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The Oporto Light Metro Project offered many hints on tunnelling in unusual conditions. This paper describes the experience gathered during the Project, excavating about 6 km of tunnels using two EPB Tunnel Boring Machines. The densely urbanised environment of Oporto City centre, the complexity of the geology and the very high variability of ground mechanical properties led to the adoption of closed mode EPB excavation for the whole length of the tunnels, including long sections of sound granite. Excavation in rock always maintaining the chamber full of pressurised muck had serious implications on the operation of the machines, which often worked up to the limit of their performance, requiring daily cutterhead maintenance in hyperbaric conditions. In those conditions, it was always required a significant amount of energy and conditioning agents (polymers and/or foam) to crush the granite and turn into highly plastic support medium, as required for appropriate EPB operation. In order to analyse the effect on TBM performance of the closed mode excavation in hard rock, the relevant data parameters gathered during the Oporto Light Metro Project have been compared with the results of the prediction models for hard rock tunnelling in open mode excavation by the Norwegian Institute of Technology of Trondheim (NTH). The results of the back-analysis showed that NTH model can lead to a reliable prediction of basic penetration (in mm/rev), since in the input data it is possible to take into account that the effect of pressurised earth paste (the thrust per cutter is reduced according to the earth pressure in the chamber). Conversely the cutterhead torque is largely underestimated, as it would be expected. Therefore it can be concluded that in Oporto granite the specific energy needed to excavate in closed mode is about 3-4 times greater than the energy demand for open mode tunnelling. Another key aspect of the Project is tunnelling in a densely urbanised area that proved to be an extremely delicate issue. Special care needs to be taken beginning at the design phase, until the final construction phase. Since the works restarted in September 2001, the implemented procedures have been demonstrated to be successful for safe tunnelling, with a minimum impact on the citizens of Oporto. Critical situations were spotted in advance, and measures undertaken in due time. The particular case of tunnelling in the low cover area beneath sensitive buildings was perhaps the most challenging and complicated section of tunnel. In such circumstance, the tunnel was excavated beneath buildings of 3-4 storeys with basements located in a commercial area of Oporto city centre, with a shallow overburden (3-4 m) from foundation level. Given the unfavourable ground conditions, real-time monitoring and ground treatment were required. The results showed that the design solution adopted was appropriate, and the execution of works and treatments were effective. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500. |
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