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Track And Cut Undercut

Work is almost complete on road improvements in the Birmingham suburb of Selly Oak. The complex project involved passing beneath a railway line and adjacent canal and brought many challenges. Adrian Greeman reports

The spring sun has been shining on some relaxed smiles around the Selly Oak new road project - asphalt is going down, drainage kerbs are being fitted and bridge columns are getting their finishes.

By the end of summer the second part of a new road system will be open in this Birmingham suburb to help improve access to the new Queen Elizabeth “super hospital” on the hill, and the adjacent Birmingham University campus.

Reaching this point has been a little more tense.

To create the new link, the second phase of a six-phase scheme, a way has had to be made beneath a massive Georgian embankment for the dual two-lane road.

Originally built 200 years ago for the Worcester to Birmingham Navigation the 15m high earthwork was widened later in the 19th century to make a 60m wide platform, fitting the Birmingham & Gloucester Railway alongside, an important connection into the city centre.

Today, the electrified twin track line is also one thread of the several West Coast Main Line routes through the conurbation.

The critical work has involved building two new bridges, for the rail and the canal, before excavating away a 30m wide cutting through the 110m base width of cutting beneath them.

There have been high temporary works extensions to the embankment and diversions for the canal, both making use of tyre bales on a large scale, delicate piling work alongside the live railway with its high voltage overhead lines, caissons, sheet piles and complex concreting.

“We had to keep engines running throughout to avoid diesel freezing”

It all culminated in a tightly scheduled three-day rail possession at Christmas, for the largest pre-made deck move yet done in Europe onto uncertain ground “and during a period of intense cold which broke meteorological records”, says John Daft, the scheme’s project manager for contractor Birse Civils, which is these days part of Balfour Beatty.

The freeze was so severe that Daft says: “We had to keep engines running throughout to avoid diesel freezing, used lots of rock salt, substituted epoxy for cementitious grouts and ‘stabled’ all the operatives locally so they could get to site.”

But that came at the end of a complex sequence of planning and construction.

First was Birse’s devising of the idea for the bridges, which were an alternative to client Birmingham City Council’s original conception of a jacked-in tunnel box, which would have been 100m long 20m wide and 10m high.

“But it would have been very expensive both to ventilate and to maintain and not a pleasant space to go through as a pedestrian,” says Daft.

Lighting would have needed constant attention too, he adds, and it would have required a lot of temporary works.

“Finally, it would have been risky given the distance from the top of the box to the sleeper base above,” adds Daft. “If the ground had moved at all you would have had to stop. Once you do that the ground would relax and starting again would be difficult.”

Birse also suggested that two bridge structures would be much more pleasing and the council agreed.

Network Rail was less enthusiastic but accepted the notion.

An initial £1.5M design contract was awarded and then a follow on £16m construction award, which, with some added in utilities work and inflation allowances, is currently just under £18M.

Separate utilities diversions which Birse managed for the client, cost £14M.

The two bridges, each with a 30m central span and two 20m side spans, would involve different methods.

For the cut it was a matter of making a diversion big enough for mainly recreational canal traffic, while excavating and then building a new reinforced concrete bridge trough on the original line.

Since the canal takes the traditional narrow boats, the diversion channel could fit in between the old line and the tracks.

“The two are close, almost touching at points on the route, but we had about 25m of space here,” says Daft.

“We made a 3m wide channel 1.5m deep which was lined on each side by a row of tyre bales two deep,” says Daft.

Design for this sustainable solution, which cut the project’s CO2 footprint, was done with consultant Pasco.

The channel was made watertight with a conventional pond liner material 2mm thick.

“A critical aspect of the decision to use bales was finding a way to remove them again,” says Daft.

Via the National Industrial Symbiosis Programme, of which Birse is a member, the contractor found a local supplier, Credential Environmental, willing to take the bales away again after use.

“frozen solid so that no boats could use it”

Victorian stop points along the canal were used to empty a section of the canal before sheet pile sides were removed to form wide diversion basins at each end of the bridge section big enough for the boat users to manoeuvre around the bend.

As an incidental Birse rebuilt the original blocking points.

After the new parallel line was refilled, a section could be dewatered for the 72m long bridge which comprises an insitu reinforced concrete trough 1.5m deep and 7.2m wide.

Design for the two bridges was by Tony Gee & Partners for Birse’s design and build contract, while Gifford did the road and other structures.

The piers were formed first as twin rows of four 1.2m piles bored by Bachy Soletanche.

These were recently revealed as top down excavation proceeded. A fair face finishing on the exposed parts is being cast with a patterned formwork.

