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The definition of a rock mass's characteristics is crucial in underground construction to avoid delays and cost overruns. This study proposes a system to quantify the economic uncertainty related to a lack of knowledge of a rock mass in the tunnel construction stage, either for tunnel boring machines or for drill and blast excavation techniques. Using a back-analysis of three actual tunnels completed in Spain (Burata, Lot 3 of the Pajares variant, and Bolaños), the study assessed the directional core drilling technique (DCD) for this purpose, comparing it with conventional boreholes. In this regard, the DCD approach reduced the uncertainty by between EUR 6.7 and EUR 12.7 for every EUR 1, while the total cost of the drilling campaign remained within a widely accepted proportion of the construction budget. Overall, the uncertainty was reduced by approximately EUR 6000 per meter of the tunnel. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the most outstanding experiences regarding the use of fibres as the main reinforcement in precast segmental linings in the metropolitan area of Barcelona. It is known that the addition of structural fibres improves, on the one hand, the mechanical behaviour of the structure during its construction, especially in cases such as the thrust of the jacks, and on the other hand it leads to a reduction of the global costs by reducing the conventional passive reinforcement. The aim of this paper consists in presenting three real experiences that are representative of the application of FRC in urban tunnels and a design methodology to take into account the structural contribution of the fibres. Two particular cases of the application of this design method are presented. In the first case, the use of 25kg/m3 of fibres has led to a reduction of 70% of the conventional reinforcement initially proposed in the project. In the second one, which was planned to employ fibres but without considering its structural contribution, the parametric study reflected the possibility of reducing up to a 38% of the rebars adding 25kg/m3 of steel fibres in the concrete mixture. In light of good results, construction companies in Spain have become aware of the advantages of using fibres in these structures and have carried out experimental stretches. This attitude has also been influenced by the approval of the new Spanish Code, which includes the FRC as a construction material with design purposes. [Copyright &y& Elsevier]

Abstract: Tunneling in urban areas demands an accurate characterization of the shallow subsurface to minimize risks during excavation. Unexpected high water inflows constitute a major problem because they may result in the collapse of the tunnel face and affect surface structures. Such collapses interrupted boring tasks and led to costly delays during the construction of the Santa Coloma Sector of L9 (Line 9) of the Barcelona Subway. A method for predicting groundwater inflows at tunnel face scale was implemented. A detailed 3D geological and geophysical characterization of the area was performed and a quasi-3D numerical model with a moving tunnel face boundary condition was built to simulate tunnel aquifer interaction. The model correctly predicts groundwater head variations and the magnitude of tunnel inflows concentrated at the crossing of faults and some dikes. Adaptation of the model scale to that of the tunnel and proper accounting for connectivity with the rest of the rock massif were crucial for quantifying the inflows. This method enables us to locate the hazardous areas where dewatering could be implemented. [Copyright &y& Elsevier]