Study On The Reinforcement Methods Of Subway Large-Diameter Shield Launching In The Sandy Clay With High Water Pressure

With the increasing accomplishments of numerous crossing-sea transportation tunnels, there aremore and more large-diameter-long-distance shield tunnel projects under construction. Inpractice, the larger the tunnel section is, the higher the risk of tunnel's entering and exiting willface. Especially in the condition of sandy soil with high water pressure, it is a big issue how toscreen out the optimal method to enhance the tunnel end and guarantee the security when largediameter shield enter and exit during construction. Based on the shield launching project ofcrossing-river tunnel construction of Nanjing Subway Line10, my dissertation, by combiningthe execution of lab tests, numerical simulation, theory study, engineering application andpractical measurements, studied and compared reinforcement methods and evaluate theirsecurity in the project. First, two typical soils, collected on the site of shield launching project,were tested in terms of thermo-physical property and the mechanical characters before and afterthe cement improvement, respectively. Second, numerical analysis and comparison among threefrequently employed reinforcement methods were carried out. The relative data were analyzedand the optimal method was then proposed. Third, combined with the adopted method inpractice, numerical simulation to shield launching was made under different constructioncircumstances. Additionally, onsite measurement research was conducted on the vertical frozenconstruction. This paper mainly focused and drew the conclusions as follows:1. Lab tests were conducted to study the thermo-physical property and the mechanicalcharacters of the two typical types of soil collected at the tunnel ends, before and aftercement improving, respectively. The thermo-physical and mechanical parameters of the soilat the ends used in the numerical analysis are suggested.2. The common used methods of soil reinforcement at the ends of shield tunnel and theirdesign procedures are compared and analyzed. The reinforcement sizes of the soil under themethods of triaxisal deep mixing and high-pressure rotary spray and its stability is checkedby analytical solution algorithm; the initial excavation of different reinforcement areas isnumerically analyzed in terms of displacement and stress field with the door demolished,and the specific reinforcement scope of these methods and related reinforcement techniquesare given.3. Combined with the numerical simulation, the scope for cup-shaped horizontal freezingreinforcement was determined. The design and operational technologies of the freezingreinforcement were explored, consisting of the analysis for the temperature fielddevelopment and distribution pattern of the big-diameter-cup-shaped frozen soil wall, andthe impact from the surrounding factors, including the temperature of saline water,coefficient of heat conductivity, the thermal capacity of the volume, the latent heat of phasechange and the original earth temperature. Research is carried out to compare thedevelopment and distribution of this temperature fields in different circumstances such ascement sandy soil, sandy soil, cement clay and clay.4. The design scheme is determined, in which the reinforcement methods of triaxial deep mixing and vertical freeze are adopted. The development and distribution of the temperaturefields of the vertical frozen soil wall is studied. Research is conducted to compare thedevelopment and distribution of this temperature field in different diameters of freezingpipe and spacings between freezing pipes.5. Comparisons are made among the three reinforcement methods studied above, andadoptable reinforcement methods are put forward; the reinforcement method applied in realproject as well as its operational techniques were introduced; and combined with adoptedreinforcement methods, numerical analysis is conducted upon the initial shield excavationunder different construction circumstances.6. Combined with reinforcement methods adopted in practice, onsite measurement research iscarried out on vertical freezing construction; the rules of the temperature change of verticalfrozen soil wall in the launching project during the phases of positive freezing andmaintained freezing are analyzed in detail to judge whether the vertical frozen soil wall hasmet the demand of design and determine the timing of hole excavation and shieldlaunching.