The official opening ceremony for the new canal connection in February was a significant event for the enthusiasts community, says Daft, “since it is probably the first new canal bridge that has been done for a long time”.

The canal had been ready on 1 December, but was “frozen solid so that no boats could use it”, says Daft.

The rail bridge, meanwhile, involved more difficulties, not least because the line had to be kept open most of the time.

The solution here was to build the bridge piers off centre, one each side of the track with a connecting concrete beam to support the deck.

The beam and the deck were built 30m to one side for a later move in.

To control any movement or settlement near the railway, piers were built inside 4.5m diameter caissons with Macrete precast concrete rings forced 22m into the ground with hydraulic jacks.

“These shafts were filled in the bottom 8m with concrete and then above that we cast 2m diameter piers,” says Daft.

Safety was an issue and screens between the work and the track 3m away were needed to protect from 0.5t swinging loads.

Rules of the route working was required and special measures were also taken, such as replacement of the elevated return current cables along the trackside by insulated wires.

At the abutments Network Rail was insistent that there be piled foundations for the bankseats, says Daft.

These would be a 1m deep slab 5.5m below track level, sitting on six minipiles.

Excavating these was a complex operation.

A sheet piled box was formed with three levels of walings on the inside.

But it was not big enough for the piling rig.

“So we cast a 300m thick slab with holes in it for the mini-piles,” explains Daft. “When that was in, the bottom two layers of support frames could come out which left space enough for the big rig to work.”

Piles were driven to 37m deep with 12m at the bottom keying into the sandstone, which underlies the area.

A top 7m of the piles was cased in steel to help transfer lateral loads.

When those were in and the rig could be removed, the frames were replaced and the temporary slab was knocked away.

“Blinding was excavated and a permanent slab 1m thick was cast at the bottom,” says Daft.

Meanwhile, a reinforced earth extension had been formed to make an additional “plateau” at the top of the embankment.

After the successful canal diversion, this also made use of tyre bales as the facing “blocks” at the front of the wall which ran for 120m and up to 15m high.

Backfill compacted in layers over geofabric totalled 35,000m3.

A small extension to some existing culverts carrying the local and temperamental Bourne Brook was necessary to make the structure.

On this working area the bridge deck, the bankseats and the crossheads could be made ready for the big push scheduled for a minute past midnight on Christmas morning.

The 70m long bridge superstructure, also designed by Tony Gee, was fairly heavy since Network Rail had rejected any voided designs, says Daft. “They are wary of possible future maintenance issues.”

The total move therefore would have to be for a concrete mass of 1600m3, weighing 4026t, which Daft believes is one of the largest moves made in Europe. “The bankseats held onto each end were 650t each themselves,” he says.

The operation was tightly planned with a sequence of power line and track dismantling operations to be done, excavation of the space for the bridge deck and then seating on the prepared piers and foundations.

But despite various rehearsals there were still complexities and uncertainties.

First was the sand gravel and clay ground within the embankment, which was not precisely known.

“No one had lifted a deck this long so there was not real basis to calculate the hogging effect as the jacks came up”

Difficult soggy conditions had been encountered during the southern bankseat foundation excavation, thought to be from canal leakage, and with such a heavy load there was in any case a danger of sticking.

A crushed stone layer with a conservative 450mm thickness was laid as a platform therefore with a geofabric in the middle.

On top, aluminium trackway was unrolled.

The combined load was to be moved with a dozen SPMT hydraulic multi-axle trailers supplied by ALE - four under the centre span and two groups of two for each side span.

The jacks in them were linked in three clusters to give a three-point lift. Things almost went wrong.

A trial lift had been planned for three days before the move to test things, but was delayed because of the SPMTs were held up for a day by iced roads.

With just two days to go there came a heart-stopping moment.

“No one had lifted a deck this long so there was not real basis to calculate the hogging effect as the jacks came up,” says Daft. “We worked out a 175mm hog, but when power was applied the jacks went way beyond that and the bridge ends still did not clear.”

Piling on kentledge in the middle eventually solved the problem.

But more worry followed as the bridge moved into position over the piers and bankseat foundations.

Six bolted connections had to be made at each corner and four more at each pier to fix down the bearings which was carried in on the deck.

“But these had never been all visible at the same time during construction to allow a very precise checking survey,” says Daft.

A slope and a two degree curve on the bridge line did not ease matters.

Fortunately, the bolts all passed through their associated holes and the lift in went well.

Track, signals and power were re-installed with 10 hours to spare at the end.

Excavation under bridges was recently completed and the last cosmetic finishes are being made for the piers.

Elsewhere along the kilometre of new road, work is finishing on a various smaller structures, including an extension to a culvert at a far end.

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