Whilst tunnels are popular in many urban areas, city developments usually require piled foundations for the construction of high-rise buildings. This leads to the obvious conflict between existing tunnels and new piled foundations. Since interaction between tunnels and piles is complex, design assumptions for new piles close to existing tunnels are often conservative. Better understanding of this problem is essential for the development of more economical designs in future.
This research aims to investigate the effects of single bored or jacked piles on an adjacent tunnel in sand. This is conducted by means of centrifuge model tests. Model piles were loaded close to a model tunnel in-flight at 75g. The model tunnel is equivalent to a tunnel of 7.5 m external diameter at prototype scale at a depth of 26.3 m. It is instrumented with strain gauges and leaf springs for the measurement of tunnel lining behaviour and tunnel deformation.
Back-analysis of the measured bending moments in the tunnel prior to being affected by an adjacent pile shows that the tunnel flexibility ratio is approximately 10. This tunnel is considered to be flexible, which is typical for most tunnels in the field. Field evidence shows that construction of bored piles close to tunnels in London Clay has only a very small effect. The centrifuge tests in this study show that the loading impact from the bored piles on the tunnel is rather limited. Even when the pile head loading is increased to the failure load (defined by a failure criterion of 10% of pile diameter), the induced bending moment increases by less than 40% of the pre-existing maximum bending moment and the associated tunnel deformation is less than 0.1% of tunnel diameter. These observations suggest the possibility of constructing bored piles very close to flexible tunnels. Contrary to bored piles, jacked piles cause a much greater impact on the tunnel. When the model jacked pile is installed adjacent to the tunnel, the induced bending moment at the tunnel near shoulder is increased by almost 100%. Even worse, the value is increased to over 400% at the crown if the pile is installed directly above the tunnel. Radial stress induced by the pile base penetration on the tunnel estimated by spherical cavity expansion theory shows that the induced radial stress reduces rapidly with distance from the pile base. The measured bending moment in the tunnel lining was found to be proportional to the estimated radial stress at the critcal location of the tunnel.
The centrifuge tests on the loading of non-displacement (bored) piles close to tunnels were simulated by finite element analysis, with the computed and measured results showing generally good agreement. Parametric studies using finite element analysis show the important influence of pile shaft interface friction and soil friction angle on the predicted influence of bored pile loading on tunnels.