干湿-冻融循环作用下季冻区膨胀土的流变特性及其微观机理

Wetting-drying and freezing-thawing cycles with climate change gradually cause deteriorating effect on the engineering properties of expansive soils in seasonal frozen areas, even leads to serious engineering disasters. However, the previous research efforts on expansive soils do not pay enough attention on the seasonal frozen areas. Thus, this project will tentatively introduce a novel concept – united expansion-frost heaving ratio to evaluate their frost heaving behavior and propose the fuzzy synthetic evaluation ranges to reasonably class the frost heaving levels of expansive soils, taking into consideration the disaster characteristics of geotechnical structures related to expansive soils in seasonal frozen areas. Especially, the effect of wetting-drying and freezing-thawing cycles on soil-water characteristic curves (SWCCs) of expansive soils will be also investigated, and a fitting model for the SWCCs will be built. Based on a damage variable, a stress-strain constitutive relationship of expansive soils, which will be applicable to both strain hardening and strain softening behavior, will be established, incorporating the effect of confining pressure and number of wetting-drying and thawing-freezing cycles. The triaxial creep tests and sheer stress relaxation tests will be conducted on consolidated-undrained expansive soil samples subjected to different wetting-drying and thawing-freezing cycles. The microstructure of expansive soils prior to triaxial tests will detected by using scanning electron microscope (SEM) tests and mercury intrusion tests etc., in order to reveal the microstructural mechanisms of creep and stress relaxation performance and establish creep and sheer stress relaxation constitutive models that reflect the effects of wetting-drying and thawing-freezing cycles. The research results can provide the scientific and theoretical base for the long-term safety and stability of geotechnical structures located in expansive soils in seasonal frozen areas.

季冻区膨胀土随气候变化经历着周期性的干湿和冻融作用，其工程性质随时间逐渐衰变，甚至引发工程灾害。然而，关于季冻区膨胀土问题的研究却鲜有报道。本项目充分考虑季冻区膨胀土场地上土工构筑物的灾害特点，探索用“膨胀-冻胀总胀率”指标评价其冻胀性的测定方法，给出划分膨胀土冻胀性等级的模糊综合评价范围；探明干湿-冻融循环耦合作用对膨胀土土-水特性的影响及其土-水特征曲线模型；引入表征干湿-冻融循环耦合效应引起应力-应变曲线初始切线模量衰减程度的“损伤变量”，建立能统一描述应变硬化和应变软化的应力-应变本构模型。重点针对不同干湿-冻融循环作用的膨胀土试样，开展三轴固结不排水试验后的蠕变和应力松弛试验；采用扫描电镜、压汞试验等手段测试三轴试验前平行试样的微观结构，揭示其蠕变、应力松弛特性的微观机理，并建立能反映干湿-冻融循环耦合效应的流变模型，为季冻区膨胀土场地土工构筑物的长期安全与稳定分析提供理论依据。

随着气候的季节性变化，冻土地区膨胀土经历着周期性的脱湿-增湿和冻结-融化作用，土中水相及其含量的变化引起膨胀土体积发生明显的胀缩变形，导致土体裂隙发育，进而造成膨胀土工程性质随时间逐渐衰变，给建（筑）物的安全和稳定带来威胁，甚至引发工程灾害。本项目成果可为季冻区膨胀土场地土工间（构）筑物的长期安全与稳定分析提供科学依据。.（1）给出了基于 “膨胀-冻胀总胀率”指标划分膨胀土冻胀性等级的模糊综合评价方法；.（2）干湿、冻融和干湿-冻融三种循环显著改变了膨胀土的大孔隙系统，但对其微孔系统影响不大；干湿、冻融循环对膨胀土既产生肉眼可见的宏观裂缝，又产生肉眼不可见的微观裂隙，对膨胀土结构的破坏最为严重。三种循环作用对膨胀土结构的损伤，是经历三种循环作用膨胀土水力-力学特性的物质基础。.（3）探明了干湿、冻融和干湿-冻融三种循环作用对膨胀土收缩特性和土-水特性的影响。三种循环作用均显著降低了膨胀土在低吸力范围内的持水能力和从饱和状态到完全干燥状态的收缩能力，但三种循环作用均未改变膨胀土的缩限和塑限；提出了一个可反映循环作用次数N影响的土-水特征曲线模型，并结合Fredlund-Xing模型和Gallipoli模型提出了一个改进的土-水特征曲面（含水比-吸力-孔隙比）模型。.（4）基于Kondner双曲线模型，以极限偏应力为归一化因子，提出了一个能考虑循环次数N影响的膨胀土应变稳定型及应变硬化型应力-应变曲线预测模型；进一步提出了一个可描述膨胀土在低围压下应变软化行为的预测模型。.（5）探明了低/高围压下，经历干湿-冻融循环作用膨胀土试样的蠕变行为及其微观机理，提出了可反映循环次数N影响的蠕变预测模型；.（6）基于本项目的研究成果，开展了膨胀土边坡在低强度持续降雨期间的水力-力学耦合稳定性的数值模拟分析。结果表明，三种循环作用下，边坡的临界安全系数位置向坡面移动，边坡破坏模式有向浅层破坏发展的趋势，且边坡坡脚处安全系数最低。