Numerical Simulations And Energy Dissipation Characteristics Of Frozen Soil With Prefabricated Holes Under Impact Loading

Frozen soil is a multi-phased and complex geological material,comprising solid particles,ice,liquid water,and gas.It has been subjected to various impact loads during cold-area engineering constructions,such as industrial tunnel excavations and coal mine shaft blasting.Thus,understanding the dynamic and mechanical properties of frozen soil has been an important research topic.Natural frozen soil contains numerous initial defects,such as holes and cracks of different sizes,either on the surface or in the interior.Therefore,understanding the influence of such defects on the dynamic mechanical behavior of the frozen soil enables experimental and theoretical guidance for soil engineering.We studied the influence of hole defects on the dynamic mechanical properties of frozen soil.Circular hole defects with 1.6,2.8,and 4.0 mm aperture were prefabricated on the frozen soil samples（with size:φ30×18 mm）.Then,the dynamic compression was investigated at different test conditions,such as temperature（-5°C,-10°C,and-15°C）and loading strain rates(600 s^-1,800 s^-1,and 950 s^-1),employing a split Hopkinson pressure bar（SHPB）device.The results indicated that the holes significantly reduced the dynamic compressive strength of the frozen soil samples.The peak stress and elastic modulus of the sample decreased upon increasing the aperture size,thereby lowering the deformation-resistance of the frozen soil.Defective samples exhibited temperature and strain rate effects.The peak stress increased as the freezing temperature decreased and the loading strain rate increased.Moreover,fractal theory was used to demonstrate the self-similarity of the sample fragment after impact crushing.The fractal characteristics were determined using fractal dimensions.We demonstrated that the impact fragments of the defective samples had notable fractal characteristics.Further,the sample fractal dimensions increased as the aperture size and temperature increased,demonstrating greater crushing.Owing to the dynamic brittleness of frozen soil and high strain rate,investigating its specific failure processes and modes（such as crack propagation and penetration）was difficult in the SHPB test.Therefore,the SHPB experiment was numerically simulated using the LS-DYNA software application and a modified Holmquist–Johnson–Cook（HJC）material model,to investigate the mechanical failure characteristics and specific failure modes of the frozen soil samples with prefabricated holes under impact loading.The simulation results were in good agreement with the experimental results.The unit failure characteristics and stress distribution of the frozen soil samples with holes were observed using the post-processing software,LS-PREPOST.Crack initiation and expansion in the defective samples were observed to begin from the hole ends that were experiencing concentrated stress.The samples were found to exhibit an X-type shear failure,which was consistent with the theoretical observations.The same as the failure rule of rock with holes.The frozen soil sample energy conversion and dissipation under various SHPB test conditions were summarized based on the one-dimensional stress wave theory and energy conservation law.The energy dissipation rate,which was calculated as the ratio of the dissipated energy to the incident energy,was used to evaluate the defective sample energy dissipation efficiency.Following the energy dissipation law,the dissipated energy of the samples with prefabricated holes was higher than that of the intact samples,under the same test conditions.The energy dissipation rate increased with increasing aperture size.Moreover,the energy dissipation rate of the defective samples increased with decreasing temperature.Finally,in order to determine the coupling effect of temperature and aperture,a dynamic constitutive model based on energy was established.According to the law of energy conversion and dissipation during impact loading,this model aimed to describe the complex dynamic stress–strain behavior of the defective samples.Our model effectively established the aperture sensitivity and temperature dependence of the defective sample,and accurately predicted the dynamic stress–strain curve of the frozen soil samples with prefabricated holes.