Modeling preferential water flow and solute transport in unsaturated soil using the active region model

Preferential flow and solute transport are common processes in the unsaturated soil, in which distributions of soil water content and solute concentrations are often characterized as fractal patterns. An active region model (ARM) was recently proposed to describe the preferential flow and transport patterns. In this study, ARM governing equations were derived to model the preferential soil water flow and solute transport processes. To evaluate the ARM equations, dye infiltration experiments were conducted, in which distributions of soil water content and Cl⁻ concentration were measured. Predicted results using the ARM and the mobile–immobile region model (MIM) were compared with the measured distributions of soil water content and Cl⁻ concentration. Although both the ARM and the MIM are two-region models, they are fundamentally different in terms of treatments of the flow region. The models were evaluated based on the modeling efficiency (ME). The MIM provided relatively poor prediction results of the preferential flow and transport with negative ME values or positive ME values less than 0.4. On the contrary, predicted distributions of soil water content and Cl⁻ concentration using the ARM agreed reasonably well with the experimental data, with ME values higher than 0.8. The results indicated that the ARM successfully captured the macroscopic behavior of preferential flow and solute transport in the unsaturated soil.     

A low-dimensional subsurface model for saturated and unsaturated flow processes: ability to address heterogeneity

A low-dimensional model that describes both saturated and unsaturated flow processes in a single equation is presented. Subsurface flow processes in the groundwater, the vadose zone, and the capillary fringe are accounted for through the computation of aggregated hydrodynamic parameters that result from the integration of the governing flow equations from the bedrock to the land surface. The three-dimensional subsurface flow dynamics are thus described by a two-dimensional equation, allowing for a drastic reduction of model unknowns and simplification of the model parameterizations. This approach is compared with a full resolution of the Richards equation in different synthetic test cases. Because the model reduction stems from the vertical integration of the flow equations, the test cases all use different configurations of heterogeneity for vertical cross-sections of a soil-aquifer system. The low-dimensional flow model shows strong consistency with results from a complete resolution of the Richards equation for both the water table and fluxes. The proposed approach is therefore well suited to the accurate reproduction of complex subsurface flow processes.     

Numerical hysteresis model for intermittent studies in unsaturated soils

In the present study, the use of one of the recent dependent domain models of capillary hysteresis in the numerical analysis of intermittent infiltration and redistribution of water in two types of soils (a sand and Rubicon Sandy Loam) has been shown. The numerical results for both the soils have been presented in terms of pressure head depth, moisture content depth and the pressure head-moisture content relationships. The capillary hysteresis model has been found to be very useful for the prediction of both wetting and drying scanning curves of various orders.     

A macroscopic model for predicting the relative hydraulic permeability of unsaturated soils

A macroscopic model for predicting the relative hydraulic permeability of unsaturated soils is proposed. In this model, pores in unsaturated soils are considered to be parallel flow tubes. The water flow in the pores is assumed to take place in the water film on the inside wall of the flow tubes. The viscosity of pore water is considered to be different from the viscosity of pure water and variable with the variation of degree of saturation. The values of tortuosity factor and pore shape factor of unsaturated soils are estimated theoretically. The theoretical model is verified using experimental data for 32 different soils. For application in engineering practice, the value of viscosity of pore water in different soils is proposed.     

裂隙岩体非饱和渗流的分形模型

基于毛细吸持理论和Sierpinski空间的假定,建立了用分维数表示的毛细压力-有效饱和度及毛细压力-相对渗透系数关系的裂隙岩体非饱和渗流分形模型。根据Monte-Carlo模拟技术由平硐资料生成三维裂隙网络,利用盒维数方法计算裂隙岩体体积分维数。探讨了分维数与裂隙渗透率的关系。结合糯扎渡水利工程实例,将模型计算结果与Brook-Corey模型对比,结果表明相对渗透率主要受最大裂隙开度的影响,当裂隙开度范围不大时,分形模型可较好描述岩体裂隙的毛细压力与有效饱和度的关系。当裂隙开度范围较大时, ,忽略最大裂隙开度,模型可以等价于Brook-Corey模型。     

