Song, Geyang
(2019)
The use of protective structures to reduce tunnelling induced damage to buildings.
PhD thesis, University of Nottingham.
Abstract
In urban areas, due to the limited underground space, tunnels are being built close to or underneath existing buried foundations. As a result, it is essential to understand the effect of tunnelling on nearby structures and buried foundations. Tunnel-structure interaction problems have been widely investigated via numerical and physical modelling as well as with field monitoring. However, the use of protective measures, such as protective walls, to reduce the effect of tunnelling on structures have received much less research attention. Protective measures are often very costly and designed in an overly conservative way. A better understanding of tunnel, soil, piled structure and protective wall interaction (TSPPWI) problems would enable more efficient and cost-effective protective measures to be adopted.
In general, this thesis involves the study of three main areas, with centrifuge models developed to study each area: (1) greenfield tunnelling in sand, (2) effect of tunnelling on piled structures, and (3) the use of protective walls to protect buildings from tunnelling induced damage. The centrifuge models focus on the two-dimensional plane-strain case where the tunnel runs transversely to the structure. A newly developed mechanical model tunnel was used to simulate tunnel volume loss within the centrifuge tests. The research makes the use of the novel coupled centrifuge-numerical modelling (CCNM) hybrid testing equipment and methodologies developed at the University of Nottingham to simulate the tunnel-structure interaction problem within the centrifuge tests. The CCNM process allows more accurate simulation of the tunnel-structure interactions by adjusting the loads applied to the foundation piles within the centrifuge model according to outcomes of a numerical model that is used to simulate the structure domain simultaneously with the centrifuge model domain. In addition, to measure the axial force along the foundation piles and bending moment of the protective walls, optical fibre Bragg grating (FBG) sensors were used during the centrifuge tests. This research represents the one of the first applications of FBG sensors in a geotechnical centrifuge.
Greenfield tunnelling centrifuge tests were carried out using the newly developed rigid boundary mechanical model tunnel. The result showed good consistency in terms of soil displacement mechanisms with data from conventional flexible membrane model tunnels. An available empirical method for describing the variation of settlement trough shape in sand was modified based on the current centrifuge results. The outcomes of this work provide a more in-depth understanding of the effect of model tunnel boundary condition and will benefit future researchers considering which type of model tunnel to adopt when developing centrifuge tests related to tunnelling.
The tunnel structure interaction tests using the CCNM approach considered two cases of structure stiffness: a zero stiffness structure (constant pile head load), and full structure stiffness (based on a five storey steel frame building with four foundation piles running transverse to the tunnel direction). These tests served as a baseline for comparison with the protective wall tests. The structure stiffness affects pile head settlement and the variation in the pile head load with tunnel volume loss. The pile located closest to the tunnel showed the most significant pile head settlement with tunnelling, and FBG data indicated the loss of end bearing force and shaft resistance along the pile during tunnel volume loss. Post-tunnelling pile jacking tests were performed and indicated that the amount of displacement required to re-mobilise the shaft resistance along the pile depends on the amount of loss in shaft resistance during tunnelling.
In the tunnel-building interaction tests including protective walls (TSPPWI), two protective walls (made out of aluminium plate) were used with different embedded depths, referred to as `short' (toe of the wall located at the tunnel spring line) and `long' (toe of the wall located below the tunnel invert). The use of a protective wall can reduce the soil movements on the building side of the wall. Results demonstrate that the ‘long’ wall provides a more significant reduction in soil movements with tunnelling than the `short' wall. During tunnelling, the protective walls reduced the amount of settlement of the building foundation piles, as well as the change in pile head load. The efficiency of the walls was quantified using the settlement of the piles with respect to the case without a protective wall. Results indicate that the `long' protective wall provides higher efficiency, but the efficiency reduces for the piles located further away from the tunnel. Despite the use of protective walls, the FBG sensors indicated that the pile shaft resistance reduced with tunnelling, and during post-tunnelling pile jacking, a more significant pile head displacement was required to fully re-mobilise the shaft resistance for the piles that lost most of their shaft resistance during tunnel volume loss. The outcomes from these tests will help engineers to refine their design of protective walls in practice, and enable more efficient and cost-effective protective measures to be adopted.
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