Dynamic Mechanical Properties and Failure Mode of Artificial Frozen Silty Clay Subject to One-Dimensional Coupled Static and Dynamic Loads

+e dynamic stress-strain relationship of artificial frozen silty clay under one-dimensional coupled static and dynamic loads is obtained using modified split Hopkinson pressure bar (SHPB) equipment. +e variation in dynamic compressive strength, dynamic deformation modulus, energy dissipation, and failure mode of artificial frozen silty clay with axial precompressive stress ratio are studied in this research. Experimental results indicate that the dynamic stress-strain curves under uniaxial state and onedimensional coupled static and dynamic loads can be divided into four stages, i.e., compaction stage, elastic stage, plastic stage, and failure stage. +e dynamic compressive strength, first-stage deformation modulus, second-stage deformation modulus, and absorbed energy density of artificial frozen silty clay present a trend of first increase and then decrease with the increase of axial compressive stress ratio, and the axial compressive stress ratio corresponding to the peak value is 0.7 in this test. In addition, there is a very similar effect of axial precompressive stress ratio on dynamic compressive strength and second-stage deformation modulus of artificial frozen silty clay. At 0.4 axial compressive stress ratio, spall phenomenon appears at circumferential direction and center position of the frozen soil specimen has no obvious failure. Shear failure appears at 0.7 to 0.9 axial compressive stress ratio, and the larger the axial compressive stress ratio applies, the more obvious the shearing surface appears; moreover, comminution failure mode appears at 1.0 axial compressive stress ratio.


Introduction
e dynamic mechanical property is one of the most important branches of mechanical property of frozen soil, which has attracted extensive attention by scholars in recent years [1][2][3].Many studies are conducted to investigate the dynamic mechanical and deformation characteristics of frozen soil using the split Hopkinson pressure bar (SHPB) equipment.e early research primarily concentrates on the dynamic mechanical properties of frozen soil under uniaxial loading state.Moreover, effects of freezing temperature, water content, and strain rate on dynamic stress-strain relationships, strength characteristic, and failure modes are studied experimentally and numerically [4][5][6][7][8].Previous studies indicate that the dynamic compressive strength of frozen soil increases with the increase of strain rate and the decrease of freezing temperature [4][5][6].In addition, studies show that stress state has significant effect on dynamic mechanical property of frozen soil [7,8].
Artificial freezing method is an environmentally friendly technique to provide temporary excavation support and groundwater control during structural construction, such as shaft and tunnel excavation, metro construction, under difficult geological and hydrological ground conditions [9,10].In fact, artificial frozen soils at this condition are subjected to static stresses arising from geostatic stress and frozen-heave force coupled with dynamic loading induced by blasting and machine impact [11,12].e mechanical behaviors of artificial frozen soil under coupled loads are obviously different compared with those under uniaxial loading state.However, not much previous investigations focusing on dynamic mechanical property of frozen soil under different stress states were found.Ma et al. [13] conducted a series of SHPB tests under active confining pressure state for artificial frozen clay and frozen sandy soil, and the effects of active confining pressure on dynamic mechanical property of frozen soil are analyzed.Furthermore, the dynamic constitutive model of frozen soil under active confining pressure are also proposed and verified.Investigations of dynamic behaviors of materials under coupled static and dynamic loads are primary on rock [14][15][16][17].Gong et al. [18] studied the effects of high strain rates and low confining pressures on the dynamic mechanical properties and failure modes of sandstone with a modified triaxial SHPB system.Moreover, the unified dynamic Mohr-Coulomb and Hoek-Brown strength criteria were established based on the obtained test results [19].
However, by summarizing previous theoretical and experimental study achievements, it can be found that researches on the mechanical properties of frozen soil under coupled static and dynamic loads have not been studied.In this paper, the dynamic mechanical and deformation characteristics of artificial frozen silty clay are investigated using the coupled static and dynamic loading test system.In addition, the dynamic stress-strain curve characteristics, dynamic compressive strength, deformation modulus, failure mode, and energy dissipation of artificial frozen silty clay under different axial compressive stress ratios are studied.
e results are of certain significance for both studying the property and energy dissipation characteristics of frozen soil and guiding the frozen soil crushing engineering.