An identification technique for evaluation of phenomenological coefficients in coupled flow in unsaturated soils

To determine moisture movement and heat transfer through an unsaturated soil under temperature and volumetric water content gradients, it is necessary to have knowledge of phenomenological coefficients of the soil. However, in unsaturated flow, i.e. flow through unsaturated soil, these phenomenological coefficients are not constants, but vary with volumetric water content as well as temperature. In this paper, an identification technique is proposed for evaluation of the phenomenological coefficients. The phenomeno-logical coefficients are first assumed to be certain kinds of functions of volumetric water content and temperature. The choice of the functional forms is based on an understanding of the physical situation, and previous knowledge of water flow in the isothermal case. The constant parameters associated with the functional forms are evaluated through the use of the identification technique. Once these phenomenological coefficients are obtained as certain functions of the volumetric water content and the temperature for a specified soil, analysis of coupled moisture flow and heat transfer in the unsaturated soil can proceed.     

Three-dimensional active earth pressure for retaining structures in soils subjected to steady unsaturated seepage effects

Acta Geotechnica - In extant studies, most of the analyses on active earth pressure focused on dry or saturated soils using two-dimensional failure mechanisms. However, in practice, soils are...     

An effective stress-based DSC model for predicting the coefficient of lateral soil pressure in unsaturated soils

Acta Geotechnica - In this study, an analytical model is developed to establish a framework for predicting the coefficient of lateral soil pressure in unsaturated soils. To this end, the disturbed...     

A model for moisture and heat flow in fractured unsaturated porous media

The present study investigates propagation of a cohesive crack in non‐isothermal unsaturated porous medium under mode I conditions. Basic points of skeleton deformation, moisture, and heat transfer for unsaturated porous medium are presented. Boundary conditions on the crack surface that consist of mechanical interaction of the crack and the porous medium, water, and heat flows through the crack are taken into consideration. For spatial discretization, the extended finite element method is used. This method uses enriched shape functions in addition to ordinary shape functions for approximation of displacement, pressure, and temperature fields. The Heaviside step function and the distance function are exploited as enrichment functions for representing the crack surfaces displacement and the discontinuous vertical gradients of the pressure and temperature fields along the crack, respectively. For temporal discretization, backward finite difference scheme is applied. Problems solved from the literature show the validity of the model as well as the dependency of structural response on the material properties and loading. Copyright © 2016 John Wiley & Sons, Ltd.     

多孔介质中非均匀流动特性的染色示踪试验研究

通过土壤染色剂进行的4组试验,对不同介质结构条件下的水流和溶质非均匀运动规律,非均匀流动变异信息分布特征关系以及全局性非均匀流动示踪方法进行了研究。结果表明,即使在相对比较均匀的介质条件下,流动也表现出明显的非均匀特性;对数正态分布能够较好的反映水流的运动分布模式,相比水流运动,溶质的运动和分布规律明显不同,表现出更多的不确定性和变异性。     

考虑吸力效应的非饱和黏土回弹模量预估模型

回弹模量是反映路基弹性支撑能力的重要指标,并受运营期间基质吸力变化的显著影响。通过3种压实黏土试样的土-水特征曲线试验和重复动三轴试验,研究了回弹模量与基质吸力的相关性,并基于非饱和土有效应力原理,提出了考虑吸力效应的回弹模量预估模型。结果表明：基质吸力与回弹模量之间具有较强的非线性相关性,可采用指数形式进行两者的拟合;含水率由最佳含水率-4%提升至最佳含水率+4%时,受基质吸力减小的影响,回弹模量降幅达到29.1%～39.0%;在 土-水特征曲线过渡区,采用以饱和度表征的有效应力参数,可较好地反映基质吸力对于回弹模量的贡献,预估效果优于采用残余含水率或进气值相关的有效应力参数;在相同饱和度的情况下,黏粒含量越多、塑性指数越高,基质吸力对有效应力的贡献比例越小。     

