A study into the behaviour of the formation level of an excavation under different unloading patterns in soft deposits
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ORIGINAL PAPER
A study into the behaviour of the formation level of an excavation under different unloading patterns in soft deposits Yuqi Li 1 & Xiaohui Chen 1 & Sam Divall 2 & Xiaodi Zhang 1
Received: 18 January 2017 / Accepted: 28 September 2017 / Published online: 9 October 2017 # Saudi Society for Geosciences 2017
Abstract The construction of basements in urban areas is often associated with the possible damage to existing structures and services. The varying construction processes inevitably lead to different stress unloading patterns and therefore the dissipation of these excess pore-water pressures may lead to non-standard deformation profiles. The three main types of basement construction processes are layered excavation (LE), basin excavation (BE) and island excavation (IE). The effect of the various unloading patterns has been investigated by a three dimensional (3D) effective stress analysis method using the developed computer program 3DBCPE4.0. An excavation of length 50 m, width 50 m and depth 9 m in a certain homogenous and isotropic saturated soft soil was modelled. This included a diaphragm wall of 800-mm thickness embedded 18 m deep into the soft soil. The different excavation deformation profiles under different excavation patterns were related to the different unloading process, the exposure time of excavation face and the dissipation of negative excess porewater pressures. The most favourable process for controlling the horizontal deformation of a retaining wall or the heave deformation of the formation level is suggested. The ground water potentials within the formation level are also presented. Keywords Excavation pattern . Finite element method (FEM) . Negative excess pore-water pressure . Ground water potential * Yuqi Li [email protected]
1
Department of Civil Engineering, Shanghai University, Shanghai 200444, China
2
Department of Civil Engineering, City University London, London EC1V 0HB, UK
Introduction Excavation will cause the deformations of retaining structure, pit base and ground surface; therefore, numerous investigations on the characteristics of excavation-induced deformations have been performed. Ou and his research group did a lot of research; they proposed an empirical method for predicting the spandrel and concave settlement profiles on the basis of a regression analysis of the field observations of settlement curves (Hsieh and Ou 1998), studied building responses and ground movements caused by an excavation using the top-down construction method (Ou et al. 2000), analysed basal heave of excavations (Hsieh et al. 2008), evaluated basal heave stability (Do et al. 2013) and investigated extensively the behaviour of excavations with cross walls (Hsieh et al. 2012, 2013; Ou et al. 2013; Wu et al. 2013). In addition, Zdravkovic et al. (2005), Kung et al. (2007a, b) and Finno et al. (2007) studied the deformation behaviour of excavation in other aspects. Excavation will also cause the variation of pore pressure due to unloading. In order to investigate variation of pore
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