Passengers experience faster and more reliable journeys after £21million Sevenoaks tunnel refurbishment: Sevenoaks Tunnel Refurbishment 2018 (9)

Wednesday 21 Nov 2018

Passengers experience faster and more reliable journeys after £21million Sevenoaks tunnel refurbishment

Region & Route:
South East
Southern
  • One of longest tunnels in southern England given biggest overhaul in 150 years
  • Work results in a 20% reduction in delay incidents through the tunnel nearly halving journey times for passengers
  • Drainage systems improved with track and sleepers replaced

Passengers are experiencing more reliable and faster journeys on one of the busiest routes in South East England as a two-year renewal of infrastructure on the two-mile long Sevenoaks tunnel nears completion.

The tunnel has been given a £21million refurbishment which has included the replacement of thousands of sleepers, the laying of several miles of new track and the installation of new drainage systems. The completion of the main works has resulted in a reduction in delays for passengers of nearly 20% with journey times through the tunnel up to 64 seconds quicker due to the lifting of speed restrictions.

The work on the 150 year-old tunnel began in 2017 and has included partially lining 13 ventilation shafts, measuring up to 120 metres deep below the Sevenoaks Weald, which has stopped water flooding the tracks below and causing delays. Engineers have also overhauled its electrical systems to make them more reliable and ensure that other lines in the area can operate if there is a power issue in the tunnel.

The work will improve reliability for Southeastern as well as Thameslink services, which are affected by knock-on delays and congestion when Southeastern’s London services need to be diverted.

The team have employed several innovations during the project, including 3D modelling to prove the suitability of special padded sleepers for reducing stress to the Victorian structure and enabling faster train speeds, while safe access methods have allowed the tunnel to stay open during most of the works to the ventilation shafts, where previously line closures would have been required.

Journeys through the Sevenoaks tunnel are now smoother and faster now that the work has allowed speed restrictions to be removed with trains able to travel at up to 90mph instead of 50mph, nearly halving journey times through it.

John Halsall, Network Rail South East route managing director, said: “We’re pleased to have delivered this project to upgrade one of the country’s oldest and longest rail tunnels using the investment provided by Government through the Thameslink Resilience Programme. Delays due to flooding had become a regular occurrence through the Sevenoaks Tunnel but we’ve been able reduce delays by a fifth, giving passengers well-earned relief to their journeys, and I’d like to thank them for their patience while we’ve carried out this vital work.”

Ellie Burrows, Southeastern’s train services director, said: “We’re very pleased that this major work on the Sevenoaks Tunnel has been carried out and will improve the reliability of services for our passengers. I’d like to thank passengers for their patience during the 15 weekend closures of the line which have been necessary to allow this work to be carried out.”

Other improvement works have also been carried out locally to maximise the use of line closures including structural improvements to adjacent tunnels, ballast cleaning, repairing small areas of worn track in more than 36 locations and the replacement of 700 yards of sleepers suffering from ‘concrete cancer’ degradation outside the southern end of the tunnel.

The project has been a major element of a £300m government-funded programme to tackle delay hotspots and improve railway reliability for the new Thameslink timetable.

Notes to Editors

Our work in detail

  • The Victorian tunnel’s drainage systems have been improved, with modern solutions installed to better manage water entering the tunnel through the side walls of its ventilation shafts.
  • Blockages and collapses in the brick built Victorian track drainage systems have been cleared and renewed with modern materials.
  • More than 2 miles of timber sleepers – which support the track running rails – have been replaced with special underpadded concrete equivalents. These sleepers are cheaper and quicker to install and help reduce wear and tear by absorbing train movement stress more effectively.
  • Electrical resilience has also been improved by reconfiguring the Sevenoaks track power supply and implementing some heavy refurbishment. Previously, when issues have arisen in the Sevenoaks area (on the Orpington - Tonbridge Route) requiring power isolations, the Bat and Ball service would also be impacted. Now the Bat and Ball route can remain open even if the electrical systems on the Main Line are compromised.

Project stats

  • £21m project.
  • Passenger journeys now quicker by up to 64 seconds through the tunnel (half the time it took before the speed restriction was lifted).
  • 19% reduction in delay incidents and 12% reduction in delay minutes since before work started.
  • More than 100,000 man hours have been clocked up by the teams.
  • More than 300 people on site per shift at the busiest times.
  • 617 yards of drainage replaced.
  • 4.2km of track renewed.
  • More than 4,500 sleepers replaced.
  • 11500 tonnes of ballast used
  • 13 of 14 ventilation shafts are being improved (shaft 4 is infilled). 4 were completed 2009/10, 4 in 2017/18 and 5 are in the process of being completed by Easter 2019 with 2 already substantially complete.
  • Shaft depths range from 30m to 120m.
  • 630m of collapsed and blocked drainage has been replaced (Christmas 2016 & 2017 and Easter 2017 & 2018) with the remaining cleared to be free flowing.

