Another year, another Christmas and yet another session on the drains of Sevenoaks tunnel. Some of you – the sharper eyed, even after the festive season – may have noticed that this is also yet another article about Sevenoaks tunnel and Christmas etc etc.

Indeed it is. It’s true, of course, that you can’t have too much of a good thing, but we’re covering Sevenoaks again mainly because we’re looking a little more deeply at the details involved in such a project. Having become practiced at drainage renewal on this site, the team involved has started to explore refinements in their methods and considered experiments with some new technology – of which more in a moment.

Photo: Tony Gee and Partners.

Photo: Tony Gee and Partners.

Recap

For those of you who can’t find your February 2017 edition of Rail Engineer magazine (issue 147), we’ll start with a bit of background.

Where is Sevenoaks and why is the tunnel a problem? Sevenoaks is south of London on the South Eastern mainline between London, Sevenoaks and Tonbridge. It is in the leafy glades of the North Downs and is also one of the most insanely expensive parts of England.

The two-mile-long tunnel is a problem because the line is very heavily used, taking traffic from equally leafy glades in the south up to (and down from) London.

It is also very wet and always has been.

When it was constructed in the 1860s, geology, as a science used in civil engineering, was in its infancy. Thus, assumptions were often made about likely strata. In the case of Sevenoaks tunnel, the assumptions were optimistic – or, not to put too fine a point on it, just plain wrong. Far from easily munching their way through homogenous sandstone, the contractor of the day discovered a good deal of water.

It has been the turn of Ian Massey, Network Rail’s project manager (building and geotech portfolios), to look after the works. As he explained: “Sevenoaks tunnel was built with a generous-sized culvert but, over the years, this began to give up the ghost.

“With blocked drainage, the inevitable happens. Silt is deposited within the culvert, so further restricting the flow. As water flows out of blocked catchpits, silt builds up within the track ballast and, before long, track top and line becomes difficult to maintain. This itself can lead to disruptive temporary speed restriction, but matters become really difficult when water levels rise up to the rails, drop track circuits, degrade sleepers and cause other electrical problems.”

Inhabitants of leafy glades do not appreciate disruption, especially when the excuse is that it’s all caused by a bit of water.

Emmanouil Tsoukalas, Network Rail’s geotechnical route asset manager, suggested an interesting theory as to why the tunnel is particularly wet. “In the vicinity, there are a couple of derelict pumping stations – evidence of early commercial water extraction. Abandoning the pumps meant that the water just went straight back into the tunnel.”

The credibility of this theory is assisted by tales of the original contractor making a shed-load of money by setting up an early Sevenoaks water company, and flogging the water to the local population.

The tunnel drain repairs have gone on for many years, if not many decades. A rescue job here, a patch there. It’s all been constrained by the intensive traffic that the tunnel carries.

The latest instalment

Over the past few years, a concerted attempt has been made to sort the drainage out once and for all. Detailed surveys of the drainage were carried out and problem areas isolated. Christmas 2017 was the penultimate session of remedial works. There will be a final push in Easter 2018, which should markedly improve the flow.

The action doesn’t start initially in the tunnel. There is the mobilisation of the site compound at the southern end of the tunnel. For such a crowded part of the South of England, the compound is in an isolated spot away from habitation. This is where the works are controlled and where the labour force and plant gains access to the tunnel.

Materials, on the other hand, are marshalled at Hoo Junction, an isolated site at the edge of the Shorne Marshes next to the Thames Estuary and the junction serving the freight-only line to the Hoo Peninsular. It is here that a train of 34, 17 for each worksite) assorted wagons is assembled, and on which the ingredients for the drainage project are loaded.

There were two distinct sites within the tunnel, separated by about 1000 metres. The order in which the materials and empty wagons arrived in the tunnel was critical, and ensuring that the train was loaded and marshalled the right way round was fundamental. It may sound obvious, but checking that loading is correct and that the route to site is unambiguous is vital. There are no turning bays handy on a railway and it really is possible for a train to deviate from a pre-set route and land up on site either back-to-front or at the wrong end of the job. Checks are necessary!

Other preparatory works involved the laying of temporary drainage pipes alongside the areas to be excavated. These were needed to take water that had to be over-pumped around the worksite. Once all the equipment arrived, these were connected to portable pumps to keep as much water as possible away from the excavations.

Naturally, there were signalling cables to protect, and these were set aside by Network Rail staff right at the start of the possession.

Communication equipment – in this case in the form of standard GSM-R radios – were tested and found to be the most practical. The main contractor, BAM Nuttall, experimented with noise reducing earphones, which lower background noise levels from machines. It is still possible to hear the machines but they amplify the human voice. The supplier was EAVE of Clerkenwell, and reports from the site were that they were very effective.

Over the years, plant suppliers have adapted their standard models to suit the peculiarities of the railway environment. One such challenge has been how to dig out a drain in the six-foot when the trench involved is parallel, but offset from the axis of the digger. Standard machines can only dig a straight trench in line with the centre of the machine. There are some agricultural machines that have the option of an offset trench, but they may not have the overall reach required on a railway site.

However, there are now road/rail diggers that have a digging boom that can be positioned directly over the line of the six-foot, well clear of the sleeper ends. The result is a very precise and productive cut.

Christmas down the drain

With the possession taken, the materials train was drawn into the tunnel, into the first worksite and half of the train separated and the wagons screwed down. The remainder of the train was taken forward to the second site and the remaining wagons were braked. The loco was then released and was taken out of the tunnel and stabled in a siding until the time came for the train to be re-assembled and taken back to the depot.

In this way, the tunnel sites were not affected by fumes from the loco. Management of such fumes that did arise from the excavators was achieved using three sets of fans and a ventilation scheme set up by the supplier of the fans, Factair. The scheme took into account the presence of existing ventilation shafts in the tunnel.

The air quality was constantly monitored and precautionary breathing equipment was available in case of rapid and unacceptable levels of pollution. The ventilation fans were positioned on road-rail trolleys at each of the sites.

Ian was very pleased that the whole project went off successfully. “Works were undertaken in 9½ hour shifts on a 24 hour basis with a shift change every eight hours to ensure adequate time for handover from one shift to another.

“On each shift there were around 40-50 site staff including operatives, machine drivers and controllers, COSSs, protection staff, safe work leaders, site compound cleaning staff and Network Rail support functions such as the local maintainer who carried out Signal and Telecom disconnections. Each one of these staff played their part in a successful delivery.

“There was a round-the-clock site canteen in the main compound to feed hot food to the staff, who were brought out of the tunnel at planned break times. Inside the tunnel, Portaloos with hot running water were provided and provisions made for drinking water.”

The works were let to main contractor BAM Nuttall via the Network Rail Infrastructure Projects Southeast Multi-Functional Framework under a NEC3 ECC Option C Target Cost contract. The design was by Tony Gee and Partners.

So, dear reader, next year there won’t be yet another instalment of drainage works in Sevenoaks tunnel because, by then, the latest scope of works will be complete. There’s always another wet and cold tunnel somewhere on the network and there’s always some poor soul who has to be down there over their Christmas break. Some might even enjoy it!

Postscript

Don’t despair, there may yet be more news from Sevenoaks! In the coming Easter possession there will be a trial of some technology that is designed to track the labour force entering, leaving and even within the tunnel. We’ll keep you posted…

This article was written by Grahame Taylor. 


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