Unsaturated Soil Layer Research Articles

Analysis of Influencing Parameters of the Improved Model for Rainfall Infiltration in Unsaturated Tailings Soil

The rainfall infiltration analysis method is an important method for slope stability forecast and prevention. Slope angle and unsaturated soil layers are not shown in the conventional Green‐Ampt (GA) infiltration model. In this paper, based on the GA model of rainfall infiltration in tailings slopes, two aspects of slope angle and the proportion of the transition layer of the wet layer are modified. The rainfall infiltration test of unsaturated tailing soil was conducted using a self‐developed large‐size tailing slope model test device. The results of GA model, improved GA model, and Richards’ equation calculation model are compared. The results show that the difference between the three models is small in the free infiltration stage, but the infiltration rate is lower than that of the GA model. With the gradual increase of rainfall time into the ponding infiltration stage, the expansion depth and infiltration rate of the wetting front of the improved GA model and Richards’ equation are greater than those of the GA model. The difference between the GA model and the improved GA model and Richards’ equation for the extended depth of wetting front increases with the increase of rainfall duration, while the difference of infiltration rate changes in the opposite trend. The results of the improved GA model and Richards’ equation to calculate the expansion depth and infiltration rate of wetting fronts are consistent, and the difference basically tends to stabilize with the increase of rainfall ephemeris. The results of the improved GA model are closer to the measured data, which can provide a reference for analyzing the rainfall infiltration pattern of open pit tailing dams and slope stability research. On the basis of the improved GA model, the influence of slope angle, rain intensity, initial water content, and saturation infiltration coefficient on rainfall infiltration was analyzed, and the analysis of parameter sensitivity indexes showed that slope inclination and initial water content had a greater influence on the model.

Impacts of Climate Change on Blue and Green Water Resources in the Xiangjiang River Basin of the Yangtze River, China

Green water resources, which are fundamental for plant growth and terrestrial ecosystem services, reflect precipitation that infiltrates into the unsaturated soil layer and returns to the atmosphere by plant transpiration and soil evaporation through the hydrological cycle. However, green water is usually ignored in water resource assessments, especially when considering future climate impacts, and green water modeling generally ignores the calibration of evapotranspiration (ET), which might have a considerable impact on green water resources. This study analyzes the spatiotemporal variations in blue and green water resources under historical and future climate change scenarios by applying a distributed hydrological model in the Xiangjiang River Basin (XRB) of the Yangtze River. An improved model calibration method based on remotely sensed MODIS ET data and observed discharge data is used, and the results show that the parallel parameter calibration method can increase the simulation accuracy of blue and green water while decreasing the output uncertainties. The coefficients (p-factor, r-factor, KGE, NSE, R2, and PBIAS) indicate that the blue and green water projections in the calibration and validation periods exhibit good performance. Blue and green water account for 51.9 and 48.1%, respectively, of all water resources in the historical climate scenario, while future blue and green water projections fluctuate to varying degrees under different future climate scenarios because of uncertainties. Blue water resources and green water storage in the XRB will decrease (5.3–21.8% and 8.8–19.7%, respectively), while green water flow will increase (5.9–14.7%). Even taking the 95% parameter prediction uncertainty (95 PPU) range into consideration, the future increasing trend of the predicted green water flow is deemed satisfactory. Therefore, incorporating green water into future water resource management is indispensable for the XRB. In general, this study provides a basis for future blue and green water assessments, and the general modeling framework can be applied to other regions with similar challenges.

