As demonstrated in the tunnel construction accident at the Heathrow Express airport, the impact of an accidental tunnel collapse on the nearby tunnels can be catastrophic. As a matter of fact, the interaction effect of multiple tunnelling is of great significance in urban environment. Indeed, the interaction effect largely depends on the relative positions of adjacent tunnels. However, three-dimensional studies on these areas are still relatively limited so far....[ Read more ]

As demonstrated in the tunnel construction accident at the Heathrow Express airport, the impact of an accidental tunnel collapse on the nearby tunnels can be catastrophic. As a matter of fact, the interaction effect of multiple tunnelling is of great significance in urban environment. Indeed, the interaction effect largely depends on the relative positions of adjacent tunnels. However, three-dimensional studies on these areas are still relatively limited so far.

The objectives of this research are: (a) to investigate the effect of a tunnel collapse on a closely spaced tunnel in sandy ground by means of three-dimensional centrifuge modelling, with the aid of numerical analysis of the centrifuge test to gain better understanding of the failure mechanism of the tunnel and; (b) to study and compare the interaction effects of twin tunnelling with different relative tunnel positions in sandy soil by using three-dimensional finite element analyses.

Catastrophic ground settlement with a volume loss up to 55% was simulated by a tunnel collapse in centrifuge. Considerable hogging incremental bending moment was induced at the pillar springline of the existing tunnel while significant sagging incremental bending moment was observed at the crown and invert. It appears that the impact of the tunnel collapse on the incremental bending moment of the nearby existing tunnel lining becomes more significant for a denser soil as well as smaller pillar width. During the new tunnel collapse, substantial increment of vertical stress and decrement of lateral stress was measured at the crown and pillar springline of the existing tunnel, respectively. Soil stress transfers from the excavated section of the new tunnel to the upper portion of the existing tunnel while the soil stress between the two tunnels reduces from earth pressure at rest to active earth pressure. It is understood from three-dimensional numerical analysis that for tunnelling close to collapse state, relatively significant plastic deviatoric strain is developed near the crown and invert of the new tunnel.

Based on three-dimensional parametric numerical analyses, asymmetric settlement profile with 1.05% volume loss is caused by diagonal tunnelling. By comparing the interaction effect of diagonal and parallel tunnelling, 38% larger volume loss is induced by parallel tunnelling with slightly shallower settlement trough. Lateral stress around the existing tunnel increases significantly during new parallel tunnelling. This results in a horizontal compression of and an increase of hoop thrust at the existing tunnel. In addition, the existing tunnel moves laterally away from the new tunnel after the construction of the new parallel tunnel. On the other hand, a comparison between diagonal and piggyback tunnelling shows that 5% smaller volume loss but with 18.8% larger maximum settlement is caused by piggyback tunnelling. Due to the reduction of the overburden owing to the new piggyback tunnel excavation, a relatively significant decrement of soil stress is computed at the upper portion of the existing tunnel. This in turn causes horizontal compression of and a decrease of hoop thrust at the existing tunnel. The existing tunnel moves upward after the construction of new piggyback tunnel.