Three-dimensional centrifuge and numerical modelling of twin tunnelling effects on piles

Abstract

Tunnel construction inevitably causes soil stress changes in the ground and hence induces ground movements. Uncontrolled ground movements induced by tunnelling may cause cracking in buildings and gas mains, or induce additional loads on piles of nearby structures. In urban cities, it is not uncommon to encounter existing piles located close to tunnel construction. Estimation of the effects of tunnelling on existing pile foundations of buildings poses a major challenge to designers and contractors. The objectives of this research are to investigate the fundamental mechanisms of twin tunnel-soil-pile interaction three-dimensionally and to quantify effects of twin tunneling on pile capacity. Two major research methodologies, namely centrifuge modelling and finite element analysis, were adopted. The advancement of tunnel excavation process was simulated in-flight by controlling volume loss (V 1 ) at 1.0% at each stage of tunnel excavation. In addition to measurements of ground surface settlement and pile settlement, bending moment and axial force induced in pile by tunnelling in different stages of construction were recorded. In the centrifuge model tests reported in this thesis, the effects of cover-to-diameter (C/D) ratios of twin tunnels on a single pile of 0.8 m diameter and 19.2 m long were investigated. In addition, the effects of different construction sequences of twin tunnelling on pile capacity at different depths were simulated and the behaviour of a 2×2 piled raft due to the construction of twin tunnels were also studied. In order to enhance our fundamental understanding of the three-dimensional twin tunnel-soil-pile interaction, systematic three-dimensional numerical back-analyses of centrifuge tests and parametric study were carried out. Three key dimensionless groups, i.e., relative location of twin tunnels with respect to pile depth (Z/L), the normalized distance between each tunnel and pile with diameter of tunnel (H/D), and volume loss due to tunnel construction are considered in numerical parametric analyses. It is found that the settlement of a pile induced by twin tunnelling is closely related to the depth of each tunnel relative to the pile. With a horizontal distance of 0.25D from each tunnel, the maximum cumulative pile settlement occurs when the twin tunnels are located at the pile toe (i.e., Test TT), owing to the significant reduction of confining stress induced by the twin tunnel excavated near pile toe. The excavation of the first tunnel results in a pile settlement of about 1.9% of the pile diameter. Similar magnitude of pile settlement is also induced by the construction of the second tunnel. Based on the displacement-failure load criterion proposed by Ng et al. (2001a), the apparent loss of pile capacity (ALPC) is about 21% after the construction of the first tunnel construction, and increases to about 36% (cumulative) after the construction of the second tunnel. The cumulative pile settlement due to twin tunnelling near the toe is about 2.2 times larger than of that caused by…