Study On Settlement Effect Of Large Diameter Shield Tunneling Under Dense Houses In Upper Soft And Lower Hard Strata

With the rapid development of Chinese subway construction scale,shield tunneling is widely used in subway tunnel engineering for its advantages of high excavation efficiency,strong geological adaptability,small disturbance to the surface environment and high safety.Subway lines generally pass through prosperous urban areas with concentrated population,and shield tunneling inevitably penetrates all kinds of existing buildings.In southern China,such as Guangzhou and Shenzhen,the complex geological conditions further increase the difficulty and risk of shield tunneling construction.Therefore,for how to guarantee the safety and stability of tunnel construction and building,it is of great significance to study the settlement influence caused by shield tunneling under the upper soft and lower hard composite strata.Based on Guangzhou Metro Line 18 PN1?PN2 shield tunnel range over long distances through the soft the hard compound formation,the engineering background of dense buildings,using theoretical analysis,numerical simulation and spot observation,mathematical statistics method,the compound formation under the earth pressure balance shield through intensive housing group of tunneling parameters and analyzing the ground subsidence caused by dual construction influence.The main research contents and conclusions are as follows:The geological characteristics of typical upper soft and lower hard composite strata in this area,construction difficulties of dense buildings under shield tunneling and main settlement influencing factors are discussed from multiple angles.Combined with measured data,the settlement influence of housing groups caused by shield tunneling,temporary shutdown,grouting reinforcement and successively under line tunneling are analyzed.Through monitoring and statistics of key parameters in the tunneling process of shield tunneling machine,the variation characteristics of tunneling parameters in various strata are analyzed,and it is found that the fluctuation changes of shield tunneling parameters are more obvious in soft strata or upper soft and lower hard strata.The fuzzy discriminant method based on specific thrust and specific torque can basically realize the classification of soft and hard surrounding rock.It can also reveal the characteristics of uneven soft and hard surrounding rock in upper and lower hard geological sections,high specific thrust and difficult control of shield attitude in tunneling process.Characteristic parameters such as maximum settlement,width coefficient and ground loss rate of surface lateral settlement trough are obtained by using Peak formula combined with specific monitoring section calculation.The modified Peck formula for three-dimensional soil settlement calculation is established by using hypergeometric method.The settlement trough calculated by this method is more consistent with the actual settlement.The asymmetry of the total settlement can also be solved and the three-dimensional soil settlement caused by double-line shield can be quantitatively predicted.Through the finite element analysis software Midas/Gts to establish the construction model of double-line shield tunnel under the building,analyze the impact of different geological composite ratio,synchronous grouting pressure,soft and hard properties of soil layer and other factors on the settlement of the surface building.The settlement of ground surface and buildings becomes more obvious with the increase of the composite ratio of tunnel section,the proportion of soft rock,the decrease of synchronous grouting pressure,the decrease of soil strength of surrounding rock and the shortening of the excavation distance of sequence line.