Experimental And Model Study On Rapid Determination Of Soil Water Characteristic Curves

China has a vast territory,and the distribution of unsaturated soils is very wide.Soil in nature exists in solid,liquid and gas three-phase state,and soil above groundwater level exists in unsaturated state.Hydraulic characteristics of unsaturated soils include soil-water characteristic curve and permeability function.Soil-water characteristic curve describes the relationship between matric suction and saturation.Soil-water characteristic curve is an important hydraulic characteristic parameter of unsaturated soil.It plays an important role in the analysis of unsaturated soil strength and slope stability.The methods we usually use to determine soil-water characteristic curve are basically measured in the equilibrium state of the sample,waiting for equilibrium time is longer,and consuming manpower and material resources is relatively high.This paper studies the principle and method of rapid testing of soil-water characteristic curve,and carries out the following four aspects of work:(1)The GEO-Experts stress-related soil-water characteristic curve pressure gauge system produced in Europe and America has been modified.After the modification,the pressure gauge has added data acquisition system,small water storage device and vacuum pump.The data acquisition system is connected with the computer at one end,and the data acquisition software matched with the customized balance is used to complete the data acquisition function.The other end is connected with the balance.The data acquisition software can collect the change data of water discharge in the sample in real time on the data acquisition software.The data acquisition software can set the acquisition time according to the test requirements,and the time interval can be from several seconds to several small ones.Time varies,usually set up 30 S to collect data once;small water storage device can store a whole pipe of water to flush bubbles and saturate the whole system of the instrument.The function of vacuum pump is to wait for the water in the small water storage pipe to be insufficient and to pump water from the external water storage instrument to make the water in the small water storage pipe enough to flush and saturate the whole instrument and pipeline.(2)A one-step flow test was carried out with the modified pressure plate instrument.The silt sample is put into the stainless steel pressure chamber of the pressure plate instrument after refitting.Firstly,the saturated silt sample is loaded with a small pressure value of about 10 KPa,so that the water overflow in the sample changes from saturated state to unsaturated state.When the water overflow in the sample does not change,the loading gas pressure is increased to 290 KPa,and the one-step flow test is carried out in the dehumidification process.Proven.Data acquisition software was used to collect the relationship curve of water spillover with time,and then converted to the relationship curve of volume water content with time.(3)A one-dimensional water transport model was established by using HYDRUS-1D software,and the curve of water spillover in samples with time was fitted by the model.The curve of the relationship between the volume water content of samples with time was basically consistent with the measured curve.Then,based on the relationship curve,initial boundary value,upper and lower boundary range of the test,material parameters of water permeability of clay plate and the one-dimensional water migration model established by HYDRUS-1D,the curve of water overflow mass changing with pressure time in the sample during dehumidification process is fitted,and the parameters alpha and n that are difficult to measure in the VG model are calculated by reverse analysis.Furthermore,SWCC in the dehumidification process of silt samples can be obtained quickly and effectively.(4)The conventional test method for silt samples has been used before the original pressure plate tester was modified to determine the dehumidification process SWCC;the unmodified pressure plate tester spent about 45 days in the test for the dehumidification process SWCC of silt samples,and the modified pressure plate tester spent about 4 days in the one-step flow test for silt samples.According to the determined SWCC,the modified pressure plate tester can quickly measure the dehumidification process of silt samples,which can save a lot of time compared with the conventional test method of the non-modified pressure plate tester.