Study On Anti-uplift Portal Frame In Control Of The Underlying Tunnel Deformation Induced By Foundation Pit Excavation

Excavation of the foundation pit over the existing shield tunnel leads to the release of ground stress and the rebound of the soil at the bottom of the pit,which leads to the uplift deformation of the tunnel structure.Anti-uplift portal frame is a kind of anti-uplift structure composed of anti-uplift piles and anti-uplift slab,which has been gradually applied in engineering in recent years.However,according to the existing engineering cases and research status,there are still some problems in the project,such as unclear interaction mechanisms between anti-uplift portal frame and stratum,unknown anti-uplift effect,structural design based on experiences and so on.Based on the rapid reconstruction project of Guimiao Road in Shenzhen,this paper studies the control mechanisms and effect of the anti-uplift portal frame on the uplift deformation of the tunnel.The tunnel deformation response laws during the construction stage of anti-uplift portal frame and the construction stage of the main project are investigated through field measurements.Based on the numerical simulation of single shaft excavation,the interaction mechanisms of tunnel-soil-pile during the construction of anti-uplift portal frame are revealed.Based on the numerical simulation of foundation pit excavation with anti-uplift portal frame,the anti-uplift effect of anti-uplift portal frame with different pile lengths is revealed,and finally the design suggestions of anti-uplift portal frame are put forward,which can provide references for the design and construction of anti-uplift portal frame in similar projects.The major concluding remarks of this study are as follows:(1)Based on the field measurements,the deformation responses of the tunnel in the anti-uplift portal frame construction stage(shaft excavation)and the main project construction stage(foundation pit excavation)are obtained: i)the vertical displacement at the tunnel crown and vertical convergence deformation of the tunnel section are obviously induced by shaft excavation,which makes the tunnel develop into a shape of vertical ellipse;ii)there is an obvious superimposed effect on the tunnel uplift when the shaft is not excavated every three,and the shaft backfill can not make the tunnel uplift fully recover(the average recovery rate of different sections is62.5%-85%);iii)in the shaft excavation stage,there is an exponential positive correlation between the normalized tunnel uplift and the unloading rate.In the stage of foundation pit excavation,there is a linear positive correlation between the tunnel uplift and the unloading rate;ⅳ)the uplift value of some tunnel sections tends to be stable before the foundation pit is excavated to the bottom,which indicates that the anti-uplift portal frame plays a role in resisting uplift.(2)Based on the numerical simulation of shaft excavation,the interaction mechanisms of tunnel-soil-pile in the construction stage of anti-uplift portal frame are revealed: i)it is clear that the tunnel deformation induced by shaft excavation is 1.4times of shaft excavation depth(about 3 times of shaft width),and then it is verified that three shafts is the most appropriate interval of shaft excavation;ii)in the process of shaft excavation,the uplift displacement of the tunnel crown is dominant,and the convergence deformation develops from the mode of horizontal ellipse to the vertical ellipse.The maximum uplift amount has a piecewise linear negative correlation with the pile length;iii)the tunnel uplift can not be fully recovered after the shaft backfill,and the recovery rate is 89%.The reason is that the horizontal stress recovery degree of soil around the tunnel is greater than the vertical stress in the process of the backfill load transferring to the soil around the anti-uplift piles through the shaft resistance and tip resistance of the anti-uplift piles;ⅳ)the mechanisms of pile-soil interaction and the relationship between pile internal force and pile length are obtained: under the same pile length,the tunnel uplift is greatly affected by the excavation area and unloading rate of the shaft,and the internal force of the anti-uplift pile increases with the increase of the excavation area and unloading rate of the shaft.There is a linear positive correlation between the axial force and the pile length,and a piecewise linear positive correlation between the bending moment and the pile length.(3)Based on the numerical simulation of foundation pit excavation,the anti-uplift mechanisms of the anti-uplift portal frame on the underlying tunnel are revealed: i)by comparing the working conditions with and without the anti-uplift portal frame,it is found that the final tunnel uplift under the two working conditions differs by 4.2mm,thus the effectiveness of the anti-uplift portal frame is determined;ii)the interaction mechanisms between the anti-uplift portal frame and soil are clarified,that is,the anti-uplift portal frame deforms to the unloading side driven by the rebound of the soil at the bottom of the pit,then the anti-uplift pile gradually exerts the pile shaft resistance from the top to the bottom of the pile and gradually limits the uplift of the anti-uplift slab.At the same time,the anti-uplift slab resists the overall deformation of the frame with its large bending stiffness,Thus,the rebound of the soil at the bottom of the pit is limited;iii)through the parameter sensitivity analysis,it is found that there is a significant negative linear correlation between the tunnel uplift and the pile length,so the increase of pile length has a significant control effect on the tunnel uplift.The increase of slab thickness has limited control effect on the uplift of the tunnel,but it can improve the integrity of the anti-uplift portal frame and reduce the deformation and internal force of the pile;ⅳ)the internal force of anti-uplift pile increases linearly with the increase of pile length and decreases with the increase of slab thickness.(4)According to the interaction mechanisms of anti-uplift pile-soil in the shaft excavation stage,and the anti-uplift mechanisms of anti-uplift portal frame in the foundation pit excavation stage,as well as combining with the results of parameter sensitivity analysis,some suggestions on the design and field construction of anti-uplift portal frame are put forward.