Effects of Three-Dimensional Cyclic Stresses on Permanent Deformation of Natural Undisturbed Clay

The cyclic stresses in subgrade soil induced by traffic loads are three-dimensional—cyclic major, intermediate, and minor principal stresses. To investigate the permanent deformation of natural undisturbed clay under three-dimensional cyclic stresses, a series of undrained true triaxial tests was carried out, using a true triaxial apparatus that could apply cyclic major, intermediate, and minor principal stresses simultaneously. The emphasis was on the effects of the cyclic stress ratio (CSR), the coefficient of cyclic intermediate principal stress (b_cyc), and the reciprocal slope of the stress path in the p–q plane (ξ) on the permanent deformation characteristics. The test results show that b_cyc reduces the accumulation of permanent major principal strain $({\epsilon }_1^p)$ remarkably. For given values of the number of loading cycles (N), CSR, and ξ, ${\epsilon }_1^p$ decreases approximately linearly with increasing b_cyc. ξ also limits the accumulation of ${\epsilon }_1^p$. Furthermore, the ratio of ${\epsilon }_1^p$ for a given value of ξ to ${\epsilon }_1^p$ for ξ = 1 decreases from 1.095 to 0.887 when ξ increases from 0.67 to 1.5, and the relationship between ${\epsilon }_1^p(\xi )/{\epsilon }_1^p(\xi =1)$ and ξ is described by a linear formula. A critical value of b_cyc is observed at which ${\epsilon }_2^p$ changes from tension to compression, that is, ${\epsilon }_2^p$ is zero at the critical value of b_cyc. The difference among the critical values of b_cyc in all the tests is relatively small, and the mean critical value of b_cyc is 0.328. Moreover, an empirical formula is established for predicting the permanent major principal strain under three-dimensional cyclic stresses.