Yu-Chao Li

Analytical Solutions for Contaminant Diffusion in Double-Layered Porous Media

Analytical solutions for conservative solute diffusion in one-dimensional double-layered porous media are presented in this paper. These solutions are applicable to various combinations of fixed solute concentration and zero-flux boundary conditions (BC) applied at each end of a finite one-dimensional domain and can consider arbitrary initial solute concentration distributions throughout the media. Several analytical solutions based on several initial and BCs are presented based on typical contaminant transport problems found in geoenvironmental engineering including (1) leachate diffusion in a compacted clay liner (CCL) and an underlying stratum; (2) contaminant removal from soil layers; and (3) contaminant diffusion in a capping layer and underlying contaminated sediments. The analytical solutions are verified against numerical solutions from a finite-element method based model. Problems related to leachate transport in a CCL and an underlying stratum of a landfill and contaminant transport through a capping layer over contaminated sediments are then investigated, and the suitable definition of the average degree of diffusion is considered.

Influences of operational practices on municipal solid waste landfill storage capacity

The quantitative effects of three operational factors, that is initial compaction, decomposition condition and leachate level, on municipal solid waste (MSW) landfill settlement and storage capacity are investigated in this article via consideration of a hypothetical case. The implemented model for calculating landfill compression displacement is able to consider decreases in compressibility induced by biological decomposition and load dependence of decomposition compression for the MSW. According to the investigation, a significant increase in storage capacity can be achieved by intensive initial compaction, adjustment of decomposition condition and lowering of leachate levels. The quantitative investigation presented aims to encourage landfill operators to improve management to enhance storage capacity. Furthermore, improving initial compaction and creating a preferential decomposition condition can also significantly reduce operational and post-closure settlements, respectively, which helps protect leachate and gas management infrastructure and monitoring equipment in modern landfills.

Stability of Slurry Trenches with Inclined Ground Surface

AbstractA method for assessing stability of slurry trenches within an inclined ground surface is presented using the Coulomb-type force equilibrium method. The influence of ground surface inclination on trench stability and angle of critical failure surface is investigated via parametric studies on an example trench. As expected, the factor of safety decreases considerably with an increase in the ground surface inclination. It is shown that it is unconservative to neglect ground surface inclination while analyzing trench stability. According to the example, no significant error in the minimum factor of safety is introduced by using the failure surface inclination angle of 45°+ϕ′/2 for trenches with an inclined ground surface. It is also found that differences between the angle of critical failure surface and 45°+ϕ′/2 may exist; therefore, the angle of the critical failure surface needs to be determined instead of using 45°+ϕ′/2 for the design of remedial measures.

Stability Analysis of Slurry Trenches in Similar Layered Soils

Slurry trenches are often used to impede groundwater flow or to contain soil and groundwater contamination. The stability of the slurry trench during excavation and prior to backfilling is a major concern in design. A horizontal slice method for stability analysis of slurry trenches, which allows consideration of soil stratification, is presented. Vertical force equilibrium for each slice of the sliding mass and horizontal force equilibrium for the whole sliding mass are considered on the basis of limit equilibrium. The factor of safety is obtained via the Newton–Raphson method and the critical slip surface corresponding to the minimum factor of safety is located by the pattern search method. The proposed method is verified via consideration of a previously reported problem. It is then applied to a full-scale field experiment in three-layer soils. The obtained minimum factor of safety and location of the slip surface of the sliding mass are in excellent agreement with the experimental measurements. Analyses of trenches having a weak layer at various depths demonstrate potential variations in slip surface inclination between soil layers.

A degradation model for high kitchen waste content municipal solid waste

Municipal solid waste (MSW) in developing countries has a high content of kitchen waste (KW), and therefore contains large quantities of water and non-hollocellulose degradable organics. The degradation of high KW content MSW cannot be well simulated by the existing degradation models, which are mostly established for low KW content MSW in developed countries. This paper presents a two-stage anaerobic degradation model for high KW content MSW with degradations of hollocellulose, sugars, proteins and lipids considered. The ranges of the proportions of chemical compounds in MSW components are summarized with the recommended values given. Waste components are grouped into rapidly or slowly degradable categories in terms of the degradation rates under optimal water conditions for degradation. In the proposed model, the unionized VFA inhibitions of hydrolysis/acidogenesis and methanogenesis are considered as well as the pH inhibition of methanogenesis. Both modest and serious VFA inhibitions can be modeled by the proposed model. Default values for the parameters in the proposed method can be used for predictions of degradations of both low and high KW content MSW. The proposed model was verified by simulating two laboratory experiments, in which low and high KW content MSW were used, respectively. The simulated results are in good agreement with the measured data of the experiments. The results show that under low VFA concentrations, the pH inhibition of methanogenesis is the main inhibition to be considered, while the inhibitions of both hydrolysis/acidogenesis and methanogenesis caused by unionized VFA are significant under high VFA concentrations. The model is also used to compare the degradation behaviors of low and high KW content MSW under a favorable environmental condition, and it shows that the gas potential of high KW content MSW releases more quickly.

