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  Effect of soil conditioning on the ground response during EPB shield tunneling in sandy ground

  Earth pressure balance shield is widely used in recent years for safety and high efficiency in the process of urban subway construction. However, there are some problems in the engineering application. For example, the ground response of excavation face is one of the major problems, which affect the security and stability of adjacent buildings and other structures. The problem of excavation face ground response has been studied by some researchers, mainly focusing on problems of failure model and supporting stress. Based on limit analysis method, the excavation face stability coefficient was put forward in the clay stratum (Broms etal.,1967). The sliding zones in the front of excavation face were assumed to semicircle shape, a quarter of circle shape and three-dimensional spherical shape of failure face respectively (Vermeer etal.,2002) and shear stress acting on the sliding surface was analyzed. Considering the tunnel excavation face collapse and uplift, the two modified failure models were put forward (Abdul-Hamd Soubral,2000), and the failure of models was made up of two rigid of truncated cone and a shear zone, which consistent with Leca & Domriuex model obtained by centrifugal model test. These previous studies do not consider the influence of soil conditioning on ground response of excavation face. The ground pressure at excavation face is balanced by that in soil filled in the opening of soil chamber and that by cutterhead of shield machine in the process of the excavation. The sandy soil cut by rotating cutter head is low plastic and has low ability of transferring a stable pressure; and so the shield tunneling efficiency is seriously affected. The soil should be conditioned so that it has good plasticity and good ability of transferring a stable pressure. The problem of soil conditioning had been studied by scholars, paying more attention to experimental research of soil conditioning. A large number of slump tests of foamed soil were performed to revealed soil plasticity (R.Zumsteg et al.,2012; Yan et al.,2010). Small screw conveyor was developed to study ability of transferring a pressure of soil conditioned with foam (Peila et al.,2013). Compression tests were performed to analyze compression performance of conditioned soil (Psomas et al.,2001). The strength and internal friction angle of conditioned soil were studied at the different foam injection ratio by improved direct shear test apparatus (Miguel Ángel PeÑa Duarte,2007). Due to the fact that both soil in the opening of cutterhead and steel board of the cutterhead contribute to the support pressure at the excavation face, it is necessary to analyze the effect of soil conditioning on the ground response of excavation face. Based on the shield tunneling project in the Section 5 of Nanchang Metro Line 1, the effect of soil conditioning on the ground response

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