Safety Technology For Construction Of Cut-and-cover Tunnel Pits On Artificial Islands In The Sea Adjacent To Existing Bridge Pile Foundations

In view of the construction problems of the existing bridge foundation adjacent to the artificial island in the sea,based on the East artificial island project of the ShenzhenChina Passage,the mechanism of the impact of the construction of the cut-and-cover tunnel foundation of the artificial island in the sea on the existing adjacent pile foundation is explained,the effect of the foundation excavation and the influence law on the existing bridge pile foundation are analyzed,the displacement and deformation law of the enclosure structure,the protection measures,the existing pile foundation and the surrounding strata during the construction process are derived,and the impact analysis of the foundation The impact analysis based on the orthogonal test method was carried out for three main engineering measures: reinforcement strength,equivalent thickness of the enclosure structure and equivalent thickness of the bridge pier protection measures,and the construction methods and targeted security measures were elaborated:(1)The current status of domestic and international research on the influence of pit excavation on adjacent pile foundations is described,and the deformation mechanism of surrounding soil and pile foundations caused by pit construction is analyzed.(2)Sea area artificial island deep soft soil environment foundation excavation deformation law:(1)pit bottom soil body shows "middle big,both sides small" larger uplift deformation,disturbance range from pit bottom to full weathering granite layer bedrock;(2)surrounding surface settlement curve shows "soup ladle" shape(ii)the surrounding surface settlement curve shows a "soup ladle" shape,the settlement trough increases gradually and significantly with the excavation depth,and the location of the maximum settlement value gradually moved to the side of the pit;steel pipe pile pier protection measures within and around part of the region of the soil settlement has significantly reduced,and the maximum settlement location to the outside;(iii)the sea deep soft soil environment pit excavation of the horizontal impact range increased significantly,in more than 3 times the excavation depth of the pit soil have a certain degree of impact The horizontal displacement of the enclosing structure in the soft soil layer is abruptly changed,and the horizontal displacement is sharply reduced along the depth direction due to the strong embedded solidification of the middle weathering rock layer at the bottom;(5)both sides of the protection measures of the steel pipe pile bridge pier show the offset to the inner side of the foundation pit and the horizontal displacement value is basically the same,with the long side showing a sinking trend and the short side showing an upward trend.(3)The deformation law of the pile foundation of the existing bridge adjacent to the foundation excavation:(1)The horizontal displacement of the pile foundation is mainly in the direction of the horizontal foundation,the deformation of the pile foundation near the side of the foundation pit is obviously larger than the pile foundation far from the side of the foundation pit,there are 2 and 3 backbending points on the displacement curve respectively,the increment of horizontal displacement is positively correlated with the excavation depth,the location of the maximum horizontal displacement occurs gradually upward with the excavation;in the direction of the foundation pit,the horizontal displacement curve of the pile foundation is "S" type,the upper part of the pile foundation deformation direction is opposite to the lower part.(2)vertical displacement of pile foundation,with the construction of the overall performance of the settlement value increases,but in the excavation process there is "uplift" phenomenon of the pile foundation,the fourth excavation generated by the vertical settlement increment is much larger than the first three excavation stages.(4)The equivalent thickness of enclosure structure,foundation reinforcement strength and pier protection measures are negatively correlated with the maximum horizontal displacement of pile foundation,and the analysis based on orthogonal test method shows that foundation reinforcement strength > equivalent thickness of enclosure structure > equivalent thickness of pier protection measures.(5)In view of the characteristics of the artificial island stratum in the sea and the pile foundation of the existing bridge underneath,the analysis proposed the pile foundation protection of the existing bridge,the pit enclosure structure,the foundation reinforcement design and the main construction method,carried out the construction deformation monitoring of the pit excavation,verified that the plan is reasonable and feasible,and met the strict deformation control requirements of the pit and the existing bridge.