Surface Uplift And Control Under Asymmetric Additional Thrust Caused By Rectangular Pipe Jacking Construction

In recent years,with the rapid development of underground projects,rectangular pipe jacking tunnel is often widely used in the underground street channel,municipal utility tunnel,underground commercial space connection channel and other projects due to higher cross-section utilization,higher mechanization and lower environmental pollution,representing the development direction of short tunnel construction technology in the future city.At present,the surface deformation caused by rectangular pipe jacking tunnel construction has attracted more and more scholars' attention.The research on surface deformation caused by rectangular pipe jacking construction mainly focuses on settlement deformation.However,in actual engineering,improper selection of construction parameters may cause excessive uplift deformation on the surface and non-uniform deformation to crack surface in the excavation section.The pipelines in the underground are damaged,and even the corners of the pipeline joints are too big to break,which affects the safety of the surrounding environment.The second phase of the underground comprehensive utility tunnel project of the New Metropolitan Area of Baotou City(the largest long-section underground rectangular pipe jacking pipe project in the country)is taken as the background,on-site monitoring and theoretical analysis methods are used to study the law of surface deformation caused by the rectangular pipe jacking in the gravel sand stratum construction.The main research conclusions are as follows:Firstly,it is found that the longitudinal deformation of the surface of the project along the axis is in a raised state through the analysis of the field measured data,but as a whole,it is still an objective law that gradually settles after the first uplift,and then gradually becomes stable.The whole process is divided into the uplift stage,settlement stage and stabilization stage.As the pipe jacking machine advances forward,the surface cross section undergoes non-uniform deformation,and the deformation value of the left side point of the axis is significantly higher than the deformation value of the right side point;Secondly,the deformation of the surface uplift caused by the excessive additional thrust on the front is obtained through the study of the influence of the four factors of the additional thrust,friction,grouting pressure and soil loss on the surface deformation,and the additional thrust of the front and rear sides of the pipe jacking machine unbalance leads to non-uniform deformation of the surface monitoring cross section;Thirdly,the over-limitation of surface uplift mainly occurs in the uplift stage.The frontal additional thrust has a significant influence on the surface uplift deformation at the stage.Comparing the different working conditions,it can be seen that when the frontal additional thrust is less than 110 kPa,the surface uplift deformation value is within the control value;Fourthly,substituting the value range of the frontal additional thrust into the theoretical analysis formula,the range of the cutter surface fitting speed and the jacking machine jacking speed within 10 mm is controlled,as long as the speed is matched by the cutter head and the pipe jacking machine is jacked up.If the point at which the speed is determined falls within the range,the deformation of the surface bulge falls within the control value;Fifthly,the additional thrust on the front is composed of the contact pressure of the cutter-head,the earth pressure at the opening of the cutter-head and the static earth pressure on the face of the cutter.The relationship between the imbalance factors between them is analyzed to obtain that the cutter-head contact pressure imbalance coefficient has little influence on additional thrust imbalance coefficient and the earth pressure unbalance coefficient at the opening of the cutter-head has a great influence on the additional thrust imbalance coefficient on the front side in the actual project.It can be seen that the displacement of the surface deformation is caused by the imbalance of the earth pressure at the opening of the cutter head;Sixthly,the jacking speed of the pipe jacking machine has little influence on the unbalance coefficient,which mainly depends on the rotation speed of the screw machine.In the construction,the rotation speeds of the left and right sides of the screw machine should be kept as consistent as possible,so that the unbalance coefficient can be close to one,and surface bulge deformation does not shift.