Just before the 23 June this year, the BBC issued the following dire warnings:

“Motorists are being warned about road closures and possible delays ahead of a rock concert in Hertfordshire. Up to 80,000 people are expected to attend the Red Hot Chili Peppers’ concert at Knebworth Park. Police said there will be a number of road closures………….”

So what?

Well, Hertfordshire’s ill fortune was a blessing for Network Rail’s Hitchin flyover project. The 23 of June had been the critical ECML possession. It had been booked for months. Complete four-line blockages don’t grow on trees. But then it was put back a week because of “that pop concert”.

The irony is that if the project had kept to the 23 June possession, the weather would have prevented a major crane lift and the project would have been set back by months!


But enough of this spoiler. It’s time for a recap on what is going on at Hitchin – and on some of the history. Hitchin station is where the line to Cambridge branches off from the East Coast Main Line (ECML). At this point the ECML has four tracks, with the slow lines at each side. Being a flat junction has meant that any train departing for Cambridge from the Down platform has to cross all of the tracks to get to the branch. So this stops traffic on the Down fast, the Up fast and the Up slow (from the North).

It has been a major operating constraint for years – for many decades in fact. The situation had been a classic case of “something needing to be done”. But there was never the money. In the 1980s, a flyunder was proposed and land was set aside – but nothing happened. It was forever a pipe dream.

Then, in 1998, a pre-feasibility study was commissioned which identified a whole raft of options with flyovers proposed to the north or south of the station. Inevitably, some of these options were gun fodder, allowing attention to be focused on the most sensible and acceptable solution which was a flyover just to the north of the station.

Forging alliances

Originally a design-and-build contract was to have been let under normal competitive tendering, but part-way through the process Network Rail decided to embark on an alliancing process in line with their change in supplier/client relationships.

Consequently, during the latter stages of the tender the focus went from price and programme to compatibility and collaborative working. There were a number of post-tender workshops where contractors – still in competitive tender – were invited up to York to meet with their counterparts in the client team. Nick Hilton, Network Rail’s senior project manager, explains, “From the ECML point of view this project is trail blazing the alliancing concept. It was the first one to be put together. We have done all the analysis of the alliance concept looking at how the cost model works and how the relationship and behavioural models work.”

At the end of the tendering process, the contract was awarded to HOCHTIEF. The day after a letter of intent had been issued, on 27 September 2011, the Hitchin Alliance arrived on site mobilising to a temporary office. They had been encouraged to put together an integrated collaborative team as soon as possible and so had gone out and done some research around the area and identified an office which was available. They remained there for 6 months, although Nick admits that “it felt longer than that!”

Making preparations

By the time Nick and Hochtief’s Julian Spiller were involved, the scheme had already been through the Transport and Works Act (TWA) process to an initial design by Arup. The line had been fixed, the levels had been fixed and, whilst the scope of the embankments, viaducts and land-take were tightly specified, there remained opportunities for the alliance to explore value engineering opportunities and economy in delivery.

Nick remembers: “We started by going through the final stages of the planning consents which had to be satisfied prior to getting access to the site. So there was close liaison with the local authority to establish the items that needed to be addressed. The first of those was the construction of a temporary road junction with the new site compound, but the local authority allowed site clearance, ground investigations, ecology and topographical surveys, all in advance of the junction works.”

The good relationships fostered with the local authority allowed the other major early item of work to be constructed – the construction of the haul route through the entire length of the site. The 2.5km haul road is necessary because no formal access is afforded elsewhere on the scheme. It sits to the side of the new railway and has to be removed at completion of the works.

Hitchin pier

With the preliminaries out of the way it was possible to focus on piers 17 and 18 (of 30) either side of the ECML. In fact, the sequence of beam lifting meant that pier 16, two spans away from the railway, was needed as well. This was completed early in the programme because the first steelwork lift was to erect the back-span ready to accept the main span in the critical possession. But more explanation of the erection sequence later…

All the piers are on piled foundations. The substrate is mainly chalk but the quality diminishes quite markedly and there’s quite a lot of soft material. As a result, piles are over 20 metres deep, and that applies to both sides of the railway.

All the viaduct steelwork is fabricated and erected by Mabey Bridge in Chepstow. The critical main span was trial erected in their works, with all the splices checked. It was originally envisaged that this span was going to be a single drop-in structure with two ‘air’ splices. As Sarah Cooper, senior engineer of designers Tony Gee and Partners (TGP) explains, “Through conversations with Mabey and HOCHTIEF, it was concluded that trying to achieve two splices in mid-air in such a short possession was just too risky. The design was changed to a single splice with a free end that lands on temporary bearings without having critical location issues.”

The span is curved, skewed, hogged and pre-cambered. It’s a real challenge for structural analysis made possible by the use of a LUSAS software analysis package.

