Behavior Of The Composite System Of Deep Mixing And Concrete Pile Wall At Deep Excavations In Soft Soils

Deep excavations are one of the key problems for the design and construction of a cut-and-cover tunnel in urban areas. The stability of the excavations in soft soils, and the choice and optimization of the supportting system for the excavations always have a heavy influence on the time schedule of construction and the magnitude of environmental disturbance due to the construction of an underground structure. Based on the analysis on the mechanical behavior of a composite system of deep mixing and concrete pile walls, a few paramters, which have a strong potential of influencing on the supporting system behaviors, are discussed, to provide technical support for the optimization of the supportting system and construction control.The results of case histories show that Increasing system stiffness and embedment of the wall into a stiff layer are key factors in limiting the deformations of supporting system and retained soils at deep excavations in soft soils. Deep mixing may have the diplex fuctions of both ground improving and limiting the deformations.The results of the analysis, with finite element code of Plaxis, on the composite system at deep excavations in soft soils show that:(1) of a composite system of deep mixing and concrete pile walls, the function of the waterproof curtain of deep mixing piles behind walls, to some extent, is effective; (2) it is not an effective measure of increasing supporting capacity of the system to increase the diameter of concrete piles; and (3) the supporting capacity of the system can be significantly improved by improving the ground below excavating bottom.A case study shows that the construction time schedule may be optimized by reducing the numbers of strut of the composite system of deep mixing and concrete pile walls at deep excavations in soft soils, provided that the deformations of the retained soils are not strictly limited. It is noted that the factor of safety of the excavations are and therefore significantly reduced under this situation, and the difficulty of the risk control of the construction increases accordingly. As the only one strut is set at the top of the walls, the bending moment of the lower part of the walls is larger than that of the excavations with two struts. The increased moment due to reducing strut may leads to wall cracking at the level of 1.5 m above formation level.