Stairway to heaven: shaft repairs at Kilsby tunnel

Change is a fact of life to which we often develop a resistance, particularly as we get older. Despite its embedded role in the modern world, 10 per cent of UK households still didn’t have the internet in 2017, mostly those with occupants aged 65 or over, while 27 per cent of adults had no mobile connection via a smartphone. For most of us, an offline existence in the 21st century is unfathomable, even if we might occasionally crave one.

It was a similar story when the last great social revolution came to Britain in the 1830s – that of the railway. Some saw the opportunities and embraced them enthusiastically; others opposed the blight it would inflict on our landscape and the prospect of undesirables being empowered to move about. And then there was the havoc wreaked during construction as drunken, marauding navvies shattered the tranquillity enjoyed by delicate villagers. In that context, ‘no pain, no gain’ is an impossible sell.

Ignorance promotes fear, and there was a lot of it about in the railway’s early days. Tunnels were perceived as death traps. “The furnace of the engine soon renders the air unfit for breathing,” claimed several newspapers in an 1834 article on Robert Stephenson’s proposed London & Birmingham Railway (L&BR), now known as the West Coast main line. Although shafts were proposed for its longer tunnels, “We are not aware whether the sufficiency of such an expedient for the purposes of ventilation has yet been ascertained by experiment.” Doom mongers were plentiful.

Not in my back yard

Like other towns along its route, Northampton did not want the railway and vigorous opposition to it was marshalled. Members of the House of Lords threw out the Parliamentary Bill in July 1832, asserting that “the promoters had not made out such a case as would warrant the forcing of the proposed railway through the lands and property of so great a proportion of dissentient landowners and proprietors.” There was no doubt where the power resided back then.

Undeterred, Stephenson surveyed a new alignment around the west side of the town, resulting in Royal Assent being granted a year later. With it came the need for Kilsby tunnel which, at 2,423 yards, would be the longest yet driven for a railway. Joseph Nowell & Sons secured the contract to build it in May 1835 at a cost of £98,988, and resident engineer Charles Lean laid the last brick on 21 June 1838. The intervening three years brought challenges, tragedy, heroism and two unique features.

Go with the flow

Some idea of the likely ground conditions were already known thanks to the engineers of the Grand Union Canal who had pushed a tunnel through the same ridge 40 years earlier. Trial pits were also excavated, generally revealing Lias shale with a few beds of rock; dry in some places, whilst elsewhere there was a considerable quantity of water.

Nothing untoward had been indicated, so it came as a severe blow when quicksand was encountered in the second of the working shafts, especially as Stephenson had plotted a course to avoid this known local difficulty. Further investigations discovered the sand to be extensive at tunnel level, shaped like a flat-bottomed basin beneath a bed of clay and cropping out on one side of the hill. The trial pits had missed it.

The impact of this discovery was immediate as the workings flooded, almost leading to the tunnel’s abandonment. Attempts were made to construct lengths of the brick lining from a raft on which men and materials were floated into position. To escape one rapid inundation, an engineer swam to the base of a shaft, towing the raft and its occupants behind him with a rope held between his teeth. The stress of it all proved too much for Nowell, who took to his bed and passed away in January 1836, leaving the L&BR to deliver the rest of the project itself.

Seven more shafts were sunk – timber cylinders being assembled to hold back the sand – and, on George Stephenson’s recommendation, pumping engines were installed. Possessing a collective power of 160 horses, they operated around-the-clock for eight months, removing 2,000 gallons of water every minute from an average depth of 120 feet.

Under this protection, engineering operations continued at numerous points along the tunnel. The thickness of the lining was increased from the planned 18 inches to more than 2 feet. In the wetter areas, bricks were washed clean of their cement within moments of it being applied so straw was used to deflect the water’s ingress.

Conditions for both miner and bricklayer must have been hideous and the dangers unsurprisingly took their toll, claiming 26 of the 1,250 men involved – a death rate of 1 in 48. Not helping matters was the bizarre recklessness of some navvies – presumably fuelled by alcohol – who attempted to jump, one after another, across the mouth of a shaft. Two or three succumbed to the inevitable.

And breathe…

Although ten working shafts were retained for ventilation, Stephenson must have entertained doubts as to whether these would provide sufficient airflow to ensure passenger comfort. His solution might seem excessive with the benefit of hindsight, but perhaps it was more about overcoming public perceptions than any genuine assessment of risk.

In May 1836, work got underway on the first of two vast shafts, 132 feet deep and 20 yards in diameter. Its lining was formed in sections – 10 feet in depth and up to 12 feet long – which were built in trenches dug sequentially around the circumference. Once one ring had been completed, the material within it was excavated and the process repeated below until the requisite depth had been obtained. It took over a year to reach the bottom. Comprising more than a million bricks, the walls are 3 feet thick and weigh 4,034 tons. Its sibling – 800 yards further south – is 100 feet deep.

