One-dimensional Water-air Two-phase Seepage Test And Its Numerical Simulation

Unsaturated soil is different from saturated soil.Because of the gas phase,the soil exhibits complex and variable characteristics in many aspects,so it is unstable and has poor predictability.Therefore,the change of water-air two-phase caused by water infiltration in unsaturated soil is the key to the study of unsaturated soil seepage.Due to the limitations of technical means,almost no set of equipment can quantitatively determine the air phase in the process of unsaturated soil infiltration.In this paper,the existing unsaturated infiltration test device is modified to develop a device that can measure the water/air two-phase pressure and the temperature and humidity changes of the soil during the test.In addition,the high-intensity digital camera is used to take photos of the entire infiltration device.The change of the wet front with time is obtained,and the measurement of multiple variation factors in the process of unsaturated soil infiltration is realized.The test data is rich and the correlation between them is strong.The main research contents are as follows:1)Developed a new type of unsaturated soil infiltration test device.Mainly using modern sensor technology for the existing unsaturated soil infiltration test device with two types of sensors:air pressure and temperature and humidity,introduces the development process of the test device and detailed description of various instruments and equipment,including the production of sensors.With the placement,microcontroller,PC control program,etc.2)The indoor one-dimensional water infiltration test was carried out under three different dry densities of 1.6g/cm~3,1.7g/cm~3 and 1.8g/cm~3.The test material was coarse sand.The results showed that the dry density had a great influence on the water infiltration process.The effect of dry density on infiltration is mainly reflected in pore size and pore connectivity.The higher the dry density,the more the humidity reaction time at the same position on the sand column is delayed,and the time it takes for the wetting front to reach the bottom is longer and longer.The results of the total pressure of water and air two phases are different.The pre-measurement of the two dry density tests of 1.6g/cm~3 and 1.7g/cm~3 is the air pressure value,and the later is the hydrostatic pressure value,and the total moisture content of the pre-test of 1.8g/cm~3 dry density test.The pressure value is abnormally high and violently oscillated.The difference between the upper and lower parts of the water content is larger,the smaller the dry density is,the larger the difference is.The final pore saturation at the same position increases with the increase of dry density.The main influencing factors are the corresponding pore size and volume under various dry densities.The smaller the pores,the greater the resistance to moisture infiltration.3)The indoor one-dimensional capillary rise test was carried out under three different dry densities of 1.6g/cm~3,1.7g/cm~3and 1.8g/cm~3.The test material and the water infiltration test were the same as coarse sand.The results showed:dry density versus moisture The infiltration has a great influence,and the influence is mainly reflected in the capillary rise stage.Before the water infiltration reaches the level with the water surface at the initial stage of the test,the total pressure of the water and gas generated is large,and the gas pressure value accounts for more than half.After the capillary action starts,the total pressure of water and gas begins to slowly decrease.At different dry densities,the pressure curves of most sensors above the water surface showed six pressure fluctuations,and the interval between adjacent two fluctuations continued to shrink.The maximum pressure fluctuations at different dry densities appear in the upper part of the sand column.The wetting fronts at the three dry densities are all fast and slow,and the final ascending height is basically the same.The final water content decreases with the increase of dry density.The upper and lower parts of the pore saturation are unevenly distributed,and the difference is larger.The larger the dry density,the larger the difference.4)Based on the one-dimensional water infiltration test and the one-dimensional capillary rise test,numerical simulations were carried out.It was found that the simulation results and the two tests differed in many aspects.The main reason was that the software could not simulate the influence of air relative infiltration.The inversion of the parameters was carried out during the numerical simulation and the permeability coefficient was obtained.In the simulation of the water infiltration test,the infiltration coefficient obtained by the inversion was compared with the relevant empirical formula,and it was found that the empirical formula was used only at the lower dry density.In the capillary rise test simulation,the obtained permeability coefficient was substituted into the Terzaghi formula and the modified Terzaghi formula and compared with the measured capillary rise curve,and the improved Terzaghi formula was found to be in good agreement with the measured curve.