The Mechanism Reseach On Retaining Structure With Long And Short Pile Combination

In the retaining design of foundation pit, equal length pile is often used to solve the stability problem. The pile length is controlled by meeting the request of whole pit stability. Because bending moment under excavation surface reduces gradually with depth increasing, bending bearing ability on the lower part of the pile is not fully performed with equal length pile. This paper contacts with the real project condition, on the premise of meeting the pit safety, reduces part of the pile length, and adopts retaining-structure form with the combination of long pile and short pile. Based on the previous research, this paper works on the following aspect.According to the relationship of stress and strain, this paper discusses the earth pressure variation on the process of excavation. Meanwhile, this paper makes correction for the earth pressure on such a process, by introducing the above relationship into the calculation of increment method. Through comparison with the real measurement data in project, the calculation result of increment method considering the earth pressure correction fits real project condition better. This method provides a reference for earth pressure calculation in the retaining design with long-short pile.Based on the stiffness equivalence principle, this paper selects the adjacent long pile and short pile as the essential calculation element, to make simplification for the pile stiffness and achieve long-short pile simulation on two dimensions. This paper analysises the pile-length-variation effect of short pile on bottom heave and deformation characteristic of retaining structure, based on two conditions with different embedded depth and different pile spacing.This paper establishes three-dimention FEM model to simulate the excavation process, and analysises the pit deformation characteristic on conditions with different pile length, different pile diameter, different pile spacing and different short pile number combination.Through analysis of FEM calculation result, it is concluded that pile length and number of short pile can be determined by the following method on retaining structure design with long and short pile combination.(1) The matching of short-pile number is in line with bending moment distributed situation on the lower part of the retaining structure. The short pile number is determined by the ratio of the long pile bending moment at the place of short pile bottom to the peaked bending moment of long pile.(2) The matching of short-pile length is determined by the upheaval security coefficient according to the standard, which preliminarily calculates the pile length of short pile. Meanwhile, combined with the bending moment distributed situation on the lower part of long pile after short-pile length reduces, it can determine the final pile length configuration of short pile.Finally, this paper makes a statement for engineering applications of long-short pile and matters needing attention on the process of construction. Adopting long-short pile construction technology can save material, reduce construction time, and generate a fine social and economic benefit.