The hazards of the January Sales are officially over. This is, after all, the February edition of the rail engineer magazine. But did you emerge unscathed? Or are you right now sitting on a can’t-leave-that-there sofa? Or trying not to open the credit card statement?

At sales time the high-street environment can be decidedly less friendly than going trackside. Everywhere you look are discounts of 50%, 60%, 70%. Two-for-one on new safety boots; offer-ends-today on Bluetooth hands-free sets; clearance of assorted left-hand-thread Whitworth nuts – how can you resist the genetic triggers of scarcity and urgency? The shop-windows are excellent displays of the marketers’ techniques.

Like all good ideas, it was only a matter of time before cross-industry fertilisation occurred. Rail engineers working over Christmas to replace the bridge at Sandhills Lane in Liverpool decided to apply these promotional methods. They chose to implement the Buy-One-Get-One-Free scenario, a common and effective sales technique which, however, is rarely presented using its acronym.

During a 103-hour possession, one old bridge was removed and successfully replaced with two new ones. This £3.4 million project was delivered by Birse Rail on behalf of Network Rail. The line was handed back to traffic three hours early.

Re: signalling

The Birse Rail team has a long history with Network Rail’s London North Western route. Their current Civils Framework Agreement is approaching its seventh anniversary. Graham Gallagher, Birse Rail’s project engineer, has been involved with the Framework throughout this time. “Sandhills is one of the most difficult ones we’ve done”, said Graham.

Mark Walker, Birse Rail’s regional framework manager, commented, “The big challenges were the interfaces and planning”. Signalling was foremost among these. The Sandhills IECC signalling centre is just beyond the bridge and controls the Merseyrail Northern and Wirral Lines. Over one hundred cables cross the structure.

“It took six months’ work just to get the cables moved”, said Walker. “On a normal re-deck project we’d start detailed planning a few months before hitting site but on this one we were heavily involved from a year ahead”.

Temporary suspension arrangements or scaffolding bridges are commonly seen on underbridge replacements. However, for this project a pair of prefabricated truss footbridges were provided by Mabey Hire and installed in April 2011. Since these temporary structures were to be in place for more than six months, a full Form A/B permanent works design approval was needed.

The footbridges, by their very nature, came with an integral safe walking route. This was a crucial consideration for maintenance since the diversions would last in some cases for nine to ten months. The walking routes allowed for easy installation of the diversions and also provided a high-level crossing between abutments during the removal of the bridge.

Diversions were carried out by the maintainer, Network Rail, from May to November. Away from the bridge, the cables occupied both cesses and were also routed within the wide six-foot. Few were as straightforward as a simple slew. Many under-track crossings existed and many cables required additional loops to be cut and spliced in.

Unidentified or redundant cables were a key concern. At the outset around 20% were thought to be unused. However, every single one needed to be proved dead and severed or diverted off the bridge deck. The maintainer treated this as an opportunity to rationalise the cabling. It is a credit to the entire team that all connections and disconnections were made without problems; the consequence of a mistake here could have been a shut-down of significant sections of the Merseyrail system.

“It was all about teamwork and regular meetings”, remarked the Network Rail scheme project manager, Dan Wilcock. “With so many different interests involved, we often had 20 to 30 people attending the weekly progress meetings”.

Upstairs downstairs

As well as services on the bridge, services below the bridge presented a problem as the road and footpath were full of utilities. Chris Spragg, Birse Rail’s assistant contracts manager, remembered, “There were high voltage cables and a cast iron gas main. We had a restriction of absolutely no additional load being imposed onto the gas pipe”.

The old deck was built in the days before bearings and the designers thought it could now be acting as a prop between the abutments. Removing the deck could allow the abutments to rotate inwards. To prevent this, 56 ground anchors each 30m long were installed into the abutments.

An earth platform against the wall would be the usual way of getting the drilling rig to the right level for the anchors. Unfortunately this would be directly over the gas main. In this case, trench boxes were used to prevent load being applied to the pipe.

Early on, it was decided to use self-propelled modular transporters for the deck replacement. This avoided the risk that high winds could delay a crane lift. No doubt this assessment was informed by the view westwards from the bridge, where the skyline is dominated by the four spinning turbines of the 10MW Port of Liverpool wind farm.

The transporters supplied and operated by ALE were originally developed for moving heavy rigs for the offshore industry. They are modular and work on the principle that you keep adding wheels until you have sufficient load distribution. Even so, the old deck weighed 450t and mathematics tells us that only an impractical infinite number of wheels would satisfy the restriction on the gas main.

Instead, ALE used 15m-long bridging mats to span the footpath and pipe. Two 14-axle transporters with a total of 112 wheels were used to drive out the old deck. The gross bearing pressure below the wheels was around 75kN/m2.

Highway code

Each axle is individually steerable and adjustable to ensure uniform load distribution. A computer sorts out all this clever stuff under the control of one operator. However, the road crossfalls, gradients and additional bridging mats made this a challenging geometry. At some locations during the drive, axles at either end of the transporter unit were at opposite extremes of the maximum 600mm stroke.

