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Wednesday, April 21, 2021

The big move

Moving out of the armchair on Boxing Day is usually difficult enough. It can take serious preparation and determination simply to fetch a top-up to go with the plate of leftover meat and stuffing. And the mere thought of shifting from in front of Eastenders is enough to bring out a cold sweat. So spare a thought for the engineers at Selly Oak who were contemplating a move in a different league – a 101-hour Christmas possession for the heaviest bridge move ever undertaken by modular transporters in the UK.

Perfect prefab

Civil engineers have been building things in one place and shifting them to another for years. This isn’t just because they keep getting the setting-out wrong – off-site construction is the current buzzword (it used to be ‘prefabrication’ until that got a bad name). A particular advantage for rail projects is speed of construction and, in this case, it was simply impossible to build the structure in situ at its intended location.

The Selly Oak New Road is a key part of Birmingham City Council’s plans to regenerate the area. Although only 1.5km long, the 10m wide single carriageway road is intended to relieve congestion in the heart of Selly Oak and link key facilities including a new hospital, a science park and the Birmingham University site. Its route passes below the BAG1 Birmingham to Gloucester railway which carries the busy ten-minute interval cross-city service and the adjacent Birmingham to Worcester canal.

The original plans called for a tunnel through the 15m-high embankment below both railway and canal. However Birse Rail and Birse Civils jointly tendered with an alternative design for the £17.5 million scheme. In conjunction with bridge designers Tony Gee and Partners, Birse proposed two individual bridges rather than a 100m-long jacked box.

This alternative design had the advantages of reducing the construction programme, mitigating the key risk of working below the railway and improving the appearance compared with adding yet another long, dark, concrete underpass to Birmingham’s road network. Having worked with Birse over many years delivering complex engineering projects, client Birmingham City Council had the foresight and confidence in the company to proceed with such an innovative and challenging ‘design & build’ project.

A fundamental principle of this proposal was to construct the railway bridge off-line and move it into position using self-propelled modular transporter units during a possession. Birse has used this technique on other projects, such as the Alderley Edge bridge featured in last February’s issue (64) of the rail engineer and which has other similarities with this project.

Selly Oak bridge move caisson shaft
A caisson shaft for the centre pier foundations

Super structure

A continuous three-span arrangement was developed through discussions with Network Rail and British Waterways to iteratively refine the design. A centre span of 30m and two side spans of 20m allow the embankment to be graded back from the road at a 27 degree slope without the need for any retaining walls.

The main structural strength is in the haunched edge beams, the depth of which varies from 4.14m at the piers to 2.84m at midspan. Due to the reversal of stresses which occurs during transport compared with the permanent condition, heavy reinforcement is needed in equal amounts for hog and sag. Up to 30 B40 reinforcement bars were provided in four layers at the top and bottom of the midspan section. The 800mm thick deck slab within has a slight fall for drainage but is otherwise flat.

The deck was constructed alongside the railway on a flat plateau 5m below rail level over five months from June 2010. This allowed the deck to be built at its final level but from the ground, simplifying construction and removing many of the hazards of working at height. Considerable temporary works were needed to create a plateau alongside the embankment. Reinforced earth walls up to 15m high used an innovative system of tyre bales to form the front face and provide edge protection as the wall was built.

The bridge was cast on temporary pad foundations on the plateau, giving just sufficient clearance to allow transporter units to edge under the main beams in each of the three spans. The principle underlying the transport arrangement is basic: keep adding wheels until the load is sufficiently distributed. Abnormal Load Engineering supplied an adaptable modular system with 12 axles on each of 12 units. The total of 144 axles distributed the overall 4,026-tonne weight of the bridge deck and temporary works to a typical pressure of 90kN/m2 below the wheels, giving a 22% over-capacity on the lift.

The axles are individually steerable through a computer-controlled interface which allows for complex manoeuvres to be undertaken with extremely accurate positioning of the bridge being carried out, even under typical site conditions. All of this flexibility was used during the move – due to the constraints of the site, the bridge could not be built entirely parallel to the railway and was moved 15m north along its length, rotated, then shifted 25m east into its final position. All this took barely a couple of hours, controlled by one operator. Who needs a PlayStation when you can hook up a joystick to one of these?

Selly Oak bridge move site
The bridge move in full swing

Selly supports

Whilst the new deck was whisked into place, the foundations to support it had been waiting invisibly since being installed over the previous nine months. Unlike with a deck replacement, there were no existing abutments to reuse and this created a particular challenge. The bridge is situated at the low point of the valley beside the Bourn Brook and thus at the highest point of the embankment – this means 15m of fill before even reaching natural ground. There is also the difficulty that the instinctive position for the new foundations is directly under the tracks and hence inaccessible, so a bit of lateral thinking was applied.

