Study On Construction Mechanics Response Of Tunnels Near Pile Foundation Of Urban Underground Loop

With the continuous acceleration of China’s modernization process,the rapid social and economic development,and the influx of a large number of people into the city,which has led to increasing urban traffic congestion.The development of the underground transportation system has become a new way to solve the problem of traffic congestion.In response to traffic congestion in urban business circles and economic high-tech zones,the establishment of an urban underground loop transportation system can connect infrastructure such as underground shopping malls,parking lots and rail transit in series,thereby effectively reducing the pressure on ground traffic.At the same time,considering the dense high-rise buildings on the surface,the development of underground space will inevitably encounter building pile foundations,which is undoubtedly a big challenge for the excavation and construction of urban underground loop tunnels.Based on this,the study of the mechanics response mechanism of the ground and building pile foundation during the construction of the urban underground loop tunnel can provide references and references for the development and utilization of urban underground space,help solve the problem of urban ground traffic congestion,and promote urban modernization..This article first analyzes the current research status of urban underground space development and tunnel construction.At the same time,combining the engineering geological characteristics of the Jiefangbei underground ring road in Chongqing and the special spatial position relationship,it summarizes the tunnel project underneath existing buildings.The main risks and difficulties in Secondly,based on the construction of the Jiefangbei underground ring road,the numerical calculation method combined with two-dimensional and three-dimensional is used to analyze the changes in the tunnel excavation method,support parameters and distance between the tunnel faces on the surrounding rock of the tunnel and existing buildings.The construction mechanics response of the pile foundation is comparatively studied,and the tunnel deformation and the settlement law of the building pile foundation and the mechanical response characteristics under the conditions of different excavation techniques,the thickness of the initial support of the tunnel,and the face of adjacent tunnels are analyzed.The main research results of this paper are as follows :(1)Through two-dimensional numerical simulation,the impact of three different construction methods,namely the bench method,the CD method and the double-side pilot pit method,on the surrounding rock and the pile foundations of existing adjacent buildings was discussed and studied.The results show that the settlement of the tunnel vault caused by the CD method is reduced by 22.91% compared with the step method,and the double-side heading method is reduced by 8.84% compared with the CD method.The settlement of the pile foundation caused by the CD method is reduced by15.38% compared with the bench method,and the double-side pilot method is reduced by 9.09% compared with the CD method.Finally,it is concluded that the double-side pilot method is a better excavation method.in conclusion.(2)Through two-dimensional numerical simulation,the influence of the change of the thickness of the initial support of the tunnel on the stability of the pile foundation of the tunnel and adjacent existing buildings is analyzed.The analysis of the five support situations of the initial sprayed concrete with thicknesses of 150 mm,200mm,250 mm,300mm,and 400 mm shows that the greater the thickness of the support parameters,the smaller the settlement caused by the surrounding rock and pile foundation.However,when the support thickness is 300 mm and 400 mm,the reduced settlement is smaller and the utilization efficiency is very low.Therefore,the thickness of 250 mm is a more reasonable and economical support thickness.(3)Through three-dimensional numerical simulation,the influence of the distance between the upper and lower tunnels on the existing adjacent buildings and upper roads is analyzed.Numerical simulation calculations were carried out for six working conditions including the distance between the upper and lower tunnel faces of 6m,12 m,18m,24 m,30m,and 36 m.The results show that when the upper and lower tunnels are simultaneously excavated,the greater the distance between the excavation faces,the smaller the impact on the existing structure,but after more than30 m,the reduced settlement is extremely small.Therefore,the two tunnels should be opened The distance between the excavation faces is 30 m.At this time,the impact on the pile foundation of the existing adjacent buildings and the upper road is small,and the construction period will not be lengthened.