Tunnelling effects on framed buildings

Abstract

The ever-increasing population in the urban environment requires continuous improvement for infrastructure, which often involves tunnel construction. To predict/mitigate tunnelling-induced damage of nearby structures, for instance, surface buildings, engineers need to be able to understand the complex soil-structure interaction mechanisms caused by excavation. Nevertheless, due to the complexity of the problem, simplifications have to be made; for instance, the frequent use of an equivalent isotropic plate to represent the surface structure, which brings a high level of uncertainty within risk assessments. This dissertation investigates the response of framed buildings with shallow foundations (rafts or separate strip footings) to tunnelling through using data obtained from 44 plane-strain geotechnical centrifuge tests. The research considered 8 elastic framed building models with varying configuration, with tunnelling- and structure-related parameters also varied during tests. The response of the frames on raft foundations was contrasted against results obtained using an equivalent isotropic plate. Results indicate that, in contrast to equivalent isotropic plates, framed buildings primarily exhibit shear behaviour and a semi-flexible response with both sagging and hogging deformation modes. Regardless of the foundation configuration, buildings with a greater width (transverse to the tunnel) showed increased structural distortions, whereas building eccentricity reduced deformations because of the freedom to tilt. Tunnels with a greater cover depth resulted in a lower level of building distortion. The structure self-weight and stiffness also played a role, particularly in relation to the formation of a gap beneath the building. The relative density of sand determined the soil shear and volumetric behaviour, resulting in a greater structural deformation in loose sand than in dense sand for a given tunnel volume loss; whereas for a similar soil volume loss (equal volume of surface settlement trough), a lower level of distortion was obtained in loose sand than in dense sand, particularly for separate footings. On the other hand, the foundation configuration plays an important role in determining the ground response to tunnelling, affecting soil displacement fields as well as the distribution of soil shear and volumetric strains. Furthermore, foundation settlements and differential horizontal displacements are experimentally confirmed to be larger for separate footings compared to raft foundations. The results also highlighted the role of footing embedment on soil-foundation interaction, with the potential of causing greater horizontal displacements and strains between footings. The obtained building deformed shapes indicate that angular/shear distortions within each bay or panel are more appropriate for quantifying framed building distortions than deflection ratios. Based on this, a relative stiffness parameter is suggested to relate maximum angular distortions to the slope of the greenfield settlement profile. Linear trends of angular distortion modification factors with relative soil-building shear stiffness were observed (in semi-logscale), and for buildings with similar values of relative stiffness, the level of distortion within framed buildings is lower for separate footings than raft foundations. Upper and lower empirical envelopes for preliminary damage assessment considering building eccentricity and foundation configuration are suggested. In addition, the efficiency of available relative stiffness parameters for the deflection ratio modification factors is confirmed. Finally, limitations of the equivalent plate approach and practical implications of the results for framed buildings are discussed