S.S.126 DIR “sud occidentale sarda”

The aim of the project was to improve the road system serving Sant’Antioco, by creating a new road link to replace the existing one between the isthmus and the island of Sant’Antioco. This road link is designed to join up with a new stretch of road that acts as a ring road to by-pass the town centre, offering a smoother, more functional link with the port of Calasetta and the regional road network.

The work along the SS126 dir is divided up into two different functional interventions:

• functional intervention no. 1/Overpass (from km 0+000 to km 1+944): demolition work and construction of a new overpass along the S.S.126 dir link with the island of Sant’Antioco to allow yachts and ships with a height of up to 18 metres to cross the road. The new structure will be grafted onto the existing stretch of the SS.126 dir near the archaeological area, and will end with a slip road linking the centre of Sant’Antioco and the future Sant’Antioco Ring Road;
• functional intervention no. 2 (Ring Road), from km 0+000 to km 3+826: construction of a new approx. 3.5 km direct road link between the bridge over the isthmus and the existing SS126 dir, which will allow drivers to avoid the centre of Sant’Antioco, for a smoother, more functional road route to the port of Calasetta.

Acoustic study. The aim of this study is to specify the location and type of the noise reduction works for the “S.S.126 DIR “SUD OCCIDENTALE SARDA” and how to construct them. DIR “SUD OCCIDENTALE SARDA”.

Noise levels were estimated using the SoundPlan model (version 8.0). SoundPlan is an elaborate noise simulation model, based on the Ray Tracing technique, which can be used to simulate the propagation of noise in complex source and orography situations. The calculation model for the road axis is the French method NMPB Routes 96, with a number of rays equal to 100 and a number of reflections equal to 5. The propagation distance has been considered up to 1000 metres. The noise level estimate takes account of traffic composition, number and speed of vehicles, the type of asphalt and the slope of the road. The high quantity of initial information, together with the level of precision set at the data processing stage yields results that are very accurate, although onerous in terms of calculation resources. The calculation model for the road axis is the French method NMPB Routes 96, with a number of rays equal to 100 and a number of reflections equal to 5.

The settings of the post operam simulation took account of the morphological characteristics of the site, the elevations in the project, the properties of the road surface and the flows relating to the project status for the year 2030 (average daily traffic for the year).

The road in the project has a single carriageway, with one lane per direction. The traffic system is composed of the main road axis and the roundabouts, considering the sections envisaged in the project.

The speed considered was 90 km/h for light vehicles and 70 km/h for heavy vehicles; speeds on the roundabouts were set at 40 km/h for all vehicle categories.

Extrapolated from the calculation model were the horizontal maps at an altitude of 2 metres (in accordance with the characteristic reception altitude of the area) and the vertical sections with standard-type conditions at the propagation line (sigma=600).

Noise barriers were designed for the points at which noise levels exceed the permitted threshold. These barriers are composed of low-alloy weathering steel. Weathering steels is part of the category of low-alloy steels defined as patinable. The main characteristic of low-alloy weathering steel is its ability to protect itself from corrosion by forming a compact passivating patina on the surface, made up of the oxides in its alloy elements, which prevents the gradual spread of corrosion. Thanks to this particular characteristic, the use of weathering steel avoids the need for subsequent zinc coating (required for the regular steels generally used for metal carpentry support structures) and varnishing (required for metal carpentry support structures and for aluminium panels); these high-environmental-impact processes have a major impact on the overall energy balance of the end product.

The base panel is composed of a posterior shell in 1 mm thick sheet weathering steel, a mineral wool sound-absorbing blanket and an interior perforated sheet (perforation percentage 35%). The panel is closed at the sides with polypropylene heads.

The sound absorption class of the panel is A5, and the sound insulation class is B3.

Air study. The estimate of air pollution dispersion caused by vehicular traffic with the road in operation was conducted by simulation using the CALINE4 air dispersion model (implemented into the MMSCaline software). CALINE belongs to the category of Gaussian models, and specifically takes account of the linear form of the source and the turbulence induced by the movement of the motor vehicles. NOx, CO and PM10 emissions were evaluated.