Experimental Research On The Verification Of Stress State Variables Of Unsaturated Soils And Permeability Of Unsaturated Remolded Q₂ Loess

This paper is supported by the National Natural Science Foundation of China which is about on the stress theory of unsaturated soil and the construction projects of Yan’an district which is about on the seepage and deformation of Yan’an District high embankment. In this paper, basing on unsaturated soil mechanics and using the improved and upgraded unsaturated triaxial apparatus and the water seepage device, we studied the verification of two stress state variables of unsaturated soil and the permeability of unsaturated remolded Q2 loess. The main research work and innovation of this paper are as follows.Firstly, the conventional unsaturated soil triaxial apparatus and the equipment of horizontal column infiltration test were upgraded and improved. we used three GDS pressure volume controller respectively control inner chamber pressure, outer chamber pressure and back pressure, which effectively improved the measuring precision of soil volume change and water content; Besides we equipped triaxial apparatus with axial loading device, which made the triaxial apparatus can control strain and stress and meet the needs of different test schemes;Lastly we used TDR moisture sensor to measure moisture content of horizontal soil column infiltration test and used DT80 remote monitoring system to collect test data, which improved the level of automation and the survey precision of moisture content.Secondly, with the improved triaxial apparatus, three forms of triaxial tests had been performed. The first is isotropic tests of unsaturated soil, inclunding 3 groups of remoulded Q3 loess and 1 group of undisturbed Q3 loess,which is under different initial dry density,different initial saturation, different initial suction; The second is 2 groups of triaxial consolidation undrained shear test of remoulded Q3 loess,which is under different deviatoric stress. In the two kinds of test above, Equivalent changing the total confining pressure, pore water pressure and pore gas pressure while stress state variables themselves do not change,We found that volume change and water content change is very small sample and can be ignored, thus the rationality of two sress state variables of unsaturated soil is proved from the aspects of deformation and change of water. The third is three groups of consolidation drained shear tests of remoulded Q3 loess which controlled the net confining pressure and suction constant. Each group contains two consolidation drained shear tests and the net confining pressure and suction are the same. After the completion of the consolidation, one of them directly drained shear test, the other one equivalent increase total confining pressure, pore water pressure and the pore air pressure while keep the net confining pressure and suction do not change, and then drained shear test. We found the shear strength, the change of volume and moisture content in shearing process are very closed. Thus, the rationality of two stress state variable of unsaturated soils is verified from deformation, moisture content change andstrength. The research results show that using two stress state variables to describe the stress state of unsaturated soil is reasonable and reinforcing the stress theory foundation of unsaturated soil mechanics. The results of this paper provide a reliable experimental basis for the theory of two stress state variables of unsaturated soils.Thirdly, In order to study the permeability of unsaturated remolded Q2 loess and the differences of permeability between remolded Q2 and remolded Q3 loess, four dry density of horizontal soil column infiltration test are performed; Dry density of the sample are 1.30, 1.40,1.50 and 1.60g/cm3; MP-406 water meter and remote monitoring system are made to monitor volumetric water content in different sections. The results show that: the permeability of the same dry density of Q2 and Q3 loess is quite different. In the same saturation, the diffusion coefficient of Q3 loess is greater than Q2 loess, and with the increasing of saturation this difference gradually decreases; For Q2 and Q3 loess, when saturation reached about 0.6, the relationship of saturation and diffusion coefficient is approximated formed by two straight line. For Q3 loess, when saturation is less than 0.6, the difference of dry density has less effects on diffusion coefficient, when saturation is higher than 0.6, there is a large difference in diffusion coefficient. For Q2 loess, this law is not obvious.