HS2 Saltley Viaduct Rebuild Begins in Birmingham

The reconstruction of a viaduct in an urban area over which transport routes are still in operation is a problem which involves almost all the fields of civil and structural engineering, and the HS2 Saltley Viaduct project shows a number of techniques that A-level Design and Technology and Physics students will come across when studying more advanced topics.

The most notable method used is the employment of a Self-Propelled Modular Transporter (SPMT) when crossing the railway line. This type of SPMT is a platform built on a number of independently steered axle lines, each equipped with hydraulic suspension, and is capable of moving very heavy loads with precision down to a millimetre. Instead of assembling the bridge span directly over the tracks, which would necessitate long periods of closure of the railway and crane work right above the live infrastructure, the span is constructed on land next to the railway. When it is finished, the SPMT lifts it and then drives it laterally into position within a short possession period. The whole process can be completed in as little as one weekend, whereas conventional crane lifting would take weeks.

The structural design of the new viaduct has to take into account loading conditions which are considerably different from those of the original. Since the deck is raised by two metres, the geometry of the approach ramps is altered, the bearing loads on the abutments change and the lateral forces caused by wind on the taller structure also increase. Because the viaduct is widened by 4.5 metres, both the dead load (that is, the weight of the structure itself) and the live load capacity required for traffic, cyclists and pedestrians rise. The foundations will either have to be strengthened or replaced in order to cope with these greater forces, and the ground conditions in the River Rea floodplain add an extra element of complexity.

The steel referred to in the design for the new building is an alloy which, when exposed to the atmosphere, develops a stable and protective layer of rust thus making painting unnecessary. This represents a decision in lifecycle engineering since painting a viaduct spanning a railway, a canal and a river would require costly access scaffolding and environmental containment measures. Although weathering steel has a higher initial material cost, it results in greatly reduced maintenance over the 120-year design life of the structure. Another feature is the incorporation of LED lighting into the steel panels, a detail that reflects current standards in infrastructure design and which enhances the safety of pedestrians and cyclists without the need for individual lighting poles.

The step-by-step demolition programme shows an example of interface management, which involves coordinating a number of stakeholders (Network Rail, the Canal and River Trust, Birmingham City Council, the utility companies and the bus operators) since their assets and operations all come together at one point. In reality, each stage of the demolition has to take into account the requirements of all the stakeholders at the same time. The period during which the canal is closed, the railway possession weekends, the utility diversions and the road traffic diversions all have to match one another, and if there is a delay in one part of the programme it can have a knock-on effect for the whole programme.

The project serves as a typical illustration of the way real infrastructure projects are delivered for those students who are studying structural engineering or construction management. Although the engineering aspects of the bridge - such as the steelwork, the bearings and the deck - are important, the logistics involved in building it in the correct location at the right time without damaging anything else are arguably even more so.