Artificial Frozen Soil Specimen Preparation.
e test frozen silty clay collected from a coalmine in Shanxi Province, China.Detailed explanations and demonstrations on the treating processes of undisturbed soil have been introduced [13].e property and particle size distribution of silty clay are shown in Tables 1 and 2, respectively.Specimen sizes for static and dynamic tests are Φ50 × 100 mm and Φ50 × 25 mm, respectively, as shown in Figure 1.[20] have shown that frozen soil in artificial freezing engineering is subjected to confined vertical and horizontal static stresses.In this research, when the frozen soil specimen is subjected to static stress arising from circumferential stress coupled with axial dynamic loading, it is defined as the active confining pressure state.As a contrast, when the specimen is subjected to static stress arising from axial precompressive stress coupled with axial dynamic loading, it is defined as the one-dimensional coupled static and dynamic loads state.e stress schematic of active confining pressure state and onedimensional coupled static and dynamic loads is shown in Figure 2. SHPB equipment has been successfully used to investigate the dynamic behaviors of materials, such as rock [21], frozen soil, and concrete.In this test, a modified dynamic testing equipment is adopted [18,19], as shown in Figure 3.It contains a launch device, a spindle-shaped striker, which has benefit of stress balance of frozen soil specimen, an incident bar, a transmitted bar, an axial precompressive stress system, and a buffer bar.e length of incident bar, transmitted bar, and buffer bar are 2000 mm, 1500 mm, and 500 mm, respectively, and the diameters of them are 50 mm.e test procedure is as follows: (1) put artificial frozen soil specimen between the incident bar and transmitted bar; (2) apply axial precompressive stress to the test specimen; (3) close the oil pipe when the value of axial precompressive stress reaches the predetermined value; (4) open launch device and make the striker impact the incident bar.

Coupled Static and Dynamic Loads Tests. Studies
According to the collected data, the dynamic stress, strain, and strain rate can be calculated through simplified three-wave analysis [22].In the process of applying axial precompressive stress, a certain amount of elastic strain energy has been deposited inside the frozen soil specimen [23], which can be calculated through where E S is the elastic strain energy deposited inside the specimen, w S is the elastic strain energy density, and V S is the volume of specimen.w S can be calculated through us, the absorbed energy of artificial frozen soil (W S ) can be calculated through the following equation: where W I , W R , and W T are the incident energy, reflected energy, and transmitted energy, respectively; E 0 , A 0 , and C 0 are Young's modulus, the cross-sectional area, and the elastic wave speed of the bar, respectively; σ T and σ R are the transmitted stress and reflected stress, respectively; and t is the duration time of elastic wave.
In addition, the concept of absorbed energy density (W) is adopted and defined as the ratio between the absorbed energy and the volume of specimen, which can be described by

SHPB Tests Design.
To investigate the effect of axial precompressive stress on dynamic strength and deformation characteristics of artificial frozen silty clay, the axial precompressive stress ratio (λ) is adopted and defined as the ratio between the axial precompressive stress (σ ac ) and the static compressive strength of artificial frozen silty clay (f c ), 2 Advances in Civil Engineering as shown in equation ( 5).In contrast, the static compressive tests of artificial frozen silty clay are also conducted.e SHPB tests design is shown in Table 3:

Static Test Results.
To ensure the reliability of test results, three artificial frozen soil specimens are prepared and tested for each case and the average value of experimental data are recorded.e static stress-strain curves are shown in Figure 4.It is obvious that the static stress-strain curve of artificial frozen silty clay at −10 °C can be divided into three stages, i.e., elastic stage, plastic stage, and failure stage.e average static compressive strength is 2.73 MPa. Figure 5 demonstrates that shear failure appears in the static test.

Coupled Static and Dynamic Loads Tests Results.
In the present study, the purpose of SHPB tests on the artificial frozen silty clay specimen is to obtain the dynamic stressstrain relationships and study the effects of axial precompressive stress ratio on dynamic performance (e.g., dynamic compressive strength, deformation modulus, failure mode, and energy dissipation).Results obtained from  dynamic impact tests are summarized in the following sections.

