Study On Mechanical Behavior And Construction Monitoring Technology Of Foundation Pit Excavation In Sand Layer

With the increase of the number and size of deep foundation pits in cities,their construction environment is becoming more and more complex.On the one hand,the effect of soil pressure on the retaining structure of foundation pit may be changed by the adjacent buildings of foundation pit,on the other hand,it will cause many inconveniences to the retaining design of foundation pit.It is of great significance to study the excavation and support process of this kind of finite soil foundation pit adjacent to buildings.In this way,not only can we find the law of stress and deformation during the construction of foundation pit with limited soil,but also can provide relevant basis for the design of other foundation pit support.Based on the foundation pit of Xisanzhuang Station of Shijiazhuang Metro Line 1,theoretical formula derivation,numerical simulation and construction site close-range photographic deformation monitoring are studied in this paper.The main work is as follows:?1?The analytical formulas of active earth pressure E_a and active earth pressure coefficient K_a of finite soil in sandy soil are derived.The effects of width-height ratio n,friction angle in soil?and friction strength parameter delta of wall and soil?on E_a and K_a are studied.It provides a theoretical basis for the design of foundation pit support in finite soil.?2?With the help of the finite difference software Flac ^3D6.0,the excavation and support of the foundation pit of the nearby buildings are simulated.The calculated deformation and stress of foundation pit are in good agreement with the measured values,and the variation law is basically the same,which proves the accuracy of the model.The analytical formula of finite earth pressure is validated.The results show that it can accurately reflect the earth pressure of foundation pit with finite soil.?3?Furthermore,the calculation model is established,and the effects of width-height ratio n,friction angle in soil?and buried depth of adjacent building foundation on stress and strain of foundation pit excavation are explored.Results found:The n is proportional to the ground settlement and the horizontal displacement of piles on the side of adjacent buildings.The higher the n,the higher the active finite earth pressure.The?is proportional to the ground settlement on the side of the adjacent building.The?is inversely proportional to the horizontal displacement of the pile.The higher the?,the smaller the active finite earth pressure.In a certain range,the ground subsidence value is proportional to the depth of building foundation,and the smaller the depth,the more obvious the change is.The buried depth of building foundation has little effect on earth pressure.?4?A close-range photogrammetric simulation experiment was carried out.It was found that the 50 mm focal length lens was used for long-range photogrammetric shooting.The larger the size of the control point sign,the higher the accuracy.Selection of 28 mm focal length lens for surface deformation photography.The larger the size of the control point sign,the lower the accuracy.The number of control points should be at least four.The maximum error of horizontal displacement and vertical displacement measured by close-range photogrammetry is 1.86 mm and 0.92mm respectively.The maximum error between surface settlement and measured value is 1.97 mm.The accuracy of close-range photogrammetry can be guaranteed,and the measurement method is simple and efficient.It can provide technical means for green construction of foundation pit engineering.