HomeInfrastructureChipping Sodbury Tunnel - A saturated environment

Chipping Sodbury Tunnel – A saturated environment

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Chipping Sodbury tunnel is situated to the east of Bristol Parkway station on the Paddington to Cardiff Great Western main line (GWML). The 2.5-mile long tunnel was opened in 1902, ensuring that the main line to South Wales would run under, and not impact on, the picturesque Badminton estate that is located above. The only evidence of the tunnel’s existence at ground level is a row of six elegant Grade 2-listed ventilation shafts which have been tastefully designed to add character to the surrounding countryside.

The tunnel was one of the last major railway tunnels to be built in the UK on the classic mainline network. It has also acquired an unenviable reputation for frequent flooding, causing passenger and freight trains to be cancelled or diverted on a regular basis.

The reason this flooding has become such a regular occurrence is due to the fact that the tunnel was constructed through an area known as an aquifer and, as a consequence, it has been a nightmare for railway maintenance engineers over the decades.

Water-bearing permeable rock

An aquifer, as readers may well be aware, is an underground layer of water-bearing permeable rock. While aquifers can occur at varying depths below the surface, those closer to the surface are not only more likely to be used for water supply and irrigation but are also topped up whenever there is local rainfall.

In the case of Chipping Sodbury tunnel, whenever there is rainfall, huge amounts of water gravitate toward the tunnel area. If the rainfall is significant, springs can emerge through the track ballast and the brick lining within the tunnel. On one occasion, the volume of water entering the tunnel was measured at 2.5 cubic metres per second.

The dilemma for the engineer who is trying to resolve this problem is that pumping water out into local streams and rivers usually means that the water goes on a short trip through the local strata and then re-emerges in the tunnel or the approaching cuttings. It is like an aqua/water roundabout. It is exasperating for train operators and a daunting challenge for engineers responsible for keeping the tunnel open.

Impact on the environment

Given the significant volumes of water involved, Network Rail continues to work very closely with the Environment Agency in designing a scheme for the tunnel and its surroundings.

Also, the Chipping Sodbury project is one of a number of schemes under the Department for Transport’s Flood Resilience Programme. This £26.5 million programme was established after extreme weather in 2012 and 2014 caused extensive disruption to the rail network. The aim of the programme is to reduce the risk of flooding at key locations in both the Thames Valley and the South West and to ensure that, when flooding does occur, train services can be resumed at a quicker rate, reducing disruption for passengers.

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Mott MacDonald was invited by Network Rail to analyse and prioritise these flooding hotspots and develop outline schemes for improvement. Chipping Sodbury was identified as the second highest priority closely behind Cowley Bridge in Exeter (issue113, March 2014 and issue 100, February 2013), where currently two schemes are being developed.

Many local initiatives and schemes have been tried out in the past, but with only marginal success. However, with the imminent introduction of the electrification of the main line from Paddington to Cardiff it has been decided that it is time to challenge nature and sort this perennial problem out once and for all.

Dissipating water

Phil Morton, Network Rail’s project manager for the tunnel’s flood alleviation scheme, explained the details of the work now being carried out. He emphasised that the focus was on finding a more effective way of dissipating the water and, therefore, dramatically reducing the need to close the tunnel.

The original plan was to carry out part of the work during two weekend blockades but, as the GWML Electrification project emerged, opportunity was taken to piggyback on a 28-day blockade that started on 19 August and lasted until 15 September. The work already planned included extension to platforms at Bristol Parkway, installing electrification equipment in Chipping Sodbury and Alderton tunnels as well as piling for electrification masts throughout the route.

In November 2016, AMCO was awarded a £2.5 million contract to carry out certain elements of the proposed scheme and Arup was also invited to develop detailed design work to support the scheme. As Philip explained, the first phase of work carried out in this blockade was completed successfully and it entailed installing a 22-metre long cross drain, using 1.2-metre outside-diameter high-density polyethylene (HDPE) plastic pipe, 3.5 metres below sleeper bottom at the west end of the tunnel. The pipe has a capacity of 866 litres per second.

Old brick culvert

Before the pipe could be installed, two sidings had to be removed. Also, temporary bridging structures were installed to support cables and services alongside short sections of the main line. The path of this cross drain is designed to intercept the Up and Down cess drains.

This wasn’t such a problem, but there is also an old brick culvert that runs through the tunnel along the six-foot, at a shallower depth than that proposed for the cross drain.

This old brick culvert has a flat, 700mm-wide base with a 900mm span to the crown of the culvert arch. Significant volumes of water are carried out of the tunnel by this structure to the local Kingrove River, so any interference with this structure could pose a significant risk.

Resin and hot water

The culvert needed to be removed to enable the new cross drain to be installed. First, CCTV cameras were used to determine the state of the culvert and resin saturated lining tubes were installed into the piping. The resin was then cured using hot water, thus securing both ends of a three-metre section.

Water flowing down the culvert was pumped into a lagoon located in the Up cess from a manhole adjacent to the three-metre section that was to be removed. The section was then taken out and the formation was excavated down to 4.5 metres, requiring the removal of approximately 2,000 tonnes of material. The plastic pipe for the new cross drain was then positioned on carefully prepared and consolidated ground.

A new three-metre precast concrete trough and arched lid was installed to replace the brick culvert. It was sealed and waterproofed before backfilling with pea shingle, 6F5 aggregate and ballast, and then reinstating the track.

Volume increase by eight

All this was completed successfully, ready for the next stages of the work. As already stated, the new cross drain, which now receives water from the Up and Down cess drains, flows into a lagoon situated in the Up cess. The volume of this lagoon is going to be increased eight times, so that it will be able to store 11 million litres of water, the equivalent of four and a half times an Olympic swimming pool.

To create this significantly enlarged sump, a large amount of spoil will have to be removed which will involve 26 spoil trains, the equivalent of more than 1,000 lorry movements. The work is planned to start in November this year and, weather permitting, will be completed by February 2018.

In addition, two new canister pumping stations will be constructed, each with an inlet diameter of around 7.5 metres and an outlet of four metres. One will be situated at the outlet of the new cross drain and will have the capacity to suck 1,000 litres of water per second. The second pump, which will have a lower capacity, will be at the outlet of the lagoon into the Kingrove stream and will be used to pump water into the river system only when the local terrain is able to cope with the capacity. It is expected that this work will be completed by May 2018.

The water running through the brick culvert in the six-foot will continue to do so and it will discharge straight into the Kingrove stream. This will reduce the amount of times the pumps cut in and out, therefore saving on energy, efficiency and maintenance. However, there is a feedback from the culvert into the new cross drain that can be used to prevent water backing up back into the tunnel when weather conditions are extreme.

A headache for previous engineers

All of this work will not guarantee that the route will never be closed in future because of flooding, but there is a clear expectation that the result will be a considerable amount of additional resilience built into the infrastructure. However, given the effects of climate change and the ever-changing state of the countryside, work like this is essential just to maintain current levels of flooding occurrences.

Having said that, it does feel that Philip and his team are going to make significant improvements to a location that has been the bane of many a railway engineer’s life over past generations. It is also an essential improvement that will help to keep train services running from London to Cardiff through Bristol Parkway.

This article was written by Colin Carr.

Read more: An interview with Network Rail’s Francis Paonessa



  1. Very interesting. Three years on, and given the wet winter Oct 2019 to March 2020, how has the Chipping Sodbury tunnel drainage scheme performed?


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