Preparing the way for Bath electrification

To the east of the city of Bath – from the Georgian Sydney Gardens, through Bathampton Junction on to Box Tunnel – the railway has recently undergone a period of intense engineering work in preparation for the electrification of the Great Western main line (GWML) by May 2017.

It was the biggest track renewal possession ever planned. 2,539 people were inducted to enter the possession and over 160,000 hours were worked. It all started on Saturday 18 July and was completed on 1 September, during which time trains to London were diverted via Bristol.

Preparation, detailed planning and consultation started over two years ago. Andy Haynes, Network Rail’s project director for the West of England, explained that this involved detailed discussions with, primarily, First Great Western (FGW) and many other groups including Historic England, Victorian Society, Georgian Society, Bath Preservation Trust and Bath & North East Somerset Council, to name just a few.

Blockades banished

Normally, closing a 20 mile stretch of the main line, from Thingley Junction to Bath Station, for six weeks would be referred to as a ‘blockade’ but, during the consultation process, the word blockade became a very sensitive issue. It implied that Bath was closed for business, which wasn’t the case. Therefore, to ensure that tourists were not put off travelling to the city, it was suggested that Network Rail should use a different description for the closure. As a result the acronym TPOD was created – Temporary Period of Disruption! So the work was carried out during a six week TPOD. You’ll get used to it.

The work involved was valued at £50 million. Starting at the east end of the TPOD, the first big challenge was to lower track levels and renew the track throughout the famous Box Tunnel, built by Brunel and opened in 1841.

The tunnel is straight and 2,937 metres long with a gradient of 1 in 100. It is unlined for about the first 800 metres at the east end, the rest is brick lined. To comply with electrification clearances, the track needed to be lowered by approximately 350mm at the east end and 100mm at the west. This meant an excavation of 400mm to 650mm, a significant undertaking in such a tunnel.

Close monitoring

Before work started, a monitoring system designed to detect any movement of the tunnel side walls was installed in Box tunnel and Box Middle tunnel, monitored by consultants AECOM and Cambrian Transport Ltd.

The equipment installed included a comprehensive system of remotely monitored inclinometers as well as some manually monitored tell-tales. There were four trigger levels monitoring structural movement: Clear (0-1.75mm), Green (1.75-3.5mm), Amber (3.25-5.25mm), and Red (in excess of 5.25mm).

If an amber trigger level were to be breached, any excavation work would be restricted or curtailed. If it was a red trigger, then work would stop immediately.

As the equipment was installed well in advance of the commencement of the work, the engineers understood how the tunnel behaved in normal circumstances. During the work, only one red was recorded but no issue was identified.

The track throughout the tunnel was more than 40 years old. It was re-laid by Babcock under its five-year plain line contract with Network Rail for this area, with ISS Labour Ltd providing safety critical and track labour resources, site welfare management, front of house management and site lighting. A conventional approach was adopted cutting rail into 27-metre lengths and using road/rail equipment to tandem lift the track panels onto flatbed wagons. The old ballast was then dug out to the required depth. The big challenge was the logistics, planning the work to coincide with the flow of engineering trains required.

Modular process

Network Rail’s programme manager Rob Blackstock and his team, working closely with Babcock and Network Rail’s National Supply Chain which provided the trains, developed a modular process which ensured that each train arrived on time, with the correct wagons in the proper place and order and facing the right way round. This sounds quite straightforward but, when you consider that the project involved 217 trains coming from five different depots on the network, the task becomes a little  more daunting. The fact that the work was carried out successfully each night on both tracks is a testament to those involved and more than 160,000 tonnes of spoil and 6km of track were successfully removed and replaced.

Having said that, it was inevitable that at least one hitch emerged and, of course, it did – in the form of drainage catch pits. A carrier drain runs down the six foot and some of the catch pits were either in poor condition and/ or misaligned while some of the pipes needed to be renewed.

Atkins was the main designer for this project and had to respond at short notice to the emerging problems associated with the drain in Box tunnel. The team also had to understand the complex tunnelling systems that exist around, alongside and under Box tunnel. The systems are complex because tunnels were originally driven to extract the Oolitic Bath stone and then, during WW2, additional tunnels and shafts were constructed by the Royal Engineers to provide shelter for ammunition and explosives.