Dynamic Stress-Strain Curves.
Figure 6 shows the representative dynamic stress-strain curves of artificial frozen silty clay under uniaxial state and one-dimensional coupled static and dynamic loads.It can be concluded that dynamic stress-strain curves can be divided into four stages, i.e., compaction stage, elastic stage, plastic stage, and failure stage under two states.Additionally, the compaction stage of the curve under one-dimensional coupled static and dynamic loads state is less obvious compared with that under uniaxial state.

Dynamic Compressive Strength and Deformation
Modulus.In this test, dynamic compressive strength (σ d ) is defined as the dynamic peak stress in dynamic stress-strain curves.e defining method of deformation modulus is as follows: (1) the first-stage deformation modulus (E 1 ) is defined as the slope of the line from the origin to the point at which the stress is 50% peak stress on the stress-strain curve; (2) the second-stage deformation modulus (E 2 ) is defined as the slope of line from the point of 50% peak stress to the point corresponding to peak stress on the stress-strain curve [24], as shown in      e mechanism behind this phenomenon can be clarified as follows: (1) If the axial precompressive stress ratio is less than 0.7, from the static stress-strain curves of artificial frozen silty clay, it can be noticed that the frozen soil specimen remains in the elasticity stage and microcracks and voids inside the frozen soil specimen will be closed.erefore, more stress waves will propagate through the specimen, resulting in the increase of dynamic mechanical parameters.(2) If the axial precompressive stress ratio exceeds 0.7, it is obvious that the internal microdamages and macrodamages will generate and develop inside frozen soil specimen, resulting in the degradation of frozen soil specimen.In this case, more stress wave will be reflected, resulting in the decrease of dynamic mechanical parameters.
Based on the above analysis, we can conclude that σ d , E 1 , and E 2 present a similar trend of first increase and then decrease with the increase of axial precompressive stress ratio.To investigate the internal relationships between the above three important parameters and axial precompressive stress ratio, the strength growth factor (σ dcf ), first-stage deformation modulus growth factor (E 1f ), and secondstage deformation modulus growth factor (E 2f ) are Advances in Civil Engineering adopted to demonstrate change rules.e definition of σ dcf , E 1f , and E 2f can be described by where σ dcλ , E 1λ , and E 2λ are the dynamic compressive strength, the first-stage deformation modulus, the secondstage deformation modulus of artificial frozen silty clay with different axial precompressive stress ratios, respectively; σ dc0 , E 10 , and E 20 are the dynamic compressive strength, the first-stage deformation modulus, the second-stage deformation modulus of artificial frozen silty clay under uniaxial state, respectively.According to the calculation methods, the σ dcf , E 1f , and E 2f with different axial precompressive stress ratios are calculated and illustrated in Figure 11. Figure 11 reveals that there is a very similar effect of axial precompressive stress ratio on σ dcf and E 2f .However, the variation trend of E 1f is clearly different from that of σ dcf and E 2f .e E 1λ reflects the deformation capability of artificial frozen silty clay at the elastic stage; however, E 2λ reflects the deformation capability at the plastic stage, which corresponds to the damage evolution process of the specimen, and many studies results have testified that the materials strength is effected by the damage evolution process [25][26][27].erefore, similar variation trend can be found about σ dcf and E 2f .

Dynamic Failure Mode.
In frozen soil crushing engineering, studying the failure mode of artificial frozen soil has important significance for improving the crushing efficiency.
e failure modes of artificial frozen silty clay under different axial compressive stress ratios are shown in Figure 12.
Figure 12 reveals that the axial compressive stress ratio has significant effect on the failure mode of artificial frozen silty clay.ree typical failure modes are found under onedimensional coupled static and dynamic loads, as described in the following: (1) At 0.4 axial compressive stress ratio, spall phenomenon appears in circumferential direction and center position of the frozen soil specimen has no obvious failure.( 2) Shear failure appears at 0.7 to 0.9 axial compressive stress ratio, and the larger the axial compressive stress ratio, the more obvious the shearing surface appears, and moreover, the comminuted area close to the incident bar is greater than that close to the reflected bar, which is caused by the attenuation of stress waves [28].(3) Comminution mode appears at 1.0 axial compressive stress ratio.

