Upon drying, the evaporation of soil water results in volumetric shrinkage and desiccation cracks on expansive soils. The presence of crack can significantly weaken the engineering properties of soil, and cause various problems in geological, geotechnical, hydraulic, environmental engineering fields. With the increasing frequency of severe drought climate, the engineering problems that induced by desiccation cracking can be more and more, worse and worse. The study of desiccation cracking is therefore very significant for revealing the intrinsic mechanisms behind this common natural phenomenon, and plays an important role in engineering practice in clayey soil areas and especially in expansive soil areas. Generally, the initiation and propagation of desiccation cracks show evident dynamic characteristics and significantly depend on soil water evaporation rate, stress state and shrinkage property. The cracks initiate at constant evaporation rate stage. Soil suction and tensile strength are the two key mechanical parameters that control the cracking behavior. Cracking is likely to occur if the tensile stress which is induced by soil suction reaches the tensile strength of soil. Intrinsically, cracking is the result of pore shrinkage. It is believed that the mechanical effect and shrinkage potential are the two necessary factors for crack initiation. It is found that the desiccation cracking behaviour is affected by many factors including soil materials, mechanical properties, structures, test conditions, and test methods. Quantitative characterization of crack patterns is required for desiccation cracking investigation, and plays very important role in understanding the cracking mechanism and constructing the relevant model. Image processing is a powerful, efficient and high-accurate tool for quantitative description crack patterns. In the next stage, more attention should be paid to the dynamic characteristics of cracking, and more work should be done on water-soil interaction, mechanical mechanism, shrinkage mechanism, large scale field test and 3D quantification technique that related to soil desiccation cracking. It is also important to study desiccation cracking behaviour on the basis of soil materials, mechanics and structures, to integrate macro-observation with micro-analysis and to construct perfect theory for characterizing desiccation cracking.
The mine development becomes somewhat a dilemma. On one hand, it provides the resources people need, while on the other hand, it results in the environmental problem and people are suffering from the diseases caused by the degradation of the environment. The situation is getting so bad in the areas with higher severity of the environment pollution that it becomes difficult in providing the drinkable water and food supplies and even worse, the endemics and the genetic diseases come on. Therefore, it becomes more and more important about how to perform the environment surveillance. One of the solutions is to figure out the range of the environmental changes based upon the investigation and the analysis about the series of the index indicators about the environmental changes. With the research conducted at Dexing Mine, Jiangxi Province, the following parameters are defined in this paper as the index indicators: the water color, the Mine acreage and the mineral residues locations, and the variation of the plant coverage within 1 Km around the water system. The methods to calculate these index indicators are also illustrated in this paper, which are based on the primary geographical information and the remote sensing data. In addition, the spatial characteristics of these index indicators are finely analyzed. All these information are helpful to the risk analysis about the mine development.
Desiccation cracking in clayey soil, which may lead to soil erosion, geotechnical engineering accidents, or even environmental pollution, is a serious problem nowadays. This research proposed a multifield coupling discrete-element model of clayey soil, in which each element represented a certain volume of soil. Meanwhile, the uneven distribution and transfer of moisture were also achieved. By establishing the relationships between water content and element radius, Young's modulus, and tensile strength, respectively, the model coupled the moisture field with the stress field. Through a discrete-element simulation of desiccation cracking in a thin clay layer, the gradual development of crack network was successfully reproduced, and the proposed model was validated. The uneven moisture distribution in the numerical specimen indicates that cracks can intensify evaporation by increasing the area of soil-air interface. Layer thickness, evaporation intensity, soil-base interaction, and compressive strength are proved to have significant impacts on crack pattern by influencing the equilibrium between desiccation shrinking and cracking. This research provides a new means to study the mechanism of desiccation cracking under multifield coupling effects.
Micro-disturbance grouting is a recovery technique to reduce the excessive deformation of operational shield tunnels in urban areas. The grout mass behaves as a fluid in the ground before hardening to form a grout-soil mixture, which highlights the necessity of using fluid-solid coupling method in the simulation of grouting process. Within a discrete element modeling environment, this paper proposes a novel fluid-solid coupling method based on the pore density flow calculation. To demonstrate the effectiveness of this method, it is applied to numerical simulation of micro-disturbance grouting process for treatment of large transverse deformation of a shield tunnel in Shanghai Metro, China. The simulation results reveal the mechanism of recovering tunnel convergence by micro-disturbance grouting in terms of compaction and fracture of soil, energy analysis during grouting, and mechanical response of soil-tunnel interaction system. Furthermore, the influence of the three main grouting parameters (i.e., grouting pressure, grouting distance, and grouting height) on tunnel deformation recovery efficiency is evaluated through parametric analysis. In order to efficiently recover large transverse deformation of shield tunnel in Shanghai Metro, it is suggested that the grouting pressure should be about 0.55 MPa, the grouting height should be in the range of 6.2–7.0 m, and the grouting distance should be in the range of 3.0–3.6 m. The results provide a valuable reference for grouting treatment projects of over-deformed shield tunnel in soft soil areas.
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