# SHPB Experiment and MPM Simulation on Dynamic Properties of Unsaturated Clay Under Impact Loading
## 1 Introduction  
Soil, a typical carrier for military or civil construction, suffers from not only conventional static load, but also unexpected dynamic load. In the past decades, the mechanical behavior of soil under static and cyclic loading conditions has been examined many times. However, accurate characterization of the soil dynamic response subjected to impact loading is still a big challenge for researchers and needs further study. This will provide better insight into various engineering applications, such as the design of dynamic compaction [1] in geotechnical engineering, the assessment of landmine impact or projectile penetration [2] in military engineering, the investigation of underground structure damage from accidentalexplosions [3; 4] in protection engineering, and the design of aseismic performance and shock absorption for structures [5] in earthquake engineering. Extensive studies have demonstrated that the strain rate dependency [6; 7; 8; 9] plays an important role in characterizing the mechanical behavior of material. Since the Split Hopkinson Pressure Bar (SHPB) [10; 11] technique was developed and applied to investigate the dynamic behavior of material, the recognition of the strain rate effect has been extended from a quasi-static condition (\(10^{-7}\)-\(10^{-1}\) s\({}^{-1}\)) to a medium or high loading rate condition (\(10^{-1}\)-\(10^{4}\) s\({}^{-1}\)). However, when the conventional SHPB is adopted to study the impact properties of soil with a low wave velocity and high energy attenuation, it is difficult to obtain valid data due to the high-frequency oscillation and dispersion effect [12; 13]. Subsequently, many beneficial attempts have been made to overcome these problems, for instance, applying a pulse shaper [14; 15] to increase the rise time of the incident wave, embedding a quartz crystal [16; 17] to capture the weak transmission signal, placing the soil sample within a confining rigid tube to remedy the lack of structure strength, thereby causing an effective dynamic response [18; 19]. Based on the above improved techniques, the SHPB testing system has been successfully used for researching the dynamic properties of various soils under impact loading. [PERSON] et al. [20] performed the SHPB test on dry sand by placing the sample in a cylindrical tube made of three different materials and discussed the influence of the passive confining level. As the confining level increases, the sand can sustain more axial stress, thereby resulting in a stiffer dynamic response. Furthermore, [PERSON] et al. [21] pointed out that the shearing resistance of sand is highly affected by gain crushing at lower confining pressures, but is negligible at high confining pressures. [PERSON] et al. [22] found that water can lubricate the soil particles, resulting in sand in an unsaturated state becoming more compressible and presenting a more obvious strain rate dependency compared to dry sand. Unlike sand, the dynamic response of clay under SHPB impact loading presents a remarkable strain rate effect [23; 24; 25]. To evaluate the rate dependency properties of compacted clay, [PERSON] et al. [24] adopted the dynamic increase factor (DIF) to carry out an analysis on the dynamic peak stress and maximum strain. The results showed that the DIFs for peak stress and maximum strain exponentially increased with the increase in strain rate, while the fitting parameters were highly affected by the compaction degree and water content. [PERSON] et al. [25] studied the effect of the consolidation state on the mechanical behavior of clay under dynamic uniaxial compressive conditions. The results indicate that the applied preconsolidation pressure contributed to the rate-sensitive characteristic of clay.