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Electric challenge for north London line

GOBLI nnwest

To call current works on London’s Gospel Oak to Barking Line a ’mere electrification’ would be gross understatement.

“Installing and hooking up the wires is the easiest bit,” Network rail senior project manager Tim Galvani says.

Network Rail is a few months into the £133M project on the 12-station line. The ‘electrification’ portion is only about £35M, Galvani says.

Four sections of track are to be lowered, four bridges will be rebuilt and a further six modernised.

The critical issue is the crowded nature of the line’s surroundings. The two-line corridor is far from roomy to begin with. Then there is also the houses backed right up against the Victorian retaining walls, with single lane bridges zigzagging overhead.

Then there are the trains. Bombadier diesel 172s used on the line have a theoretical top speed of 100mph. But just five years ago the state of the track was so poor a temporary speed restriction was set at 5mph. There have been widespread reports of overcrowding on the line, especially during a period when the nearby Victoria underground line was closed.

Nearby residents have even complained in the past that vibrations from the trains have shook and caused damage to their houses.

So it is no suprise to learn the electrification proposal has been around since at least 2008, when the British government deemed it too complex and expensive a project. In 2012 it got the green light, with £25M arriving from Transort for London and £108M from the Department for Transport.

“It’s been a long time coming – I think people are just happy to see things moving forward. I think originally the perception was a long blockade would provide negative tension, but people are realising, we don’t want a band-aid approach.”

“Our community engagement team have got their work cut out for them, keeping all the residents informed and happy,” Galvani says.

GOBLI ntracklower

GOBLI ntracklower

Where the track will be lowered along the Gospel Oak to Barking line.

Network Rail are principal contractor, Murphys are main contractor, which is includes track, civils, signals and telecom, and bridges. An Amey and Inabensa joint venture are handling the track design and introducing the 25kv power supply to the line.

New Civil Engineer is invited to the longest site of track lowering: a 1,600m long section running from near Blackhorse Road Station to Walthamstow Queens Road station. The entire stretch is to be lowered 500mm.

“There are 17 overhead structures in this one mile stretch. So it was a choice of lift 17 structures or lower the track,” says Network Rail lead engineer Neil Hamilton.

“You can see from here, it actually looks like a tunnel, there are that many bridges in quick succession,” says Galvani.

“You’ve got the Thames Water services, overhead wires, and height of the bridges. In front of us here you’ve got this steelwork lattice, which will be interfacing with a 25kV electrification system.”

So the decision to lower the track was taken. Accordingly, Victorian-era retaining walls either side of the track then had to be strengthened.

GOBLI npiling

GOBLI npiling

Early piling works were completed by Aspin Group

In one interesting section, inverted concrete arches were installed under the track for strengthening in 1958. Each beam is 600mm wide, there is 13 of them and they measure 12m from wall to wall. Crews have affectionately nicknamed it ‘The Viking Ship’.

To further strengthen this section, 100 cubic metres of concrete were poured into 24 bays to provide the additional strength needed to the existing retaining walls.

Elsewhere, the tiny clearances under and above the rail line become critical, and sections of slab track are required.

Engineers opted for ÖBB-Porr slab track system from Rhomberg emerged from Austria in 1992, but has made its way to Britain in recent years, most recently used in Queen Street station.

“The Victoria Avenue Bridge, has a Victorian sewer under the track. It literally runs 200mm below rail level – not even below sleeper level, below rail level,” says Hamilton. ”We were unable to divert that, so we had to lower the track by about 160mm there, we’ve only got about 40mm to play with, and that’s why we’ve had to go for that higher track fixity.”

As the team have moved through the Victorian-era infrastructure, a few surprises have emerged.

“There’s an 18 inch sewer that runs under Walthamstow High St we didn’t know about,” says Hamilton.

GOBLI nviking

GOBLI nviking

Source: Network Rail

A before-and-after look at the ‘Viking Ship’ — concrete inverted arches, installed in the 50s and recently infilled with concrete to support the Victorian-era retaining walls.

