Study On The Uplift Resisting Control Of Large Underground Structures Based On Groundwater-discharging Pressure Relief Method

Many cities worldwide take a comprehensive use of their urban underground space. With the emergence of large amounts of large and extra-large underground structures, the problem of hydrostatic uplift safety of large underground structures became important. The traditional passive uplift resisting approachs are complex, expensive, and technically difficult, with many common problems in underground structures such as structure durability of cracking, corrosion-resistant and anti-fatigue. Therefore, it is important and urgent to explore new green anti-floating methods for large-scale development and utilization of underground space. This paper introduced a new anti-floating method of underground structures—under drain anti-floating method.The mechanism of this innovative uplift resisting method, the theoretical analysis and calculation system, and key technical problems such as project risks and quality control were systematically studied.Firstly, by analysing the construction of a large underground parking lot in Shenzhen, this paper systematically compared the characteristics of common anti-floating methods on seven aspects, including the schedule, the life cycle cost and so on. It also conducted the in-depth analysis on the uplift resisting mechanism of groundwater-discharging pressure relief. In this way, the under drain anti-floating method was believed to be a green construction technology method that saved materials and energy, and it was argued that this method could be applied to aquitard sites in general.Secondly, this paper studied the mechanism of groundwater-discharging pressure relief and the characteristics of the underground structure, and identified four vital parameters for designing the large underground structures with the under drain anti-floating method, including the pore water pressure head at the bottom of raft foundations.Therefore, it brought up the dynamic control principles of under drain anti-floating method and the composition of the control system from the perspective of modern system control theory.The working principle and process of dynamic anti-floating system largely bases on the comprehensive real-time monitoring data and the finite element seepage analysis and calculation results, and dynamically protect the anti-floating safety of underground structures and the surrounding environment by adjusting the level of the horizontal tube elevation.Thirdly, based on the groundwater seepage theory of saturated soil and unsaturated soil, this paper established the seepage finite element anti-floating calculation model of under drain. By analysing and calculating the seepage under a variety of conditions analysis, it optimized the control interval and value of the anti-floating design parameters. In-depth discussions on two key issues were conducted on the anti-floating design, including system head loss and the impact on the surrounding environment during the operation.In addition, this paper proposed the the under drain anti-floating risk management process framework, based on the principles of project risk management. It identified and determined the under drain anti-floating failure risk indicators by the finite element anti-floating calculation and analysis. The fault tree model of the established under drain anti-floating system was built and all possible failure modes and their possibilities of occurrence leading to the under drain system failures were located and calculated, and corresponding control measures of the anti-floating risks were developed according to the degree of importance.Finally, this paper analysed the quality control of uplift resisting inverted filter layer. The constitution model of the inverted filter layer was determined, the critical control parameters were calculated, and the construction process and construction control points of the inverted filter layer were summarised. This paper integrated the eight types of monitoring data of the underground structures in both the construction phase and the operational phase, and discussed the effect of the under drain uplift resisting system, it concluded that the under drain anti-floating method had a very small impact on the surroundings, and it was economically rational and technically feasible.This paper brought up an innovative anti-floating method for underground structures. This research is theoretically and empirically important and can provide a reference for similar large-scale underground construction.