考虑路基平衡湿度状态的黄泛区中高液限黏土抗剪强度研究

Shear strength of the medium-high liquid limit clay in the Yellow River Flooded Areas considering the subgrade equilibrium moisture condition

为了给黄泛平原区冲淤积黏性土路基设计与施工提供指导,对黄泛区中高液限黏土路基的抗剪强度、平衡湿度和长期稳定性进行了研究。基于24种工况下的直剪试验,获得黄泛区中高液限黏土抗剪强度指标与压实状态的相关关系,建立土体黏聚力和内摩擦角与含水率和压实度的双因素经验关系式;通过84组滤纸试验,获得不同初始压实状态下的饱和度与基质吸力的关系曲线,分别根据MEPDG法和Frendlund&Xing模型确定土水特征曲线表达式的经验参数;进而建立黄泛区中高液限黏土路基的平衡含水率及其对应的抗剪强度计算公式;结合路基稳定安全系数,提出考虑平衡湿度的黄泛区路基临界填土高度。结果表明:以磨圆度较高的粉粒为主要骨架的黄泛区中高液限黏土,其抗剪强度指标受含水率的影响非常显著且表现出阶段性的减小规律,但内摩擦角几乎不受压实度的影响;土体的进气值非常高(199kPa以上),且随压实度的增加而提高,表明高压实度土体在高饱和度下具有很强的保水能力;平衡含水率较最优含水率增加了4%~8%,导致抗剪强度较施工状态有很大幅度的降低;对于黄泛区典型双向四车道高速公路,施工含水率和平衡含水率下对应的临界路基高度分别为20和8m。

In order to provide a guide for design and construction of clayey soil subgrades in the Yellow River Alluvial Plain,the physical and mechanical properties of the medium-high liquid limit clay subgrade including the shear strength,the equilibrium moisture content and the long-term stability were studied. Direct shear tests under 24 conditions were performed. The relationships between the shear strength indices and compaction conditions were discussed,and the empirical formulae of the soil cohesion and the friction angle were established as functions of the moisture content and the compaction degree. Based on the 84 groups filter tests,the relationships between the saturation degree and the matric suction were obtained at different initial compaction states,and the empirical parameters of the soil-water characteristic curve equations were given according to MEPDG method and Frendlund & Xing model. The equations of the equilibrium moisture content and the corresponding shear strength of the medium-high liquid limit clay subgrade were deduced,and the critical subgrade height was proposed considering the safety factor and the equilibrium moisture content. Results show that,for the medium-high liquid limit clay mainly composed of silt particle with higher roundness in the Yellow River flooded areas,the shear strength indices is markedly influenced by the soil moisture content but that the friction angle is independent on the compaction degree. The high soil air-entry value(more than 199 kPa),ascending with increasing the compaction degree,indicates that the soil with a high compaction degree has great water retaining capacity at high saturation state. The equilibrium moisture content is bigger by 4%–8% than the optimum moisture content,which results in a larger reduction of the shear strength. For the typical bi directional and four lane highway in the Yellow River flooded areas,the critical subgrade heights at the construction and equilibrium moisture contents are 20 m and 8 m respectively.