冻融软土结构性特征及其应力应变硬化/软化模型

Objective of this subject is to study the stress-strain cuvre for frozen-tharwed soft soil (FTSS) of soft foundation. By using frozen-tharwed cycle box and GDS triaxial appratus, triaxial tests of remoulded soil and unditurbed soil will be designed and conducted, considering different states such as frozen-tharwed cycle times, confining presasures and stress paths. Tested data will be further discussed in order to obtain relative mechanics parameters and experimental rules of FTSS. In order to properly understand the structural behavior, stress index will be proposed and employed to signalize the developing process and to build mathematical property set of FTSS stress-strain cuvre. Mathematical property set for harden type stress-strain cuvre consists stress index, initial tangent modulus, and ultimate stress. However, the set for soften type curve consists stress index, initial tangent modulus, ultimate stress, peak stress and inflect point stress. Based on the law of thermodynamics, energy dissipating theory and scan results of electron microscope, failure mechanism of FTSS would be investigated. Differential governing equation for strain increase rate and dissipated energy value will be deduced, which should be paied enough attention to structural potential energy dissipating, and considering soil particles leaping over and rearranging during shear process. One genral stress-strain model for FTSS will be established which is available for both harden and soften type stress-strain curve. Finally, good accuracy of the new model will be proved.

本项目以软土地基为对象，对冻融软土的应力应变曲线进行研究。通过冻融循环箱和GDS 三轴仪，进行不同应力路径的冻融原状软土和冻融重塑软土的三轴试验，研究不同冻融循环、不同围压、不同应力路径下软土的应力应变特性，获得相关的力学参数和试验规律。引入"应力因子"的概念，建立由应力因子、初始切线模量和极限偏应力组成的硬化型应力应变模型的数学特征方程，分析与揭示冻融软土的结构性特征；根据初始模量、应力因子、峰值偏应力、拐点偏应力和极限偏应力，建立冻融软土软化型应力应变模型的数学特征方程。根据热力学定律和能量耗散原理，辅以SEM电镜扫描结果，分析冻融软土的破坏机理；从剪切过程中结构势能的耗散出发，考虑颗粒的相互跨越和重新排列对能量的损耗，建立冻融软土的应力应变发展速率与能量耗散之间的控制微分方程，推导出能同时反映应力应变硬化和软化特性的冻融软土应力应变统一模型，并证明新模型的正确性。

冻融循环对滨海软土的力学性质有着重要的影响。通过固结压缩试验、无侧限抗压强度试验、三轴试验，研究了冻融循环作用下普通滨海软土、水泥改性滨海软土、纳米材料改性水泥土等三类软土的应力应变特性和破坏模式，固结压缩试验和无侧限试验表明：同一龄期下，NmCS的压缩量基本随纳米材料掺入量的增加而减少；同一纳米材料掺入量下，NmCS的压缩量基本随龄期的增长而减小，90d龄期较28d龄期最大的减小量为48.1%；纳米材料和水泥的掺入能够有效补偿冻融循环对普通滨海软土强度的消弱。三轴试验结果表明：随着冻融循环次数的增加，滨海软土的破坏应力逐渐减小，5次冻融后的破坏应力约为其原始破坏应力的42%；冻融循环次数的增加使滨海软土的软化特性逐渐降低，甚至消失，5次冻融后的应力相对软化系数逐渐趋于零。结合SEM电镜扫描结果，根据能量耗散原理、初始模量、应力因子、峰值偏应力、拐点偏应力和极限偏应力，建立了多个冻融软土软化/硬化型应力应变模型的数学特征方程。结合试验数据，给出了模型参数的确定方法，并通过曲线拟合对比证明了该方法的准确性。提出了基于干涉向量的模型精度评价方法。最后，针对不同试验，改进了试样的4种制备仪器和各自的制备方法。研究成果在滨海软土工程领域具有良好的理论意义和工程实用价值。

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