Franza, Andrea
(2017)
Tunnelling and its effects on piles and piled structures.
PhD thesis, University of Nottingham.
Abstract
Current needs for infrastructure and services in urban areas often require the construction of tunnels that may affect existing surface and buried structures. In general, the construction of new tunnels in the proximity of deep foundations raises concerns related to pile failure and associated structural damage (in both the superstructure and the foundation). Despite its practical importance, few studies have investigated the global tunnel-pile-structure interaction (TPSI) and, thus, engineers generally compensate for the lack of understanding with an overly conservative design approach.
To provide insights into the interaction mechanisms of TPSI, this research used geotechnical centrifuge testing as the main investigation method to acquire data related to both greenfield tunnelling in sands and tunnel excavations beneath piles and piled buildings. In particular, a novel method was developed to study TPSI problems through the real-time coupling of numerical and centrifuge modelling, enhancing centrifuge modelling capabilities. Furthermore, empirical and closed-form solutions were used to study the tunnelling-induced displacement fields and simplified elastic analyses were used to provide insights into the global TPSI mechanisms.
Results from the greenfield tests illustrate that ground movement prediction in sands is very complex because of soil arching effects and changes that occur as tunnels transition from relatively shallow to deep depths, resulting in highly non-linear displacement mechanisms. Results also illustrate the correlation between vertical and horizontal displacement mechanisms. In particular, the influence of soil relative density and volume loss on deformation patterns is highly dependent on the tunnel relative depth. To provide simple tools for engineering practice, empirical and closed-form solutions are proposed. Predicted ground movements provide sufficient accuracy for preliminary assessments, though limitations of these methods should be considered.
The centrifuge tests on TPSI provide experimental evidence that tunnelling-induced pile displacements are affected by [i] pile installation method (displacement versus non-displacement piles), which affects the pre-tunnelling soil state and the distribution of loads between pile shaft and base, [ii] initial safety factor of the pile foundation, which is related to pile bearing capacity and superstructure self-weight, and [iii] superstructure stiffness and configuration, which results in pile load redistribution while minimising structural distortions. In addition, results show that potential for pile failure is a critical aspect for piles with relatively low initial safety factors and that pile failure may be prevented by a limited relative reduction in the pile load due to the superstructure.
Finally, the importance of superstructure stiffness and self-weight on tunnelling-induced structural distortions is confirmed. Piled buildings respond critically to tunnelling beneath the pile tip depth in terms of flexural deformations. In general, it is shown that [iv] piles increase structural distortions compared to shallow foundations and that [v] the superstructure stiffness and self-weight decrease and increase the superstructure distortions resulting from tunnelling, respectively. Results are also evaluated within the modification factor approach; parametric analyses of elastic soil-pile-structure interaction are used to develop simple design charts that can be used to estimate horizontal strains and deflection ratio modification factors based on newly defined relative axial and bending stiffness parameters. The envelopes compare well with deflection ratio modification factors measured from centrifuge tests. Further research is needed to include the effects of soil plasticity, building self-weight, superstructure configuration and tunnel-structure eccentricity in these design charts. This dissertation highlights the improvements in the design of underground constructions that can be achieved by combining ground and structural engineering.
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