Showing papers by "Chao-Sheng Tang published in 2019"

Applicability of Microbial Calcification Method for Sandy-Slope Surface Erosion Control

Abstract: Surface soil erosion is one of the most common slope degradation processes. In this study, microbial calcification (MC), a stimulated natural biocementation process, was investigated for it...

Desiccation Cracking Behavior of MICP-Treated Bentonite

Abstract: This study aims to characterize the effect of microbial-induced calcite precipitation (MICP) on the desiccation cracking behaviors of compacted calcium bentonite soils. We prepare six groups of samples by mixing bentonites with deionized water, pure bacteria solution, pure cementation solution, and mixed bacteria and cementation solutions at three different percentages. We use an image processing tool to characterize the soil desiccation cracking patterns. Experimental results reveal the influences of fluid type and mixture percentage on the crack evolution and volumetric deformation of bentonite soils. MICP reactions effectively delay the crack initiation and remediate desiccation cracking, as reflected by the decreased geometrical descriptors of the crack pattern such as surface crack ratio. The mixture containing 50% bacteria and 50% cementation solutions maximizes the MICP treatment and works most effectively in lowering the soil cracking potential. This study provides new insights into the desiccation cracking of expansive clayey soils and shows the potential of MICP applications in the crack remediation.

A new water retention model that considers pore non-uniformity and evolution of pore size distribution

Abstract: Pore size distribution (PSD), which is usually measured using mercury intrusion porosimetry (MIP) tests, is often used to predict the water retention curve (WRC) of unsaturated soil. Existing models generally predict the drying path of the WRC only because the intrusion of non-wetting mercury in MIP tests is equivalent to air entry during drying. Moreover, the PSD changes under hydro-mechanical loads, which has a significant effect on water retention behaviour. In this study, a new model is developed to predict both the main drying and wetting paths of WRCs. Based on a single PSD at reference stress and suction conditions, we quantified the influence of pore non-uniformity on MIP test results and the main drying and wetting paths of WRCs using the new model. From the reference PSD, we determined variation in the PSD with stress and suction and incorporated this variation into modelling of the WRC. The newly developed model was applied to simulate the PSD variation and the hysteretic WRC of different soils. Based on the results, it is evident that the new model is able to capture the evolution of the PSD during drying, wetting and compression. Moreover, the main drying and wetting paths of WRCs of unsaturated soil were closely predicted.

Experimental simulation of boundary condition effects on bentonite swelling in HLW repositories

Abstract: On the basis of textural and hydro-mechanical characteristics, bentonite has been proven to be an effective buffer/backfill material for long-term containment of high-level radioactive waste (HLW) in deep geological repositories. Herein, the results of experiments performed to investigate the swelling equilibrium limit (SEL) of bentonite under various boundary conditions are presented. A special apparatus was employed to simulate various stress–strain boundary conditions, including constant volume (CV), constant vertical stress (CVS), and constant stiffness (CS). Bentonite samples were prepared with various initial dry densities ranging from 1.5 to 1.7 g/cm3 and vertically stressed to different levels. During wetting, they were subjected to different boundary conditions before the swelling strain or swelling pressure reached equilibrium. Test results indicate that stress–strain boundary conditions have significant effects on the measured swelling strain and swelling pressure of the tested bentonite. More specifically, the relationship between the sequence of swelling pressure under different boundary conditions is CV > CS > CVS, while the relationship between the sequence of swelling strain is CVS > CS > CV. In addition, the characteristics of SEL curves are governed by the initial dry density and vertical stress with the effect of dry density being more significant. Based on these results, several SEL curves were developed to index the effects of boundary conditions on swelling potential of bentonite. They can be used to evaluate the final stress and volume states of bentonite during fluid infiltration under the range of boundary conditions possible in HLW repositories.

Feasibility Investigation of Improving the Modified Green–Ampt Model for Treatment of Horizontal Infiltration in Soil

