Physical and Mechanical Performance of Frozen Rocks and Soil in Different Regions

The artificial freezing method is extensively used in the reinforcement of engineering strata in various regions for shaft excavation and subway connection channels. In this study, representative rock and soil strata from different regions were subjected to low-temperature physical and mechanical performance tests. The results show that, compared with Cretaceous and Jurassic rock and soil strata, deep topsoil and shallow coastal topsoil have high water content, low thermal conductivities, high frost heave rates, and high freezing temperatures. In addition, the results show that, as the curing temperature decreases, the uniaxial compressive strengths and elastic moduli of deep topsoil and shallow coastal topsoil increase almost linearly. The strength of the sandy soil strata is the highest, followed by the cohesive soil strata, and the strength of the mucky soil and the calcareous clay is the lowest. The strength of the frozen wall and the waterproof requirements must both be taken into account in the freezing design. Deep Cretaceous and Jurassic rocks can have high strength of more than 5 MPa under normal temperature conditions. An increase in the uniaxial compressive strength and elastic modulus with decreasing curing temperature is mainly manifested within the range from the normal temperature to −10°C. The strength can reach more than 10 MPa at −10°C, and only the strength requirements of the frozen wall need to be considered in the freezing design. At low temperatures, deep topsoil and shallow coastal topsoil are dominated by the form of compression failure. The average failure strain at −10°C is typically greater than 5%. When excavating the strata, it is essential to pay attention to the effect of creep. The failure strain of deep Cretaceous and Jurassic rocks is between 1% and 2%, and the breaking and sudden collapse of surrounding rocks should be prevented.


Introduction
e artificial freezing method freezes the water in unstable water-bearing strata using artificial refrigeration, which can increase the strength and stability of the strata and isolate the interaction between groundwater and the construction operating surface. is is a special construction technology that enables safe and smooth evacuation for groundwater engineering under the protection of the frozen wall. is method has been used extensively because of its good waterproof performance. Frozen soil mechanics has been a discipline since the publication of the first paper on frozen soil mechanics in the USSR in the 1930s [1][2][3][4]. In 1955, the freezing method was introduced into China and applied to the construction of the Kailuan coal mine shaft. It was used extensively in the construction of new wells in the central and eastern regions of China in the 1980s. Considerable experience was gained in artificial freezing engineering in the third and fourth series of topsoil strata in the central and eastern regions, and numerous studies were carried out on the physical and mechanical performance of the artificial frozen soil [5][6][7]. In the 2000s, the freezing method was gradually extended to the construction of new wells in the western region and the reinforcement of the holes of subway shields and subway connection channels in the eastern coastal area. Research on the mechanical performance of the artificial frozen soil has since been conducted nationwide [8][9][10][11][12][13][14]. e freezing method works similarly whether it is applied to new well construction or subway construction. However, the application of the freezing method, for the purpose of reinforcement in different regions of China, raises the issue that strata conditions vary greatly from region to region. New well construction in the central and eastern regions mainly involves deep topsoil, while, in the northwestern region, it involves deep Cretaceous and Jurassic rock strata. Work on the eastern coastal subway involves shallow coastal topsoil. China has conducted intensive research on the physical and mechanical performance of the artificial frozen soil in the topsoil strata in the central and eastern regions. Less research has been conducted, however, on the physical and mechanical performance of the primary frozen strata in the eastern coastal portion of the northwestern region. Most of the methods of freezing design used in this region are the same as those used in the central and eastern regions. Because of significant differences in the low-temperature physical and mechanical performance of frozen rocks and soil in different regions [15][16][17][18][19], a good grasp of the physical and mechanical performance of frozen rocks and soil in a specific region is important to ensure the correctness of the freezing design scheme and ensuring the safety and stability of the frozen wall. In this study, representative rock and soil masses in the eastern coastal region, central region, and western region were selected for comparison. Based on the result of tests of low-temperature physical and mechanical performance, a reference guide for the selection of freezing parameters for various regions in China was developed.

