Showing papers by "Chao-Sheng Tang published in 2020"

Factors affecting the performance of microbial-induced carbonate precipitation (MICP) treated soil: a review

Abstract: Soil stabilization technology based on microbial-induced carbonate precipitation (MICP) has gained widespread interest in geotechnical engineering. MICP has been found to be able to improve soil strength, stiffness, liquefaction resistance, erosion resistance, while maintaining a good permeability simultaneously. MICP processes involves a series of biochemical reactions that are affected by many factors, both intrinsically and externally. This paper reviews various influential factors for MICP process, including bacterial species, concentration of bacteria, temperature, pH, composition and concentration of cementation solution, grouting strategies, and soil properties. Through this comprehensive review, we find that: (1) the species and strains of bacteria, concentration of bacteria solution, temperature, pH value, and the cementation solution properties all affect the characteristics of formed calcium carbonate, such as crystal type, appearance and size, which consequently affect the cementation degree and distribution in geomaterials; (2) the condition with temperature between 20 and 40 °C, pH between 7 and 9.5, the concentration of the cementation solution within 1 mol/L, and high bacteria concentration is optimal for applying MICP in soil. Under the optimal condition, relatively low temperature, high pH value, and low concentration of cementation solution could help retain permeability and vice versa; (3) the effective grain size ranging from 10 to 1000 µm. MICP treatment works most effectively for larger size, well-graded sand; (4) the multi-phase, multi-concentration or electroosmotic grouting method can improve the MICP treatment efficiency. The grouting velocity below 0.042 mol/L/h is beneficial for improving the utilization ratio of cementation solution. The recommended grouting pressure is generally between 0.1 and 0.3 bar for MICP applications in sand and should not exceed 1.1 bar for silty and clayey soils.

Effects of wetting-drying cycles and desiccation cracks on mechanical behavior of an unsaturated soil

Abstract: The effects of wetting-drying (W-D) cycles on the desiccation-induced cracking of a fat clay (CH) as well as the role of soil suction in the evolution of soil strength, were investigated. Cylindrical block soil samples (50 mm in diameter and 100 mm in height) compacted to a dry density of 1.3 Mg/m3, were subjected to 3 stages of drying at a temperature of 22 ± 2 °C, the first of which had an initial water content of 25%. Three parallel samples were retrieved and tested for unconfined compressive strength (UCS) and surface fracture characteristics at moisture content levels ranging from 13% to 25%. Analyses of images of crack patterns obtained using stereological software, indicate that the pattern of increases in UCS with decreasing moisture content induced by drying, is strongly affected by the intensity of cracks on the samples. Desiccation cracking only occurred in the 2nd and 3rd stages of soil drying. During the first stage of drying, the stress-strain curve of the soil exhibited increase in slope with decreasing water content. This implies the domination of suction at the initial stages. The number of crack segments observed on samples, increases with W-D cycles, thus decreasing soil brittleness. However, increase in W-D cycles does not significantly affect the soil water retention characteristics. These findings illustrate the significant role played by soil suction in strengthening soils against the degradation effects of cracks during the early stages of drying. As drying proceeds, the proliferation of cracks eventually reduces soil strength as soil suction is overcome. Therefore, in developing structures on clayey soils that will be subjected to seasonal changes in moisture content, assessments of the balance of the effects of suction and soil desiccation cracking should be made.

Bio‐mediated soil improvement: The way forward

Abstract: Bio‐mediated soil improvement involves the usage of microbes to improve soil engineering performance through a series of bio‐geochemical processes. In particular, Microbially Induced Calcite Precipitation (MICP), a ubiquitous bio‐geochemical process that occurs in soil and results in permanent inorganic cementation between soil grains, has received the greatest research focus. While substantial progress has been made to develop MICP as a mainstream soil improvement technique, we still need to: (a) improve our understanding of the fundamental microbial, chemical and flow processes involved; (b) achieve multi‐functionality by coupling engineering performance enhancement with ecological, environmental and carbon footprint benefits; and (c) maintain ecological balance and environmental friendliness, avoid long‐term deterioration and lower the energy demand.

