Recent experience in automatic tail void grouting with soft ground tunnel boring machines

Abstract

When tunnelling is carried out using a shield machine and a segmental lining, the shield machine is made slightly larger than the lining. This difference in size is necessary to allow the lining to be erected within the protection of the tailskin of the shield. A gap around the lining is also necessary to provide sufficient 'rattling' room, to allow the shield to be steered around curves without the lining and tailskin coming into contact. Grouting of the tail void is essential to ensure an even distribution of pressure around the lining, and to minimise settlement. It is demonstrated, based on field data from Singapore, that surface settlements can be controlled more effectively by utilising pipes laid along the tailskin, than by using conventional grouting through the tunnel lining. This method of grouting is now widely adopted on modern tunnelling machines. Modern tunneling machines are capable of rapid rates of advance; on the Cairo Metro, Phase 2, advance rates of up to 34.5 m per day were achieved. Field measurements in the Netherlands have shown that the rings are subject to significant uplift (flotation) forces while the grout is fluid. If the grout cannot hold the rings in place, then the rings will move during shoving, leading to steps, lips and ring damage. To achieve a good finished product it is therefore necessary to ensure that the grout is capable of holding the rings in place against the flotation forces exerted by the grout itself. In addition, for production reasons, the grout used should be slow setting and pumpable for a long duration after mixing. For cost reasons, the grout should be designed to make the maximum use of the cheapest locally available materials. Meeting all of these requirements is not easy. The grout mixes used on four recent tunnelling projects are outlined. The four projects are: the Cairo Metro, Phase2 (Egypt), the North East Line (Singapore), a sewer tunnel in Florida (USA) and the Groene Hart tunnel (Netherlands). For the Cairo Metro and the Groene Hart tunnel, mortar grouts, without cement, have been used for general tunnelling. In Florida a kaolin/cement grout was developed to deal with particular problems. In Singapore both accelerated cement grouts and cement/sand mortars were used. Based on the experience on the four projects, and research work done as part of the 'Eupalinos' project in France, mortar grouts are only effective if the mix is carefully designed to provide a mortar with the appropriate rheology to resist the flotation forces on the ring. For the Groene Hart tunnel a performance specification has been developed, together with specific testing requirements for the grout. Both the performance specifications and the testing requirements are outlined. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.