Acid on the Jubilee: A Major Challenge, not a Soundbite

Yesterday saw a plethora of headlines hit both the London papers and the web about the Jubilee Line, as it was revealed that closures would be required in order to allow work to address problems of acid damage to the tunnel wall segments along a 90m section of the line.

This is not a new issue, with the need for repair work having been identified for some time now. Indeed the “Baker Street to Bond Street Tunnel Reconstruction” project has been on both TfL and Tube Lines’ books since at least 2011, but the overrunning Jubilee Line signalling upgrades (with which relining could not be run concurrently) and the ultimate takeover and restructuring of Tube Lines meant that it is only recently that the project has begun to move forward in force.

The work will require a number of short closures on the line between Finchley Road and Waterloo over the remainder of 2013 and into 2014. The full list of closures can be found at the foot of this article, but they mainly follow the normal closure pattern used by the likes of TfL and Network Rail – total closure over the Christmas and Easter periods, coupled with late Sunday openings and isolated weekend closures.

Perhaps unsurprisingly, much of the press (and political) coverage has followed the now-traditional “transport chaos” pattern. The Evening Standard and Metro were arguably the biggest perpetrators of hyperbole, aided by a spectacularly misleading and uninformed quote from Manuel Cortes, head of the TSSA:

This is all we need. Now it’s the wrong sort of water. Jubilee line passengers and businesses served by the line have already suffered more than their fare [sic] share of disruption without more on the way.

Their fires suitably stoked, both papers led with “THE WRONG SORT OF WATER” with others, such as the Independent, following a similar theme (the Evening Standard also, rather oddly, seems to have opted with a photo which suggests the Jubilee Line has been unchanged since about 1979).

Coverage has also focused on the total cumulative closure time (approximately thirty days) rather the particular timing of them, something which again makes good copy but is something of a sleight of hand. Comments by a number of London Assembly members have also added to the confusion. Caroline Pidgeon’s question as to whether a block closure strategy had been considered was arguably a fair one, but veiled suggestions by herself and others in the Assembly that this work represents a failure to deliver on promises of an “end to Jubilee closures” was less fair – and, as with Manual Cortes’ comments – a case where those making such comments should really know better.

So with points political being scored, and hyperbolic headlines being written, it is perhaps worth us stepping back and taking a look at what the actual issue on the Jubilee is, and what will actually happen to fix it.

To do that we must at both the details that have already begun to surface about the project, and also take a brief trip into a rather fascinating (but largely forgotten) moment in the Northern Line’s recent past.

London: A City Built on Clay. Except When it Isn’t

It’s easy to think that London is a city, at least to the north of the Thames, built on very uniform ground. The key role that the rich deposits of clay to be found beneath our feet played in the construction of the Tube network is largely public knowledge. A relatively friendly (and non-fatal) material to tunnel through, particularly when you are just learning the art, its presence helped the Deep Tube take shape, and its absence limited progress south of the river Thames for some time.

As is the case with most things, however, the truth is more complex than it seems. General histories – and the beautifully coloured-in strata most LR readers will remember drawing in Geography classes – tend to imply a more strict demarcation between geological layers than actually exists in nature.

In reality, London is far more geologically diverse than one might initially think. Chalk and sand can all be found in layers beneath our feet and, as generations of Tube builders and workers have long discovered (sometimes at fatal cost), even the widest layers of clay can hold lenses of sand and imperfections.

A Certain kind of Chemistry

It is these discrepancies that make maintenance of the tunnels through which the Tube runs such an interesting exercise. Chemistry, like death and taxes, is something that cannot be avoided, and pushing a large iron tube filled with air through a diverse mix of subsurface elements and compounds can, over time, have interesting effects.

Although extensive details as to the cause of the acid problem on the Jubilee Line have yet to emerge, it seems likely that they are broadly similar to that discovered on the Northern Line at Old Street some years back.

In 1945, engineers noticed that a section of the Northern Line just south of Old Street station had begun to show signs of damage from sulphuric acid attack.

The cause of the acid attack, which affected both running tunnels wasn’t initially clear. The tunnel section continued to be monitored though, and when cracks began to appear in the tunnel lining in the 1960s London Underground began to further investigate.

What eventually became clear was that the geology of the Old Street area was far more complex than had previously been realised. Here, it emerged, a layer of London Clay met the Woolwich and Reading beds (composed of a mix of clay and sand). Between these two was a lens of sand, through which the Northern Line tunnels actually passed. Further analysis of this sand layer showed that it was full of iron pyrites.

For millions of years water, seeping down through the clay above, had come to rest in this sand and come into contact with these iron pyrites, with no effect. The arrival of the Tube, however, changed the chemical mix. Now, each time a train passed through the tunnel, minute quantities of air were forced out between the tunnel rings, this was something to which the wet pyrites had not previously been exposed, and they reacted with the air to produce, amongst other things, small quantities of sulphuric acid. Locked into the sand lens, like the water, over time this had built up and begun to attack the tunnel itself.

Although full details have yet to emerge as to the causes of the acid damage on the Jubilee, it seems likely that they are broadly similar to those that affected the Northern Line as well. As it was not until boreholes were drilled in the eighties that the problem at Old Street was fully understood, it is perhaps not entirely surprising that such issues have ultimately manifested elsewhere on the network.

Acid Damage on the JubileeAcid Damage on the Jubilee, as highlighted by Tube Lines safety notices for some time now.

To Block or Not To Block

Whatever the cause of the acid damage, the solution for the Jubilee will ultimately be the same as that taken for the Northern Line – where damaged, the tunnel will need to be relined.

