No-one should underestimate the difficulties it presented, delivering a complex structure over the River Aire’s dynamic waters. That’s what engineers do, of course: make the improbable possible. But this was not just another railway project; it would have broader social and economic impact.

Leeds keenly anticipated its completion, the local press informing us that “to construct the new station at the requisite level and give ample field for the wonderful reticulation of lines and points, a preliminary and greater work had to be accomplished, in the erection of an extensive series of arches.”

That’s right – we’re starting our journey in the nineteenth century, not the twenty-first.

Those arches were on a vast scale, occupying seven acres and comprising 18 million bricks. Above them was built Leeds New Station, opened jointly by the North Eastern and London & North Western railways on 1 April 1869. At a cost of around £350,000, the vision of engineers Thomas Harrison and Robert Hodgson was realised by George Thompson & Co, their plucky contractor. Included in that price was a mile or so of largely elevated railway, connecting to the NER’s (North Eastern Railway) terminus at Marsh Lane. Thus, today’s route into Leeds from the east was established.

Closure of the city’s Wellington and Central stations – in 1938 and 1967 respectively – both brought significant remodelling. The ‘Leeds 1st’ capacity improvements were introduced in 2002, providing five extra platforms and more in the way of light. In all likelihood, there will be more to come, but that’s evolution for you.

Wrong direction

Stations don’t get much busier than Leeds. Look through the data from 2014/15 and you’ll find it ranks 12th in the UK with 28.8 million entries and exits – the third highest figure outside the capital and a million up on the previous year. Until recently, though, access was far from ideal, all the entrances being on the railway’s north side. That was fine for those visiting the main civic and commercial districts, but less convenient if your destination was amongst the emerging residential and business areas south of the station, beyond the River Aire. Reaching them involved a walk down Neville Street, enduring 100 yards of gloom beneath platform- supporting ironwork.

It was clear to stakeholders that further redevelopment potential could be impeded without improved station access. This drove plans for a new entrance on the south side, promoted by the West Yorkshire Combined Authority (WYCA) and Network Rail, with additional funding from the Department for Transport and Leeds City Council.

In their collective sights was a landmark structure, complementing the contemporary feel of the urban village in which it would sit. But where to put it? The only practical location involved straddling the Aire, pushed out from the station’s existing brick arches.

Aesthetically, this had much to commend it, benefiting from uncluttered sight lines and reflections in the river, with apartment blocks framing the entrance on either side. The logistical, safety and technical implications would, though, be significant.

Tasked with overcoming them was Carillion Rail, appointed by Network Rail as principal contractor in November 2014. Mott MacDonald fulfilled the engineering design Building: Network Rail GlasgowLocation: GlasgowArchitect: AHRwork whilst AHR acted as architects. The project value was £20.3 million.

Elbow room

The extent of the envelope within which the structure would have to be contained was set through a public inquiry and subsequent Transport and Works Act order, stipulating minimum clearances from adjacent buildings. Once the architect had refined plans for the internal space – optimising the layout of escalators, lifts, steps and landings based on pedestrian flow analysis – a total of just 400mm was available on either side for both the primary structure and any secondary framing. That led to the design of a closely-spaced structural grid to minimise the loading on each frame, presenting the opportunity to control curvatures more tightly by means of braced, moment-resisting hoops, similar to a diagrid.

This solution was elegant but further complicated matters. So as not to impede the river flow, the foundations for the new entrance took the form of extensions to two of the piers supporting the station’s brick arches. However, the superstructure columns could not land on these foundations due to the aforementioned spacial constraints. This created the need for a transfer deck but the height at which it could sit – and hence the depth of its beams – had to take account of a once-in-200-years flood event and the impact of climate change. Minimising the load on these beams was therefore imperative which proved another advantage of the closely spaced grid.

Beyond all this, the nature of the site imposed additional limitations. Whilst a couple of the arches were adapted to host mess facilities and office space, there was no room for a compound. Materials and equipment would instead have to be brought in by barge – beneath two bridges – from a satellite area 500 metres downstream and craned into place using a 63-metre high tower crane located in a cramped service yard behind an apartment block.

The positive effects of this were to keep construction traffic off busy city-centre streets and greatly reduce the disruption felt by nearby residents. It did, though, mean that Mott MacDonald’s design team had to have a critical focus on ‘buildability’, driving the adoption of lightweight, modular or prefabricated components.

Go with the flow

Option selection for the pier design had to account for environmental forces – which could be considerable – as well as the permanent load. The decision was made to form two piers, comprising single rows of twelve 900mm diameter piles, each 10 metres deep. These were sunk by a rig, supplied by Martello, mounted on a jack-up barge which had been towed to site from the satellite area. Partly driving this approach was an Environment Agency stipulation that a clear freeboard zone must be maintained at all times, the barge – from Pontoonworks – having the ability to lift itself up as the water level rose.

