Study On The Mechanical Characteristics And Engineering Application Of Parallel Super-large Shallow Buried Rectangular Pipe Jacking

With the rapid development of China's economy and the gradual acceleration of urbanization,rational and efficient use of urban underground space has become the focus of future urban development.Rectangular pipe jacking has been widely used because of its high utilization rate,high cost-effectiveness ratio and many other advantages.At present,there is little research on the construction theory of large cross-section parallel rectangular pipe jacking with shallow overburden in China,which needs further exploration.In this paper,based on a parallel rectangular pipe jacking project in Fuzhou City,the mechanical characteristics of large section shallow buried parallel rectangular pipe jacking under the coastal soft soil layer are studied by numerical simulation and field measurement.The main research results are as follows:(1)The mechanical effect of pipe jacking and construction control technology are summarized,mainly including: construction technology measures to reduce soil disturbance in special coastal soft soil stratum;environmental problems caused by support force,grouting and stratum loss of pipe jacking face during construction;mutual disturbance mechanism among the first pipe,the second pipe and the soil during parallel pipe jacking construction.(2)Using the finite element software to simulate the construction process of parallel pipe jacking,the vertical displacement of soil and the development and change rule of pipe jacking stress in the process of parallel pipe jacking are analyzed.The conclusions are as follows: 1)after the completion of pipe jacking,the maximum ground settlement value is located on the top of the pipe jacking central axis,and the farther away from the pipe jacking central axis,the smaller the impact.2)When the pipe jacking is completed,the final settlement value of the surface reaches 1.5 times of the maximum settlement value when the pipe jacking is completed.The position of the maximum ground settlement value is transferred from the top of the central axis ofthe pipe jacking to the joint position of the parallel pipe jacking.3)The pipe jacking stress increases rapidly at the beginning of jacking,and with the increase of jacking distance,the stress growth rate decreases gradually.(3)Combined with the finite element software,the effects of different pipe spacing and soil friction coefficient on the surface vertical displacement and pipe jacking stress are studied.The relevant conclusions are as follows: 1)when the pipe jacking spacing / pipe jacking width is less than 1,the mutual disturbance between the two pipes is very intense,and the surface settlement curve presents a "V" shape.2)When the pipe jacking distance / pipe jacking width ? 1,the interaction between parallel pipe jacking decreases gradually,and the surface settlement curve changes from "V" to "W".3)When the pipe jacking distance / pipe jacking width is more than2,the interaction between pipe jacking is almost 0,and the independence is very obvious.4)The increase of the friction coefficient of pipe and soil will aggravate the disturbance of pipe jacking construction to the soil mass and produce greater shear failure,which will increase the ground settlement and pipe jacking stress produced in the process of parallel pipe jacking construction.(4)Based on the results of numerical simulation,the optimization suggestions for parallel rectangular pipe jacking engineering construction are put forward: 1)reasonably control the ratio of pipe jacking distance to pipe jacking width,reduce the mutual disturbance between parallel pipe jacking and reduce the impact of the project on the surrounding environment.In the design of parallel pipe jacking engineering,the spacing / width of pipe jacking should be no less than 2,which can minimize the interaction between parallel pipe jacking and improve the safety of pipe jacking construction.2)Reasonably control the grouting volume in the construction,reduce the pipe soil friction force in the construction process,reduce the ground settlement and reduce the pipe jacking stress,and improve the safety factor of the construction.the disturbance between the pipe wall and the soil mass at the lower side of the pipe jacking is relatively severe,so the grouting volume at the lower side of the pipe jacking should be increased during the pipe jacking construction.(5)Based on the analysis of the monitoring data of a parallel rectangular pipe jacking project in Fuzhou,the following conclusions are drawn: due to the irregular vehicle load on the road surface during the construction process,the ground settlement caused by the construction can not be symmetrical along the central axis of the pipe jacking;the pipe diameter difference between the pipe jacking machine andthe subsequent pipe jacking will produce the formation loss,so the grouting amount should be increased in time to prevent the surface from excessive settlement;the slotting of the road will result in the formation loss The soil mass has a large disturbance,which causes a large settlement on the ground surface,which is not conducive to the construction of subsequent projects;the pipe jacking stress increases rapidly at the initial jacking,and with the continuous jacking,the lubrication of slurry makes the stress change rate gradually reduce,but the pipe jacking stress value is still slowly increasing;when the pipe jacking is going to run through the soil mass,the pipe jacking stress slightly decreases;at the same section of the pipe jacking At the position,the stress value is higher near the bottom of the pipe jacking.Through the comparison between the numerical simulation results and the monitoring data,it is verified that the numerical simulation analysis is reasonable and effective,and it will provide theoretical basis and reference for similar parallel pipe jacking projects in the future.