HomeInfrastructureMoses Gate bridge - Best laid plans

Moses Gate bridge – Best laid plans

Hidden by the big-bang schemes that commanded everyone’s attention through August was a routine track realignment at Moses Gate, a district to the south-east of Bolton. The job, which added another piece to the sprawling jigsaw of the Great North Rail Project, involved clearing a short section of the Manchester-Preston route for electrification and 100mph running.

The work had been planned as part of a blockade which shut the southern end of the line from Saturday 12 August to Monday 28 August (a bank holiday), the sharpest focus being on platform, track, signalling and overhead line works at Bolton station.

Don’t hold back

From Farnworth, three-quarters of a mile to the southeast, the railway passes through a cutting on its approach to Moses Gate station. Immediately before the platforms is an overbridge – replaced in 1968 – carrying the main A6053 Bolton Road and part of its junction with the A575. The structure is skewed by 55 degrees and, as a result, extends for some 35 metres.

Until recently, the wedge of land on its south (Down) side between the road and the railway had been retained by a wall 18 metres long. Beyond this, a similar length of slope was battered back and soil-nailed in 2015. Since around that time, water had been recorded coming through both the wall and adjacent bridge abutment, causing track maintenance issues. United Utilities conducted tests to determine the source; these proved inconclusive, but it was reported that no leaks could be found from the water main that crosses the bridge under the pavement on the east side.

Photo: Four by Three.

Nevertheless, this had become enough of a problem for a remediation scheme to be pushed forward as part of the enabling works for electrification. Network Rail contracted the Buckingham Group to deliver it, the firm having previously delivered platform works at Moses Gate.

The intention was to remove the Down line, install new drainage and dig out the wet bed associated with the water ingress. Both tracks would be slewed, the Up by 100mm and the Down by 150mm to resolve a gauging issue caused by the retaining wall which, to assist further, would have its brickwork scabbled back over a distance of about five metres. At the station, the platform copings also had to be reset to follow the new track alignment.

Critical path

To ensure the stability of the retaining wall was not affected by the drainage works, excavations were limited to 300mm below sleeper level. In addition, arrangements were put in place to monitor the wall every two hours. On the first Saturday afternoon – with good progress being made – signs of movement were recorded. This became significant over a six-hour period, with the wall being pushed towards the track by 140mm. The southernmost section of bridge parapet cracked and slipped, prompting its removal to prevent any risk of it falling onto those below. But at eight o’clock that evening, the decision was taken to withdraw the workforce on safety grounds. Whilst this was the only tenable option, it came with the potential for repercussions at other sites along the line.

During the following Monday night, the blockade plan required six engineering trains to pass through Moses Gate on the Down line to service Amey Sersa S&C renewals in Bolton. Fortunately, no more movement of the retaining wall was recorded in the 24 hours after work was suspended; this allowed ballast to be tipped at the toe – thus resisting any further movement – whilst some of the brickwork defects were stitched and grouted. Thereafter, the Down line was relaid and slewed into the six-foot by 160mm in order to maintain the correct gauge. The trains passed through safely at 5mph.

By this stage, it was already clear that the retaining wall would have to be removed so the process of designing a solution got underway immediately. Plans started to emerge during Monday 14th August, led by consultants Tony Gee and Partners from their offices in Manchester and Hong Kong, an approach that ensured continuity of effort around the clock.

Photo: Four by Three.

On the morning of Thursday 17th, the site team emailed United Utilities to express concern at the volume of water coming from the abutment. An initial requirement was to excavate material from behind its southeast corner; as part of this process, the buried services crossing the bridge had to be exposed. With the tarmac removed, progress was made using a Vac-Ex suction system. At around 16:45, a water main – passing a couple of metres from where the work was taking place – burst suddenly.

Full flow

Within half-an-hour, 140 metres of railway looked more like a canal. The influx of water from the lower part of the abutment was now considerable, whilst significant quantities were also being discharged from weep holes cored by the team through the retaining wall to relieve the pressure. As new paths were created, longitudinal fractures opened in the brick and stonework; a movement of 240mm was recorded and a large bulge developed. Mud was deposited on the track, washed out from behind the wall where a void was created.

The main road had to be closed and local residents were left without water until the early hours of the following morning. Although the supply to the main was quickly turned off, difficulties with a valve meant the flow did not fully stop until Saturday afternoon – to address this, pumping equipment had to be brought in.

Despite the depth of the inundation, the water had gone from the railway within 24 hours – testament to the efficacy of the new drainage. However it had caused the tracks to lift, meaning they would have to be re-laid before a 100mph train service could be introduced.

Now what?

The dismantling of the retaining wall continued in tandem with the excavation at the southeast corner of the bridge abutment. The team’s expectation – based on the construction drawings – was that concrete fill had been poured behind the abutment wall – a four-feet thick masonry structure – to help support the deck slab. However, it became increasingly apparent that there was no concrete: the load was largely being carried by the stonework.

Photo: Four by Three.

Although investigations revealed the abutment’s condition elsewhere to be generally good, this discovery added a substantial new element to the design and site works – the requirement to stabilise that corner of the bridge. In all likelihood, it was this that pushed the programme beyond the blockade’s end-date of Monday 28 August, partly resulting from the need to erect a trestle – blocking the Down line – to temporarily prop the deck. Recovery from the burst water main was well in hand and could probably have been finished on time.

Moving forward

Whilst the rest of us were enjoying an uncharacteristically pleasant August Bank Holiday Monday, staff from Network Rail and Buckingham were getting to grips with the abutment reconstruction. Having dismantled a section of the masonry wall approximately four metres in length, formwork was assembled in advance of the following day’s first concrete delivery from Hanson. Once poured and cured, the process was repeated, building up in three lifts of 1.5 metres to meet the bridge deck. By the end of the week, this task was complete.

To replace the retaining wall, five precast concrete cylinders, 2.5 metres in diameter, were assembled in an excavation at the toe of the cutting slope and filled with concrete. A sixth may be installed as part of the permanent design, which is still being developed. The railway was then cleared, allowing Buckingham to restore and tamp the tracks on the new 100mph alignment.

Services between Manchester and Bolton resumed on Wednesday 6 September, albeit with a 20mph speed restriction imposed through the site. This was soon raised to 50mph, first on the Up line – on 7 September – and on the Down the following day. The tracks will need to be renewed before 100mph running can be introduced. Trains started serving Moses Gate station again on Monday 11 September after Story Contracting had attended to reinstate the copings and platform surfaces.

Photo: Four by Three.

All together now

Work continues on site, but the immediate priority – getting passengers on the move again – has been accomplished. The water main is being diverted to the other side of the bridge, but this is complicated by the shallow depth of cover available. Partly as a result, it cannot yet be confirmed when the highway will be reopened for vehicular use. Provision for pedestrian access has been in place throughout. Obviously, the disruption impacts both on locals and those for whom the main road was regularly travelled. Ongoing dialogue with the Council ensures all parties share a common purpose; there is a clear intent to get the job done as quickly as possible.

But this is the nature of unpredictable events – they pose challenges that are rarely quick to resolve: design, procurement, logistics, manpower. All these have tested the team from Network Rail and Buckingham as an everyday task escalated first into a local difficulty, then an emergency situation. They were helped out by the staff from Story, who would otherwise have been dealing with the platform copings, and a UPAC piling team, redeployed from a site near Farnworth Tunnel. More hands make lighter work. As is so often the case, the railway really comes together when it’s up against it.

Thanks to Olivia Boland, Network Rail’s scheme project manager, for her help with this article.

Written by Graeme Bickerdike


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