OK, so what would be your idea of an ideal Christmas? In the warm? In the dry? With friends and/or family around you? (The slight qualification is needed as you can choose your friends. You can’t choose your family.) Perhaps a comforting drink or two and the sure knowledge that all the celebrations will be followed by a relaxing comatose state – with everyone going away soon afterwards.

That’s one idea. The alternative is to spend Christmas and four succeeding days up to your ankles in freezing water, bathed in artificial lighting, and being occasionally rained on by equally icy streams of water – with no alcohol and no occasional snoozes.

Strangely, there’s a waiting list for the latter. Such attractions have to be booked months in advance. David Jarman, Network Rail project manager, had his name down back in the spring of 2016. The venue of his special Christmas was Sevenoaks tunnel, the site of drainage work on the South Eastern main line between London, Sevenoaks and Tonbridge.

Speed restriction

The project had all the pre- requisites. It was in the depths of a two-mile-long tunnel. There was plenty of cold water involved, issuing both from the ballast and from the tunnel roof. But, in all seriousness, these were the very reasons for such an extraordinary exercise at an extraordinary time.

Sevenoaks tunnel is a vital link in the route to Hastings from London. The public timetable alone shows something like 27 trains run towards London before 09:00 on a weekday morning, with the first one passing through the tunnel a few minutes before 05:00hrs. The evening peak is just as busy.

Trains scoop up bleary-eyed commuters from beyond the conurbations of Tonbridge, Hildenborough and Tunbridge Wells. And all this traffic runs through a tunnel which is very wet and on track that has required frequent maintenance to keep it open. A speed restriction has been imposed because of the deterioration of the track due to water at times running at sleeper level. Two miles of 50mph track instead of 70mph has to be addressed.

An old problem

David had the job of sorting out the worst sections of a failed six-foot drain. Work was to be encompassed by Network Rail’s South East Route’s multifunctional framework contract and the planning of the complex logistics was led by the principal contractor BAM Nuttall. The designer was Tony Gee and Partners.

BAM was selected due to its proven delivery capability of tunnel drainage schemes in the South-East and the ability to draw successfully upon important lessons learned from previous similar projects.

About 260 metres of drain was identified by Network Rail’s South- East Route’s Asset Management team as being either blocked or, in places, non-existent. It’s not as if this was a new problem. Drains within Sevenoaks tunnel have given trouble for many years and several attempts have been made to improve the situation. But piecemeal repairs have never taken the problem by the throat. It was time for a bold approach.

The trouble is that bold approaches can take time and, with the intensity of traffic both during the week and at weekends, time was never on offer – except, of course, at Christmas.

Excavation

At 01:15 on Christmas Eve, a possession of both lines through the tunnel was taken. This was due to be given back four days later at 04:00 on 28 December. During that time, twelve shifts – three per day – were worked by a labour force of approximately 40 people per shift.

Not all the drain was excavated at a time. Two worksites were set up so that plant, machinery and labour could be used efficiently. Discreet lengths, which varied between 10 and 30 metres, were opened up so that the job would not present a risk of overrun if serious problems were encountered.

Just one engineer’s train was involved, with wagons planned to be strategically placed in the train to allow the two work teams to operate. The locomotive was shut down between moves but, even so, air quality was monitored regularly and fans provided by Factair were used to give a constant flow of fresh, if not very fresh, air to the worksites.

The drain was originally a 450mm diameter brick barrel with catchpits at irregular intervals. The task of the machinery, supplied by Total Rail Solutions, was to break out the roof of the barrel – where it existed – and to allow the excavation of spoil and obstructing material before laying the Aqua pipes. New catchpits were constructed every 20 metres, although some of the existing catchpits were retained depending on their condition and on whether they fitted the 20-metre interval.

Such were the historic drainage problems within the tunnel that there was already a six- inch over-pumping main in operation which had its own power supply. This was kept going along with additional temporary pumps to help keep the water under control whilst pipes were laid.

The long haul

As with many tunnel sites, access was not straightforward. It would be tempting to look at Sevenoaks station as the logical operations centre but, in practice, there was very little space available with only limited access for road-rail vehicles.

A site near Sevenoaks Weald village was chosen for site offices and for track access, although this was half a mile from the southern portal. All personnel travelled to site from this access point, many using road/rail personnel trolleys.

Christmas works attract a good deal of media attention, but only if they go badly. For this reason, David and his team adhered to the strict discipline of four-hourly reporting to the South Eastern route control even though the logistics of doing this could be challenging without disrupting productivity.

Although the aim of delivering 260 metres of drainage installation in the 100 hour timeframe was not achieved, after twelve shifts in unrelentingly unpleasant and wet conditions, more than half of the defective sections of six foot drain were repaired. David is keen to emphasise that his team and the contractor BAM Nuttall, worked stoically throughout to achieve the end result.

The possession was given back on time and, following some final drainage installation works in spring/summer 2017, the tunnel should be dry enough for new track to be laid during the next financial year. There is a real prospect of line speed being restored through the tunnel, so giving less disruption to the intense timetable.

So, if you fancy the alternative Christmas, get your name down early for the next tunnel job on the list.

Written by Grahame Taylor