Differences Of Physical And Mechanical Properties Of Undisturbed Soft Clay Before And After Freezing And Thawing

With the development of urban underground space in soft soil area, artificial freezing method is widely used in soft soil area of underground construction projects for its superior reliability and technical suitability, such as construction of tunnel connecting passage and pumping station, reinforcing soil for departure and reception of shield, metro repair works, etc. And when the artificial freezing method is used in construction, due to the effect of freezing and thawing of soil, the physical and mechanical properties of the soil are changed. Through the analysis of the physical and mechanical properties of the undisturbed soil and the frost-thawed soil. Through the difference analysis of the physical and mechanical properties of the undisturbed soil and the frost-thawed soil, we can reasonably predict and control the frost heave and thawing settlement of soil and provide the basis for relevant engineering design, which promotes the popularization and application of artificial freezing method at urban underground space.In this paper, it takes Ningbo clay as the research object, which is considered the typical soil layer in the soft soil area. Selecting the freezing temperature and thawing conditions as influencing factors, we conduct the frost heave and thawing settlement test and laboratory test of the frost-thawed soil and its undisturbed soil. Before and after freezing and thawing, the differences of the physical properties of water content, dry density, specific gravity, critical moisture content, void ratio, permeability and the mechanical properties of compressibility, shear strength of the undisturbed soft soil are studied. The main research results are as follows:(1) The freeze-thaw action leads to the redistribution of moisture content in the unidirectional freezing samples. The moisture content of upper sample decreases, and the moisture content of the middle and lower part of the sample increases. The lower the freezing temperature, the smaller the effect of freeze-thaw action on the water content redistribution of soil. Compared with the forced thawing, the effect of freezing and thawing of natural thawing on the soil water content redistribution is more intense.(2) The freeze-thaw action makes the dry density of the soil decreased. For the dry density of the samples, the effect degree of freeze-thaw action increase gradually from top to bottom. The lower the freezing temperature, the less the dry density of soil sample is affected by the freeze-thaw action.Unlike the freeze and natural thaw action makes the soil dry density decreased, the freeze and forced thaw action makes the dry density of the middle and lower part of the sample decreased and the upper of the sample increased. Compared with the forced thawing, the effect of freezing and thawing of natural thawing on the soil dry density is more intense.(3) Gravity does not change before and after freezing and thawing, which has nothing to do with the freezing temperature and thawing condition of the freeze-thaw action.(4) The plastic limit, liquid limit and plastic index of the soil are all increased after freeze-thaw action, which increase with the increase of the freezing temperature. The freeze-thaw actions of different thawing conditions have little effect on plastic limit, liquid limit and plastic index of the soil.(5) The freeze-thaw action makes the void ratio of soil increased. The effect of freeze-thaw action on the void ratio of the lower of the sample is greatest, the middle part is the second, and the upper is minimal. The higher the freezing temperature, the greater the effect of freeze-thaw action on the void ratio of soil. The void ratio of natural thawing soil is larger than that of undisturbed soil. However, the void ratio of the middle part and the lower of the forced thawing soil is larger than the undisturbed soil, the void ratio of the upper of the forced thawing soil is smaller than the undisturbed soil.(6) The soil permeability increases after freeze-thaw action, permeability coefficient increases significantly, which increase 1~3 times. The permeability of frost-thawed soil increases with the decrease of freezing temperature. Permeability and void ratio are positively correlated, dry density and permeability are negatively correlated.(7) The freeze-thaw action makes the compressibility of the soil increased. For the compressibility of the samples, the effect degree of freeze-thaw action increase gradually from top to bottom. The lower the freezing temperature, the smaller the effect of freeze-thaw action on the compressibility of soil. The freeze-thaw action of natural thawing makes the compressibility of soil increased, the freeze-thaw action of forced thawing makes the compressibility of the middle part and the lower of sample increased, which makes the compressibility of the upper of sample decreased. The effect of freeze-thaw action of natural thawing on soil compressibility is stronger than the effect of freeze-thaw action of forced thawing.(8) After freeze-thaw action, breaking strength of undisturbed soil decreases, cohesive force is reduced significantly, and internal friction angle increases slightly. The lower the freezing temperature, the smaller the breaking strength of the soil, the smaller the cohesive force of the forced thawing soil, the bigger the internal friction angle.