Coupled Thermo-Hydro-Chemo-Mechanical Modeling for Geoenvironmental Phenomena

Coupled thermo-hydro-chemo-mechanical (THCM) behavior of soils is important in a number of geoenvironmental engineering applications including nuclear waste disposal, landfill engineering, freeze-thaw cycles and fate and contaminant transport. As part of ongoing research and development in this field a number of key developments have been undertaken at the Geoenvironmental Research Centre (GRC), Cardiff, UK, with particular focus on the numerical simulation of these phenomena. This paper outlines some of these developments set within the context of the numerical finite-element model developed at the GRC — COMPASS, and discusses the theoretical and numerical implementation and comparison of numerical simulations with experimental data. In particular the development of the effects of temperatures elevated above 100°C on gas pressure and water phase change and consequently the moisture movement; frozen ground behavior with particular reference to seasonal freeze-thaw cycles with frost heave and ice segregation possibly leading to solifluction; reactive thermo-hydro-chemical-mechanical models including geochemical behavior with focus on the osmotic potential; and High Performance Computing (HPC) have been included.

One-Dimensional Transient Analytical Solution for Gas Pressure in Municipal Solid Waste Landfills

AbstractLandfill gas (LFG) is generated in municipal solid waste (MSW) landfills because of the decomposition of degradable components. Gas pressure within the landfilled MSWs is of importance as cover or landfill stability may be affected by gas pressure. This paper presents a one-dimensional transient analytical solution for gas pressure profiles in a MSW layer. This solution considers the variation of the LFG generation rate with respect to depth and decay of the LFG generation rate with time. The authors used the specified outward gas flux bottom boundary condition to simulate horizontal LFG collection systems and basal leachate collection systems serving dually as LFG collection systems. The presented solution also gives the outward gas flux from the MSW layer surface, which may be used to assist in the prediction of air quality near landfills. An application of the presented solution shows that averaging the LFG generation rate throughout the MSW layer results in a remarkable overestimation of gas p...

Freezing soil effects on earth-contact heat transfer

The inclusion of soil freezing and snow cover within the context of a building energy simulation is explored. In particular, a method of including soil freezing within the simulation of heat flow from a building to the neighbouring foundation soils is considered. Non-linear thermal conductivity and heat capacity relations are explored that account for the effect of soil freezing. In addition, the work also considers latent heat generated by phase change that occurs as the soil water temperature reduces and ice forms. A simple approach to represent the insulating effect that snow cover may have on the net heat flow at the ground surface is also provided. The approach is illustrated by application to the simulation of a full-scale ground heat transfer experiment performed by others. The results provide a first indication of the potential significance of the inclusion of ground freezing within the context of modelling heat transfer from a full-scale monitored building. Overall transient temperature variations are shown to be dependent on ice content and latent heat effects. Non-linear, ice-content dependent, thermal conductivity and heat capacity are included in the work. Good correlation between measured and simulated temperature variations has been achieved. Practical application: The principle application relates to the assessment of heat transfer from buildings through to the underlying foundation soils. In addition, the work is of direct relevance to a wider range of applications, for example, ground source heat pumps, thermo-active piles and foundations. The model lends itself to application with respect to utilisation of ground energy within the context of pavement design, particularly, in relation to the alleviation of winter ice problems. It also applies within the context of assessment of urban heat island phenomenon and heat and moisture transfer beneath freezer foundations.

Analytical Solution for One-Dimensional Diffusion of Organic Pollutants in a Geomembrane–Bentonite Composite Barrier and Parametric Analyses

AbstractGeomembrane–bentonite composite barrier is known as one of the most reliable and effective technologies for containing underground contamination. Analytical solution for one-dimensional diffusion of an organic solute in the three-layered composite barrier is presented for the case in which the barrier is keyed in aquitard. Parametric analyses were conducted to investigate the effects of partition coefficient and installation location of high-density polyethylene (HDPE) geomembrane, retardation factor, and thickness of bentonite slurry wall on the barrier performance using this analytical solution. The analysis results demonstrate that the containment performance of the composite barrier depends much on the partition coefficient of HDPE geomembrane over the target pollutant. The HDPE geomembrane exhibits an excellent resistance to the hydrophilic organics, which have a low partition coefficient. However, the hydrophobic organics, which have a high partition coefficient, can diffuse readily through ...

A Simplified Third-Type Inlet Boundary Condition Solution for Contaminate Transport Through Slurry Cut-off Walls

Low-permeability slurry trench cut-off walls are commonly constructed as barriers for containment of subsurface point source pollution or as part of seepage control systems on contaminated sites. Scholars made achievements on design methods for first-type inlet boundary scenarios, but there are still lacking on third-type inlet method, as the choice of inlet boundary conditions is still controversial. A method to estimate wall thickness in slurry wall design in such boundary condition is proposed based on previous design method for first-type inlet. An error analysis is performed to investigate the impact of this assumption by comparing the results obtained by the proposed method to those from an analytical solution. For breakthrough criterion C* of less than 0.1, which covers common practical situations, the relative error is not greater than 15%, demonstrating that the proposed method is suitable for design. An example is also presented to illustrate the procedure of implementing the proposed method for slurry wall design.