ALE carried out a trial run a week before the main possession. Although a swept-path analysis had been carried out, this checked that there were no unforeseen issues with clearances or street furniture – a traffic island and four street lights had already been removed. It also checked that critical clearances were as expected below one of the cable bridges, which had been set just high enough to allow the replacement decks to be driven in below it.

Any load in excess of 40t being moved on a public highway is classed as an abnormal load, and this one was no exception. ALE submitted a Special Order to the highways authority for each of the moves in order to gain consent from the council and Merseyside Police.

Two new Us

The old deck was built for four tracks but now only carries two, and suffered from a life-expired timber deck. The tracks are pushed to the edges of the bridge by the adjacent island platform at Sandhills station. This gives a wide unused central section: in railway terms, the six-foot; but actually more like 40-foot. The 20m width of the old bridge gave a dark tunnel-like appearance for road users.

Since the station precludes any future additional lines or slews, only the track-carrying sections of the bridge were replaced. Two nearly-identical U-type decks were installed with a wide light-well between them, making a dramatic difference to the feel of the road below. Fabrication was by the Lanarkshire Welding Company.

The decks span 22m between bearings, which is longer than the maximum 20m length for Network Rail’s standard detail. The decks are square, with bearings on wide cill beams which accommodate the 15 degree skew of the decks to the road. The preliminary Form A design was carried out by Tony Gee & Partners. Detailed design was carried out in-house by Network Rail’s civils design team led by Bob Standring.

Standring commented, “This was the first replacement underbridge we’ve designed since the team in Manchester was formed in April 2010”. The standard low-maintenance and low-construction depth features of the U-deck were kept, which allowed the old height restriction of 16’ to be removed.

Deeper webs and thicker flanges were needed to suit the increased length. Bob Standring stated, “Being co-located with the Birse Rail framework team really paid off for this design, given the number of interfaces that had to be considered”.

The decks are 3.3m wide between webs, the maximum for this type. Despite this, horizontal tolerances are extremely tight because of the curvature of the track, the station on one side and pointwork on the other. Standring recalled, “The decks had to be positioned to suit the track alignment. We designed for a 25mm installation tolerance”.

Achieving these tolerances in practice was complicated, again, by the gas main. The decks were driven in with cill beams already attached. Ideally, the twin 8-axle transporters used for the new decks would have been positioned close to the abutments to ease the fine positioning of the 270t load. However, the gas restriction meant the units had to be positioned towards the centre of the road. Obviously this magnifies a small adjustment of the transporter into a larger movement at the deck ends.

“The skill of the operator on the night was absolutely key to achieving the tolerances”, Chris Spragg commented. “In the end, we were within 3 and 6mm on one deck, and 11 and 16mm on the other”. In addition to aligning the cill beams, the clearance at track level was checked using targets on the deck. In particular, checks were made at the critical pinch points generated by the end – and centre-throws of the train vehicles.

Station approach

Another tight area was the interface with the station building. The artistically-curved shape of the ticket hall reflects the £6.7M refurbishment completed on 7 July 2008, and which gained the Station of the Year award at the National Transport Awards in October 2011.

The undersides of the old beams were around 2.5m below rail level due to the depth of the beams. Thus a 3m-deep hole was needed to release them and this meant excavating below the new station building. Owing to the poor ground conditions, the new ticket office had been built on piles and thus could not be undermined. A temporary works check was carried out assuming a shallow soil angle and partial exposure of the piles, but in the event the soil stood at 85 degrees.

In addition to working round the building, signals ML53 and ML55 close to the platforms had to be removed, and later reinstated, to allow for the excavation. The end of the platform ramp also needed to be temporarily removed. Precast L-shaped units, cast by Shay Murtagh, were installed to reinstate the gap between the two new decks, and will ultimately be faced with brick cladding. The civils and permanent way works were carried out by Stobart Rail.

Kirk Taylor, the Managing Director of Stobart Rail commented:

“I am immensely proud of all Stobart Rail employees who gave up their Christmas, to safely and successfully deliver the civils and P-way aspects of the Sandhills Bridge Replacement. A great collaborative achievement, equally shared by all parties involved”.

The station was open for the last 48 hours of the possession, somewhat ironically allowing passengers to buy tickets for the bus replacement service. “Barriers separated the worksite from the platform”, recalled Gallagher, “but we’re not usually used to having the public so close to our works”.

Shopping around

Shoppers like to ”buy one get one free” because they appear to get a good deal. In reality the success of the promotion relies on the price of the ”one” item taking into account that two items are being sold.

However, on this project the two-for-one deal has paid off for all concerned. Certainly the price reflects what has actually been provided. But it also represents an economic solution, providing the minimum square metreage of new bridge while removing unwanted redundant areas, using standard low-maintenance details and modern installation techniques.

The new bridges also have combined walkway and cable routes cantilevering from their sides. The maintainer could be forgiven for wishing for 70% off or two-for-one on the number of diversions – as they start the long task of diverting those one hundred signalling cables back into their permanent homes.