Abstract
The soil-water characteristic curve (SWCC) is an important hydraulic parameter for modeling water flow and contaminant transport in the unsaturated soil. However, direct measurement of the SWCC is still difficult. The commonly used measuring instrument in the laboratory is the pressure plate extractor. In this paper, the original pressure plate extractor has been improved based on the one-step outflow test. Compared with the original pressure plate extractor, three devices, water storage, data acquisition system, and bubbles scouring device, are added to the improved pressure plate extractor. The improved equipment eliminates the problems of long test time consumption and inaccurate test data. A one-step outflow test is performed utilizing the improved pressure plate extractor. The test data (the amount of spilled water and the time of pressurization) are substituted into the one-dimensional moisture migration analysis software HYDRUS-1D. The parameters (α, m, and n) are identified by the numerical inversion method in the inversion module of the HYDRUS-1D, which are associated with the VG model to fit SWCC. The fitted SWCC is highly consistent with the measured SWCC, which is obtained from the conventional test of the original pressure plate extractor. Results confirm that the improved pressure plate extractor not only saves considerable time but also effectively measures the SWCC. The improved pressure plate extractor also involves a simple operation. The influence factors of controlling the geometry of SWCC are also discussed, the results of the discussion confirm that the geometry of SWCC is directly controlled by pore distribution, and the consolidation pressure is an indirect factor.
Highlights
Knowledge of infiltration characteristics is necessary for hydrological studies in different soils [1]. e soil-water characteristic curve (SWCC) reflects the process of water entering the soil, which presents the relationship between the matrix suction and water content. e hydraulic conductivity function [2, 3], volumetric variation of the soil [4,5,6,7], shear strength in the unsaturated soil [8, 9], and attributes of the water content in the subsurface [10,11,12,13] can be computed using the SWCC. is relationship plays an important role in the research and practical engineering related to unsaturated soils
In the weighing outflow method (WOM), due to high pressure and a long testing time, bubbles are accumulated on the back of the ceramic plate, and water is occupied by bubbles; the water is pushed out, and the amount of spilled water is overestimated. ere are errors in both methods. erefore, determining the SWCC quickly and accurately is a key problem to be solved
Ng and Pang [27] studied the SWCC of remolded soil and natural soil, and the results show that the SWCC of a remolded sample is very different from that of a natural sample with the same initial soil density and initial water content
Summary
Knowledge of infiltration characteristics is necessary for hydrological studies in different soils [1]. e soil-water characteristic curve (SWCC) reflects the process of water entering the soil, which presents the relationship between the matrix suction and water content (such as saturation or volumetric water content). e hydraulic conductivity function [2, 3], volumetric variation of the soil [4,5,6,7], shear strength in the unsaturated soil [8, 9], and attributes of the water content in the subsurface [10,11,12,13] can be computed using the SWCC. is relationship plays an important role in the research and practical engineering related to unsaturated soils. Wang and Linc [19] used a pressure plate extractor and GDS unsaturated triaxial apparatus to study the influencing factors, such as types of soil and net mean stress, using the Fredlund five-parameter model through least squares to fit the SWCCs. Lee et al [20] introduced an improved automatic flow pump system, which measured the transient suction response by using the balance method during the drying and wetting cycles to obtain the soil-water retention curve (SWRC). E aim and innovation of the paper are to improve the original pressure plate extractor which does not meet the one-step outflow test, so that the improved pressure plate extractor can meet the requirements of one-step outflow test, and an experimental research is carried out, to achieve the purpose of obtaining the SWCC of silt quickly and effectively, and influence factors of controlling the geometry of SWCC are discussed; the result states that the pore distribution of sample is a direct factor that affects the geometry of SWCC, which is changed by the consolidation pressure to affect the geometry of SWCC
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