On 20 August 1838, the directors and their friends breakfasted at Birmingham station before heading south on the first ever rail journey to London Euston. They paused at ‘The Great Shaft’, which no doubt took their breath away – rather the opposite of what was intended. There to cheer them on – perched high above – were some of the men who had driven Kilsby tunnel at a final cost of around £320,000.

According to one of the engineers, the shafts “are perfect masterpieces of brickwork, and are found fully to answer the purpose for which they were intended, leaving the tunnel entirely free from any offensive vapour immediately after the transit of each train, and their magnitude can only be estimated by standing in the tunnel and looking upwards.”

Those who erected scaffolding for recent brickwork repairs in the shaft had the opportunity to do just that.

Hands on

That project first appeared on AMCO’s radar in autumn 2016 as part of its LNW South CP5 Renewals framework with Network Rail. Works to the lining within the tunnel itself were also planned.

Initially the company was asked to determine the condition of both large ventilation shafts – Nos. 11 and 12 – which involved specialist engineers from XEIAD carrying out tactile surveys by rope access during four Saturday night ‘Rules of the Route’ possessions. While the outputs of these were a detailed examination including scheme drawings, budget constraints meant that repairs were prioritised in the deeper shaft where defects were greater in number.

Generally, AMCO’s preferred means of access for shaft repairs is to install a cradle, suspended from above. But the castellated shaft turrets at Kilsby are Grade II* listed, which would have created difficulties in terms of tying in a scaffold from which to support the winches. Also, the cradle would have been colossal – probably not what you’d want to be hovering over the West Coast main line.

The only other practical option was a full scaffold, bringing every part of the brickwork within touching distance. Network Rail agreed with this approach. Four scaffolding contractors submitted prices; Abbi Access Services got the job.

Ups and downs

Abbi commissioned RDG Engineering to design the scaffold and a work scheme whereby it was erected in two phases. The first required low-level structures to be built either side of the railway from which a crash deck would span the two tracks, together with a staircase on the shaft’s east elevation to a sealed doorway in the protection wall at ground level which AMCO reopened. This work was progressed in Saturday night possessions with the overhead line equipment isolated, materials being transported by RRV from the main compound, located a mile south of the tunnel.

Thereafter, the remaining scaffold could be assembled in what effectively was a high street environment – safely separated from the railway, all the components being lowered by hoist from a secondary compound squeezed between the shaft and the adjacent A5.

Constructed using Plettac Metrix equipment, the scaffold was founded on timber base pads laid on the ballast, an approach designed by COWI UK following ground investigations. The crash deck – comprising X-beams and support transoms overlain with scaffold boards and plywood – was carried by seven towers on the Up side and eight on the Down, the latter being required due to the presence of two signalling cabinets.

The deck sat 780mm above the 600mm OLE exclusion zone, four temporary bonds being applied – to a design by PBH – for earthing purposes between the scaffold and OLE stanchions. A central vent allowed air pushed upwards by passing trains to escape without lifting the deck, although their entry into the tunnel was still felt by anyone standing on it.

During the possessions, Abbi built three of the towers to full height and used them to support the staircase by means of ties and diagonal wind bracing. Thereafter the remaining towers were constructed radially to create 25 single-lift rings, each two metres in height. The whole structure was tied into the brickwork using M16 threaded rods, secured by Minova Lockset resin, which were then subjected to load testing.

Directly above the tracks, three infill towers had to be erected at both ends of the shaft, initially standing on the deck. However, after two lifts had been constructed, pairs of CL25 brackets and ladder beams were installed to transfer the load from each tower into the masonry.

Take two

Having spent many weeks establishing the means of access, the repairs themselves were remarkably routine: breaking out and recasing areas of hollow brickwork (accounting for about one-fifth of the shaft’s surface area), extensive repointing and the removal of 180 years’ worth of accumulated soot and vegetation. It’s the sort of high volume activity that’s undertaken every midweek night without anyone batting an eyelid.

But what made this job spectacular were its setting and the complexity of the platform on which the workforce toiled. The red-brick shaft turret offered no clues as to the sight that would be revealed after walking through the innocuous doorway in its side.

Dave Thomas, AMCO’s contracts manager, endured a sleepless night or two. “As soon as you start scaffolding over the West Coast main line – with a linespeed of 110mph and live overheads – in midweek day shifts while trains are running, you need a lot of confidence in your design, in your scaffolder and in God!” he reflected.

Network Rail’s project manager, Ellen Dean, said: “The successful installation of the full scaffold enabled our team to conduct additional examinations. This has resulted in Network Rail including additional scope into the repairs, reducing the maintenance in the shaft as well as creating efficiencies for the client, and reducing the requirement to revisit the shaft for many more years to come.”

Northampton’s refuseniks succumbed to the railway age in 1845 when the London & Birmingham drove a cross-country route to Peterborough. Whilst several later tunnels have full-width shafts – Morley and Bramhope being examples from the 1840s – none boast Kilsby’s dimensions. This is a project that will only be repeated when the tunnel’s other big shaft receives the same attention.

Read more: Read the May issue of Rail Engineer here