Study On Structural Parameters Optimization Of Large Section, Long Distance And Large Angle Oblique Pushing Box Culvert

With the continuous development of urban construction,railway and highway cross-roads are increasingly frequent.Box culvert jacking method,especially large section and long distance box culvert jacking method,has become a vital construction method for crossing existing traffic lines due to its prominent advantages of small stratum interference and wide application scope.With the section and span increase of the structure,the stress situation and influencing factors of the structure become more complex,which brings significant challenges to the construction and operation of the project.In view of the relatively little research on jacking engineering of the large section box culvert in China,especially the phenomenon rarely involved in jacking construction process,this paper mainly carries out the following research based on the jacking engineering of the oblique box culvert crossing Beijing-Guangzhou Railway under Zijingshan Road in Zhengzhou City.(1)Combined with engineering practice,the maximum jacking force is calculated by empirical formula method and theoretical formula method respectively,and conservative estimation is made.Taking the relatively large values of the two methods as the final jacking force design values,the maximum jacking force required for the section length of 18 m,26 m,36 m and 55 m box culverts is finally obtained as follows: 133931 kN,144279 kN,163019 kN and 229662 kN.(2)By means of numerical simulation,four different segment lengths(18m,26 m,36m and 55m)as well as five different jacked length(0,1/4,2/4,3/4 and 1 times of the total segment length)are selected.The numerical simulation aim is to establish a comparative model of box culvert jacking construction process and to study the displacement and stress variation law of structure with different segment lengths,different jacking stages in jacking construction process.The results show that the longer the length of the box culvert is,the larger the horizontal displacement gap between the left and right side walls will be.Moreover,the deviation of the jacking direction will be easily changed.At the same time,the longer the length of the box culvert is,the larger the vertical displacement gap between the front and back ends will be.This situation is so-called the "tapping" phenomena,which will cause hidden dangers to the construction.(3)Eight comparative numerical analysis models with different train load and structural thickness conditions were established to study the stress and displacement of the structure in the formal operation stage.The results show that the maximum settlement of the railway subgrade decreases with the increase of the structure thickness.That is to say,the thicker the box culvert structure is,the smaller the corresponding subgrade settlement is.However,with the further increase of box culvert thickness,the maximum settlement of subgrade decreases gradually.(4)This paper recommended that the length of the structure segment should be 26 m considering the factors such as jacking direction deviation,construction safety,project cost and project cycle.With the consideration of the subgrade settlement,deformation,structure safety,construction difficulty,and engineering cost,it is recommended to select the thickness of the side wall and roof of the structure should be 1.2m,and the thickness of the bottom plate should be 1.3m.