Dense shotcrete structures in road tunnel construction

The standard case of a road tunnel in Germany envisages a double-shell structure with sealing and drainage, whereby as a rule the outer shell, the temporary securing, is made of shotcrete and the inner shell, the final securing of the structure, is made of reinforced or unreinforced formwork concrete. According to the "Richtlinien zur Ausstattung und dem Betrieb von Straßentunneln" (RABT) [22], longer structures should be designed as twin-tube tunnels, with one tube per traffic direction, or - under certain boundary conditions - as a single-tube structure with two-way traffic and a parallel rescue gallery. By definition, such rescue tunnels have a smaller cross-section than the main structure, but according to the current regulations, their shell structure must be identical to the main cross-section. In the area of such rescue tubes, but also in the case of crosscuts and complex sections of the structure in terms of formwork (e.g. niches or caverns), single-shell shotcrete structures made of waterproof shotcrete can theoretically be regarded as an economical alternative to the usual construction method described because they are simple and inexpensive to produce. However, the problem here is the punctual ingress of water that can be observed in the event of water ingress, especially through individual areas of the shotcrete shell, primarily in the area of crack zones of the shell, sections of disturbed structure (keyword: splash shadow) or via the head plates of the rock anchors inserted as securing means. The resulting problems currently prevent the planned use of such structures, since there are no practical and generally valid specifications for the production of durable and dense shotcrete structures. Within the scope of this research project, procedural and material-technological improvement approaches for dense shotcrete constructions are to be developed and experimentally verified with regard to their problem-specific functionality.