Further information

‘Concrete cancer’ – Alkali Silica Reaction (ASR), more commonly known as "concrete cancer", is a swelling reaction that, given sufficient moisture, occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates.  This reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate (hydrate). This hygroscopic gel swells and increases in volume when absorbing water: it exerts an expansive pressure inside the siliceous aggregate, causing spalling and loss of strength of the concrete, finally leading to its failure. The ‘concrete cancer’ effect is exacerbated when steel reinforcing within a concrete slab begins to rust due to water ingress through failure fissures. As the steel rusts it expands, displacing the concrete around it, accelerating the failure process.

Replacing timber sleepers with concrete equivalents has saved time and money. We had initially opted for timber railway sleepers to lower the stress transferred to the tunnel itself. However, the need to cut wooden sleepers to size threatened the schedule and budget. Concrete sleepers with under sleeper pads were thought to reduce load transfer to the structure but this had never been fully explored.

Thanks to 3D modelling, we discovered concrete sleepers with under sleeper pads would be more effective at lowering stresses imposed on the tunnel invert brickwork than the wooden sleepers previously utilised for this purpose.

Working in tunnels presents unique challenges, including ventilation. Dust can cause problems for productivity and pose potential health hazards. We had no issues with our ventilation system thanks to the hard work of our specialist ventilation team who provided us with fume monitors and additional staff to evaluate fume levels at the site and ensure all of our staff returned home safely every day.

About the Thameslink Resilience Programme

The work in Sevenoaks tunnel is part of the Thameslink Resilience Programme. This is a £300m government funded programme to tackle delay hotspots and improve reliability across the South East route. The funding will also allow:

  • Additional staff to be employed to maintain the railway as passenger numbers and service frequencies grow
  • Digital Railway technology such as incident management, traffic management and automatic route setting to be introduced in the most congested areas such as on the approaches to London Victoria and London Bridge
  • Data-driven asset interventions at delay hotspots such as the Sevenoaks tunnel
  • As part of the £300m programme, major work will take place on the Brighton Main Line between Three Bridges and Brighton at weekends in 2018 and for nine days in February 2019.

About the South East route

The South East route is Britain’s busiest and most congested route, connecting London and its southern and southeastern suburbs with Kent, Surrey, Sussex and mainland Europe.

Every day, about 5,000 passenger trains run on the route across almost 2,000 miles of track.

Almost 68,000 passengers travel on this route into central London during the peak hour of 8am to 9am each weekday. Many more travel to key interchange points such as Clapham Junction and East Croydon, or commute to and from important regional centres such as Brighton, Eastbourne, Crawley, Maidstone and the Medway Towns.

Such strong demand for the South East route means we’re working hard to increase capacity and reliability for all users.

About Network Rail

We own, operate and develop Britain's railway infrastructure; that's 20,000 miles of track, 30,000 bridges, tunnels and viaducts and the thousands of signals, level crossings and stations. We run 20 of the UK's largest stations while all the others, over 2,500, are run by the country's train operating companies.

Every day, there are more than 4.7 million journeys made in the UK and over 600 freight trains run on the network. People depend on Britain's railway for their daily commute, to visit friends and loved ones and to get them home safe every day. Our role is to deliver a safe and reliable railway, so we carefully manage and deliver thousands of projects every year that form part of the multi-billion pound Railway Upgrade Plan, to grow and expand the nation's railway network to respond to the tremendous growth and demand the railway has experienced - a doubling of passenger journeys over the past 20 years.

We are building a better railway for a better Britain.

Follow us on Twitter: @networkrail Visit our online newsroom: www.networkrailmediacentre.co.uk

Contact information

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About Network Rail

We own, operate and develop Britain's railway infrastructure; that's 20,000 miles of track, 30,000 bridges, tunnels and viaducts and the thousands of signals, level crossings and stations. We run 20 of the UK's largest stations while all the others, over 2,500, are run by the country's train operating companies.

Every day, there are more than 4.8 million journeys made in the UK and over 600 freight trains run on the network. People depend on Britain's railway for their daily commute, to visit friends and loved ones and to get them home safe every day. Our role is to deliver a safe and reliable railway, so we carefully manage and deliver thousands of projects every year that form part of the multi-billion pound Railway Upgrade Plan, to grow and expand the nation's railway network to respond to the tremendous growth and demand the railway has experienced - a doubling of passenger journeys over the past 20 years.

Follow us on Twitter: @networkrail
Visit our online newsroom: www.networkrailmediacentre.co.uk