Construction Mechanics Behavior Analysis Of Pile-strut Supporting Structure Of Subway Deep Foundation Pit In Lanzhou Special Red Sandstone Formation

With the implementation of “One Belt and One Road” strategy in China,the urban development has entered the fast lane in the northwest of China.In recent years,the large-scale construction of rail transit is an important part of the great development of urbanization.In the process of construction,special strata such as special red sandstone are encountered,which seriously affects the process of subway construction.Therefore,how to ensure the safety of subway deep foundation pit under the premise of ensuring the speed of subway construction is of great practical significance as one of the severe issues faced by urbanization infrastructure construction.In this paper,the deep foundation pit drilling occlusive pile + internal support of Dongfanghong square subway station was relied on in Lanzhou special red sandstone stratum.The dynamic numerical inversion analysis model was established by combining the monitoring test of deep foundation pit construction site and based on the theory of displacement forward inversion analysis.Taking the in-situ monitoring data as the target value,the construction mechanical behavior of the support structure under each working condition was analyzed by inversion.The research results provide valuable reference and theoretical support for the government and enterprises.The main research contents and conclusions are as follows:(1)Using the principle of equivalent stiffness,this paper analyzed and discussed the advantages and disadvantages of the calculation method for the stress and deformation of the drilling occlusive pile + internal support structure in deep foundation pit engineering,which method belongs to the underground continuous wall supporting system.The inversion analysis and calculation method suitable for deep foundation pit pile support structure was proposed on this basis.It provided a reliable guarantee for the establishment of dynamic numerical inversion model and data analysis;(2)In view of the deep foundation pit of Dongfanghong square station in the special red sandstone stratum,the monitoring scheme and monitoring method were developed.The horizontal displacement of the pile top,the inclination of the pile body,the vertical settlement of the building,the settlement of the adjacent underground pipeline and the surface settlement were monitored in real time.The maximum monitoring values were all smaller than the corresponding control values,indicating that the drilling occlusive pile + internal support structure could effectively ensure the safety of subway deep foundation pit construction in Lanzhou special red sandstone stratum;(3)Combined with the complex deep foundation pit project of Dongfanghong square station,different excavation and support conditions were divided according to the construction process,and the most suitable constitutive relationship was selected to conduct numerical simulation analysis of the whole construction process;(4)Taking the in-situ monitoring data of pile support structure as the target value,unknown quantities of surrounding soil were calculated by inversion.Until the inverse objective function reached the minimum accuracy requirement.The results of inversion calculation and field monitoring showed that the two results were similar and the change trend were basically the same.It is shown that the results of the inverse analysis of the excavation and support process of foundation pit by using the dynamic numerical model and the life-or-death element method can provide effective guidance for the design and construction of deep foundation pit;(5)According to the reasonable finite element dynamic numerical model,the internal force and deformation of the supporting pile wall were calculated.The dynamic variation law of the internal force,deformation and support force of the wall with each step of excavation and support of the foundation pit were summarized.Furthermore,the mechanical behavior mechanism of foundation pit construction was improved,which provided theoretical basis for “dynamic design and information construction” of foundation pit engineering.