The presentation started with a number of dramatic photographs of sinkholes in Singapore and Canada, demonstrating the damage to existing infrastructure that can occur, usually in public places. It was noted that sinkholes tended not to be picked up by monitoring points, as usually they occurred between these points. They were also usually associated with large ground movements at the tunnel face (rather than being due to collapses or other ground movements after tunnelling had been completed). To distinguish between normal settlements and sinkholes, it was proposed that sinkholes be defined as settlements in excess of 150mm - such movements can be seen (i.e. detected without instrumentation), and represent a volume loss larger than about 10% for a typical 6m diameter subway running tunnel. The experiences in Singapore, supplemented by data from Canada, showed that, whilst risk assessments of EPB tunnels almost invariably record a "negligible" risk of collapse, this is not borne out by experience. Table 1 shows that 56 incidents of large settlements occurred in 77km of tunnels, an average of one incident every 1.4km. A number of published cases from other countries also suggest that the risks are not "negligible" -particularly where faces are in mixed ground, or where geological interfaces are close to the tunnel route, or when the TBM has just been launched.
展开▼