Performance of rocking foundations on unsaturated soil layers with variable groundwater levels

An emerging foundation design scheme, termed a rocking foundation, has the potential to limit loads transmitted to the superstructure, with the potential trade-off of increased settlements and rotations at the foundation level. These soil deformations can be significant for foundations with low vertical bearing capacity factors of safety. In this study, a set of dynamic centrifuge tests was performed to assess the response and to develop design considerations for a rocking foundation embedded in soil with varying degrees of saturation. Foundation design procedures dependent on degree of saturation are developed to predict moment capacities and initial rotational stiffness. Experimental data were used to validate the reliability of these procedures, regardless of the degree of saturation in the underlying soil. As the degree of saturation reduced, the moment capacity of the foundation increased, potentially increasing the load transmitted to the superstructure. Further, foundation settlements and rotations in the saturated soils can be reduced by lowering the water table depth in the underlying soil layer, which is beneficial to the performance of rocking foundations. Consequently, the degree of saturation of the supporting soil should be considered in the foundation design to predict the foundation settlement–rotation response and to assess properly the superstructure ductility demands.

Semi‐analytical solution for one‐dimensional consolidation of a two‐layered soil system with unsaturated and saturated conditions

AbstractSemi‐analytical solutions to one‐dimensional (1D) consolidation of the two‐layered soils are developed in this paper that can consider a two‐layered soil system with unsaturated upper soils and saturated lower soils. In this case, the upper soil layer remains the water‐air two‐phase flows, while the lower soil layer only involves the flow of the water phase. Based on the assumptions made in the 1D consolidation framework for saturated and unsaturated soils, the analytical solutions in the transformation space for the consolidation model are derived using a decoupling technique and the Laplace transformation. Subsequently, following the Crump algorithm, the numerical inverse Laplace transformation is carried for the related semi‐analytical results that can analyze the consolidation characteristics of the two‐layered soils in the time domain. Furthermore, two typical cases for saturated soils in the literature are shown to examine the methodology and final solutions. Finally, results of a case study in this paper indicate that the excess pore pressures within the unsaturated soil layer near the interface are significantly affected by the water flow in the saturated soil layer, and that the final primary consolidation settlement of soils would be overestimated if the unsaturated condition of upper soils is ignored.

A review of the current state of process-based and data-driven modelling: guidelines for Lake Erie managers and watershed modellers

Elevated phosphorus (P) loading from the watersheds draining into Lake Erie, particularly from agricultural (53%) and urban (43%) sources, is identified as one of the main drivers of the severe eutrophication. In this study, we present a comprehensive evaluation of 11 process-based models to characterize the water cycle as well as nutrient fate and transport within a watershed context, and to find a robust and replicable way to optimize the modelling strategy for the Lake Erie watershed. Our primary objective is to review the conceptual/technical strengths and weaknesses of the individual models for reproducing surface runoff, groundwater, sediment transport, nutrient cycling, and channel routing, and to collectively guide the management of the Lake Erie Basin. Our analysis suggested that the available models either opted for simpler approximations of the multifaceted, nonlinear dynamics of nutrient fate and transport, and instead placed more emphasis on the advanced representation of the water cycle or, introduced a greater degree of biogeochemical complexity but simplified their strategies to recreate the roles of critical hydrological processes. Notwithstanding its overparameterization problem, the MIKE SHE model provides the most comprehensive 3D representation of the interplay between surface and subsurface hydrological processes with a fully dynamic description, whereby we can recreate the solute transport that infiltrates from the surface to the unsaturated soil layer and subsequently percolates into the saturated layer. Likewise, the physically based submodels designed to represent the sediment detachment and erosion/removal processes (DWSM, HBV-INCA, HSPF, HYPE, and MIKE SHE), offer a distinct alternative to USLE-type empirical strategies. The ability to explicitly simulate the daily plant growth (SWAT and APEX) coupled with a dynamic representation of soil P processes can be critical when evaluating the long-term watershed responses to various agricultural management strategies. Drawing parallels with the (sub)surface and sediment erosion processes, a more complicated physically based approach, e.g., the dynamic wave model provided by MIKE SHE (coupled with MIKE URBAN or MIKE HYDRO) and SWMM may be more appropriate for realistically simulating the pressurized flow and backwater effects of water routing in both open channels and closed pipes. While our propositions seem to favor the consideration of complex models that may lack the commensurate knowledge to properly characterize the underlying processes, we contend this issue can be counterbalanced by the joint consideration of simpler empirical models under an ensemble framework, which can both constrain the plausible values of individual processes and validate macroscale patterns. Finally, our study discusses critical facets of the watershed modelling work in Lake Erie, such as the role of legacy P, the challenges in reproducing spring-freshet or event-flow conditions, and the dynamic characterization of water/nutrient cycles under the nonstationarity of a changing climate.

