Study On Axial Mechanical Characteristics Of Reinforced Concrete Pipes During Longdistance Jacking In Soft Stratum

Pipe jacking,with the advantages of fast construction,lower noise and limited impact on the ground traffic,has been widely used in many fields including sewage pipelines,water supply pipelines,drainage pipelines,electricity cable tunnels and utility tunnels.The complexity of the application environment makes the pipe jacking gradually to develop a trend of long-distance and large-diameter,which increases the risk of structural damage of pipes during jacking.However,existing researches on the mechanical characteristics of pipe segment in jacking process are unilateral,and the corresponding conclusions are not universal,there is a lack of systematic technical guidance in related engineering construction.Therefore,the thesis aimed to study the axial force characteristics of long-distance reinforced concrete pipe jacking,and the theoretical analysis,numerical simulation,model test and field monitoring were comprehensively applied.The calculation of jacking force,transmission law and influencing factors of axial stress,and the stress distribution characteristics of pipe segments under different compression states were studied.The main research work and achievements are as follows:(1)Considering the different stages of vertical earth pressure change in trap-door tests,the corresponding forms of developed shear bands were proposed and the calculation formulas of vertical earth pressure were deduced considering the deflection of major principal stress.By comparing the loss volume of soil around the pipe to the expansion of soil between the slip plane,the vertical earth pressure at the top of the pipe under different conditions was given.The pipe-soil contact conditions were judged by comparing the grouting pressure with the normal pressure around the pipe,and the calculation methods of frictional resistances under different pipe-soil contact conditions were put forward.(2)Through the numerical simulation,the influence of different activation modes of the main jacks on the axial force transfer between the initial pipe segments was studied.When there were only a few amounts of activated jacks,it was found that obvious local stress concentration occurred in the 1# pipe segment closest to the jacks,while the subsequent pipe segments are under eccentrically compression,and the stress pattern of pipe segments 10 m away from the jacks was no longer changing.The stress in the middle was significantly different from that on both sides of pipe head face,and the axial load calculated by using the average stress in the middle of the pipe segment was closer to the actual.(3)The axial strains of a total of 54 pipe segments were monitored in the field,it was found that the pipe segments were all under eccentric compression during the jacking process.The real-time attitude adjustment in the vertical direction of the pipe jacking machine played a leading role in affecting of the axial strain on the pipe segments in the subsequent 20 m.The axial strains changed obviously when the pipe segment passed through the section where the shape of tunnel cave changed rapidly,the compressive strain accumulated on the inside of the trajectory curve.The variation trend of the axial force of pipe segment was consistent with that of the jacking force,and the pipe segments near the launching shaft were more sensitive to the fluctuation of jacking force.The frictional resistances during crossing different stratum were calculated through the axial stresses of pipe segments at different positions,then the jacking force was back analyzed in combination with the numerical simulation.It was found that the pipe-soil contact range was closer to 1/3 during jacking.(4)The compression test of the plywood was carried out,the stress-strain curve of the plywood could be divided into two distinct stages.It was feasible to calculate the axial force through the deformation of the plywood,the change of compression zone and the position of critical strain on the plywood should both be considered in the analysis.The model test under deflection load and numerical simulation of the pipe segment were carried out,and the results were relatively close to each other.The 3# pipe segment was in an eccentric compression state,and the inelastic strain appeared firstly in the inner wall on the eccentric compression side,which was prone to a local pressure damage.The 2#pipe segment was under the diagonal compression,and the inelastic strain occurred on the inner wall at 90° and 270°,also on the outer wall at 30° and 150°.There was a risk of simultaneous local compression failure and diagonal shear failure of the 2# pipe segment.(5)LVDT displacement sensors,film pressure sensors and surface-mounted strain gauges were used to monitor the gap development between adjacent pipe segments and the stress distributions in the pipe during jacking.The stress distribution was consistent with the calculated result according to the variation of gap.The axial load of the pipe segment calculated according to the deformation of the plywood was close to the actual.The stress monitored by the thin-film pressure sensor was relatively small,which was influenced by the effect of contact.