Local settlements and sinkholes due to EPB tunnelling

Most of the tunnelling for the North East Line in Singapore was carried out using earth pressure balance (EPB) shields. Settlements were generally well controlled; however, there were occasional large settlements, exceeding the design estimates, sinkholes or losses of ground. Most of the larger surface settlements, sinkholes and losses of ground can be related to the use of insufficient face pressure. Other documented experience of settlements over EPB shields is also reviewed and the same pattern of occasional, but large, settlements or sinkholes is found repeatedly. The factors associated with such settlements are also common in many of the cases reviewed.