| The artificial freezing method is the use of artificial refrigeration technology to reduce the temperature of the soil, the formation of the temporary freezing reinforcement can achieve the purpose of improving the strength and stability of soil. In recent years, freezing method is applied to the construction of cross passage of metro tunnel in coastal soft soil area, which highlights its good sealing performance, high strength, safety and other advantages. However, in the construction of cross passage of metro tunnel with freezing method, the freeze-thaw circulation will affect the engineering properties of soil, resulting in different dynamic properties of frozen-thawed soil and non-frozen-thawed soil under cyclic loading. The further study on dynamic properties of frozen-thawed soil is necessary, which can provide the basis for controlling long term settlement after the construction of freezing method. In this paper, the dynamic properties of frozen-thawed soil under metro loading are studied by dynamic triaxial test, including the development law of dynamic pore pressure and strain. Frozen-thawed soil is tested to find the effect of freezing temperature, freeze-thaw cycle and initial consolidation degree of thawed soil on the dynamic properties, the dynamic models are proposed according to the experimental results. Combined with scanning electron microscopy (SEM), the microstructure of soft clay before and after freeze-thaw action, before and after cyclic loading is analyzed qualitatively, then the quantitative analysis of the microscopic structure is used to explain the weakening effect of freeze-thaw and the macroscopic deformation mechanism of frozen-thawed soil under cyclic loading. Through the analysis and summary, the conclusions are as follows:(1) In order to ensure the comparability of the results of laboratory tests, and to reduce the influence of the difference of the homogeneity of test soil samples on the test results, remolded clay is used in this paper. Based on the principle of vacuum preloading, a multi-connected vacuum preloading device is developed. By controlling the moisture content and density of soil samples to prepare uniform remolded soil for laboratory dynamic triaxial test. The multi-connected vacuum preloading device is mainly composed of four parts:vacuum pump, vacuum diversion volume, soil preparation tank and intelligent control system. Quantitative analysis of microscopic scanning tests on soil samples shows that the distribution of the pore area of horizontal and vertical cross-section in each heights is more consistent, the uniformity of the remolded soil is demonstrated.(2) Bias-sine wave is applied to simulate the subway loading, the development of pore pressure of frozen-thawed soil is similar to non-frozen-thawed soil during loading, which including three stages:rapid growth phase, slow growth phase and stable phase, and the pore pressure will reach the stable value around 10000 cycles. After the rapid growth stage, the strain of frozen-thawed soil enters a slow growth stage and the growth rate gradually decreases. It can be inferred that the strain will be stable after enough loading cycles. Under cyclic loading, the development rate of pore pressure and strain of frozen-thawed soil is faster than that of non-frozen-thawed soil. This is because the freeze-thaw cycle will destroy of internal structure of the skeleton, cause the increase of pore volume, resulting in the weakening of the soil structure.(3) For frozen-thawed soil, different freezing temperature and freeze-thaw cycles have influence on degree of damage to structure, which will lead to different dynamic characteristics. The experimental results show that the lower the freezing temperature, the faster the development rate of pore pressure and axial strain, and the greater the stable pore pressure and accumulated strain. Multiple freeze-thaw cycle will aggravated the destruction of soil structure, and the pore pressure and strain of secondary frozen-thawed soil develop faster and the stable value is larger. The reason is that the frost heave is more significant during freezing with low temperature, the growth of ice crystals destroy the grain skeleton, and the perforation of small pores lead to the appearance of large pores. In the process of melting, the pore shape and bonding of particles are difficult to restore to the original state which will lead to the weakening of structure, and the destruction of structure after secondary freeze-thaw cycle will be more obvious.(4) The redistribution of soil skeleton in the process of melting will finally lead to a new balance stage. and the meltinu rate of ice crystals is lager than the rate of dissipation of excess pore pressure. Before the formal operation of the subway, if the consolidation of frozen-thawed soil is not fully completed, the initial consolidation degree will affect the dynamic characteristics of foundation during the subway operation, and causing a large cumulative settlement. The undrained loading test results show that under cyclic loading, the lower the initial consolidation degree, the faster develop rate of pore pressure and strain, and the larger stable values. It will be better to consider the initial consolidation degree for the analysis of long term settlement of frozen-thawed soil during the subway operation.(5) Combined with the microscopic scanning electron experiment, the microstructure changes before and after freeze-thaw action, before and after cyclic loading are studied to reveal the dynamic characteristics of frozen-thawed soil from the microcosmic angle. After freeze-thaw action, the degree of particle agglomeration is obvious, flocculated structure appears, and the number of large pores increases significantly. The lower the freezing temperature, and the more freeze-thaw cycles, the greater the proportion of large pores, which will lead to more obvious structure weakening effect under cyclic loading. Under cyclic loading, the pore structure of soil has the tendency of compaction, the degree of particle breakage increases, which will lead to the increase of small pores and the decrease of large pores. The macroscopic dynamic characteristics are explained form the microcosmic angle. |
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