Deformation characteristics of overconsolidated clay sheared under constant and variable confining pressure

Abstract

The traffic-induced settlements of clay subgrade are usually investigated based on one-way cyclic triaxial tests. Most one-way triaxial cyclic tests on clays have been carried out on normally-consolidated samples, with a constant confining pressure and under undrained conditions. However, it has been found that the varying stress field induced by traffic loading consists of not only cyclic axial stresses, but also cyclic horizontal stresses and shear stresses. Sometimes part of the subgrade soil is overconsolidated, rather than normally-consolidated, and the drainage of pore water is usually allowed under long-term traffic loading. Recognizing this, a series of one-way cyclic triaxial tests was performed on 30 specimens under partially-drained conditions, and the emphases were placed on the coupling effects of the overconsolidation ratio, the cyclic stress ratio, and the cyclic confining pressure on the deformation characteristics of the saturated clay. The test results show that increasing the amplitudes of cyclic confining pressure will cause a remarkable acceleration in the accumulation of both the permanent volumetric strain and the axial strain, whether the sample is overconsolidated or normally-consolidated. A linear relationship is found between the permanent axial strains from tests with variable confining pressure and their counterparts obtained from tests with constant confining pressure, regardless of the OCR and CSR values. A quantitative relationship, which considers the effects of the OCR values, is also established between the increments in cyclic confining pressure amplitudes and the increments in permanent axial strain.