With the piles in place and adjustable waling beams welded to their casings, specialists from DiveSafeUK fitted 36 precast concrete panels – sealed into the river bed – to effectively act as cofferdams. Designed by Carillion’s in-house group technical services, these were then filled with mass concrete to provide working platforms on which rebar cages for the permanent pile caps could be constructed.

Cantilevered from the river bed, the two piers are thus designed to act independently – tied simply together at the top by the deck – but able to deflect within predefined limits under hydraulic forces from the Aire, lateral forces due to wind loading and any eccentric vertical loads imposed by the structural columns.

Onwards and upwards

Completion of the foundations lifted the project out of the water and allowed the steelwork assembly phase to get underway. It had two distinct elements: the outboard part over the river – accommodating the lifts, steps and escalators – and the new concourse spanning three electrified lines. Whilst the former could be progressed during normal working hours, the latter required around 30 overnight track possessions and isolations of the OLE.

With the beams for the transfer deck craned into place, sacrificial steel formwork was inserted between them to create another working platform, avoiding the need for temporary works or propping in the river. In addition, two upstand trusses were installed to support the entrance lobby which extends back under one of the arches to connect with a road bridge passing through the piers. It’s from this bridge that passengers have step-free access.

With reinforcement laid on the working platform, the deck’s concrete slab could be cast, allowing a predetermined sequence of structural members, hoops and bracing to be brought to site and positioned, progressing upwards and outwards from the face of the old brickwork.

Question time

Having safely and successfully overcome one major risk – working in the river – Carillion’s construction team was then confronted by another, the project’s interface with the operational railway. This involved widening the station’s footbridge, first removing the glass façade and modifying the existing structure before installing the permanent steelwork and deck which, in its temporary state, was designed to act as a crash deck to protect the railway below.

Key to assembling the new steelwork were two trusses, both two metres deep and more than 20 metres in length. Due to cranage restrictions, the longer one had to be lifted in two parts and temporarily propped. The concourse floor was suspended from these trusses; installing them below floor level proved impossible as there was less than 600mm available to achieve the required headroom over the railway.LeedsStation0413

Establishing support for the trusses’ southern ends was relatively straightforward – by landing columns on the pier extensions. It was a different story, though, at their north end; here we find the only points where there is vertical loading on the 1869 arches. Two columns were needed – one of them new, the other pre- existing from the ‘Leeds 1st’ rebuild. Both land on Platform 15.

Understanding the effect of these columns would obviously be crucial. Prior to work starting, a point-cloud survey had been carried out in the area around the new entrance, from which a 3D Revit model was generated – one of several that were brought together under BIM principles to aid the design work. From this, a number of sections were taken through the quadripartite vault where the brick arch and road bridge intersected. Thrust-line analysis revealed that there was insufficient capacity to carry the extra loads from the trusses, a conclusion verified by Ramboll using finite element analysis as part of a Cat 3 check.

Overcoming this issue necessitated the design and installation of steelwork to provide additional strength. This had two elements to it: vertical support for the quadripartite vault – secured about a metre into the brick piers at springing level using 40mm-diameter Cintec anchors – and horizontal propping in the adjacent spans to spread the thrust across the piers.

Great divide

With the trusses in place, the supporting beams for the concourse were suspended and sacrificial formwork again used to create a working platform from which the slab would be cast. A temporary screen was then erected which combined with the crash deck to completely separate and protect the three electrified lines below from the construction activities. This meant that the roof supports and concrete deck pouring, as well as installation of the glazing and anodised aluminium cladding from Lakesmere, could all be dealt with during daylight hours. The safety benefits of that don’t need any explanation.

Work was brought to an end prior to Christmas 2015 with fitment of the lifts, escalators and station furniture, together with the side-span footbridges which offer stepped access from the riverbanks. Passengers were invited to use the new entrance from Sunday 3 January. Since then, the crane site and barge loading area have been reinstated; life for the local residents – which was never unduly disturbed – has returned to normality.

Social climbing

What the collaborative efforts of Carillion, Mott MacDonald, Network Rail, AHR and WYCA have delivered here is a distinctive piece of architecture. But it serves a very practical purpose too, relieving congestion around the original concourse by drawing an estimated 20,000 passengers a day in the direction they really want to go. That could save regular commuters 50 minutes per week through shorter walking distances. Surely that’s priceless. And heading that way are the running costs – much reduced through a clear design emphasis on material optimisation and making future maintenance needs easier to fulfil.

But we shouldn’t overlook the bigger picture. Leeds is now thriving – somewhat against the odds – having skilfully plotted an exit route from several decades of social and industrial decline. The city’s landscape is unrecognisable, but the only way is up. The station’s striking new southern entrance helps to maintain that trajectory, just as the efforts of Harrison and Hodgson did back in the 1860s.