Mechanical behavior of fiber-reinforced, chemically stabilized dredged sludge

Abstract: In order to improve the mechanical properties of dredged sludge, short polypropylene fibers were used as physical reinforcement, while cement and fly ash were used as chemical stabilizers. Different mass percentages of fiber (i.e., 0%, 0.05%, 0.1%, 0.2%, 0.4%, and 0.8%), cement (i.e., 15%, 20%, and 25%), and fly ash (i.e., 15% and 30%) were added to dredged sludge at two initial high water contents (i.e., 92% and 120%) and evaluated using physical experiments. The unconfined compression test was performed on samples after curing them for 28 days. The results show that inclusion of cement and fly ash can significantly increase the dry density and reduce the water content of dredged sludge after curing, consequently enhancing the unconfined compressive strength (UCS). It is found that chemical stabilization can increase the stiffness and brittleness of dredged sludge. However, excessively high fly ash fraction in the mix inhibits strength development in cement-stabilized dredged sludge. The inclusion of fiber decreases the initial stiffness and enhances the UCS, strain at failure, and residual strength of dredged sludge. The UCS increases with fiber content and then decreases slightly, with the optimal fiber content being 0.1%. Fiber reinforcement generates a distinct transition zone in the stress–strain curve, which enlarges with increasing fiber content. The contribution of fiber reinforcement to the strength of dredged sludge is more pronounced at relatively lower water content. The inclusion of fiber can temper the brittleness of chemical-stabilized dredged sludge with more ductile behavior by inducing a ‘bridging’ effect. This effect helps in reducing loading-induced cracking of the stabilized sludge.

Drying-induced soil shrinkage and desiccation cracking monitoring with distributed optical fiber sensing technique

Abstract: Monitoring of drying-induced volume shrinkage and desiccation cracking in clayey soils is of great importance in geological and geotechnical engineering. Compared with conventional strain monitoring methods providing discrete measurements, the Brilliouin optical time domain reflectometer (BOTDR) technique enables continuous measurement of the distributed strain generated along optical fibers. In this study, a physical model test is conducted to investigate the feasibility of monitoring drying-induced soil shrinkage and desiccation cracking using BOTDR. Three optical fibers with different surface protections (thermoplastic polyester elastomer (TPEE) jacket, nylon jacket, and acrylate coating) are tested and compared. Experimental results validate that BOTDR is applicable for the direct strain monitoring of desiccation cracking soils. Monitored strain values are strongly influenced by water content, soil cracking, and fiber types. The strain measured by the optical fiber reaches only several micro strains when the soil is over-saturated, gradually increases with the decreasing water content and the increasing soil-fiber interfacial shear stresses, and drops rapidly after the occurrence of decoupling between fiber and soil resulting from the mature development of desiccation cracks. The optical fiber with acrylate coating is not suitable because of its fragility and the poor interfacial coupling with the soil. Optical fibers covered with TPEE jacket or nylon jacket are both applicable for soil strain monitoring, with the former one more sensitive to water content variations. The study is the first attempt to apply the BOTDR technique for the direct and continuous monitoring of drying-induced soil shrinkage and desiccation cracking process. It is expected bring new insights into the fundamental understanding of volumetric shrinkage and desiccation cracking in clayey soils.

Effects of microorganism within organic matter on the mechanical behaviour of solidified municipal dredged mud

Abstract: Municipal mud consists of organic matter naturally deposited in a microbial-rich environment, and its common pre-treatment in the laboratory is normally different from that in situ. In this study, ...

Effects of Biochar on the Compression and Swelling Characteristics of Clayey Soils

Abstract: In recent years, biochar has been widely used in environmental and geotechnical engineering applications, but minimal studies have been done on its influence on soil engineering properties. In this study, the effect of biochar on the compression and swelling characteristics of two clayey soils, namely PKE and XS, was studied by conducting standard consolidation tests and no-load swelling tests. The compression curves and swelling parameters of modified clays with different biochar content (0, 0.5, 1, 2%, w/w) and biochar particle sizes (< 0.15, 1–2 mm) were obtained. The microstructure of the biochar-clay mixture was analyzed by scanning electron microscopy (SEM) test. The results showed that: (1) the compressibility of clayey soils can be effectively reduced by adding biochar, and this effect is more significant with the increase biochar percentage; (2) the effect of reducing soil compressibility by fine biochar was better than coarse biochar; (3) under the same pressure, the settlement rate of soil increased with the increase of biochar content; (4) the incorporation of biochar had no obvious influence on the no-load swelling behaviour of PKE while it has significantly increased the swelling of XS (low expansive soil). Additionally, the fine-grained biochar has a significant effect than the coarse-grained biochar on clayey soils.

Desiccation Cracking Behavior of Clayey Soils Treated with Biocement and Bottom Ash Admixture during Wetting–Drying Cycles:

Abstract: Desiccation cracking considerably impairs the hydraulic and mechanical properties of clayey soils that are critical to the long-term performance of infrastructure foundations and earth structures. ...

