Research On Friction Characteristics Of Pipe Jacking And Drag Reduction Measures

With the acceleration of urbanization and the shortage of land resources,more and more cities are turning their attention to underground engineering.Among them,pipe jacking is a commonly used trenchless tunnel construction technology in underground engineering.The resistance in the construction process mainly comes from the friction between the pipe and the soil,and the friction resistance gradually increases with the increase of the jacking distance.With the needs of the project,the pipe jacking is developing in the direction of large diameter and long distance,resulting in greater friction,so the drag reduction effect directly affects the pipe jacking construction.If the drag reduction effect is not good,it will make the jacking pipe difficult to push in,and at the same time may cause damage to the pipe section.Therefore,it is necessary to study the drag reduction characteristics of pipe jacking and the development law of friction and jacking force during jacking process.Relying on a pipe jacking project in Hangzhou as the background,this paper summarizes and analyzes the measured data,indoor tests and model tests,finite element numerical simulation on drag reduction characteristics,friction and jacking force changes with jacking distance,and excavation on the surface with the jacking.The main research contents and results include:(1)Summarize the measured data of multiple pipe jacking projects and multiple indoor test data,and analyze the friction resistance characteristics.It is found that the friction resistance ratio is relatively large at the beginning,and gradually decreases with the increase of the jacking distance ratio,and finally Stable at a certain value,the final friction ratio of most actual pipe jacking projects is stable between 0.4 and 0.6,and the final friction ratio of most indoor tests is stable between 0.1 and 0.2.(2)Analyze the performance of the mud through laboratory tests and verify the drag reduction effect of different proportions of mud with model tests.A mix ratio with a better drag reduction effect in silt is obtained.The mix ratio is bentonite with a mass fraction of 8%,0.3% soda ash,0.2% CMC and 0.05% PAM.(3)Use ABAQUS finite element software to simulate the jacking process of the pipe and compare it with the monitored value and empirical value,analyze the development law of friction and jacking force with the jacking distance,and obtain the simulation value of unit friction and the monitoring value.It gradually decreases with the increase of jacking distance,and finally stabilizes;when the jacking distance is not large,the simulation value of jacking force,the monitoring value and the empirical formula value are closer;when the jacking distance is large,with drag reduction full play of the mud effect,both the simulated and monitored values of the jacking force reached the peak value,but the empirical formula value still increased linearly,with a certain deviation from the monitored value,and the greater the jacking distance,the greater the deviation.(4)The settlement of the soil directly above the pipe jacking along the axis and the cross-section is analyzed through numerical simulation.After the analysis,it is found that in the direction of the excavation cross-section,as the distance from the axis coordinate decreases,the settlement continues to increase.At the axis,the settlement value reaches the maximum;along the direction of the excavated pipe jacking,the excavated part of the surface has a larger settlement,and the unexcavated part in the front has a smaller settlement.The soil in front of the pipe jacking machine has a slight uplift,and then it settles,gradually stabilize.