Construction Monitor And Numerical Simulation Analysis Of A Deep Foundation Pit’s Pile-anchor Support Structure In Lanzhou

With the rapid development of economy and the pace of urbanization continuing to accelerate in China, the quantity of various tall and super high-rise buildings and municipal engineering increase continually, and the exploitation scale of underground engineering is becoming larger and larger. A lot of deep foundation pit with complex technology thereupon appears. The design and construction of foundation pit enclosure structure not only has become an important part of controlling engineering quality, schedule and cost, but also is one of the key issues deciding the success or failure of the entire construction project. Therefore, how to design safe, reliable, economical and reasonabal Supporting structure form become difficulty and emphasis of foundation pit theoretical research and engineering practice. Among many foundation pit supporting forms, pile anchor support form has been widely used in foundation pit engineering because of its construction technology being simple, cheap cost and strong stratum applicability. Like the other support forms, the calculation theory of pile anchor support structure lags far behind the engineering practice of foundation pit. Therefore, the study of the force and deformation of pile anchor support structure during excavation has great significance for theory and practice.Based on foundation pit engineering theoretical research and engineering practice, with a Lanzhou’s deep foundation pit engineering background, this thesis mainly researches on:(1) Reasonable selection of supporting structure form, calculating the deformation, internal force and stability of enclosure structure by Lizheng deep foundation pit software during foundation pit construction, and also analyzing and comparing calculation results with the measured values.(2) Using MIDAS/GTS software to simulate the whole process of foundation pit distribution excavation, and analyzing the indices including internal forces, displacement, and the surface settlement around foundation pit.(3) By the monitoring field data, focusing on the placement and internal force of support pile, top beam horizontal displacement, and the change law of surface settlement, also, the monitoring field data directing the construction on site to make the whole foundation pit is controlled throughout the excavation process, meanwhile, part of The measured values being selected to analyze and compare with the computing results.(4) Using common engineering design and analysis software to analyze the effect of embed depth, pile spacing, pile diameter, additional load on the ground and anchor pre-stress on deformation of foundation pit, in order to provide theoretical guidance for the optimization of Lanzhou area deep foundation pit supporting structure system. Based on research and analysis of the above problems, the research results and conclusions are as follows:(1) A foundation pit design is described in detail in Lanzhou City, meanwhile, a typical support section is selected for calculation.The displacement and internal force distribution graph of supporting structure is presented in all kinds of construction conditions. The choice of calculation results and design parameters selection provides theoretical basis for subsequent other foundation pit design and construction.(2) With the increase in excavation depth, under the joint effects of pre-stress anchors and top beams, the upper displacement curve of pile shaft appears positive bend, while the lower reversed bend. After each row of pre-stress anchors being tensioned and locked, pile shaft would generate reverse direction displacement(pit inside). Then, with earthwork excavation inside of foundation pit, the displacement of pile shaft again develops towards the direction of pit inside.Therefore, the support should be conducted in time according to the design requirement to reduce the deformation of pile shaft.(3) In the cantilever construction Phase of supporting pile, the pile shaft bending moment curve appears variation trend of being big in the middle, and small at the tips.In the pile anchor support process, the change curve of support pile bending moment with depth presents "S" shape, and the bending moment values gradually increase with the increase of excavation depth. The action position of the maximum bending moment also appears moving down with the increase of excavation depth.(4) The horizontal displacement of the middle part of foundation pit is largest, and the horizontal displacement of the corner part is smallest. The impact of pit space effects are verified, namely pit angle imposes more constraint on the terminal part than the middle part of foundation pit. The impact of spatial effect of foundation pit should be considered during foundation pit optimization design process. The surface settlement around foundation pit shows that the distance is closer to support pile, the settlement values are smaller, and vice versa.Surface settlement is a triangle pattern.(5) By analyzing and comparing finite element calculation results with the measured results, the variation trend of them is basically accordant.It shows that numerical simulation computation results can well embody the deformation law of deep foundation pit enclosure structure. Therefore, numerical calculation is feasible to foundation pit research.By comparing the results of LiZheng computed and the monitoring field data, the former are all bigger than the later. It shows that the design of the foundation pit engineering tends to be conservative, so further design optimization exists.