Numerical investigation of the performance of engineered barriers in reducing flood risk

In this paper, 2-dimensional, hydro-mechanically coupled finite element analyses are performed to assess the performance of an engineered barrier aimed at reducing flood risk in urban environments. The barrier consists of an unsaturated compacted soil layer and a drainage layer of a coarse granular material, constructed on top of the natural soil, in this case London clay. The barrier is vegetated so that its water storage capacity is renewed after each rainfall event. Sophisticated boundary conditions are used to simulate the effect of precipitation and evapotranspiration. The rainfall water infiltration and the initiation of water run-off during intense precipitation events are simulated. The effect of the hydraulic properties of the unsaturated soil layer on the performance of the system is investigated by means of parametric analyses. The effect of precipitation rate and geometry of the barrier is also discussed. Design recommendations for the properties of the compacted layer and the dimensions of the system are given at the end of the paper.

Robust crop water simulation using system dynamic approach for participatory modeling

A system dynamic model, SMITUV (System Dynamic Modeling of Infiltration, Solute Transport, and Root Water Uptake in Vadose Zone) was developed for simulating transient soil water flow, solute transport, and root water uptake in crops under water and salinity stress in a multilayered unsaturated soil layer. SMITUV solves the mixed form one-dimensional Richards’ equation with a sink term for water transport and root water uptake and advection-diffusion equation for solute transport. It simulates salinity and water stress on root water uptake as a function of osmotic and matric potential. SMITUV is based on fundamental physical principals and is robust enough to handle different scenarios and “what-if” runs and also has tools that are crucial for growers to understand the salt-water-soil system. The model is designed to be a planning tool for strategic desalination methods and field management measures that optimize irrigation water use for flood irrigated arid region crops.

Expected Environmental Effects of Long-Term Tritium Supply—Lessons Learned

Fusion reactors are considered an almost unlimited source of power for the future, with tritium [deuterium-tritium (D-T) mixture] being the most convenient fuel for these facilities. Due to tritium-specific features and its self-radioactivity there are a number of issues when handling a large amount of tritium for long-term operations of fusion reactors. This paper presents research results of tritium behavior in the D-T mixture and in the environment during operations at tritium facilities. The results provide the basis for an assessment of the future environmental effects of long-term operation of a fusion reactor/tritium facility. The results of experimental and modeling studies include tritium reactions with carbon oxides under radiolysis, radiolysis of DTO, tritium dispersion in atmosphere under rough terrain, tritium oxide washout from atmosphere, tritium oxidation in soil with soil bacteria, tritium oxide migration in the unsaturated soil layer, and tritium uptake by plants, crops, and lichens. Both experimental and observational data have been used to develop models to predict tritium behavior in the environment under similar conditions. The models have been verified and validated.

Adsorption of Dye on a Tunisian Unsaturated Layered Soil: Physical and Numerical Modeling

The main objectives of this study is to model the transport of methylene blue (MB) in homogenous and layered soils which has been studied experimentally by fixed bed column. The effect of soil stratification has been studied through numerical investigation based on the coupled solute transport model in three-layered soil. The effect of some significant parameters such as flow rate, initial concentration of MB, thickness of each layer and the numbers of layers on the breakthrough curves have been undertaken. A finite element analysis model was employed to predict the transport of MB in soils. A two dimensional model based on Richards equation and advection-dispersion equation coupled with adsorption model has been developed and an analytical model has been used to predict the dispersivity. The results shows that the soil heterogeneity has a significant effect on methylene blue (MB) adsorption through unsaturated layered media and the effect of the parameters of the upper layer of the soil is more significant than those of the lower layer but this effect can be controlled with the choice of the thicknesses of each layer.