A coupled two‐phase fluid flow and elastoplastic deformation model for unsaturated soils: theory,implementation, and application

Although numerous numerical models have been proposed for simulating the coupled hydromechanical behaviors in unsaturated soils, few studies satisfactorily reproduced the soil–water–air three‐phase coupling processes. Particularly, the impacts of deformation dependence of water retention curve, bonding stress, and gas flow on the coupled processes were less examined within a coupled soil–water–air model. Based on our newly developed constitutive models (Hu et al., 2013, 2014, 2015) in which the soil–water–air couplings have been appropriately captured, this study develops a computer code named F²Mus3D to investigate the coupled processes with a focus on the above impacts. In the numerical implementation, the generalized‐α time integration scheme was adopted to solve the equations, and a return‐mapping implicit stress integration scheme was used to update the state variables. The numerical model was verified by two well‐designed laboratory tests and was applied for modeling the coupled elastoplastic deformation and two‐phase fluid flow processes in a homogenous soil slope induced by rainfall infiltration. The simulation results demonstrated that the numerical model well reproduces the initiation of a sheared zone at the toe of the slope and its propagation toward the crest as the rain infiltration proceeds, which manifests a typical mechanism for rainfall‐induced shallow landslides. The simulated plastic strain and deformation would be remarkably underestimated when the bonding stress and/or the deformation‐dependent nature of hydraulic properties are ignored in the coupled model. But on the contrary, the negligence of gas flow in the slope soil results in an overestimation of the rainfall‐induced deformation. Copyright © 2015 John Wiley & Sons, Ltd.     

A thermo‐hydro‐mechanical model for unsaturated soils based on the effective stress concept

A simple thermo‐hydro‐mechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way. Copyright © 2010 John Wiley & Sons, Ltd.     

新安江模型中河网汇流参数Cs的一种计算方法

新安江模型河网汇流参数Cs主要取决于河网形态及地貌特征规律,针对PUB（Prediction in Ungauged Basin）问题,从物理机理分析的角度,构建描述河网汇流动态变化的微分方程并给出适当的定解条件,用解析法得出微分方程的解,从而探索出在无实测水文资料地区适用的Cs计算方法。选取中国湿润地区和半湿润地区25个流域,统计并分析了各流域次洪模型的Cs计算值和率定值。结果表明,从总体来看计算值和率定值基本上相等;不论流域是否分块,提出的Cs计算方法均适用;当其他参数保持不变,虽然Cs取计算值时所得到的次洪模拟结果略逊于取率定值的模拟结果,但误差在可接受的范围之内,也能取得良好的模拟效果;如果令Cs取计算值,再重新率定其余的各项参数,能够使模拟精度得到进一步提高。采用该法可推求无实测资料地区的汇流参数Cs值,使模型在无资料地区的应用成为一种可能。     

Study on the selection of unsaturated flow model for the different types of soil and soft rock

Precise estimation of unsaturated hydraulic properties of porous media is indispensable in various study areas, such as analyzing the moisture flow, the drying process occurring from the surface, and the pollutant migration beneath the ground surface. Although many empirical/theoretical models describing the unsaturated hydraulic properties have been proposed by several previous researchers, the best model for the different types of soil/rock may not be identical. Thus, the model selection process and the estimation technique of the parameters included in the models should be developed. In the present study, the inverse technique based on the transient evaporation change was investigated to select the model and estimate the model parameters. The experimental work was based on a relatively low permeable soft rock and a relatively high permeable sandy soil (Toyoura standard sand). Experimental equipment was developed to precisely measure the evaporation rate for the high permeable sandy soil. The Genetic Algorithm (GA) was adopted in the inverse technique as an optimization tool. In order to simplify the problem, only the drying process from the saturated condition was considered. It was established that the information concerning the transient evaporation change could be used for the model selection and parameter estimation. Further, the saturation distribution could be used for the selection of the models. The present study provides important information for the development of the model selection process.     

