The spacing of PVDs is an essential factor affecting the consolidation effect of vacuum preloading. For exploring the influence of spacing of PVDs on the impact of sludge drainage consolidation, FeCl3, a commonly used inorganic coagulant, was used to pretreat the sludge. In the experiment a vacuum filtration test was carried out to determine the optimal addition amount of FeCl3, and then the landfill sludge was pretreated according to the FeCl3 optimal addition amount. And two different spacing of PVDs were used to carry out a vacuum preloading contrast test. Then, the drainage and settlement were recorded, and water content and vane shear strength (VSS) were measured after the experiment. Finally, Mercury intrusion porosimetry (MIP) was carried out to explore the pore characteristics of the sludge further. The main conclusions are as follows: After conditioning by FeCl3, the sludge's flocculent structure was destroyed, the intracellular water was released, and the effect of drainage capacity was significantly improved. After the spacing of PVDs is halved, the average volume reduction ratio and shear strength increases, and the effect of advanced dewatering and volume reduction of sludge improved significantly, leading to a better consolidation effect. In the progress of vacuum drainage consolidation, halving the spacing of PVDs results in large pores transformation into small pores, and the range of drainage consolidation is greatly expanded.
This study investigates the reduction of groundwater level in the soil deposit below the ground surface under vacuum consolidation. Theoretically, when the applied vacuum pressure is less than the air-entry value (AEV) of the soil deposit, the deposit will be in a saturated state and the groundwater level in the pre-fabricated vertical drains (PVDs) will not go below the ground surface during the vacuum consolidation process. In this case, the groundwater level is not a phreatic water level. If the applied vacuum pressure is larger than the AEV of the soil, part of the soil deposit can become unsaturated and the groundwater level in the PVDs can go below the ground surface. Further, the results of a large-scale vacuum consolidation model test using clay confirmed that during the consolidation process, the water level in the mini-PVD was always above the top surface of the model ground. Therefore, an attempt to measure the groundwater level in a vacuum consolidation area composed of clay deposits may not be worthwhile.
The freezing process of salinized soil is a complex, dynamic, and interactive hydro–thermal–salt–mechanical (HTSM) coupled physical phenomenon. In many recent studies, soil was assumed to be saturated, and the theoretical models established were based on the framework of saturated soil, in which the influence of the vapor phase in freezing soil was neglected. In this paper, by considering the effect of vapor flow on heat movement and the relation between saturation and void ratio, an improved mathematical model will be established based on previous research. This improved hydro–thermal–salt–mechanical (IHTSM) model simulates the dynamic process of water migration, heat transfer, vapor flow, solute transport, and deformation. The numerical simulation implemented by the IHTSM model under the typical conditions of unidirectional freezing will be compared with previous research to verify the model's validity, and the various characteristics of the curves and their physical meaning will be analyzed by comparing them with the previous research. The dynamics of temperature, mass moisture content, displacement, salt content, volumetric vapor content and saturation degree in soil column will be discussed for the salinized soil during the freezing process. The results indicated that this improved model could provide a reference for the destruction process analysis of the harsh geological environment in cold, arid, and saline areas.
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