Practice And Research On The Control Of Confined Water In The Ultradeep Foundation Pit Projects

During the construction of underground projects, the disposal of underground water is a nodus and due to the influence of the confined water to the ultradeep foundation pit, besides considering the construction dewatering, the dewatering design for the foundation pit confined water should be conducted according to the checking calculation for the confined water resistant stability of the foundation base. Because of the complexity of the geological conditions and hydrogeological environment, the diversity of the retaining and protection design for foundation excavation, the variance of the deep confined water's embedding conditions, the dissimilarity of the foundation pit drainage's boundary condition and the discrepancy of the engineering technical personnel's knowledge on the confined water, the dewatering plans made by different designers vary greatly, the inputs and the implementation results for the projects usually rest on the likes and dislikes of the designers and there is no unified standards. In recent years, as the further development of the underground space, the foundation pit engineering accidents caused by the confined water are nothing new. Therefore, it is necessary to carry out the special engineering research on the dewatering design and engineering techniques for the ultradeep foundation pit confined water.Combined the practice of the five ultradeep foundation pit confined water dewatering projects done by our company (MCC20), through typical case analysis and technical summarization, with necessary theory study, the topic of this dissertation is to find out the design and construction law of the confined water dewatering project, perfect the design theory, develop the engineering techniques and provide reference for the future similar projects. The main work includes: the analysis for the distribution and engineering hazard of the confined water, the construction summarization and technical analysis for the typical confined water dewatering cases, and the type selection study on the confined water dewatering plan.Main conclusions: the confined water's extent of harm to the projects is closely related to the water head, coefficient of permeability, layer thickness and the relative location to the foundation pit; it is an efficient method to solve the confined water's sudden piping problem utilizing the deep well for depressurization dewatering and can control the ground subsidence effectively by controlling the dewatering depth and shortening the dewatering time according to the principle of 'pumping as per requirement, controlling the dewatering depth'; if the confined aquifer is cut by the confining curtain of the foundation pit and has no vertical connection with other aquifers, its influence to the stability of the pit base can be left out of account, however, the confining effect of the enclosure should be paid enough attention to during the excavation of foundation pit; as for multilayered confined water, the dewatering plan should preserve the independence of each aquifer as best as one can and control the dewatering depth according to the requirement, which is simple, easy, and economically reasonable; whether to choose the interior dewatering or exterior dewatering should base on the comprehensive analysis of such factors as the hydrogeological condition, the size of the foundation pit, the way of retaining and protection and excavation and the environmental requirement, and it should be determined by the technical and economic comparison.