Test Plan
2.1. Sample Sources. Samples were obtained from shallow coastal topsoil in the eastern coastal city of Fuzhou, representative deep topsoil in the central Huainan mining area, and deep cretaceous and Jurassic rock strata in the Dongsheng mining area in western Inner Mongolia. e samples were numbered D1-D9. Basic information about the condition of each one is provided in Table 1.

Test and Methodology.
e types of physical and mechanical performance tests conducted are described as follows: ( e uniaxial compressive strength of frozen rocks and soil (σ) can be calculated as follows: where F max is the maximum measured test force, N, and A is the cross-sectional area of the calibrated sample, mm 2 . e elastic modulus of frozen rock and soil (E) can be calculated as follows: where ε 1/2 is the sample strain corresponding to half of the uniaxial compressive strength (%).

Results of Physical Performance Tests
e physical performance parameter values of the different rock and soil strata were determined from the test results and are summarized in Tables 2 and 3. e following conclusions were drawn from the test results: (1) e water content of the topsoil strata is high (>18%), while that of the rock strata is low (<10%). Since these rock strata were more recently formed and are poorly cemented, they will have exhibited argillation, disintegration, and other phenomena in a short time after being in contact with water. Because of the low water content of the rocks, the freezing temperature of the rock strata is also lower than that of the topsoil strata. e freezing temperature of the rock strata is in the range of −1.4 to −2.6°C.
(2) e topsoil strata have a frost heave ratio greater than 1% at −10°C, and it is necessary to take into account the effect of frost heave on the surroundings. e frost heave ratio of the sandy mudstone of the D6 group was 0.56%. Except for that, all the other three groups of rock strata have small frost-heaving ratios. e frost heave ratio of the Jurassic rocks in group D9 was 0, which indicates that not all of the rock and soil mass can undergo frost heaving when the temperature is below zero. If there is not enough water to participate in freezing of the rock and soil mass, the particles and ice inside will follow the thermal expansion and contraction mechanism, and thus the rock and soil mass will frost shrink rather than freeze.
(3) e thermal conductivity of the strata other than the sandy soil strata was approximately 2.0 W/(m·K), while the thermal conductivity of the rock strata was greater than 3.0 W/(m·K) at low temperatures. As the temperature drops from normal to −10°C, the thermal conductivity of different rock and soil mass increases significantly. In the increase of the topsoil, the increase in the thermal conductivity can be approximately 30%, and the thermal conductivity of the rock strata can increase by approximately 20%. e temperature field of the rock strata and the sandy soil strata in the topsoil develops relatively quickly at low temperatures, which is conducive to freezing. In contrast, in the remaining topsoil strata, especially the mucky soil and calcareous clay, which have relatively low thermal conductivities, the freezing temperature field develops relatively slowly at low temperatures.   (2) e strength of the rock strata also increased with decreasing curing temperature, but the range of increase was different for the rock strata than for the topsoil strata. For the two groups D6 and D9, which were from the mudstone rock strata, the strength increases were only 5.51% and 5.79, respectively, when the temperature decreased from normal to −5°C. In contrast, for the two groups D7 and D8, which were from the sandstone rock strata, when the temperature decreased from normal to −5°C, the strength increased by 66.98% and 24.00%, respectively, which are considerably greater increases than those of the mudstone rock strata groups. When the temperature dropped from −5°C to −10°C, the strength increase of the mudstone rock strata was greater, and the sandy mudstones (D6 group) were 51.97%. In contrast, the strength increases of the sandstone rock strata (D7 and D8) declined by 26.63% and 23.76%, respectively. When the temperature decreased from −10°C to −15°C, the strength increase of all of the remaining groups declined, except for the coarse sandstone in group D7, whose strength increase was greater than when the temperature changed from −5°C to −10°C. (3) e strengths of topsoil strata groups with different properties varied greatly. At the same low temperature, the strength of the sandy soil strata was the highest, followed by that of the cohesive soil strata in the mucky soil and the calcareous clay. For example, at −10°C, the strength of the mucky soil in the D1 group and the calcareous clay in the D4 group was only approximately 3 MPa, while that of the medium coarse sand in the D2 group was 4.51 MPa, a difference of more than 50%. e strength of the frozen wall and the waterproof requirements both must be taken into consideration in the freezing design. e strength of the Jurassic strata was slightly higher than that of the Cretaceous strata, both of which were more than 5 MPa under normal temperature conditions. e strength of the mudstone in group D9 was as high as 14.68 MPa. When the temperature was −5°C, the strength of the rock strata in each group was greater than 7.8 MPa; when the temperature dropped to −10°C, the strength in each group was greater than 11.2 MPa. Because of the high strengths of the Cretaceous and Jurassic strata, the waterproof requirements of the frozen wall should be the primary concern in the freezing design. (4) e trends in the elastic modulus of frozen rocks and soil with changes in temperature were consistent with those for the uniaxial compressive strength: the elastic moduli of both the frozen rock and soil groups increased with decreasing curing temperature. When the curing temperature decreased from the normal temperature to −10°C, the increases in the elastic moduli were relatively large, whereas when the curing temperature was less than −10°C, the increases in the elastic moduli were small. e elastic moduli of the topsoil strata were between 80.16 MPa and 153.34 MPa at −10°C; the elastic modulus of the rock strata was considerably larger and can reach more than 300 MPa at normal temperature.
As shown in Figures 1 and 2, the uniaxial compressive strength and elastic modulus of the topsoil strata increased almost linearly with decreasing temperature. While the rock strata exhibited the same trend of increases in strength parameters with decreasing temperature, the parameter values did not increase linearly with change in temperature and actually varied greatly mainly because of the presence which is mainly caused by the existence of many fissure surfaces in the rocks and the differences in the strength parameters values of the different samples. e variations in the uniaxial compressive strength and elastic modulus with a temperature of the topsoil strata can be expressed by the following linear equations: σ � a + bT and E � c + dT, where a, b, c, and d are coefficients whose values are determined by testing and T is the temperature,°C . e fitted equations are shown in Table 5, which also shows that the correlation coefficient for each is greater than 0.995.

