Deformation Characteristics Of Large Span Silt Foundation Fit Group And Deformation Control Of Special Joints

With the release of the "14th Five-Year Plan" and the 2035 transportation country vision goal,the construction of urban underground traffic lines is accelerated,and the size of the deep foundation pit of the urban highway tunnel is getting bigger and bigger.Especially in the coastal and riverside areas,the soil layer is mostly silty soil with high water content and low strength,so it is particularly important to explore the deformation law and control measures of large-span mud foundation pit.In this paper,based on the foundation pit project of the comprehensive tube gallery tunnel in Heng jiang Avenue,Nanjing,the deformation characteristics of large-span silty foundation pit group and the deformation control measures of special joints of silty foundation pit are studied by combining the methods of literature research,site monitoring and numerical simulation.The main contents and conclusions are as follows:(1)Based on the analysis of foundation pit monitoring data,the deformation characteristics of retaining pile of silty large-span foundation pit are summarized,and the relationship between the maximum displacement of foundation pit and the depth of excavation is given.The settlement curve,value range of settlement and influence range of settlement of soil around foundation pit are summarized.The change rule of axial force of internal braces and the influence of erection and removal of steel braces on axial force of concrete braces are summarized.(2)Based on the MIDAS GTS/NX model,the influencing factors of the deformation of the silty foundation pit were studied.The following conclusions were drawn: the increase of the length-to-width ratio would lead to the increase of the deformation of the foundation pit,but when the ratio was greater than 2,the sensitivity of the horizontal displacement of the long side of the retaining structure to the change of the size of the long side of the foundation pit decreased;When the retaining pile insertion ratio increases,the horizontal displacement of the foundation pit retaining structure decreases,but the displacement of the top of the retaining pile increases.Therefore,the optimal insertion ratio range is 1.36～1.44.The thickness of silty soil on the surface is positively correlated with the horizontal displacement of retaining pile.When the thickness of silty soil is less than 5m,it has a greater impact on the pile top,and when the thickness is greater than 7m,it has a greater impact on the basement.(3)Based on numerical simulation,the influence of the construction sequence of the foundation pit group on the deformation of the foundation pit is studied.The main conclusion is:adjacent foundation pits should not be excavated at the same time.The fewer the number of foundation pits excavated at the same time,the smaller the horizontal displacement of the foundation pit;In the foundation pit group,the deformation of the foundation pit excavated first will increase due to the construction of the foundation pit excavated later,and the foundation pit excavation later will be less affected by the excavation of the foundation pit first;the timing of removing the plugging piles between the foundation pits will increase.Later,the smaller the final deformation of the foundation pit,the plugging piles should not be removed before the second layer of support is erected.(4)A model of the special-shaped foundation pit was established,and the influence of the construction of special nodes of the foundation pit on the deformation of the foundation pit was analyzed.The main conclusion is that the maximum horizontal displacement point of the foundation pit will shift from the center of the foundation pit to the position of the sun angle.The positive angle reinforcement of the mixing pile is effective,and the larger the solid area,the smaller the displacement of the positive angle;when the deep and shallow pits of the foundation pit are reinforced with bored piles,the larger the pile length,the greater the impact of the open surface.The smaller the displacement is;when the foundation pit is replaced,the displacement of the retaining pile will increase,and the displacement of the retaining pile can be controlled by reducing the spacing and erection depth of the supports.