Amongst this complex of tunnels, the project team found a horizontal shaft running under and across the tunnel. When the track was lowered it would have exposed the Barlow rail cap that covered the shaft. Well in advance of the work starting, Atkins had to prepare a design to make this safe and then Hochtief was contracted to construct a concrete cube with a central duct to allow water to flow through. This work was completed in about four weeks. The contingency built into the plans was adequate for dealing with this issue even though additional pressure emerged when it was realised that the water flowing through the pipes ended up in a lake owned by the musician Peter Gabriel.

Three shifts were planned around the clock to maximise the use of the TPOD but with only two shifts in the tunnel. This was because there are up to ten different species of bat that either live in the tunnel or very close by. David Coles, an ecological expert employed by Network Rail, ensured that the bats were not adversely affected by the work and one of the controls introduced was to not carry out work in the tunnel during the night shift. Given that there were no service trains running through the tunnel anyway, the bats probably considered the TPOD to be a very restful and enjoyable period.

In order to protect the workforce (and the bats), RVT Rentavent was employed to ensure safe working conditions were maintained within the tunnel at all times. Powerful fans, located at each end of the tunnel in the cess area, were connected to flexible ducting running into the portal where high-speed jets of air created a venturi effect, moving large quantities of air right through the tunnel. Conditions were monitored constantly by on site engineers and consistently good air quality was maintained throughout the project.

Georgian pleasure gardens

We now move west to Dundas Aqueduct, where more track was re-laid and lowered. A similar monitoring system was installed here and a bogus amber alarm did force the team to curtail the excavation for a while. However, the lost time was eventually recovered and they moved on to Bathampton Junction. To renew the junction Network Rail worked with Colas, its S&C Alliance partner, in a truly integrated team led by Colas’ Said Lahssioui.

The alliance replaced eleven S&C units using tilting wagons to deliver the modules and a Kirow crane to lift and place them. Included in this piece of work was the lowering of track by 150mm to 300mm through the unique and beautiful Georgian pleasure gardens known as Sydney Gardens, an activity watched closely by the local Bath inhabitants.

Great care had to be taken, not only for the substantial retaining wall that separates the railway from the Kennet & Avon canal but also to protect two Grade 1 listed bridges – one a stone overbridge and the second a unique iron footbridge, both designed by Brunel. A similar monitoring system to the one installed in Box tunnel was used for the retaining wall but, fortunately, no issues were recorded. In total, 350 structural monitoring devices were used to monitor the tunnels, bridges and walls during the works.

TXM provided the plant used by Babcock and A P Webb supplied the plant for the work associated with Colas. Both suppliers were able to provide plant in good operational order throughout the TPOD, demonstrating how much improvement has taken place in recent years to ensure that plant reliability is of the highest order. The innovation that plant suppliers have collectively introduced to the rail industry in recent years has made such a difference to efficiency and output. This project is a good example.

Modelling for the future

Andy Haynes is not only responsible for this project but for all route modernisation schemes between Swindon and Bristol. He has a BIM 4D modelling team based in Bristol which is slowly building a comprehensive picture of the infrastructure. A significant amount of additional information has been added as a result of this project. For example, seven miles of cabling was stripped out of Box tunnel and new cable installed.

The BIM modelling is enabling the team to view a ‘fly through’ of Bath showing the OLE masts in place. This technique can be viewed from the perspective of a resident’s window and, given that the new chairman of Network Rail Sir Peter Hendy lives close to the railway in Bath, this might prove to be a very useful tool. Already, minor adjustments are being made to ensure that structures are positioned in the least noticeable position.

In total, 12 miles of track has been re-laid and lowered with an extra two miles re-laid using the high output relaying train. It is work that would have to be carried out in this planning period anyway and it helps to maximise the use of the TPOD and minimise disruption in the future.

Andy Haynes was very pleased with the performance of the contractors involved and the safe and responsible approach adopted by all concerned. He also wanted to acknowledge the helpful cooperation provided by the many interested parties but in particular First Great Western and those who travel on its trains. Possession was handed back 10 hours early at a line speed of 60mph rather than the planned 50mph.

The next step is to install the OLE, acquire the new train sets and enjoy the revitalised service in 2017. Having said that, let us not forget those who built the railway in the first place when conditions were quite different from today. More than 100 men were killed building Box tunnel and one tonne of candles were required each week to enable workers to construct it. Conditions are different now, and maybe in another 175 years, it will be robots which do all the work!