A case study on the behavior of shield tunneling in sandy cobble ground
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ORIGINAL ARTICLE
A case study on the behavior of shield tunneling in sandy cobble ground Z. X. Zhang • H. Zhang • J. Y. Yan
Received: 2 July 2011 / Accepted: 22 September 2012 / Published online: 2 October 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract This study investigates the behavior of shield tunneling based on a complete case record of an excavation in sandy cobble ground. Both numerical and analytical analyses were conducted to estimate the surface displacement along the tunnel axis and the surface-zone affected by the shield tunneling. This paper presents the performance of the Peck method in forecasting the transversal surface settlement trough caused by the shield tunneling in a sandy cobble layer. In particular, this study has made a comparison with the vertical surface displacement monitored during the advancing excavation to the settlements estimated by using the Peck method and by conducting a threedimensional finite element analysis. The effective measures to provide the necessary face support pressure and to reduce the formation of sinkholes and ground settlements in sandy cobble ground were also investigated and discussed. Keywords Shield-driven tunnel Settlements EPB Numerical simulation FEM
Z. X. Zhang (&) H. Zhang Department of Geotechnical Engineering, School of Civil Engineering, Tongji University, Shanghai 200092, China e-mail: [email protected] J. Y. Yan Shanghai Shentong Metro Co., Ltd., Shanghai 200092, China J. Y. Yan Computational Geotechnics Group, Institute for Soil Mechanics and Foundation Engineering, Graz University of Technology, Graz 8010, Austria
Introduction In the western parts of China, multiple shield-driven tunnels will be constructed in sandy cobble ground. In such conditions, one of the main goals of shield tunneling is to adequately support the soil and to minimize the deformations during and after construction. This is especially the case in urban environments, where a tunnel collapse or extensive deformations can be catastrophic, and even limited soil deformations may damage buildings (Broere 2001). To prevent this, construction projects must control the surface settlement and limit the risk of tunnel-face failure by continuously supporting the face during excavation. A lack of sufficient face support leads to tunnel-face instability (Clough et al. 1983; Hsiung 2009). In extreme cases, the collapse propagates up to the ground surface, generating a surface depression, especially in sandy cobble ground, as shown in Fig. 1. A collapse occurred in sandy cobble ground during the excavation of Metro line 1 in Chengdu in China, where abundant large cobbles and boulders and relatively few fine sands create a great challenge for managing the shield tunneling (Xu and Song 2007). Sandy cobble ground is a typically unstable layer that widely exists in the southwest regions of China. It has minimal cohesion and is characterized by its loose structure, unequalsized grain and large interspaces between particles. The sandy cobble ground is more likely to lose
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