Study On Mechanical Response And Calculation Model Of Underpinning Pile Foundation Structure Under The Condition Of Subway Crossing

The pile foundation underpinning project of Jiaozi interchange of Chengdu rail transit line 13 has the characteristics of a large number of new underpinning piles and the superstructure is sensitive to deformation.In the later stage,the double track tunnels of lines13 and 30 will pass through the underpinning structure.In the underpinning design,it is very important to select the appropriate pile foundation underpinning scheme and determine the pile length of the new underpinning pile.Therefore,it is of great engineering significance to study the pile length design method with both safety and economy,and select the appropriate underpinning scheme according to the actual engineering.Taking the underpinning structure in the No.7 foundation pit of the project as the main research object,this paper studies the mechanical response of a single pile during subway shield excavation under different pile tunnel distance,pile top load,tunnel lining and pile length by means of numerical simulation and field investigation,puts forward a new method for pile length design of underpinning pile,and compares and analyzes the mechanical response and load transfer mechanism of active and passive underpinning systems,The reliability of underpinning system after double track excavation of line 13 and line 30 and the applicability of the new pile length design method are verified.The main work and research results are as follows:(1)A new design method of pile length of underpinning new pile considering the excavation effect of subway tunnel is proposed.The analysis and calculation show that the mechanical characteristics of the pile when using this method are not different from those using the traditional method,which can meet the requirements of safety and reliability,and can effectively reduce the length of new pile,save quantities,save cost and shorten the construction period.(2)By studying the mechanical characteristics of piles during Subway Excavation under different conditions,it is found that the increase of pile tunnel distance can effectively reduce the disturbance of subway excavation to piles;The pile top load has a great influence on the vertical stress characteristics of the pile,but has little influence on the horizontal stress characteristics of the pile;When the tunnel lining has not been constructed,the vertical displacement,axial force,horizontal displacement and bending moment of the pile body increase significantly after subway excavation.(3)For the underpinning structure of No.7 foundation pit,the calculation is carried out according to the active and passive underpinning respectively.The results show that the active underpinning can effectively eliminate the early settlement of the underpinning structure,so as to reduce the impact on the settlement of the superstructure and ensure the smoothness of the bridge deck.After the existing pile is broken,the axial force of the new underpinning pile changes little,and the load transmitted from the pier is directly transmitted to the new underpinning pile through the underpinning bearing platform,and the existing pile will no longer participate in the load sharing;The passive underpinning cannot eliminate the early settlement of the underpinning structure.After the later settlement is superimposed with it,the structure will produce large vertical displacement,which is difficult to meet the strict deformation requirements of the superstructure.After the pile is broken,the axial force of the new underpinning pile increases,and the rest of the existing pile still bears part of the load.(4)Based on the study of the mechanical response of the underpinning system during the construction of line 13 and line 30,it is found that there is stress concentration at the connection between the bearing platform,pier and pile,but the mechanical response of the underpinning system is within the limit,indicating that the new stress system bears the upper load better.During the excavation of double track tunnel of line 13,J24-XZ1 is inter tunnel pile and J24-XZ2 is tunnel side pile.Due to the vertical superposition effect and horizontal offset effect of inter tunnel pile,the vertical displacement and additional axial force of pile top of J24-XZ1 are about twice that of J24-XZ2,and the maximum horizontal displacement and maximum bending moment of pile body of J24-XZ2 are about 4 times that of J24-XZ1.(5)By studying the mechanical response of J23-XZ2,which is designed as a short pile according to the new method of underpinning pile length design in the actual project,and comparing it with the case designed as a long pile under the same conditions,the results show that the long pile is slightly better than the short pile in displacement control,but it has little advantage.The short pile can meet the requirements in the project.(6)By comparing the numerical simulation results with the field measured values,it is found that the change trend of the two is consistent,the numerical difference is very small,and the coincidence is high,which proves the rationality of the numerical simulation calculation in this paper.