In order to analyse the reinforcement of recast segments for tunnels under severe geological conditions, theoretical and experimental studies were carried out by the University of Applied Sciences in Fri burg, Switzerland. The aim of these studies was to evaluate the influence of steel fibres on the behaviour of a tunnel lining which can be described as a complex structural system containing different recast concrete segments and a multitude of longitudinal and transverse joints. The experimental studies consisted of two parts. In the first part, tests were carried out on connecting joints between recast segments. The joints are indeed highly solicited, and the bursting of the concrete at the locations of the joints could have a significant effect on the stability of the overall structure. In the second part, full-scale tests were carried out on whole tunnel segments coming directly from the recasting site. The experimental studies show good behaviour on the part of the steel fibre reinforced concrete under various combined loads. They highlight the major impact of the high concentrated compression forces at the joints between adjacent segments. Fibre reinforced concrete appears particularly well-adapted for recast tunnel segments subject to high compression loads in severe geological conditions.
The Oenzberg Tunnel, located on the main line between Berne and Zurich, was built by the Swiss Federal Railway (CF) within the scope of the development of railway infrastructure. At a distance of approximately 80 m from its east end, this tunnel crosses another railway tunnel. The proximity of the two tunnel structures as well as the unfavorable geological conditions caused additional loads to be imposed on the recast tunnel lining segments and, in particular, on the joints between the recast segments. Various reinforcement solutions were studied and, after numerous discussions, the use of steel fibre reinforced concrete (AFC) was considered. Added in sufficient proportions, steel fibres can increase the tensile strength of concrete and improve the ductility of concrete structures [9,10]. Moreover, these fibres could eliminate the need for complicated reinforcement cages near the joints and thus contribute to the overall economy of construction [1,2,7]. To study the behavior and the effectiveness of the AFC for tunnel lining segments, the CF commissioned the University of Applied Sciences (OAS) Fri burg, with a comparative experimental study. Three alternative reinforcement solutions were selected for this comparison:
concrete reinforced with steel bars (standard cages),
Dram (glued fibre, high l/D ratio) steel fibre reinforced concrete, 60 kg/m3,
mixed solution AFC with Dram (glued fibre, high l/D ratio), 30 kg/m3, and reduced bars reinforcement.
These three solutions were the subject of an experimental program which included, on the one hand, a study on the joints between the tunnel lining segments (fig. 3) and, on the other hand, tests on full-scale tunnel lining segments (fig.1)
For the construction of tunnels using recast tunnel lining segments, detailed attention must be given to the design of the joints.