Abstract: Water infiltration in soil is a complex process that still requires appreciation of interactions among three phases (soil particles, water and air) to enable accurate estimation of water transport rates. To simulate this process, the Green–Ampt (GA) model and the Modified Green-Ampt (MGA) model introduced in the paper “A new method to estimate soil water infiltration based on a modified Green–Ampt model” have been widely used. The GA model is based on the hypothesis that the advance of the wetting front in soil under matric suction can be treated as a rectangular piston flow that is instantaneously transformed after passage of the infiltration front, and the MGA model does not contain the influence of pore size change. This cannot accurately reflect the soil moisture change process from unsaturation to saturation. Due to soil stratification and other inhomogeneity, predictions produced with these models often differ widely from observations. To quickly obtain the soil moisture distribution after passage of the wetting front for horizontal infiltration, an improved modified Green–Ampt (IMGA) model is presented, which estimates the soil moisture profile along a horizontal column in a piecewise manner with three functions. A logarithmic function is used to describe the gradual soil saturation process in the transmission zone, and two linear functions are used to represent the wetting zone. The algorithm of the IMGA model for estimating the water infiltration rate and cumulative infiltration is configured. To verify the effectiveness of IMGA model, a lab model test was performed, and a numerical model was built to solve the horizontal one-dimensional Richards equation using the finite–element method. The results show that the IMGA model is more accurate than the GA and MGA models. The horizontal soil moisture profiles obtained by the IMGA model are closer to the measured data than the numerical simulation results. The relative errors of the MGA and IMGA models decrease with an increase in infiltration time, whereas that of the GA model first decreases and then increases with infiltration time. The primary novelty of this study is nonlinear description of soil moisture content distribution, and derivation of unit transfer coefficient.

Characterization of Soil Moisture Distribution and Movement Under the Influence of Watering-dewatering Using AHFO and BOTDA Technologies

Abstract: The infiltration and distribution of water through unsaturated soil determine its mechanical and hydrological properties. However, there are few methods that can accurately capture the spatial distribution of moisture inside soil. This study aims to demonstrate the use of actively heated fiber optic (AHFO) and Brillouin optical time domain analysis (BOTDA) technologies for monitoring soil moisture distribution as well as strain distribution. In addition to a laboratory model test, finite element analyses were conducted to interpret the measurements. During the experiment, the fine particle migration was also measured to understand its influence on soil hydraulic conductivity. The results of the experiment indicate that (i) for a soil that has never experienced a watering-dewatering cycle, water infiltration can be accurately calculated using the Richards’ equation; (ii) migration of fine soil particles caused by the watering-dewatering cycle significantly increases the hydraulic conductivity; and (iii) two critical zones (drainage and erosion) play significant roles in determining the overall hydraulic conductivity of the entire soil. This study provides a new method for monitoring the changes in soil moisture, soil strain, and hydraulic conductivity. The observations suggest that the effect of fine particles migration should be considered while evaluating soil moisture distribution and water movement.

Experimental study of moisture evaporation process with different soil characteristics

Abstract: Evaporation of water from soils induces soil cracking, salinization and degradation, especially in arid irrigated areas. The factors that influence water evaporation from soil includes external type, referring to atmospheric conditions and interior type, covering surficial soil characteristics and water content conditions. In spite of the numerous assessment that have been made on the influence of atmospheric conditions and soil characteristics on evaporation of moisture from soils, many aspects still require further investigation due to soil complexity. In this study, laboratory-based evaporation tests were performed on clayey soil and three kinds of quartzite sands of different particle sizes using the same environmental conditions. Three different tests were performed on slurry samples to evaluate the effects of soil characteristics on its free water evaporation process at room temperature (20-22°C) and relative humidity (50 ± 2%) conditions: soil thickness, particle size of mixing sand and mixing sands ratio. It is shown that larger soil sample thicknesses accelerate water evaporation rate and extend the constant evaporation rate stage. Higher sand mix proportions lead to larger evaporation area and soil porosity, allowing higher evaporation rate and longer duration of the first stage and the starting of the falling rate stage at lower water content. However, soil particle size was not found to have a significant impact on evaporation rate on per unit weight of added soil basis.

Desiccation Cracking Behavior of Soils

Abstract: The formation of desiccation cracks on soil surface is a common natural phenomenon as it is subjected to drought climate. The presence of cracks can significantly affect the hydro-mechanical behaviour of soil, and results in various geotechnical problems. With increasing frequency of severe drought climate, better understanding of soil desiccation cracking behavior is becoming an increasingly significant issue. In this investigation, laboratorial desiccation tests were performed on soils by simulating long term drought climate. A camera was mounted above the specimen to monitor the initiation and propagation behaviour of desiccation cracks. Image processing technique was employed to quantify and characterize the obtained crack patterns. It is observed that the desiccation cracking generally takes place in three stages: main-cracks firstly start on soil surface and form main-clods; subsequently, main-clods are split into several sub-clods by sub-cracks; after the size of all clods is stable, cracking terminates and the final crack pattern is formed. It is shown that the image processing technique is efficient to accurately determine the geometrical parameters of crack patterns during drying, including crack width, length, clods area, etc. The introduced density function is effective in describing the distribution characteristics of the geometrical parameters of crack patterns.