Accurate characterization of soil dynamic response is paramount for geotechnical and protective engineering. However, the impact properties of unsaturated cohesive soil have not been well characterized due to lack of sufficient research. For this purpose, impact tests using the Split Hopkinson Pressure Bar (SHPB) were elaborately designed to investigate the dynamic stress-strain response of unsaturated clay with strain rates of 204 similar to 590 s(-1). As the strain rate increased up to 500 s(-1), a lock-up behavior was observed in the plastic flow stage, which can be explained as the breakage and rearrangement of soil gains under a high level of stress. Further, the strain rate dependency of the dynamic strength was quantitatively characterized by the Cowper Symonds (CS) model and the CS coefficients were determined to be the intercept of 55 and slope of 0.8 in the double logarithmic scale of Dynamic Increase Factor (DIF) and strain rate space. Furthermore, the SHPB test was reproduced using a modified Material Particle Method (MPM), which involves an improved dynamic constitutive model for unsaturated soil considering the strain rate effect. The simulated stress-strain curves basically agree with the experimental results, indicating the feasibility of MPM for investigating the dynamic properties of unsaturated soil under SHPB impact loading.
Uncompacted saturated loess retains its residual pore structure without artificial compaction, making it highly sensitive to environmental changes such as dehydration-rehydration cycles. This study investigates the dynamic characteristics of uncompacted saturated loess in the Xi'an area, where infrastructure projects are commonly affected by the soil's instability. Dynamic triaxial tests were conducted under varying confining pressures and dehydration-rehydration cycles to examine the dynamic stress-strain relationship, dynamic modulus, and damping ratio variation. The methodology involved multi-stage loading using dynamic triaxial equipment, with cycles of drying and rehydration applied to replicate field conditions. A hyperbolic tangent function was used to model the dynamic stress-strain behavior, and structural parameters m1 and m2 were introduced to quantify the soil's stability and variability. Key findings show that dynamic stress increases with dehydration-rehydration cycles, while dynamic modulus and damping ratio decrease, especially during the initial cycles. The results provide critical insights into the behavior of uncompacted saturated loess under dynamic conditions, offering practical guidelines for managing soil stability in infrastructure projects across the Xi'an region.
Soil nonlinear behavior displays noticeable effects on the site seismic response. This study proposes a new functional expression of the skeleton curve to replace the hyperbolic skeleton curve. By integrating shear modulus and combining the dynamic skeleton curve and the damping degradation coefficient, the constitutive equation of the logarithmic dynamic skeleton can be obtained, which considers the damping effect in a soil dynamics problem. Based on the finite difference method and the multi-transmitting boundary condition, a 1D site seismic response analysis program called Soilresp1D has been developed herein and used to analyze the time-domain seismic response in three types of sites. At the same time, this study also provides numerical simulation results based on the hyperbolic constitutive model and the equivalent linear method. The results verify the rationality of the new soil dynamic constitutive model. It can analyze the mucky soil site nonlinear seismic response, reflecting the deformation characteristics and damping effect of the silty soil. The hysteresis loop area is more extensive, and the residual strain is evident.