High-Speed Railway Bridges in Italy: the Modena Viaducts

High-speed railway bridges in Italy: the Modena Viaducts

In the year 1991 the Italian railway authority assigned the design and construction of infrastructures and plants for the Italian High Speed Railway between Milan and Naples to the concessionaire TAV. Within this program, TAV assigned the design and construction of the 182-km section between Milan and Bologna to a consortium.

The 4.89-billion Euro lump-sum contract included global warranty on time, cost, and quality from the consortium. The work was divided into 19 lots (13 lots of civil works, 3 lots of railway infrastructure, and 3 lots of technological plants). A sub-consortium was assigned the 39.5-km civil work lot from 142+685 to 182+148 km, the 8.0-km duplication of the Modena-Mantova line, and the new stations of Modena and Soliera.

This contract included the 24.8-km Modena Viaducts, the 9.2-km bridges of the Modena East and Lavino Interconnections, additional 1.8 km of bridges crossing four rivers and six railroads, 0.4 km of cut-and-cover tunnels and tens of box-culverts. Tasks also included 25.0 km of railway embankment inclusive of base geo-synthetic protection, anti-capillarity layer, super-compacted upper layer, and sub-ballast layer in asphalt concrete.

The sub-consortium assigned the construction of Modena Viaducts with a contractual deadline of 30 months. The author, as contractor's bridge technology consultant, wrote the quality-assurance and quality-control specifications for the entire project (civil works and special equipment) and performed the independent design review of special equipment.

The railway alignment runs through the suburbs of Modena, hence mitigating the visual and acoustic impact of the new infrastructure was a major concern. The great number of obstructions along the route (highways, railways, local roads, farm roads, rivers, irrigation channels) and the poor mechanical properties of the soil suggested using the use of long viaducts instead of embankments. Bridges would facilitate water flow during floods of the River Po and would avoid hydraulic problems from the shallow water-table in a great number of underpasses.

The Modena Viaducts include five main structures, all twin bridges supporting ballasted single tracks: the 2.1-km Brenner Viaducts over Brenner Highway A22, the 7.1-km Modena Viaducts over the new railway lines of Modena West Interconnection and the River Secchia, the 2.4-km Secchia Viaduct over the River Secchia, the Modena-North Industrial Area and the Modena-West Interconnection, the 1.5-km Panaro Viaducts over the River Panaro, and the 1.0-km bridges of Modena-West Interconnection over rivers and local roads.

High-speed railway viaducts of such length, and so near to the ground, posed unusual challenges in terms of visual and acoustic impact. The 3.5-m U-section encloses the noisy portion of the train, acts as a sound barrier and confines the train in the improbable event of derailment. Twin single-track bridges provide route redundancy (during maintenance of one track the other track can be in full-speed service) and the spans are lighter, which simplifies construction.

A box girder would have a similar depth to meet the dynamic requirements of high-speed railway bridges, but ballast, track, and sound barriers would add to the total depth. Because of legal noise restrictions, sound barriers may be up to 4-m tall in Italy. They are structurally demanding and expensive due to high-speed train suction, and they are to be maintained. The U-section improves the visual impact of the bridge, avoids the sound barriers, lowers the vertical profile of the track, provides support for systems functions, and simplifies inspection and maintenance because most concrete surfaces are directly accessible.

The architecture of the viaducts was enhanced with an elliptical shape for the cross-section's webs, engraved with wide horizontal bands that catch the sunlight at different angles. The rounded shape of protruding bearing plinths was mirrored into pier-caps that allow easy inspection of bearings, span retaining systems, anti-seismic devices and rainfall drainage systems. The columns are simple cylinders devoid of rustication.

Despite the elegant architecture, the cost of the casting cells for the precast spans was low because of the small area-to-perimeter ratio. A U-section is much easier to cast in one pour than a box girder, and girders with open section do not require collapsible inner tunnel-forms that have to be extracted through the end support diaphragms.

Prefabrication of reinforcement is simpler and the concrete surface to be hand-finished is minimized (only the top of webs). The Modena Viaducts comprise 755 simply-supported fully-precast spans and 9 cast-in-place three-span continuous bridges: 713 precast U-spans are 31.5-m long, 28 units are 29-m long and 14 units are 24-m long. The continuous bridges are 136-m long and the three spans are 40, 56, and 40-m long. For architectural homogeneity, all spans have the same U-section.

Construction of the 24.8-km Modena Viaducts required full-span precasting of 755 simply-supported spans and in-place casting of 9 three-span continuous bridges in only 30 months under global warranty on time, costs, and quality. The paper provides exhaustive coverage of how such a record-breaking goal has been met.

Custom-designed special construction equipment was used for most of the project. Defining the quality-assurance and quality-control specifications for the design, fabrication, and site operations of special construction equipment took almost 1 year. Analysis of methods, risks, and their mitigations was performed for every major activity. Contingency plans were identified and also prequalified.

Performance requirements, technical specifications, and design criteria were identified for special construction equipment and all construction processes. Equipment design was subject to full independent checking. Fabrication of equipment (traceability of materials, welding, dimensional control, systems and plants, controls, site assembly, and load testing) and site operations were also ruled by specific quality assurance and control procedures.

Published on IABSE Structural Engineering International in April 2011, Industrialized Construction of Large-Scale High Speed Railway Projects: The Modena Bridges in Italy provides exhaustive coverage of the design and construction of the project. The quality control and quality assurance procedures implemented for this record-breaking set of bridges are also discussed. The paper is available for free download through the link above.

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