Research On Circumferential Mechanical Properties Of Large Diameter Reinforced Concrete Pipe Jacking

With the increasingly prominent traffic and environmental problems brought by urbanization,the development and utilization of urban underground space has entered a stage of rapid development.As an efficient,safe and environmentally friendly trenchless pipe or tunnel construction technology,pipe jacking method has been increasingly used in urban underground engineering such as municipal pipeline,utility tunnel,and underground passages due to its small impact on the surface environment and wide range of applicable engineering conditions,while reinforced concrete pipe is one of the most commonly used pipes in pipe jacking construction.With the increase of construction demand,the single jacking length of pipe jacking method is getting longer and longer,the cross-sectional size is getting larger and larger,and the crossing stratum is getting more and more complicated.However,the relevant technical measures and theoretical basis of pipe jacking in complex stratum are not perfect.Additionally,because the effect of jacking force,slurry pressure,axis deviation,stoppage and geological conditions,the mechanical behaviors of pipe are very complex during jacking process.Most of the calculation models of pressure around the pipes in the existing standards only consider water and earth pressure,which is different from the actual situation.This thesis aims to reveal the mechanical properties of large diameter reinforced concrete pipe during jacking and improve the relevant theories.Based on a pipe jacking project in fractured rock stratum and a pipe jacking project in weak stratum,the contact state of the pipe and soil,the calculation model of the contact pressure,the mechanical characteristics of the pipe during jacking,the mechanism of the pipe stability failure and the evolution process were studied by using the methods of theoretical analysis,numerical simulation and field test.The main research contents and conclusions are as follows:(1)The excavation convergence and grouting expansion of the tunnel wall under deep and shallow burial were analyzed,and a method for determining the contact state of the pipe and soil considering the tunnel wall deformation and overcut annulus was proposed.Considering the influence of stratum properties and grouting state,the contact pressure distribution models of the pipe in stable and unstable stratums with different slurry filling states and different grouting pressure were established.The deflection states of the pipe during jacking were analyzed,and the distribution model and calculation method of earth counterforce around the pipe under different deflection states were proposed.(2)The applicability of Hertz theory,Persson theory and improved Winkler elastic foundation theory in different stratums was investigated by numerical simulation.The results show that the contact pressure calculated by Hertz model and Persson model are close,and they are applicable to soil and soft rock stratum.In addition,even when the contact angle is greater than 30°,Hertz model is also available,and its calculation method is simpler than Persson model.The calculation accuracy of the improved Winkler elastic foundation model depends on the determination and correction of the subgrade coefficient.The improved Winkler elastic foundation model is applicable in both soil and hard rock stratums with the selection of a suitable pipe-soil contact angle.(3)The mechanical properties of reinforced concrete pipe jacking in a fractured rock stratum were studied through field tests.The results show that in the fractured rock stratum of the overlying soil layer,the earth pressure at the pipe crown can be calculated by combining the Terzaghi theory with the height of the collapsed rock mass.There is a significant negative correlation between the slurry pressure around the pipe and the jacking force.In the stratum with high permeability,choosing an appropriate lubricant injection interval can effectively improve the lubrication efficiency and reduce construction cost.In the synchronous grouting area,the contact pressure and slurry pressure of the pipes mainly show a law of increasing from crown to bottom,while the magnitude and variation of the contact pressure and slurry pressure in the same parts of different pipes are similar.(4)The mechanical properties of reinforced concrete pipe jacking in a weak stratum were studied through field tests.The results show that at the beginning of the secondary grouting,the slurry firstly penetrates and diffuses,and fills the overcut annulus after forming a stable filter cake in the tunnel wall until it fills the overcut annulus.Subsequently,the grouting pressure fluctuates with the secondary grouting.After the completion of single grouting,the slurry pressure decreases rapidly,and the pressure reduction rate also decreases rapidly.After 20～30 minutes of grouting,the decreasing trend of slurry pressure reduction rate slows down significantly.After 50～70 minutes of grouting,the rate of slurry pressure reduction is gradually stabilized.The variation of contact pressure around the pipe is closely related to the pipe-soil-slurry contact state.In the pipe-slurry contact area,the contact pressure is mainly controlled by the slurry pressure,which rise during grouting and decreases after grouting stops.In the pipe-soil contact area,the contact pressure is mainly affected by buoyancy,jacking force,and pipe deflection,and the above factors usually vary greatly during jacking,resulting in large fluctuations of contact pressure.When the pipe is not deflected or the deflection is small,the jacking force has little effect on the contact pressure.On the contrary,the contact pressure increases with the increase of deflection angle and jacking force.(5)Based on the characteristics of pipe stability failure on site,the influence factors of pipe stability failure were analyzed,and the judgment method and evolution process of pipe stability failure were proposed.Combining the field test data with the proposed theory,the mechanical properties of the pipes and the soil during the development of pipe stability failure were analyzed.Moreover,the treatment measures for pipe uplift stability failure were analyzed,and the calculation methods for ultimate displacement and deflection angle of pipe were proposed.(6)The effects of stratum parameters,jacking force,grouting pressure and deflection on the pipe stability failure were studied by numerical simulation.The results show that the effect of the slurry pressure on the floating instability of the pipe is the greatest,when the slurry pressure is greater than the critical pressure,the deformation of the soil increases rapidly.The periodic action of jacking force will reduce the strength of soil around the pipe.In addition,excessive jacking force or deflection angle will promote the development of instability.