Spatial-temporal variations in blue and green water resources, water footprints and water scarcities in a large river basin: A case for the Yellow River basin

Blue water (surface and ground water) and green water (water stored in unsaturated soil layer and canopy evapotranspiration from rainfall) are the two sources of water generated from precipitation and communicating vessels that define the limits of water resources (WRs) for both human activities and ecosystems. However, the blue and green evapotranspiration in irrigated fields and their contribution to blue and green water flows have not been identified in studies conducted on blue and green WRs. In addition, information on intra-annual variations in green water footprints (WFs) and water scarcities is limited. In particular, there is a lack of information on water consumption obtained from hydrological model-based blue and green water assessments at the basin scales. This study quantified, taking the Yellow River Basin (YRB) over 2010–2018 as the study case, the inter- and intra-annual variations in blue and green WRs, WFs and water scarcities at sub-basin levels. WRs and WFs were simulated using the Soil and Water Assessment Tool (SWAT) model. The results revealed that the annual average blue and green WRs of the YRB were 119.33 × 109 m3 yr−1 and 296.94 × 109 m3 yr−1, respectively, over the study period. The blue and green WFs of the crops in the middle reach were significantly larger than those of the crops in the upper and lower reaches. There were visible spatial heterogeneities in water scarcities across the basin. Several areas in the middle reaches were subject to both blue and green water scarcities (at least modest level) for a minimum of three months a year. Furthermore, the norther region of the YRB was subject to significant and severe blue water scarcity for the whole year.

Semi-analytical solutions to consolidation for unsaturated soils by vertical drains under arbitrary time-dependent loading

This paper presents general semi-analytical solutions to equal strain consolidation for unsaturated soil layer by vertical drains, and the well resistance is considered. Firstly, the coupled governing equations are transformed into ordinary differential equations (ODEs) by Laplace transform and introducing two new variables, and combined with boundary conditions to obtain solutions. On this basis, the semi-analytical solutions in the time domain are achieved by performing inverse Laplace transform with Crump’s method. Secondly, the correctness of the semi-analytical solutions are verified by comparison with the existing analytical solutions of radial consolidation for unsaturated soils under constant loading. Finally, the effect of well resistance factor, permeability coefficient ratio ka/kw and load parameter on the average degree of consolidation was analyzed under exponential loading.

Integral transform analysis of radionuclide transport in variably saturated media using a physical non-equilibrium model: application to solid waste leaching at a uranium mining installation.

The Generalized Integral Transform Technique (GITT) was employed to simulate transient one-dimensional flow in variably saturated porous media, as well as radioactive waste transport within different layers (a solid waste pile, nearby soil, and a granular aquifer) towards the edge of a uranium mining installation under institutional control. Computational codes, written using the Mathematica software system, were implemented and tested for solution of the coupled advection-dispersion equations for an arbitrary number of daughter products within a radioactive chain migrating in saturated and unsaturated soil layers. The computer simulations were verified in great detail against results obtained using the built-in routine NDSolve of the Mathematica platform and the HYDRUS-1D software system. The present work reports the main results for 238U chain radionuclide transport using data extracted from a safety assessment of solid waste repositories at a uranium mining and milling installation in Caetité, state of Bahia, Brazil, operated by INB (Indústrias Nucleares do Brasil). Concentration evolutions of the various radionuclides obtained with the simulations were analyzed for five different cases to explore variations in the infiltration and recharge rates, the effect of assuming physical equilibrium or non-equilibrium transport conditions, and of different initial concentrations of some of the radionuclides.

Physical Modeling of Stone Columns in Unsaturated Soil Deposits

Abstract This article focuses on evaluating the load–settlement response of circular footings on unsaturated soil layers improved with stone columns using 1-gravity (1-g) physical modeling experiments. The initial (preloading) conditions in the soil layers were varied by compaction to the same dry density but different initial degrees of saturation. An effective stress analysis calibrated using direct shear experiments was found to satisfactorily predict the measured bearing capacities of unimproved soil layers, considering a change in failure mode for soil specimens at certain initial degrees of saturation. As the bearing capacity of the unsaturated soil layers increased, the amount of improvement gained by incorporating stone columns decreased. Bulging deformations of the stone column exhibited a close relationship with the bearing capacity, with smaller amounts of bulging in soil layers with low initial degrees of saturation that also have high bearing capacity. The stress concentration ratio increases with increasing initial degree of saturation, indicating that stone columns carry a greater fraction of the applied footing stress for soil layers closer to saturated conditions.

