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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.

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