“We bought the hill”

Asked where the fill for the embankments is coming from Julian looked out of the site office window and gestured, “Over there!” He was looking at a particularly bald Hertfordshire field.

“We’ve bought the contents of the hill off the landowner so that we can extract the chalk and then reinstate the hill back to farmland afterwards – with ‘sympathetic softening’ of the profile.” The benefit is that this removes the need to bring all the embankment fill in by road. 120,000 cubic metres equate to about 25,000 lorry movements.

Bottom ballast still has to come in by road, unfortunately. As part of the feasibility study, getting rail borne ballast in was found to be untenable logistically. But the top ballast will be delivered by rail.

Overall there is about 2.5km of new railway with 1.5km embankment and 1km of viaduct. Subcontractor Volker Rail will install the track and overhead line equipment, with Atkins doing the rail systems design.

Either side of the railway, although the design of the viaduct is generally similar, the sites are distinctly different. The northern half (13-spans) is predominantly in an open green/brown field site; whereas the southern viaduct construction (15 spans) is in a very constrained area between the existing ECML and adjacent landowners and natural features.

At the very southern end, where the new chord diverges from the ECML, there is a section of reinforced earth structure incorporating part of the old redundant Bedford Line.

And so back to that weekend and the events leading up to it….

Beams and splices

The main beams were delivered in the preceding two weeks to be gradually assembled at the track side. The 1200 tonne Sarens crane mobilised on the Tuesday evening prior to the weekend and was built up over the next three days.

In parallel, the team had spent considerable time and effort in jacking and adjusting the free cantilevered end of the receiving beams, to ensure best possible fit when the “possession” beams were offered up to form the splice.

By Friday evening the beams were there, the falsework was fixed and signed off, the cranes were in, certificates were all in order and everything was ready to go. By the time the EMJ permanent formwork panel and the cantilever falsework had been secured the total weight of the span was about 300 tonnes.

The connection to the lifting eyes lifted the span on a horizontal flat plane, but the pre-installed back-span wasn’t horizontal. On Saturday there was a trial lift to trim the beam in two directions to get the best fit.

But then there was the weather. The wind was monitored all week. The lift – not the crane – would become unsafe at a wind speed of more than 16mph as the span would act as a sail. The forecast was showing gusts of 20mph every day through to the week after the possession.

Strong winds and heavy rain

That continued right up to the Saturday. It was still showing 20 – 25mph gusts throughout the night and throughout the next day. On his way to site, Nick was only 5 miles away when he hit a violent rain storm with trees whipping all around. Had he come all this way for nothing?

Down in the briefing room, the atmosphere was tense. The team from Mabey Bridge were asked for their thoughts. They’d just got the latest forecast. Winds 4 to 5 mph steady, gusting to 12mph. Really? Where had that come from?

Within an hour of the possession being taken it was almost dead calm with not a cloud in the sky. Incredibly, the conditions were near perfect for the lift.

Work started by 00:30 and everything was wrapped up around 05:30. The actual bridge lift started at 01:30 and was completed in two hours with the structure landing within 3mm of design. They even had enough time to put a primer coat on the splice.

With the expected adverse weather the team never would have dreamt it was going to happen like that. They had started to discuss contingency arrangements – a new possession, bringing the crane back, talking to the train operators. Everyone had arrived on site fully expecting to stand around for 12 hours before going home with the span sat solidly at the track side.

More to come

With the main span in, there is now a pretty intense programme through to the end of November when the Cambridge junction goes in. At Christmas, the turnout from the main line to the viaduct will allow a track-laying train to enter the site for two weeks of track installation. The layout will be commissioned next June. Driver training takes place thereafter and the line will be brought into full use for the December timetable although it’s probable that it will be used from August onwards.

What about the alliancing arrangements? Julian admits, “This is our first experience of working as a colaborative partner under BS11000. It’s a different way of working for both parties and a massive learning curve bearing in mind the different cultures from which we come.

“What we’ve done is throw the combative behaviours away by working collaboratively. Basically we’re a single organisation rather than running under company lines – which explains the Hitchin Alliance logo. Nobody considers they work for Network Rail or HOCHTIEF – we all work for the Alliance. All the decisions we make are ‘best for project’. And we manage all the risks effectively as a single company.”

Nick adds, “It widens the opportunity for people to learn what each organisation actually does. Experience from this version of alliance will be put in the Network Rail knowledge store to ensure that the model is continuously improved.”

So, in the end the Red Hot Chili Peppers concert was a sell out, a good time was had by all, the traffic was indeed awful – especially for those going home and……. there were heavy rain showers and strong winds.

The Hitchin possession cancellation happened so that all those rail passengers (both of them) could get home, but maybe, just maybe, could it have been because someone in the Alliance wanted to go to the concert?

 Time lapse video here.