By Phil Marsh

The inside story of the Severn Tunnel electrification project courtesy of Network Rail

Published: 9th October 2016

Britain’s longest rail tunnel provides problem-solving engineering challenges to team!

Britain’s longest rail tunnel provides problem-solving engineering challenges to electrification team!

Network Rail has been carrying out preparatory electrification works inside the four and a half mile long Severn Tunnel for the last year carrying out as much enabling work as possible. This was to ensure the 40-day blockade of Britain’s longest rail tunnel between 12 September and 21 October goes to plan.

See for full details .

Thanks to Network Rail, was granted full access to the ongoing project at the halfway stage.

The blockade is a 24/7 operation with 450 staff working two ten-hour shifts every 24 hours totalling 18,000 man-days worked by the end of the blockade to install the overhead conductor rail system alongside track enhancement work. The Network Rail management team is an additional 90 people working over 6 weeks 24/7 with support drawn from across The Greater West electrification programme and Crossrail. The STAD team comprises Network Rail, ABC Electrification formed by AMCO, Arup, Aecom, Babcock and ISS.

The Severn Tunnel conductor rail system, tested at the Old Dalby test track, has been designed and installed to be able to accept trains running at 125mph along should the opportunity arise. Currently, Severn Tunnel has a linespeed of 90mph as do the adjacent Patchway tunnels on the English side of the route to South Wales.

Transferable workforce skills

The Severn Tunnel electrification workforce has been recruited from former mining and steel industries alongside former armed forces personnel and University graduates on the line of route with apprentices also engaged.

Network Rail Wales has worked with Coleg Y Cymoedd at Nantgarw to open an ‘electrification school’ with an indoor electrification training room

with full scale Series One wired overhead line used on the open route and the Rigid Overhead Conductor Rail System used in tunnels.

The blockade has also enabled other electrification works to be carried out– but not so much as to jeopardise the planned handback dates.

The blockade has also included open route OLE and 3 track lowering projects to ensure gauge clearance for the overhead electrification. The two Patchway tunnels have been track lowered to allow conductor rail to be installed in the tunnel roof. These tunnels have provided the opportunity for the project team to demonstrate their problem-solving prowess.

Draining challenges

They found some surprises in the Patchway down tunnel in the form of drainage sumps which were not in the historical records, plus a void behind a section of the tunnel wall and unexpected buried cross-bracing under the track.

This brought an early slippage in the first 10 days of the work programme but this had been recovered by the half-way point in the project. This was attributed to “some agile thinking and phenomenal teamwork” said the Network Rail Project director Dan Tipper, who added that the tunnel monitoring by Aecom had been enhanced after these challenges had emerged.

Severn Tunnel Flyer

In the Severn Tunnel, supplies for the work and workforce are taken to the worksites by three Road-Rail Vehicles (RRVs) hauling trailers which also ferry staff to and from work – ‘The Severn Tunnel Flyer’! One of the trailers even carries a mobile toilet and buffet for everyone’s convenience!! The workforce uses mini-diggers and mobile elevated platforms in the tunnel all equipped with suitable lighting.

The scale of the work in the Severn Tunnel is enormous with 7000 Auto Transformer Feeders (ATF) anchors being installed with associated cleats and cable clamps. The innovative 14kms of overhead conductor rail will be supported by 1648 drop tubes. The workforce is installing over 20 conductor rail anchors and 240 metres of conductor rail, (in 12 metre lengths) per

shift and during the visit it was clear to see how fast the teams were working.

The conductor rails are made from extruded aluminium to give a long life against corrosion and also to reduce the weight hanging from the tunnel roof.

To make sure that progress is as safe and fast as possible, bespoke plant and machinery has been developed by AMCO. For example, the ATF drilling rig has three 600mm drill bits each costing £1500, which operate simultaneously taking one minute for each pass. The drilling has used 35 of these and a vacuum extractor is attached to them to minimise dust and pollution to the workforce in the tunnel while in use.

To be continued elsewhere

The drilling rigs will next be used in Chipping Sodbury and Box Tunnels plus the Newport ‘Old’ tunnel housing the relief lines just to the west of the Newport station.

The Severn tunnel is a hostile place with soot and grime inches thick in places plus many very wet areas with water running down the walls at places. It was easy to see where the soot scrubber had been used on the roof removing 40 tons of 80 years of steam age residue before the blockade during a number of 52 hour possessions.

The last sections of conductor rail will be installed after the redundant NRN radio system aerials have been removed, but the 7000 metres of NRN cable has already been removed from the tunnel. When the last sections of conductor rail are installed, the local GSMR will be switched off so careful planning has taken place not to disrupt the live railway when this


Pride and joy in project team

The project team were asked what the best part of the blockade has been so far. The reply by Anthea Dolman-Gair and Dan Tipper on behalf of the team was that the team had an immense amount of pride in their work, the ability to solve problems by teamwork and innovation. The success of the blockade was put down to careful and detailed planning, the design, development, careful recruitment and surmounting the anticipated and emerging engineering challenges.

Half-way through the blockade, despite the early setbacks, everything was on schedule and no negative comments had been received from passengers or the public about the works.

Historical engineering and safety envy?

Brunel and his successors would be proud of the achievements made 130 years after the tunnel opened for traffic. Not only in the physical works but in the safety regime in place to protect everyone, it is testament to the impressive detailed planning that has so obviously gone into the blockade.

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