Dynamic Energy Dissipation.
Absorbed energy density (W) is one of the most important parameters in frozen soil crushing engineering [29].e absorbed energy density under different axial compressive stress ratios are calculated based on equation ( 4), as shown in Figure 13.With the increase of axial compressive stress ratio, the absorbed energy density first increase and then decrease.e average absorbed energy density increases from 0.179 J/cm 3 to 0.417 J/cm 3 when the axial compressive stress ratio increases from 0 to 0.7.In other words, the absorbed energy of frozen soil specimen under 0.7 axial compressive stress ratio is 2.33 times larger than that under uniaxial state.However, when the axial compressive stress ratio reaches 1.0, average absorbed energy density decreases to 0.266 J/cm 3 , which shows 36.21%lower than that under 0.7 axial compressive stress ratio.
e results indicate that change in the axial compressive stress of frozen soil appropriately can improve the crushing efficiency and reduce the energy consumption.
is finding is of great value to design the crushing parameters in frozen soil crushing engineering.
is phenomenon demonstrates that, in this research, the capacity of frozen soil specimen resistance is the biggest at about 0.7 axial compressive stress ratio, leading to absorb more energy to reach the failure state.

Conclusions
e dynamic mechanical properties of artificial frozen silty clay are tested using modified SHPB equipment, and the effects of axial compressive stress ratio on dynamic stressstrain curves, dynamic compressive strength, deformation modulus, energy dissipation, and failure mode of artificial frozen silty clay are investigated in this research.e main conclusions are as follows: (1) e dynamic stress-strain curves under uniaxial state and one-dimensional coupled static and dynamic loads can be divided into four stages, i.e., compaction stage, elastic stage, plastic stage, and failure stage, and the compaction stage of the curve under coupled loads is not obvious compared with that under uniaxial state.

Advances in Civil Engineering
(2) With the increase of axial compressive stress ratio, the dynamic compressive strength, first-stage deformation modulus, second-stage deformation modulus, and absorbed energy density of artificial frozen silty clay under one-dimensional coupled loads are higher than those under uniaxial state.Both of them present a trend of first increase and then decrease with the increase of axial compressive stress ratio, and the axial compressive stress ratio corresponding to their peak values is 0.7 in this test.In addition, there is a very similar effect of axial precompressive stress ratio on dynamic compressive strength and second-stage deformation modulus of artificial frozen silty clay.(3) ree typical failure modes are found under onedimensional coupled static and dynamic loads.At 0.4 axial compressive stress ratio, spall phenomenon appears at circumferential direction and center position of frozen soil specimen has no obvious failure.Shear failure appears at 0.7 to 0.9 axial compressive stress ratio, and the larger the axial compressive stress ratio applies, the obvious the shearing surface appears; moreover, the comminuted area close to the incident bar is greater than that close to the reflected bar.Comminution mode appears at 1.0 axial compressive stress ratio.

Figure 1 :Figure 2 :
Figure 1: Artificial frozen silty clay specimens for static and dynamic tests.(a) Static test specimen.(b) Dynamic test specimen.

Figure 7 .
According to the defining methods of strength and deformation modulus, the experimental results of artificial frozen silty clay under different axial precompressive stress ratios are shown in Figures 8-10.e figures reveal that (1) dynamic compressive strength, first-stage deformation modulus, and second-stage deformation modulus of Launch device Striker with double-tapered shape Incident bar Artificial frozen soil specimen Axial precompressive stress system Buffer bar Transmitted bar

Figure 13 :
Figure 13: Dissipated energy density of artificial frozen silty clay with different axial compressive stress ratios under coupled loads.

Table 1 :
Property of silty clay.

Table 2 :
Particle size distribution of silty clay.

Table 3 :
SHPB test design of artificial frozen silty clay.