Study On Strength Characteristics And Constitutive Relationship Of Artificially Frozen Soft Soil In Underground Engineering Construction

As a safe and environmentally friendly method of stratum reinforcement,the artificial freezing method has been widely used in various construction of underground projects.The correct understanding of the strength characteristics of artificial frozen soil is the key problem that needs to be paid attention to in actual design and construction of the freezing method.According to the actual formation process of artificial frozen soil during the construction process of the freezing method of urban underground engineering,this paper carried out the on-load freezing triaxial tests which restored the specimen to the initial in-situ stress state and the pre-freezing triaxial tests which freezed the specimen in low temperature box without load in advance.And the effects of long-term consolidation on the strength characteristics of artificial frozen soil before freezing construction were explored.At the same time,based on the on-load freezing mode,the stress characteristics of unloaded and then loaded of artificial frozen soil during actual excavation and support process were considered.The corresponding unloading and reloading triaxial tests were designed to study the effects of unloading stress path and unloading ratio on the strength characteristics of artificial frozen soil.Finally,based on the modeling idea of the Duncan-Chang model,the artificial frozen soil constitutive model considering the influence of temperature and confining pressure was established,and the variation law of different model parameters was analyzed.(1)In the range of the test temperature and confining pressure,the stress-strain curves of artificial frozen soil in both freezing modes are strain hardening type,and their strengths increase linearly with the decreasing temperature and the increasing confining pressure.The strength increase caused by the change in unit pressure(100kPa)and unit temperature(1 ℃)is about 3.6% to 6.4% and 17.5% to 22.0%,respectively.The strength of artificial frozen soil basically conforms to the Mohr-Coulomb strength criterion,the cohesive force increases remarkably with the decreasing temperature,while the internal friction angle shows a change characteristic which increases first and then almost unchanged.Under the same temperature and confining pressure,the strength of on-load artificial frozen soil is always higher than that of pre-freezing artificial frozen soil.The difference between the two increases with the increasing confining pressure and the decreasing temperature,and the maximum difference is more than 20%.(2)The initial unloading effect has a certain weakening effect on the strength of artificial frozen soil.When the unloading ratio is relatively small,the strength of artificial frozen soil decreases unconspicious compared to the artificial frozen soil that didn’t experience initial unloading.However,with the increase of the unloading ratio,the strength weakens more obviously,and the strength decreases by 12%～21% under the reduced p stress path,and the strength decreases by 15%～25% in the constant p stress path.And the greater the unloading ratio,the greater the difference in strength between the two.The initial unloading effect has a great influence on the cohesive force of artificial frozen soil,but has little effect on the internal friction angle.(3)According to the modeling idea of Duncan-Chang model,an artificial frozen soil constitutive model considering the influence of temperature and confining pressure was established by establishing a quantitative relationship between model test parameters and temperature and confining pressure.The calculated results by the model agree well with the experimental results,and the relative error is within 10%.The initial unloading effect will increase the model test parameters to a certain extent,and the greater the unloading ratio,the greater the deviation from the results obtained by the conventional loading path.