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

Abstract

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 (bcyc), and the reciprocal slope of the stress path in the p–q plane (ξ) on the permanent deformation characteristics. The test results show that bcyc reduces the accumulation of permanent major principal strain (ε1p) remarkably. For given values of the number of loading cycles (N), CSR, and ξ, ε1p decreases approximately linearly with increasing bcyc. ξ also limits the accumulation of ε1p. Furthermore, the ratio of ε1p for a given value of ξ to ε1p for ξ = 1 decreases from 1.095 to 0.887 when ξ increases from 0.67 to 1.5, and the relationship between ε1p(ξ)/ε1p(ξ=1) and ξ is described by a linear formula. A critical value of bcyc is observed at which ε2p changes from tension to compression, that is, ε2p is zero at the critical value of bcyc. The difference among the critical values of bcyc in all the tests is relatively small, and the mean critical value of bcyc is 0.328. Moreover, an empirical formula is established for predicting the permanent major principal strain under three-dimensional cyclic stresses.