A study of the correlation between electrical resistivity and matric suction for unsaturated ash-fall pyroclastic soils in the Campania region (southern Italy)

In the territory of the Campania region (southern Italy), critical rainfall events periodically trigger dangerous fast slope movements involving ashy and pyroclastic soils originated by the explosive phases of the Mt. Somma-Vesuvius volcano and deposited along the surrounding mountain ranges. In this paper, an integration of engineering-geological and geophysical measurements is presented to characterize unsaturated pyroclastic samples collected in a test area on the Sarno Mountains (Salerno and Avellino provinces, Campania region). The laboratory analyses were aimed at defining both soil water retention and electrical resistivity curves versus water content. From the matching of the experimental data, a direct relationship between electrical resistivity and matric suction is retrieved for the investigated soil horizons typical of an ash-fall pyroclastic succession. The obtained relation turns out to be helpful in characterizing soils up to close saturation, which is a critical condition for the trigger of slope failure. In such a regime, the water content and the matric suction have small variations, while electrical resistivity variations can be appreciated in a larger range of values. For this reason, besides suction measurements on very small soil volumes through classical tensiometers, our analyses suggest the direct monitoring of in situ electrical resistivity values as an effective tool to recognise the hydrological state of larger and more representative soil volumes and to improve early warning of dangerous slope movements.     

A dual-continuum model (TOUGH2) for characterizing flow and discharge in a mechanically disrupted sandstone overburden

Underground hard coal mining usually disrupts the mechanical equilibrium of rock sequences, creating fractures within minor permeable rocks. The present study employs a dual-continuum model to assess how both fractured and porous sandstone media influence the percolation process in postmining setups. To test the approach, the software TOUGH2 was employed to simulate laminar fluid flow in the unsaturated zone of the Ibbenbüren Westfield mining area. Compared to other coal mining districts in Germany, this area is delineated by the topography and local geology, leading to a well-defined hydrogeological framework. Results reveal good agreement between the calculated and measured mine water discharge for the years 2008 and 2017. The constructed model was capable of reproducing the bimodal flow behavior of the adit by coupling a permeable fractured continuum with a low-conductivity rock matrix. While flow from the fractured continuum results in intense discharge events during winter months, the rock matrix determines a smooth discharge limb in summer. The study also evaluates the influence of individual and combined model parameters affecting the simulated curve. A detailed sensitivity analysis displayed the absolute and relative permeability function parameters of both continua among the most susceptible variables. However, a strong a priori knowledge of the value ranges for the matrix continuum helps to reduce the model ambiguity. This allowed for calibration of some of the fractured medium parameters for which sparse or variable data were available. However, the inclusion of the transport component and acquisition of more site-specific data is recommended to reduce their uncertainty.

     

Numerical evaluation of the suitability of the equivalent porous medium model for characterizing the two-dimensional flow field in a fractured geologic medium

The equivalent porous medium (EPM) method is an efficient approximation method for groundwater yield analysis considering the equivalent permeability in a fractured geologic medium (FGM). The EPM method is widely used in many practical hydrogeological problems from local to regional scales. However, when calculating water head and velocity distributions, the suitability of the EPM model remains insufficiently evaluated. The suitability refers to the head error caused by the application of the EPM model. The smaller the error, the better the suitability. In this study, the influence of fracture geometric attributes on the suitability was quantitatively studied in numerical simulation experiments, and the EPM model simulation results were compared to those obtained with the discrete fracture network (DFN) model. The results indicated that the suitability decreased with increasing fracture spacing. When the fracture spacing was smaller than 0.6 m, the influence of an increase in the fracture spacing on the suitability was obvious. For the same fracture spacing, the suitability generally increased with increasing trace length. When the spatial variability of fracture aperture is not considered, the change of the fracture aperture did not impact the suitability. The fracture orientation slightly impacted the suitability, which can be ignored. An evaluation standard based on fracture parameters is proposed to estimate the suitability of the EPM model, which provides a scientific basis to ascertain whether this model can be applied to a given site to solve the head-related hydrogeological issues with FGMs.