非饱和土双剪新强度准则及工程应用

The true triaxial test results for unsaturated soils indicate that the intermediate principal stress does significantly influence the unsaturated soil strength. However, most existing shear strength formulae of unsaturated soils used today are based on the Mohr-Coulomb strength criterion. As is well known, the Mohr-Coulomb strength criterion does not consider the intermediate principal stress effect and thus can not reflect the actual stress states and the true strength characteristics of unsaturated soils. Therefore, a true triaxial strength criterion of unsaturated soils in accordance with the actual stress conditions in the field is urgent need to be established in order to improve the theoretical basis and accelerate the process of engineering applications. Based on the stress state variables and strength characteristics of unsaturated soils, a new twin-shear strength criterion under complex stress states is derived by means of the twin-shear stress theory, and is validated by the rigid and flexible true triaxial tests. Then, a new formula of shear strength under plane strain conditions is presented to obtain the analytical solutions for the earth pressure of retaining wall, ultimate bearing capacity of foundation and safety factor of slope. The obtained analytical solutions are conducted a feasibility analysis and are compared with the corresponding results of the earth pressure, ultimate bearing capacity and stability chart for unsaturated soils. Finally, a parametric study is undertaken to investigate the effect of various parameters involved in the proposed analytical solutions. This study can obtain a series of originally new results, which has innovative and scientific significance in theory as well as a good application prospect and economic benefits in engineering practice.

非饱和土真三轴试验结果表明：中间主应力对其强度影响显著，但现有的非饱和土抗剪强度公式多是基于Mohr-Coulomb强度准则而建立的，不能反映非饱和土的真实应力状态和强度特性，急需建立符合工程实际受力状况的非饱和土真三轴强度准则，完善非饱和土的理论基础，并加快非饱和土强度理论的工程应用进程。本项目拟依据非饱和土的应力状态变量和强度特性，利用双剪应力理论建立复杂应力状态下非饱和土双剪新强度准则，分别进行刚性和柔性真三轴试验验证，继而推导平面应变条件下非饱和土抗剪强度新解，并将其应用于非饱和土挡墙土压力、地基极限承载力和边坡安全系数的解析计算，对所得解析解进行可比性分析、挡墙土压力实测与地基极限承载力模型试验验证以及边坡安全系数图的比较，最后探讨各因素的影响特性。本研究可取得一系列具有独创性的新成果，在理论上具有较好的创新性和科学意义，在工程实践中具有良好的应用前景和经济效益。

非饱和土强度理论及其应用研究具有重要的理论意义和良好的发展前景，但现有的非饱和土抗剪强度公式多是基于Mohr-Coulomb强度准则而建立的，既不能反映非饱和土的真实应力状态和强度特性，也不能充分发挥非饱和土的强度潜能和自承载能力。本项目对非饱和土双剪真三轴强度准则及其工程应用进行了系统的研究，所得结论对非饱和土工程的设计与施工具有重要的指导意义。主要研究内容与成果，包括：.(1) 将非饱和土抗剪强度公式分为结合土-水特征曲线、数学拟合、分段函数、总应力指标及其他形式等5类，分析抗剪强度公式特点及研究不足，并指出吸附强度表达式的不同导致了抗剪强度公式的多样性。.(2) 分析已有非饱和土真三轴仪的试验结果，总结现有非饱和土真三轴仪及其试验研究的不足，同时指出Mohr-Coulomb强度准则和外接圆Drucker-Prager准则对非饱和土真三轴试验结果的不适用性。.(3) 利用双剪单元体模型建立的非饱和土统一强度理论为一系列强度准则的集合，其极限线覆盖了所有外凸区域，饱和土的统一强度理论以及非饱和土的Mohr-Coulomb强度准则均为其特例，能适用于各种不同特性的非饱和土，并得到非饱和土刚性和柔性真三轴试验的验证。.(4) 基于非饱和土平面应变抗剪强度统一解，合理考虑中间主应力、基质吸力及分布、强度非线性、侧压力系数等影响，建立了岩土三大问题(挡墙土压力、地基承载力与边坡稳定性)及其相关工程问题的解析解，并得到相应模型试验结果的验证。.(5) 所得非饱和土挡墙主动及被动土压力、抗倾覆稳定系数，地基临界荷载、极限承载力、路堤填筑高度和边坡安全系数等问题的解析解具有广泛的理论意义，可以适用于更多实际工程情况。中间主应力、高低基质吸力及分布、强度非线性和侧压力系数的影响显著。