The single-tube 'Santa Caterina' tunnel is located near the Caltagirone residential area and extends for about 900 m (760 m in natural tunnel under the mountain and 140 m in artificial tunnel). After an artificial tunnel section of about 60 m, the infrastructure continues in open air at the North entrance side and ends with a trench section between bulkheads on large diameter piles at the South side; this is followed by 80 m of artificial tunnel, completely underground to reduce the landscape impact.
In compliance with safety requirements (ANAS Guidelines 2009), the tunnel has two escape routes 300 m apart and at each entrance. The area morphological context directed the design towards the choice of two escape routes with separate routes, 34.5 m and 332 m long respectively. The escape routes exit at their respective mouths at a single square. In general, the work has a reduced coverage (with a maximum value of 30 m) and is inserted in a surface context characterized by the presence of some houses along the route at a variable distance from its axis; moreover, it crosses mainly sand formations with calcarenitic levels and, at times, marly clays. The water table level is usually found below the excavation of the inverted arch, with some stretches higher up to the height of the walls. Considering the geological-geotechnical context, the contained covers and the presence of numerous buildings, the project has provided for stabilization and consolidation interventions (mainly with jet-grouting columnar treatments on the cap, the piers and the excavation face). Where the contact between the argillitic and sandy units moves towards the cap, the jet-grouting columns (which are not very successful in clayey soils) were replaced with metal and/or fiberglass elements. The length of the advance field from 6 m , 9 m and 12 m, characterizes the different sections of excavation and consolidation. The different typological sections were applied along the axis in order to ensure proper stabilization and limit surface subsidence. For the construction phase, both inside the tunnel and on the surface, constant monitoring of deformation parameters and settlements was foreseen, together with the corresponding potential effects on the buildings present (risk analysis procedures for buildings). Attention and warning thresholds have been identified which, if exceeded, trigger specific countermeasures already planned in the design phase (following the development of scenarios and risk analysis).
The entrance works were the subject of an in-depth study of the implementation problems, since the low slopes of the slopes, in correspondence of which the attack of the natural excavation takes place, has determined the adoption of important bulkheads, necessary for the artificial tunnels entrance construction: in particular, for the North tunnel portal was chosen a solution to 'flute beak' extremely effective from the architectural point of view, able to allow a better masking of the structures compared to the landscape context.