Thermal effects on water retention behaviour of unsaturated collapsible loess

Abstract: Temperature has a significant influence on water retention curve (WRC) because temperature affects surface tension of water and volumetric behaviour of soil. However, in previous studies on thermal effects on WRC, the difference in suction-induced volume change of soil specimen at various temperatures is always insignificant. With increasing temperature, the wetting-induced collapse of loess increases. This study aims to investigate thermal effects on WRC of collapsible loess. A loess from Shaanxi province, China, is tested. Wetting–drying tests were carried out on compacted loess specimens at temperatures ranging from 5 to 50 °C. Thermal effects on water retention behaviour of collapsible loess are analysed. During the wetting process, volumetric water content at a given suction at 50 °C is 20% smaller than that at 5 °C. This is because when temperature increases from 5 to 50 °C, surface tension of water decreases by 10% and wetting-induced volumetric contraction increases by three times. During drying, the air entry value (AEV) of loess decreases with increasing temperature at a rate of 0.16%/°C. The retention capability of unsaturated loess decreases with increasing temperature. For the tested collapsible loess, with increasing temperature, a combined effect of smaller water surface tension and larger wetting-induced collapse results in a prominent decrease in volumetric water content of loess. Moreover, the decrease of AEV induced by smaller surface tension is partially compensated by effects of larger wetting-induced collapse on AEV.

Laboratory characterization of sandy soil water content during drying process using electrical resistivity/resistance method (ERM)

Abstract: Drought-induced evaporation can reduce soil water content and significantly alter soil hydro-mechanical behavior. Understanding the temporal and spatial distribution characteristics of soil water content during evaporation is of great significance for evaluating the encountered geotechnical and geo-environmental problems in arid or semi-arid regions. In this study, an electrical resistivity/resistance method (ERM) with a high spatial resolution of centimeter-level was developed for a small-scale laboratory test and applied to quantitatively characterize the evaporation-induced water content variations along a depth gradient. A total of 8 groups of initially saturated sandy soil columns (84 mm in diameter and 290 mm in height) were prepared, and eight pairs of mini electrodes (3.5 mm in diameter) were installed in each soil sample with a vertical distance of 30 mm. The soil columns were subjected to continuous drying. The changes in soil electrical resistance at different depths were monitored by the electrode couples. The gravimetric water contents at different depths were also measured at the end of drying. It is found that soil water content decreases exponentially with increasing electrical resistance. Based on the obtained data, a calibration relationship between soil gravimetric water content and corrected electrical resistance was well established with consideration of temperature effect. This relationship was validated successfully by the experimental results, indicating the feasibility of the developed ERM to characterize the soil water content dynamics during the drying process. Besides, the drying process with the movement of the evaporation front was discussed. The results of this study demonstrate the good performance of ERM in the estimation of temporal and spatial variations of soil water content and its potential application in arid or semi-arid regions with frequent droughts.

An experimental application of electrical resistivity/resistance method (ERM) to characterize the evaporation process of sandy soil

Abstract: Because of global climate change, drought occurs in increasing intensity and frequency across the world. Drought-induced evaporation can reduce soil water content and cause progressive desiccation cracking, which is responsible for various geotechnical and geo-environmental problems. This study aims to develop an ERM with a high spatial resolution and study the ERM performance in the characterization of soil water content dynamics from a quantitative point of view in small-scale evaporation tests, in order to support the future study of desiccation cracking in field investigations. An environmental chamber filled with initially saturated sand was designed for the evaporation test. As the chamber was subjected to drying, the evolutions of soil electrical resistance and water content at different depths were monitored continuously by the installed sixty electrodes and six time domain reflectometers (TDR) sensors, respectively. Experimental results indicate the good correlation between the measured soil electrical resistance and the water content. As evaporation continues, soil electrical resistance increases exponentially with decreasing water content. The variations of soil electrical resistance present an evidentially delayed effect along with the depth. A calibration relationship between the recorded soil electrical resistance by ERM and the water content measured by the oven drying method was established. Afterwards, the variations of soil water content at different depths were estimated based on the developed calibration relationship and compared with the TDR results. Besides, a numerical approach combining a soil-atmosphere interaction model and a coupled hydro-thermal model was employed to study the evaporation-induced variations of soil water content. The estimated soil water contents by ERM were further compared with the results obtained using the numerical method. The evaporation process with the movement of the evaporation front in the studied soil sample was also discussed in depth. This study presents that the developed ERM is effective to record soil moisture dynamics, especially in the near-surface zone, in small-scale evaporation tests. It also provides insights on the potential of ERM in field applications to characterize soil hydraulic responses to drought climate.