全吸力范围内非饱和黏性土的强度及其预测

During lower humidity season, particularly in the arid and semi-arid regions, the soil water moisture is rather low near the ground surface, and the soil suction is high. Many civil and hydraulic engineering projects are related to the strength of unsaturated soils at high suction. A present, there is a lack of experimental data of unsaturated soil strength at very high suction. In addition, the recent prediction method for unsaturated soil strength used the assumed ratio of capillary water to absorbed water with suction, is not based on measured experimental data. Therefore, the current knowledge of the prediction cannot satisfy the requirement from engineering practice. The applicant's research group has studied preliminarily the water retention and mechanical behaviour of unsaturated expansive soil at high suction recently. On basis of the preliminary study, this project is planning to study the water retention and strength characteristics of three typical soils (silt, non-expansive clay, and expansive clay) in the full suction range. For low suction, water retention and triaxial shear tests are conducted by using the axis-translation technique for controlling the soil suction. For high suction, the suction is imposed on the soil specimens by using the vapor equilibrium method generated from saturated saline solutions, the strength of unsaturated soils at high suction is obtained from triaxial shear tests on constant water moisture specimens after the equilibrium. In the same time, the nuclear magnetic resonance (NMR) tests are carried out on the soil specimens with different suction from low to high values to determine the absorbed water. A prediction method will be proposed for strength of unsaturated soils in the full suction range according to the summary of the water retention behaviour (including the change in absorbed water with suction) and strength characteristics.

在相对湿度较低的季节，尤其在干旱和半干旱地区，地表土的含水率很低、吸力极高，许多土木和水利工程涉及到高吸力下的非饱和土强度问题，而目前的现状是缺少高吸力下黏性土强度的实测数据。另外最新的非饱和土强度预测方法中使用的毛细水和吸附水的比例随吸力的变化规律只不过假设而已，没有实测数据，因此现有研究成果无法满足工程实践的需求。申请人课题组近年初步进行了高吸力下非饱和膨胀土的持水和力学特性试验研究，在此基础上计划系统地开展全吸力范围内三种典型土（粉土、非膨胀性黏土、膨胀土）的持水和强度特性研究。用轴平移技术控制低吸力范围内的吸力进行三轴剪切试验；用饱和盐溶液蒸汽平衡法施加高吸力，并进行等含水率的高吸力下三轴剪切试验，测得高吸力下的非饱和土强度。同时，用核磁共振（NMR）技术实测土样的吸附水含量。根据全吸力范围内持水（含吸附水）特性和强度特性，总结提出全吸力范围内非饱和土强度的预测方法。

以细砂、粉土、黄土、膨胀性和非膨胀性黏土为主要试验材料，进行全吸力范围内持水特性和动静力学特性试验研究和强度预测研究，主要成果如下。.1) 全吸力范围内膨胀性与非膨胀性黏土的持水特性 .利用压力板法和饱和盐溶液蒸汽平衡法研究了全吸力范围内孔隙比或干密度对珍珠土(非膨胀性粘土)和南阳膨胀土(非膨胀性粘土)压实样持水特性的影响。结果表明，以含水率与吸力关系表示持水曲线的话，当吸力超过某一值（珍珠土：200 kPa; 南阳膨胀土：500 kPa）时不同孔隙比压实试样的持水曲线几乎重合，此结果可用压汞试验结果从微观上加以说明。.2) 全吸力范围内膨胀性与非膨胀性黏土的强度特性.利用上述持水特性开发了高吸力下吸力控制非饱和土三轴试验方法，并对原状黄土、不同干密度珍珠土和南阳膨胀土压实样进行常吸力（等含水率）的三轴剪切试验和基于轴平移技术的三轴试验，得到全吸力范围内原状黄土、压实膨胀性和非膨胀性黏土的强度和变形特性。还对压实细砂和粉土用直剪试验仪进行全吸力范围内的慢剪试验，得到强度随初始饱和度（吸力）的变化规律。.3) 全吸力范围内非饱和土强度预测方法.对文献中常用的非饱和土强度预测方程在全吸力范围内的适用性进行了详细地分析和讨论。在此基础上提出了一个能较好预测全吸力范围内非饱和粉土或粉质黏土的强度计算方法。.4) 全吸力范围内单峰和双峰持水曲线参数的确定.基于修正FX拟合方程，提出了单峰和双峰持水曲线基本参数（如进气值、残余吸力值和过渡区曲线的斜率等）的确定方法。利用单峰和双峰持水曲线的试验数据，对此方法的可行性进行验证。.5) 全吸力范围内非饱和粉土动力变形特性.以非饱和粉土为研究对象，利用饱和盐溶液蒸汽平衡法和吸力可控的动三轴仪进行动力变形试验，得到了全吸力范围内非饱和粉土试样在净围压和吸力基本不变条件下的骨架曲线、动弹性模量和阻尼比，研究了吸力对非饱和粉土动力变形特性的影响。