Optimization Of Pipe Jacking Layout Of Pipe-roof Pre-construction Method Based On Surface Settlement Control

The pipe-roof pre-construction method is a new undercutting construction method in the construction of underground pipeline through the existing buildings,without affecting the use of the original construction(structure)on the basis of underground pipeline construction,and can cross roads,railways,rivers and lakes,as well as various important buildings.The permanent supporting structure is constructed by the pipe-jacking curtain in advance,which can guarantee the stability of soil during earthwork excavation.At present,there is no detailed theory to guide the construction of pipe curtain prefabrication method,and there are few domestic cases.Taiyuan railway station underpass project adopts pipe curtain prefabrication method for construction.At the same time,underpass railway station in use is the first application in China.Its design concept has no relevant specifications and other technical documents,so it can only refer to foreign related projects,and its construction process is more.there are few construction projects of dense row large diameter pipe jacking group in China,and the related theoretical research is insufficient,and the trend of jacking force change and ground settlement change during the construction of dense row pipe jacking group of pipe curtain is different from that of single pipe jacking.In this paper,the jacking force formula in the process of pipe jacking is obtained through theoretical analysis.According to the field measured data and model test,the jacking force change and surface settlement in the process of pipe jacking are studied.Besides,different pipe curtain arrangement schemes are compared through numerical simulation,and a more reasonable pipe curtain arrangement scheme is selected.The main research results are as follows:(1)The overburden earth pressure of pipe jacking can be obtained through theoretical analysis and the jacking force formula can be improved to better predict the jacking force of pipe jacking under deep and shallow buried conditions in practical engineering.According to the field data,the influence factors of pipe jacking force are studied,and it is found that the geological conditions where the pipe jacking is located have a great influence on the pipe jacking force.According to the field data,the deep and shallow buried state of pipe jacking in the pipe curtain pre-construction method is divided,and it is considered that H/D=3.4 is the critical point between deep and shallow buried pipe jacking,when H/D<3.4,it is shallow buried pipe jacking,and when H/D>3.4,it is deep buried pipe jacking.It is also found that in the process of jacking,the jacking force increases with the increase of the contact area between pipe and soil,and the proportion of pipe and soil friction in the jacking force increases gradually.In this project,the soil friction of the pipe accounts for 67% of the total jacking force after jacking.(2)By studying the influencing factors of soil deformation of densely arranged largediameter pipe jacking,the calculation formula of soil surface settlement in vertical direction and horizontal direction is revised to make the results more in line with the actual situation.In pipe jacking layout,the Angle of two adjacent pipe jacking should not be too large,which will lead to uneven pipe jacking force and increase ground settlement.The ground subsidence is minimum at 0 degrees,which should not be greater than 45 degrees in actual layout.The best jacking sequence of three pipe jacking is the construction of two sides of pipe jacking first,and the construction of middle pipe jacking last,so as to cause the minimum surface settlement.(3)There are mainly round section and rectangular section of pipe curtain.Through simulation analysis,it is found that for shallow buried tunnel,the ground settlement caused by rectangular section is less than that caused by circular section,but the overall pipe curtain stress is greater than that caused by circular section,and the pipe jacking force in the construction process is also greater than that caused by circular section.However,the surface settlement will increase slightly when the quasi-circular section is applied to the shallow buried tunnel due to the larger radian of the upper and lower row of pipe jacking,but the force of the whole pipe curtain is more reasonable,and the jacking force of the pipe jacking is 20% less than that of the quasi-rectangular section in the construction process.(4)Through simulation analysis,reducing the number of lower rows of jacking pipes and increasing the spacing of lower rows of jacking pipes,it is found that 19 pipes jacking pipes is only 4mm more than the actual surface settlement of the project,but for the whole project,the overall construction cost and construction period can be reduced,and 19 pipes jacking is more reasonable.