The annual Foot in Mouth Award – yes, there really is such a thing – is bestowed on the public figure who utters the year’s most bewildering sound bite. Writes Graeme Bickerdike
UKIP’s Godfrey Bloom triumphed effortlessly in 2013, offending most of the developing world and all of womankind in just half-a-dozen choice phrases.
An unlikely previous recipient was former US Secretary of Defence Donald Rumsfeld who generally enjoyed a favourable reputation for his media performances. He was, though, occasionally prone to a touch of the John Prescotts, as evidenced by his notorious briefing on Iraq’s weaponry in 2002: “There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that
we know we don’t know. But there are also unknown unknowns. There are things we don’t know we don’t know.” Tortuous perhaps, but those words will resonate with any engineer whose carefully planned works have been undone with spanners thanks to voids in their knowledge, amongst them the team charged with replacing Spring Bank West bridge in Prescott’s political home of Hull.
And with that convoluted preamble, so ends my pitch for the 2014 Award.
Movers and shakers
Opened in July 1885, the snappily- titled Hull Barnsley & West Riding Junction Railway and Dock Company aimed to smash the monopolistic dominance of the North Eastern Railway which controlled every train heading into Hull. Congestion was stifling prosperity and local businessfolk were up in arms about it. The solution, engineered by William Shelford, involved driving a railway through the Wolds to link Yorkshire’s coalfields with a new deep water dock on the Humber’s north bank.
The line crossed Spring Bank West – now a key artery into the city – via the bridge that forms the subject of this story. Set at a skew of 56 degrees to the road, the structure comprised three 39-metre longitudinal girders and a trough deck, with a shallow construction depth of just 650mm. Whilst this ironwork was almost life- expired, two other issues exerted greater influence on the decision to reconstruct the bridge.
Intermediate support for the girders was provided by six cast iron columns – three either side of the main highway which narrowed from dual to single carriageway to squeeze between them. With precious little breathing space when larger vehicles passed, the highest bridge-bash risk ranking of double-red had been attained despite a row of concrete protection units being placed along both kerbs as mitigation. The columns had succumbed to wayward traffic on more than one occasion.
Also identified were problems with the substructure. Corers carrying out investigation work on the abutments had previously described the brickwork as the softest they had ever encountered. The lively nature of the bridge at the deck ends – resulting from the skew and the absence of trimmer beams – was probably aggravated by this weakness. Temporary works, in the form of triangular stiffeners, were successful in reducing the movement, but with docks traffic expected to rise from the current level of around eight trains daily as economic prospects brighten, it was seen as important to provide a longer-term solution, one which must make passive provision for reinstatement of a second track.
Room for manoeuvre
Network Rail appointed J Murphy & Sons as principal contractor for the reconstruction late in 2012, responsible for assembling the specialist team which included Cass Hayward as civils designer and Mabey Bridge to manufacture the deck. Acting to constrain options for the new structure was the S&C of Walton Street Junction, 40 yards to the north on a falling gradient. This brought with it the need to minimise any track lift. Also, the existing soffit level of 4.986 metres above the road had to be maintained, with no scope to drop the roadway due to a surfeit of buried services. As a result the chosen solution involves a single 33 metre clear span with reinforced concrete abutments wrapped around the existing ones, a 450mm track lift and a direct fix of the rails to the steelwork using Pandrol Vipa baseplates. Cantilevered walkways feature on both sides, incorporating cable troughs below their FRP gratings.
Mabey Bridge regards the new deck as amongst the most complex it has ever undertaken. Working 24/7 due to tight timescales, the structure was fabricated at its Chepstow factory and trial-erected there following the development of a critical assembly sequence, encompassing its 315 tonnes of plate steel and 9,272 bolts. It was then stripped down and shipped to Hull where it took seven weeks to build up again. The trimmer to main girder connections demanded that the deck was set out accurately in plan above Shay Murtagh’s precast concrete imposts. Working at height, the unique build process proved challenging, with a bolting-up sequence having to be devised that ensured correct alignment of all the connections.
Corridor of uncertainty
Site mobilisation got underway in May 2013, with the main work scheduled for a 77-hour possession over the August bank holiday. When the project first appeared on the radar four years ago, the land adjacent to the bridge on its south side was just scrub; since then though it had been acquired by Keepmoat for housing development. Fortunately this had not advanced as quickly as anticipated such that building work could be rephased, allowing the area closest to the railway to be occupied by a compound.
An immediate priority was to issue notices for the diversion of services, making way for the piles that would support the new abutments. On the south side of the road, plans and trial pits had indicated the need to move one gas main, electricity and street light cabling, together with a 150mm water pipe. Yorkshire Water believed it also had two 600mm mains running parallel with the columns on either side of them, well out of the way. However Murphy’s own investigations discovered both these large mains to be nearer the abutment, with one of them beneath the 150mm pipe. Due to the density of services, the only way to divert this was to first move the other 600mm main into the road.
As this phase of work concluded, brickwork was exposed which gave every sign of being the abutment toe. This too would have to be removed prior to the piling, but it soon became clear that this was actually the face of a concrete cap hiding another collection of services. Although these were redundant they still had to be taken out, in addition to the actual abutment toe below them.
