从变形、水量变化和强度三方面验证非饱和土的两个应力状态变量

为了验证非饱和土两个应力状态变量的合理性,对常规非饱和土三轴仪进行了改进,采用3个压力–体积控制器分别控制孔隙水压力、内室压力和外室压力,提高了量测体变和含水率的精度;用加荷器施加轴向荷载,可以方便地控制偏应力。在笔者前期用各向等压试验验证非饱和土的两个应力状态变量的基础上,使用改进的非饱和土三轴仪和重塑Q_3黄土做了两类验证试验:第一类是两组控制净围压、偏应力和吸力的固结不排水剪切试验,第二类是3组控制净围压和吸力的固结排水剪切试验。在第一类试验中,等值改变(增加或减少)总围压、孔隙水压力和孔隙气压力而保持净围压、吸力和偏应力本身不发生变化,发现试验过程中土样的体积变化和含水率变化均非常小,可以忽略不计。第二类试验的每一组包含两个控制净围压和吸力分别相同的固结排水剪切试验,其中一个试样在固结完成后直接进行排水剪切试验,另一个试样在固结完成后等值增加总围压、孔隙水压力和孔隙气压力而保持净围压和吸力本身不发生变化的条件下进行排水剪切试验,发现二者的抗剪强度、剪切过程中的体积变化、排水量和广义剪应变都分别十分接近,可以认为两两相等。研究结果从变形(包括体应变和剪应变)、水量变化和强度3个方面说明描述非饱和土的两个应力状态变量是合理的,进一步夯实了非饱和土力学的应力理论基础,为非饱和土的两个应力状态变量理论提供了牢靠的试验依据。

Verification of rationality of two stress state variables of unsaturated soil from deformation,moisture change and strength

In order to verify the rationality of two stress state variables of unsaturated soil, the conventional triaxial apparatus is improved: three pressure-volume controlles are employed to respectively control the pore water pressure, inner and outer chamber pressures so as to improve the measurement precision of volume change and water content; and the axial load by loading derice can conveniently control the deviator stress. Based on the previous verified results of isotropic tests by the authors, the improved unsaturated soil triaxial apparatus and remolded Q3 loess are used to conduct two types of experiments: one is two groups of consolidation undrained shear tests under the net confining pressure, deviator stress and suction, and the other is three groups of consolidation drained shear tests under the net confining pressure and suction. In the first type of tests, with the equivalent change (increase or decrease) of the confining pressure, pore water pressure and pore air pressure, and without the change of the net confining pressure, suction and deviator stress, it is found that the volume change and moisture change are very small and can be neglected. In the second type of tests, each group contains two consolidation drained shear tests with the same net confining pressure and suction. Among the tests, one is that after the completion of the consolidation, the direct drained shear tests are performed; the other is that after the completion of the consolidation, with the equivalent increase in the total confining pressure, pore water pressure and pore pressure and without the change of the net confining pressure and suction, the drained shear tests are performed. It is found that their shear strengths, volume changes in the shearing process, generalized shear strains and drainages are very close and can be considered to be equal. The use of two stress state variables to describe the stress state of unsaturated soil is reasonable in terms of deformation, moisture change and strength, which consolidates the mechanics theory of unsaturated soil and provides a reliable experimental basis for the theory of two stress state variables of unsaturated soil.