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Wednesday, August 4, 2021

Did the earth move?

A scary sight – the rails disappear below a mountain of red earth, rounding the corner on the Up line four miles from Barrow. But this collapse at Lindal Tunnel in Cumbria was intentional and trains were halted as QTS Group performed £0.5 million of stabilisation work on behalf of Network Rail. When the lines reopened after a three-week blockade, the tunnel was 25m shorter and over 24,000 tonnes of rock and clay had been removed.

Rock cover

The tunnel’s south portal was unusual: instead of a typical crenellated entrance embedded deep within a cutting, a natural thin rock-shelter projected out across a side-slope. The unlined limestone provided a mere 2-3m cover over the crown of the tunnel. This unbalanced arrangement proved weak and, over the years, an ever-increasing assortment of concrete beams and rock anchors had been added.

With tunnel movements continuing, further stabilisation was needed. However, so much stitching and ironwork had already been installed there appeared to be little sound rock left to fix anything else to. Instead Network Rail decided to amputate the weak section and cut back the tunnel to sound rock.

Major surgery like this does not happen overnight. Initial programming suggested that five weeks would be needed. Negotiations with the train operating companies established a three-week window from 27th March to 18th April, coordinated with the works at Arnside Viaduct on the same Barrow-Carnforth line.

One of the measures taken to ensure the railway could be reopened as soon as possible was to protect the track in situ during the demolition. Critical to this was a 3m berm of earth pushed 35m into the tunnel, calculated to absorb the maximum impact of a 1.5m3 block of limestone falling 6.5m as the tunnel crown was demolished. The earth berm sat atop timber road-plates and steel plates to protect the rails and the cable routes. Below this, two layers of geotextile prevented contamination of the existing ballast, with additional stone creating an even platform at rail-top level.

Blind shaft

Works were complicated by the presence of a blind shaft close to the south end of the tunnel – one of the four construction shafts. Surveys were carried out in 2006 during Mott MacDonald’s initial design to determine the scope of work. Geophysical surveys by Zetica used a combination of ground resistivity imaging, electromagnetic profiling and in-tunnel ground penetrating radar to establish the position of the shafts within the lined section of tunnel at the north end. The proposed design called for the new unlined south portal to be positioned just in front of a brick-domed blind shaft.

Five years later, with funding confirmed for the works to proceed, a risk review by the delivery team identified uncertainty about the rockhead level at the blind shaft. Additional surveys were carried out three weeks before the blockade and the rock was found to be higher than previously thought. Network Rail and QTS identified a value-engineering opportunity to remove the shaft rather than infilling with concrete, to eliminate this maintenance liability.

Mott MacDonald had been retained by Network Rail as designer and to provide engineering site support, due to its previous work on the tunnel. A swift redesign turned the change to advantage, as it meant that the rock could be cut back beyond the shaft yet retain the same slope profile in the clay above. The latter was essential as the slopeworks had been carefully designed to fit within the land boundaries. The rock is cut to 70 degrees at the portal, with the clay following a 1 in 2.5 slope.

Dig for victory

The dramatic red earthworks rise 25m vertically from rail level to skyline when I visit site during the blockade’s middle week. The distinctive colour is caused by the local haematite iron ore which was largely responsible for the development of industry and railways in this area.

In charge of the dig is QTS site manager Keith Robinson. “Logistics are complicated by the restricted access,” says Keith. “We have up to ten wagons on a 20-minute turnaround to the tip and reckon on up to 25 loads per wagon per day.” The landfill site is a local quarry.

Two haul roads were agreed through painstaking discussions with the neighbouring landowners. Even so, all the soil must be handled to the top or bottom of the slope. Two 20t excavators and a 13t machine work the top end while two 35t excavators serve wagons at the bottom. “We are shifting up to 3,400t per shift” says Keith, “and in all we will need around 1,200 vehicle movements.” The weather remained mostly fair, so all operations were completed during the 12-hour day shifts without needing to use the planned contingency of night work.

The sequencing has needed some thought. “We couldn’t risk sitting any plant on the old tunnel roof, so first that had to be demolished. We’ve then had to infill again with soil to create a berm for the excavators to work off to reach the rest of the slope. Later on, we’re going to have to sort the materials as we can recycle the rock but have to dispose of the clay,” Keith says.

Once all the muck is cleared away, there will be extensive rock faces exposed – these include both the new portal face and the existing rock cutting on the approach. Over 500 rock bolts are being drilled and fixed to stabilise the rock, then rock netting installed to cover the surfaces. All this is being done by QTS’s in-house roped-access specialists. Network Rail took advantage of the blockade to instruct additional stabilisation work by QTS along the length of the cutting adjacent to the tunnel.

Lindal’s big hole

Ultimately, landscaping will patch over the raw scar of the earthworks and the upheaval will fade from sight – as have a couple of earlier convulsions that Lindal has known.

Driver Thomas Postlethwaite was somewhat surprised one Thursday morning in September 1892 when a 60-foot wide hole opened up below his engine without a great deal of warning. He jumped clear as Furness Railway No.115, a Sharp Stewart 0-6-0 tender locomotive, along with the main lines and a number of sidings, disappeared without trace into the workings of the Parkside mine. As far as is known, the loco is still down there.

The tunnel has also had previous modifications. It was built in 1851 for a single track as an economy, “in consequence of the monetary crisis of that period”, according to a contemporary report. Little changes! However, within four years the rapidly increasing mineral traffic meant two tracks were needed. By this time it had also been realised that a minimum-clearance single-track tunnel was highly undesirable both for operational and maintenance reasons as it gave no chance for the train exhausts to disperse. Despite the contractor’s best efforts to promote a second parallel single-bore tunnel as being easier and cheaper to build, the railway company instructed the existing tunnel to be symmetrically widened to take two tracks.

The railway company also mandated that works had to be carried out without disruption to its operations. Amazingly, working in the 9pm to 5am ‘white period’ slots which were allowed, contractor William Tredwell completed the delicate enlargement operation, with 9,601 trains passing through the tunnel during his works.

The expanded tunnel was considerably wider and higher than the former single-track tunnel and also 100 yards shorter. These changes probably explain the unusual arrangement at the south portal where originally a greater rock cover would have been provided to the smaller single-bore tunnel.

So the current works by QTS and Network Rail may be correcting a historical quirk. Regardless of that, they have certainly reduced the maintenance burden on this stretch of line, with light once more visible through Lindal Tunnel.

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