Study On The Method Of Obtaining Soil-water Characteristic Curve Based On Conventional Geotechnical Test

Our country has a vast geological margin and rich and varied types of soil samples.Unsaturated soils in nature exist in a three-phase state of solid,liquid and gas,and are also considered by some scholars to be composed of a solid phase,a gas phase,a liquid phase and a partition interface(shrinkage film)between the liquid and gas phases.In practical engineering,most of the engineering problems encountered mostly belong to the category of unsaturated soils.Therefore,the study of unsaturated soil properties has very important theoretical and engineering practical significance.One of the basic elements in the hydraulic properties of unsaturated soils is the soil-water characteristic curve.Soil-water characteristic curves are of great importance in solving practical engineering problems such as shear strength of unsaturated soils,slope stability and rainfall infiltration landslides.Most of the soil-water characteristic curves are measured based on the specimen in equilibrium,which is a long test time,high specialization of test equipment,and complicated test operation steps.Therefore,in this paper,the soil-water characteristic curves of Jiangxi red soil,granite residual soil and swelling soil are studied based on conventional geotechnical tests(triaxial and shrinkage tests).The soil-water characteristic curves of paths.The main research contents are as follows.(1)Research on the theoretical method of measuring soil-water characteristic curves based on conventional geotechnical tests.The pore ratio-water content curve(shrinkage curve)is obtained through the dry shrinkage test,and the pore ratio-effective stress relationship curve is obtained through the conventional saturated triaxial consolidation and drainage test,and the pore ratio is used to characterize the deformation of the specimen,and any data point is selected to match the shrinkage curve of the dry shrinkage test using the pore ratio to obtain the corresponding absorbing stress and volumetric water content of unsaturated soil,and the effective saturation parameter is substituted into the unsaturated soil The effective saturation parameter is substituted into the effective stress equation to calculate the suction force of the matrix corresponding to different volume water content,and multiple data points are obtained,so that a soil-water characteristic curve with high accuracy is obtained.(2)Soil physical and mechanical index test.In order to determine the feasibility of the soil-water characteristic curve measurement method based on conventional geotechnical tests,three representative soils,namely laterite,granite residual soil and swelling soil,were selected,and basic physical and mechanical tests such as specific gravity,particle analysis,limiting moisture content and optimum moisture content and maximum dry density were conducted on the specimens to obtain the basic physical and mechanical indexes of the three soils.The laterite is mainly composed of sand grains,with large and uniformly distributed grains,and the soil samples have low liquid and plastic limits,which are widely distributed in Jiangxi area and belong to typical low plasticity pulverized sandy soil.Granite residual soil particles are small,weathering is more serious,plasticity index is relatively small,belongs to clay soil.Swelling soil constitutes soil particles fine,rich in hydrophilic minerals,swelling soil absorbing water and losing shrinkage characteristics are more obvious,the free swelling rate of soil samples is large,as a medium swelling soil.(3)Experimental study of soil-water characteristic curve under no vertical load.Three kinds of soil samples with different dry densities were tested for drying and shrinkage,and the shrinkage curves were obtained.Based on the SLB-1 saturated triaxial instrument,the soil consolidation and drainage tests were carried out on the three soils to obtain the pore ratio-effective stress relationship curves,and the soil-water characteristic curves were calculated by combining the shrinkage curves with the unsaturated soil theory.The soil-water characteristic curves measured by GEO-Experts pressure plate instrument were compared and analyzed,and the soil-water characteristic curves based on conventional geotechnical test measurements were very close to the results measured by the pressure plate instrument.The dry shrinkage test shows that after the dry shrinkage reaches the residual evaporation stage,the final volumetric water content of the specimen almost does not change anymore,and the residual volumetric water content can be determined by this method.(4)Experimental study of soil-water characteristic curve under vertical load.Under two sets of vertical loading conditions of 50 KPa and 100 KPa were applied,dry shrinkage test and consolidation drainage test with main stress difference were conducted for the three soils respectively.In the consolidation drainage test,the drainage volume of the specimens was more than that without the main stress difference applied,and the corresponding shrinkage curve and pore ratio-effective stress relationship curve were obtained.The soil-water characteristic curve under vertical load was calculated using the effective stress equation for unsaturated soil and found to be close to the results measured by GEO-Experts pressure plate meter.The drying shrinkage test shows that: the vertical load has a small effect on the saturated volume water content and the inlet pressure value of the specimen,and when the vertical load is larger,the smaller the pore ratio of the soil,the slower the evaporation rate of soil water,the longer the time to reach the residual stage,and the residual volume water content increases with the increase of the vertical load.(5)Experimental study of soil-water characteristic curve of moisture absorption process.In order to investigate whether the conventional test can accurately measure the soil-water characteristic curve in the process of moisture absorption,the syringe is used to inject water for the test of humidification and swelling,and the vernier caliper is used to measure the diameter and height of the current specimen,which can obtain the pore ratio-water content relationship in the process of humidification.The backpressure saturation wetting test was performed using saturated triaxial,and the water intake under each backpressure was recorded to obtain the effective stress-porosity ratio curve during the backpressure saturation.The hygroscopic soil-water characteristic curves were obtained from the calculation of the effective stress equation of unsaturated soil.Comparing and analyzing the soil-water characteristic curve measured by GEO-Experts pressure plate instrument,the two results are close.