The Deformation And Control Of The Subway Viaduct Affected By A Road Deep Foundation Pit Using Pile Foundation Underpinning Technology

In order to save the cost of urban construction,the integrated construction of utility tunnel and road foundation pit has gradually become a new trend.When the deep foundation pit of the road crosses the existing subway viaduct at a small intersection for a long distance,the pile underpinning method is usually adopted before the foundation pit construction to avoid or reduce the influence of construction on subway operation.However,there are few studies on the influence of pile underpinning on existing bridges,and studies considering the combined effect of pile underpinning and road deep and long foundation pit have not been reported.Therefore,it is particularly important to study the deformation law and control of subway bridge structure under the influence of pile foundation underpinning and successive construction of deep and long foundation pits on roads.Based on a road foundation pit with a depth of 20.2 m and a length of 495 m underneath a subway viaduct project,theoretical calculation,numerical simulation and field monitoring are combined to conduct research.The main results are as follows :(1)Through theoretical calculation and numerical simulation,the underpinning parameters are determined,and the key technologies of pile foundation underpinning construction are summarized.(1)Through the load calculation of the upper part of the underpinning pile,the design value of the jacking force of active underpinning is 59500 k N – 60600 k N.2)The empirical parameter method is used to calculate the vertical ultimate bearing capacity of single pile,and the range of underpinning pile length is determined to be 70 m – 77 m.The optimal pile length is determined to be 75 m through numerical simulation.3)Through the calculation of local compression bearing capacity,the selection and plane layout of jack are determined.(2)Through numerical simulation,the construction processes of pile underpinning and foundation pit excavation are simulated respectively,and the deformation laws of bridge and track structure are obtained : 1)The pile underpinning stage : the maximum vertical deformation of the substructure of the bridge is 1.904 mm,and the maximum lateral deformation is 1.620 mm,both of which occur at the piers at the active lifting.2)Foundation pit construction stage : the maximum settlement of the substructure of the bridge is-2.982 mm,and the maximum lateral deformation is 2.530 mm,all of which occur at the piers near the foundation pit of the road.3)The maximum deformation of the track structure is similar to that of the bridge substructure,and the location is the same.(3)Through the comparative analysis of the numerical simulation results and the field monitoring data,it is found that although there is a certain deviation between the two,the deformation trend is basically the same,indicating that the numerical simulation analysis method can better reflect the deformation of the bridge and track.(4)Through the numerical model and changing the values of the retaining structure parameters including the embedded depth insertion ratio,pile diameter and pile spacing,the sensitivity of the pier deformation near the road foundation pit to the retaining structure parameters is systematically analyzed.The sensitivity of the pier deformation to the foundation pit parameters is as follows: the embedded depth insertion ratio of the retaining pile > the diameter of the retaining pile > the pile spacing.