Abstract
In urban areas, engineers often need to assess tunnelling-induced displacements of piled structures and the resulting potential for damage. This paper presents an elastic study of tunnel-pile-structure interaction through Winkler-based Two-Stage Analysis Methods (TSAMs), focusing on structural displacements resulting from tunnel excavation beneath piled frames or simple equivalent beams. Comparison of results with 3D finite element analyses shows that the simple TSAM models are able to provide a good assessment of tunnelling-induced building displacements. Parametric analyses highlight the role of tunnel-pile interaction and the superstructure (stiffness, configuration, and pile-structure connections) in the global response of the tunnel-soil-building system. In particular, the effect that key parameters have on deflection ratios and horizontal strains are investigated. Results illustrate how piled foundations increase the risk of structural damage compared to shallow foundations, whereas structural stiffness can reduce building deformations. Flexural deformations are predominately induced by tunnel excavations beneath piles whereas horizontal strains at the ground level are negligible when a continuous foundation is included. Furthermore, it is illustrated that results based on buildings modelled as equivalent beams can differ considerably compared to when they are modelled as framed structures. Simple design charts are provided to estimate horizontal strains and deflection ratio modification factors based on newly defined relative axial and bending stiffness parameters which account for the presence of the piles.
Highlights
In urban areas, the increasing demand for infrastructure and development of services has resulted in tunnel construction and deep excavations taking place in close proximity to buried infrastructure and building foundations
Various studies have considered the effect of excavations on either a building with shallow foundations or piles connected by a rigid cap, the understanding of how the tunnel-pile interaction affects the response of buildings is still not well understood
Many useful tools for tunnel-pile interaction analysis have been developed using the elastic framework. These are based on a two-stage procedure: (1) the greenfield soil displacements caused by tunnel excavation are estimated analytically, through closed-form expressions (Loganathan and Poulos, 1998; González and Sagaseta, 2001; Franza and Marshall, 2015), or numerically using software based on the finite element (FE) or finite difference (FD) methods; (2) the analysis of the full system, including soil, foundation and superstructure, is carried out considering the foundation subjected to a system of external loads that would, in the absence of the included structure, reproduce the greenfield soil movements
Summary
The increasing demand for infrastructure and development of services has resulted in tunnel construction and deep excavations taking place in close proximity to buried infrastructure and building foundations. Kaalberg et al (2005) and Selemetas (2005) suggested three zones where pile head settlements may be larger than (zone A), equal to (zone B) or smaller than (zone C) the greenfield surface settlements (see Fig. 1) These agree qualitatively with results obtained by other researchers who used centrifuge testing to study the problem (Jacobsz et al, 2004; Marshall and Mair, 2011). The authors suggested an upper limit of the normalised pile head settlement depending on the normalised horizontal pile offset to the tunnel centreline Use of this upper limit would lead to an over-conservative assessment of tunnelling-induced deformation in piled buildings. Radius from the tunnel axis) are likely to settle more than the surface, whereas piles outside this area generally settle less than the surface This causes a narrowing of the pile head settlement profile with respect to the greenfield surface settlement trough, leading to an increased potential for building damage. Two simple design charts for evaluating the piled building deflection ratios and horizontal strains are proposed
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