基于吸应力的非饱和黄土强度理论与试验研究

Based on strength theory of suction stress put forward by Lu Ning, unsaturated loess with different moisture content were tested through the conventional shear test for the determination of effective friction angle and relation curve of suction stress and moisture content, and then the suction stress function and effective friction angle were used to express strength of unsaturated loess. On this basis, a more simplified test technology was put forward, that was large cross-section tensile test or double–end direct shear test with no normal stress applied under different moisture content were used to determine the suction stress, and coupled with a group of conventional shear test the suction stress function and effective friction angle can be determined.It is proposed to design and make the tensile apparatus and the double-end direct shear apparatus for the test determination of suction stress, and then the results will be compared with that of traditional shear test in order to verify the applicability of the suction stress strength theory of unsaturated loess and feasibility of the proposed testing method. This research tries to step forward in the direction from strength theory of unsaturated soil to its practical application.

本项目以非饱和黄土为研究对象，以LU Ning提出的吸应力强度理论为基础，通过不同含水率常规剪切试验，测定吸应力和含水率的关系曲线及有效摩擦角，用吸应力函数和有效摩擦角表达黄土的非饱和强度。在此基础上，提出更为简化的测试技术，即通过不同含水率试样的大截面拉伸试验或不施加正应力的双面直剪试验间接测定吸应力，再结合一组常规剪切试验确定吸应力函数和有效摩擦角。拟设计制造黄土拉伸仪和双面直剪仪用于吸应力测试，将测试结果与传统剪切试验结果进行对比，以验证吸应力强度理论对非饱和黄土的适用性和项目提出的测试方法的可行性，并建立基于吸应力的非饱和黄土强度数学模型。本项目的研究可为非饱和土强度理论向实用化方向发展奠定基础。

非饱和土强度参数的确定是其能够用于工程实际的关键。目前公认的非饱和土强度理论有Bishop 的单变量理论、Fredlund 的双变量理论以及Lu Ning 的吸应力强度理论。其中吸应力强度理论基于吸应力特征曲线，可直接由常规试验得出，回避了测定基质吸力的问题，便于在工程实际中推广。本项目基于Lu Ning吸应力强度理论，通过三轴CD常规实验测得L5黄土的强度参数，绘制出吸应力特征曲线，验证了吸应力对表达非饱和黄土强度性质的适用性。为了更加快捷的测定黄土吸应力，研制了黄土拉伸仪和双面剪切仪进行大断面原状黄土拉伸试验和双面剪切试验。在大断面原状黄土拉伸试验中，发现黄土实际抗拉强度不是吸应力，两者之间存在一个线性关系，且与含水率的大小有关；双面剪试验测定的是土正应力为0情况下的有效粘聚力，依据Lu Ning 的吸应力强度公式，拟合出吸应力与体积含水率之间的关系曲线和数学表达式，最终得到抗剪强度与体积含水率之间的数学表达式，建立基于吸应力的非饱和土强度模型。