Simulation of the thermo-hydraulic response of energy piles in unsaturated soils

This paper focuses on the simulation of the coupled heat transfer and water flow in unsaturated soil layers surrounding a solitary energy pile undergoing heating and cooling cycles typical of a field-scale energy pile. The results indicate that heating leads to drying of the soil surrounding the energy pile, which has been shown in previous studies to result in an increase in axial capacity. During cooling, the degree of saturation was observed to recover to the value present before the start of heating initially, however, it will not recover in the following years. Which will lead to a cumulative effect after several cycles of heating and cooling. Heating and cooling cycles lead to an overall reduction in the thermal conductivity of the subsurface, reducing the heat transfer from the energy pile but also leading to greater storage of heat in the subsurface surrounding the pile.

Simulation of temperature and moisture fields around a single borehole ground heat exchanger: effects of moisture migration and groundwater seepage

This paper aims to investigate the effects of moisture migration and groundwater seepage on the heat transfer capacity of ground heat exchangers in stratified soils. A three-dimensional unsteady groundwater flow and heat transport model was established using finite volume method. Sixteen cases with different model considerations and initial soil conditions were simulated based on the proposed model. A group of 8 cases considering only transverse moisture migration and another group considering both transverse and longitudinal moisture migration were compared. The heat and moisture fields after 30 days of operation reveal that considering the change of saturation caused by vertical moisture transfer, the soil temperature field will be affected, but borehole outlet temperature was less influenced. The absolute value of outlet temperature difference between corresponding cases in the two groups is only about 0.2 °C. The position of groundwater seepage and arrangement of unsaturated soil layers with different degrees of saturation on heat transfer capacity of vertical ground heat exchanger were further explored. The results show that the longitudinal moisture migration would be made more influential by the existence of seepage layer, because the average relative deviation of inlet and outlet temperature difference between the corresponding cases of Group 1 and Group 2 was 1.34% when setting seepage layer and was 0.44% when without seepage layer. Heat transfer performance of borehole heat exchanger is also affected by the location of seepage layer. The average relative deviation of inlet and outlet temperature difference between the reference case and cases with seepage in the top, middle and bottom layers is 34.18%, 25.08% and 16.82%, respectively. The arrangement of unsaturated soil layers also has a certain effect. When the soil layer with low degree of saturation is located in the upper layer of soil, heat transfer capacity is better.

Influence of initial conditions on the liquefaction strength of an earth structure

Liquefaction has been the major source of damage for structures and infrastructures in most recent earthquakes, as it induces loss of strength and stiffness in soils, resulting in settlement of buildings, landslides, and failure of pipelines and earth dams. Such a phenomenon is primarily associated with saturated cohesionless soils and strong-motion seismic events able to induce pore water pressure build-up. Even though the degree of soil saturation is strictly related to the oscillation of groundwater table and to the interaction flows (namely precipitation and evaporation) between the exposed surface and the atmosphere, the initial distribution of pore water pressure is commonly simplified in liquefaction potential analysis and the presence of unsaturated soil layers is almost always neglected. To try to fill this gap, this work investigates the effect of the initial distribution of pore water pressure on the liquefaction strength evaluation, considering the potential presence of unsaturated soil layers. The work uses as case-study a well-investigated inhabited levee damaged by the 2012 Emilia earthquake. The initial distribution of pore water pressure within the dyke is carried out by solving the Richards equation in steady-state and transient conditions, assuming as boundary conditions the evolution of potential flows recorded at the site in the time period including the earthquake event. The cyclic resistance of unsaturated soils is considered during the execution of dynamic analyses in effective stress conditions. Results of the analyses show that initial conditions influence the liquefaction strength of earth structure and that neglecting the cyclic resistance of unsaturated soils is not a conservative assumption.

