Leeds Station is the second busiest railway station outside London and sits at the heart of the region’s economy. More than 100,000 passengers use it each day. Increasing passenger demand, congestion to the existing northern entrance and an aspiration to reduce journey times to the expanding south of the city resulted in the need for the new Leeds Station Southern Entrance (LSSE). The new entrance is expected to serve up to 20,000 passengers daily.
The southern entrance is located over the River Aire, where residential towers, an electrified railway and a pedestrian bridge all constrained the site.
This, however, was the ideal location to meet the project objectives of accessibility from the south and to create a landmark building. An innovative approach was adopted to deliver the required solutions, this included:
- use of prefabrication and barged delivery to site
- strengthening of the existing Victorian masonry viaduct and station roof
- use of overwater ground investigation to inform the pier design
- tailored working methods in and around the three operational railway lines
- FE analysis of the soil structure interaction for the pile design use of bespoke lifting equipment.
Many of the innovative solutions for the project lie unseen below the new building. This includes the foundations and galvanized steel that form the ground floor concourse.
The foundations comprise two concrete piers supported on 24,900 mm diameter piles socketed into bedrock below the river bed. Over water ground investigation proved essential to confirm the depth to rock; its highly fractured nature led to a change in piling methodology prior to construction.
Advanced soil structure interaction, verified by finite element analysis, was undertaken together with consideration of flexural rigidity of the composite pile sections.
The piers support a concourse deck formed from a grillage of 600 mm deep steel beams, which support the superstructure, transferring column loads to the new piers. Fifty percent of the concourse is constructed below the existing viaduct and above the river. Up-stand trusses supporting the new floor and escalator pit span through one of the barrel vaults, transferring load to new foundations outboard.
The new super-structure, comprising a curved steel diagrid frame, exposed internally, is partially supported by a Victorian masonry viaduct and the existing station roof.
A complex assessment of these structures was undertaken to determine capacity, including measurement of strain under existing rail load using innovative video gauging. Strengthening of the existing quadripartite ribs involved the erection of 7.5 tonne steel arches within the confined space of the viaduct, anchored to the existing piers with 1 m long embedded bolts.
Hot dip galvanizing
Hot dip galvanizing was chosen as the method of corrosion protection to all steelwork located above the River Aire forming the ground floor concourse, as well as the bridges linking the concourse to the river banks.
In order to achieve the required performance the specification included grit blasting the steel prior to galvanizing in order to achieve a thicker coating and hence additional protection.
It was felt that galvanizing offered benefits over a painted finish in terms of life to first maintenance and the overall durability afforded to steelwork located in such a harsh environment.