In a similar manner to the Alderley Edge bridge, the new foundations were created outside the footprint of the bridge deck with the work area fenced off from the railway. This allowed the majority of the works to proceed during normal working hours without interrupting train operations. The abutment bankseat foundations were constructed in 7m deep sheetpile cofferdams, using minipiles topped by a pilecap at each corner of the bridge.

The centre supports were rather more complicated as these needed to be constructed from rail level but would ultimately be exposed as the piers once the embankment was excavated.  Caisson shafts with an internal diameter of 4.5m were formed from interlocking precast units and were sunk to a depth of 22m, from November 2009 to March 2010. The bottom 7m from founding level was filled with concrete to a little below future road level, 15m below the railway. Above this, 2m diameter reinforced concrete columns were cast within the caissons. Bearing plinths were cast on the column heads – and the last 5m to rail level was thin air.

During the possession, an 80m length of embankment was removed down to plateau level in a 9,800m3 muckshift to make way for the new bridge. The upper 5m of the caissons were left standing proud. These empty sections were rapidly dismantled to allow the bearings to be placed on the column heads which were now revealed.

However, the foundations were spaced much wider than the relatively narrow twin-track deck. Crossheads were cast integrally with the bridge deck to span the 15.6m between bearings at the centre piers. At the abutments, the bearings were positioned below the main beams and the bankseats were constructed to allow them to be transported into place along with the deck. A system of temporary steel ties and props secured the bankseat to the deck with the bearings sandwiched in place between.

Trial lift

These 650-tonne bankseats caused a few last-minute headaches. Following good practice, a trial lift was scheduled in advance of the possession. As the transporter units took the load at the centres of the spans, the heavy bankseats became dumbbells causing the centre of the bridge to lift and the ends to droop. Although the actual deflections closely matched the predictions made by the designers, a combination of flexibility and tolerances in the temporary works led the bankseat units to rotate slightly.

This rotation meant that the extremities of the bridge were barely clearing the ground. The deck could not be lifted further as the jacks were reaching the limit of their ±300mm stroke and some tolerance was needed to allow for irregularities in the ground during the move. The problem was resolved by implementing pre-planned contingency measures, including adding 125 tonnes of rail ballast to the centre span and arranging for the transporter route to be raised by an additional 100mm.

The other bridge

One of the advantages of Birse’s alternative scheme was that both rail and canal bridges could be constructed simultaneously, offering a programme saving. The canal bridge has a very similar form and also has three reinforced concrete spans. Of course, it had its own intricacies – not least having to cast the entire canal trough in one 650m3 concrete pour lasting 12 hours to avoid construction joints and ensure water-tightness.

Unlike the rail bridge, the aqueduct was constructed in its final position. However, this required a temporary diversion of the canal through a 3m wide channel adjacent to the railway. Although the two bridges are structurally independent, they were critically linked by the programme. The space occupied by the canal diversion was needed for plant access and to stockpile the embankment spoil during the ‘big dig’.

This area was also needed to divert the signal and telecoms cables during a night-time possession in the week before Christmas and protect them below the working area for the excavators – clearly they could not remain on the embankment when it was removed. In advance, a 40m length of slack was spliced into the cables in a ‘loop bay’ to the south of the bridge.

The overhead lines were detensioned and slewed over to temporary supports during the possession by Border Rail, resulting in a considerable time saving compared with a full dewiring. The bridge was designed to provide OLE supports at identical locations to the previous masts to avoid having to change any of the overhead line arrangements.

Selly Oak bridge
The bridge in action

Keeping watch

So of the 101-hour possession, a percentage in the low single digits was dedicated to the highlight of the actual move. This happened to coincide with the period on Boxing Day afternoon when people were starting to consider taking a brief constitutional – and the adjacent completed canal bridge provided a fantastic observation platform for the activities. At times up to 60 members of the public were gathered to watch the operation.

Communications were noted as a key success on the project and the signed partnering agreement on display at reception in the site office has clearly played its part. Steve Longden, Network Rail’s Programme Manager, commented that “The relationship between the respective project teams has been excellent and has ensured that this prestigious project was successfully delivered to programme during the Christmas period. The end result is a great credit to the staff of Birse Rail/Civils and also demonstrates how Network Rail can not only protect its own operations and infrastructure but also proactively assist the outside party achieve its goals in an efficient and effective manner.”

Modern technology was also used to keep in touch during the possession itself, with webcams providing a minute-by-minute update. With the works proceeding well – final handback was 10 hours 45 minutes early – you could forgive the project managers for ‘checking up’ using a laptop without moving from the comfort of their armchairs. Is that where they were? Not on your life. They were out there with their site teams, making sure that the move was smooth and would be remembered as a job well done.

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