At the rail engineer we like to celebrate success. So we are pleased to report on the Bishop’s Grange underbridge reconstruction, winner of Civil Engineering Achievement of the Year at this year’s National Rail Awards on 15 September.
First an admission though – when the shortlist was announced, no-one on the editorial team had heard of the project. A bit of research showed why. Underbridge redeck – minor road – one out, one in – bit of a skew – heavy load transporters. It all seemed to be bread-and-butter stuff.
But that’s where this project, or rather this project team, did something a bit special – challenging assumptions and pushing the boundaries.
Rather than take the proffered concept design, contractor May Gurney’s tender submission proposed an alternative, more cost-effective structural form. And in the drive towards the 24/7 railway, they have staked a claim for the fastest mainline bridge reconstruction on the London North East region.
Between Durham and Chester-le-Street, the East Coast Main Line skirts the exquisitely-named village of Pity Me. An unclassified minor road heads north and passes below the twin-track electrified railway close to Bishop’s Grange.
The rail underbridge, a half-through girder design with trough decking, had a high skew of 62 degrees and was life-expired. And, minor road or not, replacing the bridge deck required a closure of the 115mph main line.
Pressure from the train operating companies put a maximum of 43 hours on the available shut-down. The scheme was tendered on this basis in late 2009 including a pre-booked possession for January 2011.
An agreed Form A preliminary design for a western region-style box-girder was provided to suit the high skew, requiring 18m-long girders despite a road width of only 7.5m.
Rather than simply price-up a conforming bid, May Gurney took the initiative to develop an alternative submission in addition. This “non-compliant” response proposed a design change to a U- deck type, offering lower cost, reduced construction depth and improved maintenance by designing-out the confined spaces associated with box girders.
Each U-deck carries a single track, allowing the construction to be shallower than the wider conforming two-track design. In turn, this allowed a standard headroom of 5.03m to be provided over the road by planing off just 200mm from the high side.
However, the U-deck cannot cope with such a high skew. Working with their designers Pell Frischmann, May Gurney’s tender proposed a 55 degree skew by intentionally making the bearings non-parallel with the abutment. This allowed the deck to have a smaller skew than the road with the mismatch accommodated by a wider cill beam.
In the spirit that epitomised the project, once the contract was awarded on 8 December 2009, May Gurney revisited the design to see if it could be done even better. Further work brought the deck skew back to 50 degrees, now at the upper limit, but within the range, of Network Rail’s standard underbridge details.
While the alternative design still required a new Form A to be produced and agreed, using the standard details removed much of the programme risk for gaining those approvals, a significant advantage.
Steve Richardson, May Gurney’s project manager, says:
“We were extremely conscious of the deadline of the pre-booked possession, so we worked closely with Network Rail to look at timescales and design parameters.”
Design to build
Heavy load transporters from ALE were fundamental to the construction method. Critically, these allowed the entire replacement to be carried out below the isolated overhead line equipment which remained in place throughout. They also removed any risk of high winds delaying the programme.
There have been 8-hour bridge replacements before, but they have tended to be of smaller, square structures on less critical routes.
“Forty three hours is exceptionally tight for a bridge of this kind”, says Steve Richardson.
“We looked at time-saving in everything in the design”.
As much as possible of the revetment walls were left insitu behind the abutments, reducing the amount of new ballast retention and backfill needed. Just a 20mm gap each side was allowed to squeeze the new deck in. With these tight tolerances, a little strategic adjustment with a lump hammer was needed on the night!
The cill beams were connected to the deck and driven in as one unit, making neat use of the anti-uplift devices at the bearings for the attachment. Temporary tie-downs were positioned to avoid conflicting with the waterproofing and track, making their removal a non-critical event later in the sequence.
Further time was saved by doing the ‘drive out’ and the ‘drive in’ with ballast in place, both as the old deck was removed and with the first layer of ballast already installed on the new deck. The high, narrow embankments made this a major consideration since they prevented the normal lay-out of ballast on the cess.
Over 200t of stone was needed to reballast the deck trackwork, with additional road-rail machines being used to transport bagged ballast from the nearest access points up to a mile away.
Network Rail played a critical role in the delivery too. This included the key enabling work of slewing the lineside cabling onto a temporary services bridge.
Operations staff tracked the progress of the last trains due over the structure from the day prior to the possession, allowing the electrical isolation to be taken two hours in advance of the possession rather than after commencement.
Finally, with the area handed back six hours early by May Gurney, Network Rail’s maintenance team installed the track and tested the signalling to allow the railway to be brought back into service.
The award judges commented:
“The winning entry truly demonstrated excellence in both design and execution”.
Although May Gurney’s contract value was a fairly modest £910k, the short timescales of the design phase and possession required a high level of attention to detail which paid off with a successful project. Everyone involved with the project was pleased with the way it had gone.
“The plaudits May Gurney and Network Rail are receiving for designing, planning and the implementation of this project are well deserved,” commented Kirk Taylor, Managing Director of Stobart Rail who themselves had 17 pieces of plant and 40 men contributing to the successful delivery of the scheme.
The team has set what is believed to be a new record for a main-line reconstruction. But today’s record is tomorrow’s target – and the train operators will surely soon be questioning whether 43 hours, or better, can be achieved not just this time, but every time.
So perhaps in a year or two, our editorial team will be justified in overlooking a 43-hour rebuild as the norm.