Steady-state analysis of heat–moisture–salt coupling in unsaturated layered soil

Based on the theory of porous media, the multifield coupling problem of heat–moisture–salt in unsaturated layered soil under the steady condition is studied considering that the pore of unsaturated soil is filled with liquid water, dissolved salt, water vapour and dry gas. Based on the conservation of mass and energy equations, the paper discusses the mass conservation equations and energy conservation equations for water, gas and salt in unsaturated soils. Considering the one-dimensional steady-state problem, the state equations are obtained by selecting temperature, pore air pressure, pore water pressure and salt solution concentration and their derivatives as the state variables. Under the condition of the boundary values of adjacent soil layers in a given unsaturated soil, the non-linear variable coefficient differential equations are solved by using the shooting method and the validity of the model is verified by comparing with existing steady-state experimental results. Based on numerical examples, the differences in temperature field, pressure field and salinity field between unsaturated layered soil and homogeneous soil are analysed.

Axisymmetric Consolidation of Unsaturated Soils with Vertical Drain with Radial and Vertical Drainage

In this paper, an analytical solution is presented to study the axisymmetric consolidation of a vertical drain inserted in a finite unsaturated soil layer. In the derivation, the decoupling process is employed to convert the governing equations of excess pore-air and pore-water pressures into essentially the same homogeneous partial differential equations (PDEs). Later, the solution with time is obtained by employing the separation variable method. Additionally, validation exercise is conducted by degenerating current solutions into that for the consolidation of vertical drains in saturated soils. Finally, the changing regularity of excess pore-air pressure, excess pore-water pressure and normalized settlement is studied against the ratios of air-water permeability, the ratios of radial-vertical permeability and depth. It is concluded that the solution presented in this paper is reliable and the research method has a reference to more complicated consolidation problems.

Generalized analytical solution for 2D plane strain consolidation of unsaturated soil with time-dependent drainage boundaries

This paper discusses the 2D plane strain consolidation problem of unsaturated soil considering the time-dependent drainage boundaries. A set of time-growing exponential functions are firstly used to describe the changes of excess pore-pressures at the drainage boundaries of an unsaturated soil layer. Then, a generalized analytical solution for the 2D plane strain consolidation under these boundaries and constant initial excess pore-pressures is obtained using the finite Fourier transform and Laplace transform methods. Further, the comparisons are presented to verify the accuracy of the proposed solution. It is shown that the proposed solution is more general in the drainage boundary, and in a good agreement with the analytical solution available in the literature and the finite difference solution. Finally, a worked example is presented to investigate the consolidation characteristics under four changing drainage parameters of time-dependent drainage boundaries. The average degrees of consolidation for both air and water phases as well as that for the soil layer are presented and discussed.

A modelling study of rainfall-induced shallow landslide mechanisms under different rainfall characteristics

Rainfall-induced shallow landslide is a common geological hazard around the world that can pose serious threat to both lives and property. Previous studies suggested that rainfall induced shallow landslides always occurred either above the bottom of a downward wetting front in infiltration, or below a rising water table. This paper proposed a different mechanism of shallow landslides in a modelling approach. The comprehensive physics-based Integrated Hydrology Model (InHM) and the infinite slope stability model were employed on a virtual slope to simulate the hydrologic response and estimate the slope stability. Different failure mechanisms with various characteristics were investigated under diverse rainfall scenarios (i.e., various combinations of rainfall depths, durations and temporal patterns). The results showed a novel mechanism of shallow landslides: a significant vertical change in saturated hydrologic conductivity causes the accumulation of infiltrated water, and subsequently leading to an increase pore pressures and landslides in unsaturated soil layer. More importantly, this kind of landslide would bring larger landslide volume even with smaller total rainfall depths. This study expanded our understanding about landslides and had important application in alleviating the loss of lives and property.