Research On The Mechanical Characterristics Of Piles In Row Of Foundation Pit Of Subway Station In Chengdu

As a kind of retaining and protecting structure, piles in row are widely used in building foundation pit and metro excavation pit in Chengdu. However, it was found that the internal forces of retaining structures obtained from in situ measurement are considerably lower than the calculating results of mechanical model adopted in design. In this paper, to study the mechanical characteristics of retaining system of foundation pit, following work has been done:(1) The construction monitoring of deep foundation of Zhandongguangchang station in line2of Chengdu metro is made, the datum concerning horizontal displacement of the top beam, the horizontal displacement and internal forces of the piles, as well as the axial force of the steel pipe braces were measured and the characteristics of deformations and forces of the retaining structure were analyzed.(2) With Abaqus, a famous FEM software, based on the mechanical model in which the interaction between pile and soil were taken into account, the deformation and internal forces of the pile, the earth pressure, the axial force of the support and the deformation of the soil between piles were calculated and compared with the monitoring results.(3) Furthermore, the influence of space between two piles, the stiffness and strength of soils, the influences of temperature change of steel pipe brace on the internal forces and deformation of the retaining structures were analyzed.The key conclusions are drew as following:(1) All values of deformation and internal forces obtained from in situ measurement are all less than the allowable values, which indicate the foundation pit is in safety. The stress of reinforced bars in piles is far less than the allowable value, which means the design is conservative.(2) Generally, the results calculated from Abaqus agree with the results of in situ measurement, which shows that the mechanical model adopted in the paper is reasonable.(3) When the space between two piles comes to4.2m, the maxim horizontal displacement reaches to43cm, which means failure of the soils between the piles. Therefore, the ultimate space between is suggested as4m around.(4) If the foundation pit is in the clay stratum, the sensitivity of the deformation and internal forces of retaining structure to the parameters of the soil will be ψ>C>E; As for sandy cobble, it is ψ>E. Generally, internal forces and deformation of the pile will increase with the decrease of the stiffness and strength parameters of the soils, which means undervalue of the parameters will result in a conservative design and is one of important reason for low stress of reinforced bar in the piles.