“Then there’s one Victorian sewer we did know about that runs from old sidings, back in the 50s… and we don’t know about the inlets that are coming in to that, so as we move along the site we’re almost having to redesign many of the Thames Water existing services.”

In such a tight and busy corridor, to remove some uncertainty form the project the team is utilising 4D project management software, which shows plant movements over time and prevents conflicts.

“We’ve developed something that’s used a lot in the construction industry, but not so much in rail,” Galvani says.

“We spent a lot of money in the early stages with site investigation. To remove as much of that uncertainty as possible, but there’s always going to be things you can’t plan for.”

As the track lowering runs through stations, naturally the station and its assets need to be lowered as well.

At Walthamstow Queens Road station Galvani explains: “So we’re underpinning the footbridge so that it is fully supported at the new platform levels. And we’re lowering the formation, the ground level, of the platforms themselves – platform edges, tactiles. Along with new telecoms, new lighting.”

To hold up the new overhead line masts there are 483 piles for the project, steel tubular and 610mm diameter.

Goblinblackhorse

Goblinblackhorse

Crews pour the concrete slab – the base for the Rhomberg slab track.

Another elements to note is the troughing, about 9.65km worth, much of it asbestos, requiring the usual special treatment.

Thankfully there’s only two signals that needed to be moved, and one new banner repeater (a pre-signal, to warn a driver on a blind bend that they’re approaching a signal).

“Also, lot of the overhead line masts were designed in such a way that they were built further away from the running edge, so they didn’t create any problems with our signal sighting,” Galvani says. The distance was pushed out from 1650mm to 2500mm.

“It’s safety by design – installing things a bit further away, and a bit bigger. It’s more expensive but better in the long term.”

Elsewhere on the line are Victorian brick arch viaducts, up to 5m high, and a tunnel at Crouch Hill, while at Walthamstow Queens Road is a deep cutting.

“We could have torn down these retaining walls and rebuilt them, at a large amount of money,” Hamilton says.

The sections of ballast go on a geotextile layer and 100mm of sand, to help maintain the integrity of the ballast from clay pushed up from underneath.

GOBLI nseast jpg

GOBLI nseast jpg

The view heading south east from Walthamstow Queens Road Station.

In life expectancy, before any maintenance is required, ballast should last 30-35 years. The slab track should be 60 to 80 years.

“From a quality control point of view, (slab track) is done in the factory, it’s quicker to install, less maintenance, it lasts longer. But it is four-and-a-half times the cost. High speed rail is all slab track, but here, on a commuter route, it’s not justified, “ Galvani says.

The slab track “lego blocks” as Hamilton calls them, (5t, 3m x 5.6m) are lifted in, placed and grouted on to the pre-poured concrete slab. “In terms of whole-life cost, I believe slab track is the way to go,” says Hamilton. “I like ballast, because I can move it. But if you put it in the right place to begin with, you don’t have to move it around. And get people out on to the track in potential danger.”

The project’s labour cost is in excess of 1.2M man hours. “Which is huge obviously. What we’ve done to get that number down is do as much prefabricating as possible,” Galvani says.

The extent of the work has demanded a “phased” blockade beginning in June this year, with a full closure from September 24 until early February 2017. Work is currently going on in some sections 24 hours a day, intermittently. To avoid conflict with commuter services engineering trains are often coming in at night.

“We had to weigh up what was the best, easiest and cheapest option, to do in the timeframe we’ve got. Initially this was going to be a two-year project working every weekend,” says Hamilton.

GOBLI nnwest

GOBLI nnwest

Looking northwest along the line, past Walthamstow Queens Road Station.

 

The current two-car diesel units will be changed to four-car electric units from 2018.

“A lot of the stations were originally 12-car back in the 50s, which then got shortened under the Silverlink [operator 1997 - 2007] days ,” Galvani says.

To improve on frequency – to get more than four trains an hour – Network rail are considering improved signalling for Control Period 6, 2019 to 2024.

Test trains will run from the end of March, then the infrastructure for the electric trains is expected to be complete by end of June 2017.

Source: Network Rail

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