Compounding the delays brought by these tribulations were difficulties with remedial works to restrain the old abutments, involving the installation of 48 soil nails which were each expected to take around 0.3 tonnes of grout.
The reality proved frustratingly different. Grout loss was so great – particularly at a lower level – that the average quantity needed for the south abutment was 5 tonnes per nail, with a peak of 16 tonnes, whilst the north side nails averaged 5.5 tonnes, peaking at 18 tonnes. The impact was considerable. “We were supposed to install three soil nails a day, but we were only getting in one,” recalled Darryl White, Network Rail’s scheme project manager. “So you can see that for every week we worked, we lost two in the programme.”
Many avenues were explored in an effort to claw back time. Weekend shifts were instituted, as was overnight grout pumping. But, at the end of July, the decision was taken to replan the core works. Traffic management restrictions imposed by the city council effectively ruled out three weekend opportunities in October as they coincided with Hull’s annual fair, one of the biggest in Europe. The next available window was a 56-hour possession over Christmas, extended to 57 hours following discussions with the docks. Spring Bank West would be closed for a two- week period either side of this.
Up against it
Whilst the postponement bought some breathing space and allowed the resumption of normal weekday working, several challenges still remained. Piling, undertaken by Van Elle, was the next critical operation but suffered constraints imposed by the limited headroom under the deck. Here, a smaller rig had to be deployed installing 450mm diameter piles whereas 600mm was possible outside the bridge footprint. A total of 23 piles per abutment were sunk through clay to the chalk below, a depth of around 15 metres. The value of the soil nails was demonstrated by the nuts forming part of their head assembly: having been hand-tight when fitted, they had become absolutely solid after just one day of piling.
Each abutment took two weeks to complete, starting on the north side where space was most restricted. Having constructed the reinforcement cage, PERI’s time-saving modular brace frames were used to support the substantial formwork demanded for the concrete pour, secured in position by Dywidag anchors cast into the pile cap. The system suitably impressed Murphy’s senior site manager Mark Simpson. “If you think about pouring 150m3 [SUPERSCRIPT 3] in one go, you’re talking about a 400-tonne load. This was effectively a freestanding structure and it didn’t move a millimetre.”
Storm before the calm
The p-way realignment – including the 450mm lift and a 500mm slew – extended across the bridge onto both the docks branch and the steeply-graded Walton Street chord. It was delivered by VolkerRail over three summer weekends, as originally planned. Other possessions were secured in the run-up to Christmas to complete tasks such as the installation of back-of-wall drainage and ballast retention. S&T cables were spliced, extended and transferred to a series of scaffold towers on the bridge’s west side, all to save time during the core works which got underway at 19:30 on Christmas Eve, after the last train had passed. Having been stormy over the preceding days, the weather – that ultimate known unknown – was on the team’s side, the wind subsiding sufficiently to remove any doubt about the vital crane operations.
The intention was for four self-propelled modular transporters (SPMT) to jack up and move the old deck to stillages in the compound. To prevent any significant deflection, beams had been welded across its ends and web stiffeners installed prior to the lift. However the deck was found to weigh slightly less than expected – around 200 tonnes – resulting in the outer two transporters not taking any load. As a result, the configuration was changed to only use two, positioned centrally. This resulted in the 200-metre journey lasting an hour longer than anticipated.
As they become part of the furniture, we increasingly take for granted the advantages brought by SPMTs. Controlled here by one man and two joysticks, these four – supplied by ALE – carried the new 500-tonne deck and imposts to their intended resting place in two hours. As they did so, the brickwork of the old abutments was broken down to the level of the new ones and the ground excavated to accommodate a 7.5 metre geogrid ballast transition.
Lunchtime on Christmas Day saw operations start for the Ainscough crane, with 18 lifts to complete comprising precast concrete wing wall units and ballast retention behind the imposts and at the walkway ends. Four run-on/run-off slabs were also positioned to take the skew out of the structure; the holes for their Vipas were drilled in situ to avoid any alignment issues. Baseplates had already been fixed on the deck and brought to approximate level by fitting slave rails. The track was then relaid and tamped across the transitions using an RRV attachment. With an additional shift arranged at Hessle Road box to assist in track circuit testing, the possession was handed back at midnight on Boxing Day, five hours ahead of schedule.
Spring Bank West was reopened on 31 December, a day early thanks to careful planning and five more days of collective exertions, gifting a little New Year cheer to the locals. The old deck was jacked down, cut up and sent for recycling during January.
All’s well that ends well
As it mostly benefits freight services, very few column inches have been devoted to Spring Bank West’s new £4.2 million bridge, the media preferring to focus its attention on things bigger, shinier and more passenger- facing. But this has been a complex project, considerable in scale. That it eventually came together was a function of tenacity, teamwork, hard graft and a willingness to adapt.
To borrow another of Rumsfeld’s proclamations, delivered this time with his feet on the ground: “Amidst all the clutter, beyond all the obstacles, aside from all the static, are the goals set. Put your head down, do the best job possible, let the flak pass, and work towards those goals.” Those involved here did just that, overcoming challenges that far exceeded what was apparent when the go-button was pressed.
Photos courtesy of Murphy