| Along with the continuous development of infrastructure construction in our country,the process of urban modernization increases continuously,the reconstruction or transformation of existing buildings are usually located in the city center,the surrounding environment is more complex.The excavation of deep foundation pit will cause the release of internal stress in the foundation pit and the formation of a hollow surface around the foundation pit,easy to disturb the surrounding environment,which brings strict requirement for the deformation control of foundation pit.Because of its advantages of high bearing capacity,good site adaptability,convenient construction and small disturbance,the micro steel pipe pile supporting system is widely used in soil and rock composite strata in Qingdao area.In view of the complex mechanical characteristics and mechanism of the micro steel pipe pile,the supporting system design theory of the micro steel pipe pile is not perfect,and its supporting performance and mechanical mechanism need to be studied.Based on the deep foundation pit project of Office Building No.25 Longjiang Road,Shinan District,Qingdao City,this paper carried out field tests on the bending moment of micro steel pipe piles and stress of prestressed anchor cable in the deep foundation pit of mixed fill soil layer,obtained the distribution characteristics and change rules of the bending moment of front pile and back pile and the internal force of prestressed anchor cable along the depth,and determined the stress characteristics of front pile and back pile under different excavation depths.The time-varying law of the reverse bending point position and bending moment of the pile body of the composite supporting system of micro steel pipe pile,prestressed anchor cable and internal support is revealed,the bearing mechanism of double-row micro steel pipe pile is clarified,and the applicability of double-row micro steel pipe pile in deep foundation pit engineering of mixed fill soil layer is verified by finite element simulation and pit site monitoring results.The research results have important reference value for the theoretical research and design calculation of deep foundation pit in soft stratum.The main work and research results of this paper are as follows:1.By combing the research progress on the application of micro steel pipe pile composite supporting structure in foundation pit engineering,the action mechanism,deformation characteristics and failure forms of micro steel pipe pile-anchor-brace supporting structure are analyzed,and the calculation model of micro steel pipe pile composite supporting structure and other stiffness conversion methods are summarized.2.Through in-situ testing of pile bending moment and anchor cable internal force on 3groups of double-row miniature steel pipe piles and 1 group of prestressed anchor cable in deep foundation pit engineering of mixed fill soil layer,the distribution characteristics and change rules of bending moment of front pile and back pile and stress transfer characteristics of prestressed anchor cable under different excavation depths are clarified.The bearing mechanism of micro steel pipe pile-anchor-brace combined support system in deep filling soil layer is revealed.The results show that with the increase of excavation depth,the pile bending moment near the excavation face increases significantly within the range of 1.0m,and the depth range of pile bending moment change caused by the completion of excavation is about 1.35H(H is the depth of excavation).The bending moment of the front pile is larger than that of the outer pile.The maximum positive and negative bending moments of the pile tend to increase,and the position of the maximum decreases gradually.The bending moment of the pile body is in Sshaped distribution.The reverse bending points of pile body are located at 0.5m below steel support and excavation face respectively.The stress is highly concentrated in the range of0.44l(l is the length of the anchorage section)at the front end of the prestressed anchor cable,and decreases along the depth of embedment.The measured stress values of prestressed anchor cable at different depths decrease slowly with the increase of excavation depth.3.Based on ABAQUS finite element simulation and field test results,the variation law of horizontal displacement of soil mass and pile body and distribution characteristics of pile bending moment after the completion of foundation pit excavation are clarified,and the design parameters of pile diameter are optimized.The results show that the finite element simulation results are basically consistent with the field test results.The maximum positive bending moment of the front pile is about 1.4 times that of the back pile,the maximum negative bending moment is about 1.3 times that of the back pile,the absolute bending moment of the pile body is not more than 6.25 k N·m,and the maximum horizontal displacement of the foundation pit is not more than 2.1 mm.The horizontal displacement of foundation pit can be reduced and the bending moment of pile body can be increased by increasing the diameter of micro steel pipe pile.When the excavation depth is small,increasing the pile diameter has limited effect on improving the supporting performance of the micro steel pipe pile.4.Combined with the monitoring data of the foundation pit,the deformation control effect of the micro steel pipe pile-anchor-brace system on the foundation pit is analyzed,and the supporting performance of the combined supporting system is verified.The results show that the monitoring data of foundation pit are stable and the values are within the safe range,which indicates that the support system has strong applicability in the mixed fill soil layer. |
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