlation for cyclic degradation of natural clayey and sandy soils at different strain amplitudes

Cyclic degradation of soil significantly affects its behavior in seismic analyses, especially at relatively large strain levels. Beyond the threshold shear strain for degradation, soil properties, including its stiffness and damping, may change dramatically with each loading cycle. The present study aims to extend the investigation by probing the effect of cyclic degradation in greater detail, interpreting results from cyclic tests conducted at an advanced laboratory. In this regard, the results of 77 cyclic tests on natural sandy and clayey soils were analyzed. It was observed that the threshold strain for degradation is 1–2 × 10⁻⁴ for clayey soils and 3 to 4 × 10⁻⁴ for sandy soils. Furthermore, a correlation is proposed between degradation parameters and strain, which is useful for predicting the shear modulus for a desired loop number in cases where only the shear modulus of the 1st loop is available. To ensure the accuracy of this correlation, two additional tests were conducted in the laboratory. The predictions showed satisfactory agreement with the measurements. It was observed that after 15 loading cycles, the shear modulus of the soil decreased to 30–40 % of its initial value at the first cycle. This degraded modulus could be estimated using the correlation proposed in this study, with a 10 % margin of error. This validation supports the applicability of the proposed correlation.