Abstract
Abstract Moisture content distribution in soil is of great importance for understanding rainfall-induced slope failure and roadbed settlement. This study aims to develop a moisture migration model that improves the quantification of moisture content at an arbitrary point of the soil at any time for the whole process of infiltration under steady rainfall. The model was derived from the Richards equation using the flux-concentration relation, which was validated by numerical solutions calculated by Hydrus-1D software to evaluate the performance of the model. Results showed good accuracy and high adaptability for the moisture migration simulation of a wide range of soil types, and is applicable for short-duration and long-duration steady rainfall. Moreover, it can also reflect the stratification phenomenon for soil profile wetting by infiltration. Our analysis indicates that the flux and surface volumetric moisture content together can bound the boundary conditions of rainfall infiltration, which presents a shift from constant-flux to constant-concentration during long-duration steady rainfall. The migration rate of the wetting front in the later stage of infiltration positively correlates with rainfall intensity under the constant-flux condition, while it finally stabilizes at Ks/(θs − θi) under the constant-concentration condition (i.e., Ks – saturated hydraulic conductivity, θs – saturated volumetric moisture content, θi – initial volumetric moisture content).
Highlights
Rainfall is one of the most significant triggering factors of slope failure and roadbed settlement (Lee et al 2009; Hedayati & Hossain 2015; Wu et al 2015)
The relative errors mainly increase in the stages of pressure infiltration transition (PIT) and pressure infiltration stabilization (PIS), which is mainly due to the function of F deviating slightly from the theoretical lower limit function in the late stage of rainfall for these two soil types
The relative error of soil moisture content calculation in the same infiltration depth is basically controlled at 2%, and it remains reasonable to use Equations (14) and (30) to calculate the soil moisture content distribution in each stage of rainfall infiltration
Summary
Rainfall is one of the most significant triggering factors of slope failure and roadbed settlement (Lee et al 2009; Hedayati & Hossain 2015; Wu et al 2015). With the increase of wetting front depth, long-duration rainfall leads to matric suction decrease and results in the reduction of shear strength for unsaturated soils (Fang & Esaki 2012), and in addition it leads to the poor durability, lack of stability, and uneven settlement of the roadbed. The investigation of the influence of rainfall infiltration on moisture content distribution has three approaches: field monitoring, numerical simulation, and analytical methods. The Richards equation (RE) (Richards 1931) is the governing equation to describe rainfall infiltration through unsaturated soil, which is the theoretical basis for numerical simulation and analytical analysis. It is a highly nonlinear partial differential equation that is difficult to solve directly.
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