Failure mechanisms of sand under asymmetrical cyclic loading conditions: experimental observation and constitutive modelling

Abstract

In practical engineering, natural soil deposits often sustain an initial driving force prior to cyclic shear, owing to earthquakes, traffic and waves; such asymmetrical loading conditions may significantly affect the liquefaction susceptibility and failure mechanism of sand. To understand the typical cyclic liquefaction responses, comprehensive asymmetrical cyclic loading tests were conducted on sand samples subjected to either compressional or extensional static stress. The results indicated that different stress conditions can result in three distinct failure mechanisms: flow liquefaction, cyclic mobility and residual deformation accumulation. According to the experimental observations, an anisotropic sand model was developed within the framework of the anisotropic critical state theory. The model employed a fabric-dependent dilatancy, and accounted for the effects of the fabric evolution and accumulated loading index on the plastic hardening, in order to better reflect the cyclic degradation of the plastic modulus. The predictive capacity of the model was confirmed through undrained monotonic test results for samples with different densities. Comparisons between the model responses and experimental results indicated the excellent capabilities of the developed model in terms of capturing the typical deformation, strength and fabric characteristics of different cyclic failure mechanisms of sand under either symmetrical or asymmetrical loading conditions.

Hypoplastic model with fabric change effect and semifluidized state for post‐liquefaction cyclic behavior of sand

“…It is seen from the figure that N f increases with D r but decreases with CSR . The simulated cyclic liquefaction resistance curve is steeper than that of the experimental data, as observed in many other constitutive models; see, e.g 11,34 . Overall, the model can broadly simulate the cyclic liquefaction resistance of sand under different densities.…”

“…As shown in Figures 9 and 10, the rate of pore pressure accumulation in the experimental tests decreases first and then increases during the pre‐liquefaction stage. This phenomenon has been widely observed in many other experimental studies 1,34,53 . Unfortunately, this cannot be captured by the hypoplastic model, with which the simulated pore pressure accumulates with a monotonically increasing rate.…”

“…As mentioned above, for many other constitutive models aiming to simulate the large post‐liquefaction shear deformation of sand, for example, 9,33,34 their model parameters are often difficult to determine because the cyclic response for the pre‐liquefaction and post‐liquefaction phases is modeled in an integrated rather than separate manner. As a result, the model response in these two phases is often influenced by the same constitutive ingredient, and the associated model parameters cannot be calibrated separately.…”

“…In the elastoplasticity framework, some attempts have been made to improve the simulative performance of pore pressure accumulation. Li 9 and Pan et al 34 . multiplied the plastic modulus of the model by a function of the accumulative plastic strain.…”

“…When the stress state is located above the neutral phase line, the model predicts a perfectly plastic response until the deviatoric strain reaches a boundary defined in the deviatoric strain space. Another widely used approach is to make the plastic modulus a function of the accumulated plastic strain, and thus it degrades with loading, thereby enabling the model to reproduce the increasing deviatoric strain amplitude 5,9,33,34 . Although these models can produce acceptable predictions, as demonstrated by the comparison between the experimental results and model simulations, they still have some apparent drawbacks.…”