Comparative Analysis of Failure Characteristics.
e failure stress and failure strain are two important failure parameters of the uniaxial compressive strength test for frozen rocks and soil. e failure stress is the peak value of stress in the stress-strain correlation curve when the frozen soil is damaged, that is, when the corresponding strain of the sample's uniaxial compressive strength is the failure strain. However, some frozen rocks and soil samples exhibit a plastic stage after loading, in which the samples are not damaged or show extreme values of stress even when the strain reaches 30% or higher. When this occurs, 20% of the axial strain in the uniaxial compressive strength test is generally taken as the failure strain, and the corresponding stress is taken as the failure stress. erefore, in this test, when the applied force reaches its peak value, loading continues until one of the following three conditions exists: the strain increases by 5%, the force decreases by 15% from the peak value, or the sample axial strain reaches 20% even though the force is still increasing. If any of these occurs, the test is stopped. ese protocols were used to obtain the stress-strain correlation curves for the frozen rocks and soil in the different strata.

Comparison of Failure Stress and Failure Strain of Frozen Rocks and Soil in Different Strata.
e stress-strain correlation curves of the second sample in each group tested at −10°C were selected for comparative analysis, and the   Advances in Civil Engineering failure stress and failure strain of each sample were calculated, as shown in Figure 3 and Table 6. e samples of mucky soil in group D1 and clay in group D3 exhibited compression failure, and the stress-strain behavior was of the elastic-strain-hardening type. e stress in the mucky soil in group D1 increased steadily up to 20% strain. e strain corresponding to the peak stress of the clay in group D3 was 12.26%. e strain then continued to increase by 5%, while the strength only decreased by 0.2 MPa. e medium coarse sand in group D2 and the sandy clay in group D5 also exhibited compression failure, but their stress-strain behavior was of the elastic-strain-softening type. e failure    strains of the medium coarse sand in group D2 and the sandy clay in group D5 were 5.28% and 9.05%, respectively, indicating that a higher sand content corresponded to a smaller failure strain. Although the stress-strain behavior of the calcareous clay in group D4 was also of the elastic-strainsoftening type, after the sample reached the peak stress, the strain remained steady at only approximately 3% until monopitched shear failure occurred because of the weak structural plane. e freezing strength of the rock strata was higher than that of the topsoil strata, both of which exhibited split failure. e stress-strain behaviors of both were the compaction-elasticity-softening type, manifested in three   e failure strains of three samples in each group under different temperature conditions were averaged and compared. It was concluded that, for the topsoil strata, the temperature had no effect on the average failure strain of the D1 mucky soil. is was mainly because of the large pore size of the sample, which increases the stress in the sample during compression. e average failure strain remained at 20% under different temperature conditions. However, the temperature had a greater influence on the average failure strain of the remaining four groups of samples. e average failure strain decreased with decreasing temperature, the plasticity of the strata samples of the elastic-strain-hardening type declines, while the brittleness of these samples increases. Nevertheless, all of the failure strains were greater than 5% at −10°C. ese results indicate, that during excavation, the strata should be supported in a timely manner to control the creep deformation of the strata. For the rock strata, the average failure strain, which was between 1% and 2%, was slightly affected by the temperature. is indicates that, during excavation, temporary support to the strata should be provided to prevent the sudden collapse of surrounding rocks. e average failure strains of the samples in each group at different temperatures conditions are shown in Table 7.

Conclusions
By means of laboratory tests of frozen rock and soil samples from the eastern, central, and northwestern regions of China, the differences in the physical and mechanical performance of the different rock and soil strata in the different regions at low temperatures were examined. e main conclusions are as follows: (1) e thermal conductivities of the different strata were found to differ. e thermal conductivities of the mucky soil and calcareous clay in the topsoil strata were relatively low, and the development rate of the frozen wall temperature field was relatively slow, whereas the thermal conductivity of the rock strata was high, and the frozen wall temperature field developed relatively quickly, which is conducive to freezing. When calculating the thickness of the frozen wall, the effect of the freezing temperature of rock and soil mass should be taken into consideration.
(2) At low temperatures, the temperature has a greater impact on the uniaxial compressive strengths of different rock and soil strata, and the strength increase with decreasing temperature occurs primarily when the normal temperature drops to −10°C.
For the topsoil strata, the uniaxial compressive strength and elastic modulus increased almost linearly with decreasing temperature. At the same temperature, the strength of the sandy soil strata was the highest, followed by the cohesive soil strata, the mucky soil, and the calcareous clay. e strength and waterproof requirements of a frozen wall should be taken into account in the freezing design. For the rock strata, because of its high strength, the design of the frozen wall should mainly consider its waterproof requirements. (3) At low temperatures, the topsoil samples subjected to uniaxial compression failed predominantly in compression, whereas the calcareous clay was affected by the presence of a weak structural plane and exhibited monopitched shear failure. e rock exhibited splitting failure. e stress-strain behavior of the mucky soil and cohesive soil strata was mostly the elastic-strain-hardening type, while that of the calcareous clay and sandy soil strata was of the elastic-strain-softening type.
e stress-strain behavior of the Cretaceous and Jurassic rock strata was of the compaction-elastic-softening type. (4) For the topsoil strata, the average failure strain was greater than 5% at −10°C. For on-site excavation, early the strata should be supported in a timely manner to control the creep deformation of the strata. For the Cretaceous and Jurassic rock strata, the average failure strain was in the range of 1% to 2%. Temporary support of these strata should be performed in time to prevent surrounding rocks from suddenly breaking and collapsing during onsite excavation.
Since the physical and mechanical properties of frozen rock and soil in different soil layers in different regions of China are very different, the samples tested in this study were obtained from the deep topsoil in the central region, the Cretaceous and Jurassic rocks strata in the western region, and the shallow coastal topsoil in the eastern region, where artificial freezing is widely used. e results of this study can provide guidance for the characterization of low-temperature physical and mechanical properties of soils in other regions. However, specific low-temperature physical and mechanical property tests are still needed to accurately determine the low-temperature physical and mechanical properties of soils in other regions.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper. 8 Advances in Civil Engineering