It’s a simple answer, but one that requires a complex engineering solution. Although Caroline Pidgeon may have questioned why a block closure approach was not being sought, the fact is that on pure passenger numbers a process of short closures and work within engineering hours, if practical, will ultimately affect fewer passengers. For although much has been made of the disruption to businesses that might be caused by a loss of weekend and non-peak traffic, the reality is that the route of the closure is actually well served by alternatives where the key leisure destinations are involved, whilst preserving a more direct service for commuters is arguably more useful.

Indeed this was equally true for the problems at Old Street. By the late 1980s it had become clear that relining the Northern Line tunnels here was necessary and, even with lower passenger numbers on the network at the time, a similar decision was reached – short sharp closures would be less disruptive to traffic than one long closure.

Rethreading the needle

With the “what” and the “why” out of the way, we can thus finally turn to the “how” – how do you reline a whole section of running tunnel, in engineering hours, without disrupting or closing the whole of the line?

Sadly although we know where the answer to this question can currently be found, much of the detail remain unclear. The where, it appears, is beneath Charing Cross.

Possessing now-disused tunnels built to the same pattern as the rest of the central section of the Jubilee Line, Charing Cross presented a perfect test bed for the engineering solution. It is thus here, over the last few months, that engineers have been working to put together a machine that can carry out the relining process.

New ring replacement machinery at Charing CrossNew ring replacement machinery at Charing Cross
A view which gives a wider view of the processA view which gives a wider view of the process

So far, the pictures above represent our only guide to the process that will be used. It does appear, however, to be similar to that taken on the Northern Line, so we can arguably head back to Old Street once again to get some idea as to what the process is likely to entail.

Returning to Old Street

From the 1960s onwards, London Underground put into place various measures at Old Street to try and correct or mitigate the continuing problem of acid damage. In 1963 almost 2500 gallons of sodium hydroxide was pumped into the ground around the affected area to try and neutralise the acid. The sheer levels of acid in the area, and the composition of the surrounding earthworks, prevented this from being effective though.

Vertical metal strapping was added to affected tunnel segments to give them more strength, and for a time this seemed to be effective. By the late eighties, however, it was clear that degradation was still continuing, and a more permanent solution was required. Investigations into what such a solution might be began, and its need was further reinforced when drillings carried out as part of the investigations themselves caused further cracking to appear in a number of tunnel segments.

Strapping on the Northern Line south of Old Street in the 1960sStrapping on the Northern Line south of Old Street in the 1960s, note the evidence of acid damage at the bottom of the tunnel

Soon it became clear that the only solution was effectively to start again. The existing lining over a 90m section of both running tunnels would need to be entirely removed and relined, this time with chromium duplex steel segments designed to resist the acid build up (indeed the resulting order for 750t of chromium duplex steel represented the largest in the world at that time).

Examples of the new tunnel segments, stacked before installationExamples of the new tunnel segments, stacked before installation

In order to lay those new tunnel segments Underground engineers faced the same problem as those on the Jubilee do today – they needed the ability to get into the running tunnels, get set up quickly, carry out the work required and then vacate before trains began to run again. Accepting that it was impossible within engineering hours to both remove and replace a tunnel segment, they also realised that their supporting machinery would need to remain in place during regular running, and then be returned to during the next engineering period or closure.

The result was the creation of a custom tunnelling shield. This could be manoeuvred into place around the section that needed to be replaced, and would support the surrounding tunnel wall while replacement took place. During operating hours, the shield would be left in place and trains would run through it.

Moving the Shield into placeMoving the Shield into place

It was a clever solution to the problem, and the photos of the work currently underway at Charing Cross, along with the limited information that has emerged so far from TfL, seem to suggest a broadly similar approach will be taken with the Jubilee.

Moving the Mess

One luxury that those working on the Jubilee line will not have, however, will be in the area of spoil removal and site access.

On the Northern Line it was identified at an early stage that site access – both for people and material – would prove problematic, especially given the limited period in which work could take place. Luckily, however, there was a solution. Running parallel to the tunnel sections in question was a disused siding tunnel. By sinking a shaft in the car park of a nearby school and building a connecting tunnel, the siding could be accessed and used as an entry and exit point.

Just what solution to this problem will be found for the Jubilee remains to be seen. Both men and material will still require access, and a new more creative solution to this problem will thus likely be required.

More than just a soundbite

Overall, as can be seen from the above, the acid issues facing the Jubilee Line represent far more than just a simple niggling engineering issue, or an excuse for another soundbite or emotive headline.

If the previous experience on the Northern Line is any guide, they represent an enormous test of planning and engineering skill. Keeping a running railway working underground whilst relining the very tunnels it passes through is an incredibly complex task, whatever the full extent of the work turns out to be, and it’ll be interesting to see how the challenge of doing so is addressed.

“The Wrong Kind of Water” may make a nice headline, but it’s about as far off the reality as it is possible to be.

The Announced Closures

The closures announced for 2013 are:

  • Sunday 16 June and Sunday 6 October
  • Bank holiday Monday 26 August
  • Weekend of 12 and 13 October
  • Entire Christmas period 25-30 December
  • Three further Sunday — dates to be announced — when service won’t start up to 11 am.

The closures announced for 2014 are:

  • Sundays of 12 January, 15 June, 17 August and 16 November
  • Weekends of 1 & 2 February and 8 & 9 March
  • Saturday 1 March
  • Four-day closure at Easter 18-21 April
  • Entire Christmas period 25-30 December
  • Further 17 late start to services on Sundays
Written by John Bull