Properties of negative creep and its constitutive model for soils

The deformation over time under a constant pressure is creep. In general, the creep refers to compressive deformation, for example, the post-construction settlements after filling valleys in high-fill engineering. However, in order to get a high-fill ground, sometimes soils are excavated from high places. The expansion of soils after unloading cannot be stable in a short time, but continues to develop. This time-dependent expansion is defined as the “negative creep”, while the traditional compressive creep is named as the “positive creep” in this paper. Moreover, the test results show that as time is prolonged, both the positive and negative creep tend to be stable. Therefore, we can assume that in e-lnp space, when the time for the positive or the negative creeps is long enough, the state point will reach a straight line, which is parallel to the normal compression line. This line is named as the normal stable line. Based on the above concepts, a new one-dimensional formula, which can reasonably describe the negative creep effect of soils, is proposed. By combining the formula with the unified hardening (UH) model, a new UH model considering negative creep is established. Finally, the predictions of the new model are compared with the experimental results of triaxial undrained positive and negative creep tests to validate the new model.