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Mission possible

Inland waterways; Standedge Tunnel Reopening Britain's longest canal tunnel, once regarded as a permanently abandoned structure that was uneconomic to repair, epitomises British Waterways' New Canal Age. David Hayward reports from deep beneath the Pennine

The word 'impossible' is one civil engineers dislike and seldom have to resort to saying. A bitof rock bolting, sprayed concrete lining and silt removal from a canal tunnel beneath the Pennines would also not seem to warrant anywhere near such a drastic admission.

But, just a few years ago, the £5M repair scheme now under way inside British Waterways' Standedge Tunnel west of Huddersfield, was officially labelled 'the impossible restoration'. For Standedge is no ordinary tunnel.

At 5.2km, it is by far BW's longest; at 197m above sea level it is the highest on the canal network and, routed 194m beneath bleak Marsden Moor, it is also the deepest.

Yet even such daunting statistics do not deter engineers; despite adding 10,700t of 2m deep silt and revealing the structure as a narrow, 2.1m wide largely unlined tunnel abandoned 56 years ago entombing rockfalls totalling 3000t. Such figures serve only as an engineering challenge.

What dubbed the scheme 'impossible' was not so much its engineering impracticality but a lack of cash, need and, most of all, will.

'Until about ten years ago there was no interest in this remote, inaccessible and abandoned tunnel which a then inward looking BW saw only as a liability,' says contracts manager for the repair work David Child. 'And without the hard work of the Huddersfield Canal Society and local authorities, it is likely the restoration would have remained impossible.

'But in the 1990s there was a sea change in attitude and political will. We started looking at the wider potential of reopening routes like the Huddersfield Narrow Canal - once an important transpennine link for which unblocking Standedge Tunnel at its summit is a key element.'

Such canals, said BW, could provide 'corridors of opportunity' along the entire 32km route, generating not only leisure development but also retail and commercial enterprises.

With the Huddersfield Narrow classed as a 'remainder' canal, maintained only for its role as a drainage channel, BW could only contribute £430,000 - the equivalent of it maintenance expenditure - towards the £32M needed to reopen it. But now, armed with £29M of partnership cash, mainly from English Partnerships and the Millennium Commission, BW is adding the canal to its national list of 322km of waterway restored over the next few years.

Yet, for this remote part of the Pennines, the 'New Canal Age' should arrive a bit earlier when, around Easter next year, the route from Huddersfield to Ashton-under-Lyne near Manchester will be completely reopened.

To achieve this aim, more than a quarter of the canal's 74 locks, cascading down either side of the tunnel, are being refurbished. To the west in Stalybridge town centre, an 800m infilled section is being recarved between streets and buildings. And in Huddersfield to the east, 320m of the original route - now lying buried beneath two later mills - is being unearthed as a cut and cover tunnel.

But, as during the canal's 133- year active life, it is the highest point - Standedge Tunnel -that lies on the refurbishment project's critical path.

Blasted through Pennine sandstones and mudstones during the early days of black powder dynamite, the 189-year old tunnel remains in good structural condition. It was after its closure in 1944 - for financial rather than engineering reasons - that most damage occurred.

Contractor AMCO's prime challenge during its 18-month refurbishment contract begun last May, is not the tunnel itself but what lies in it. Canal closure did not stop this 5.2km conduit acting as a natural drain, receiving both runoff and silt from the sandy peat hills above. But it did stop that silt flowing away either side down through the locks.

The result, after the tunnel was drained for repairs, is a 2m layer of fairly stiff jelly-like deposit in the invert which gave AMCO an important decision to make - dig or suck.

The standard solution would have been to cut out the silt by hand or excavator, removing it from the tunnel by rail mounted skips. But the contractor, noting the fineness of the silt particles and the problems in keeping out some 30 l/sec drain water, opted instead to pump it out.

'We did not know of silt ever being pumped like this over such long distances and we had no idea if it would flow successfully,' recalls AMCO project manager Keith John. 'But, after some teething problems, the system is working well and we are now exceeding our programmed rate of clearing 200m of tunnel per week.'

Teething problems involved the use of a sophisticated suck and blow pump which simply lacked sufficient power. Now the contractor has resorted to standard sludge pumps and should have all 16,000m3 of silt out by the end of this month, four weeks behind programme.

But the temporary setback is not critical as follow on permanent repairs inside the cleaned up tunnel have been accelerated to keep the overall contract on schedule. And AMCO still plans to finish all tunnel repairs by late summer, four months ahead of the official December target.

The £1M mucking out operation includes desilting pumped effluent when it reaches one of the tunnel portals. Here it is fed through a mobile treatment plant, boasting vibrating screens, hydrocyclones, centrifuges, plus settling tanks and holding lagoons formed in the dewatered canal bed. The end result is sludge cake suitable for landfill and effluent clean enough to return to the adjacent River Colne.

Beneath the silt lies further debris, some 3000t of loose rock - the result of at least four major rockfalls from the tunnel crown. But here the refurb team hopes to economise by taking advantage of the tunnel's unusual profile.

Standedge was built ovoidal shaped - narrow and deep. Its narrowness was to economise on excavation and its 8m to 10m height - double the norm in places - was to allow increased water storage. The tunnel, located at the canal's highest point, is its 'summit pound', feeding fresh water supplies down either side of the route.

The result is an average water depth of 2.5m, roughly 1m more than is required for the boats. The water level will be lowered to accommodate today's taller craft, but only by 250mm so that some of the fallen rock beneath can be left in the tunnel and simply spread out along the invert.

A working platform, erected mid height along the tunnel, allows silt removal from the invert to continue during installation overhead of rock bolts and mesh to strengthen about half the tunnel crown and sides. Stainless steel is used for both the 5000 bolts and the mesh so that, hopefully, little further maintenance will be needed during the rescued tunnel's new 125- year design life.

The 45% of the tunnel that is already lined with either masonry or brickwork requires only pointing. Shotcrete, along with mesh and bolts, is needed to line a total 2km of the crown especially around rock fall areas.

The usual nightmare of difficult access to such a long, narrow and confined construction site is not an issue at Standedge. The reason for such an easy journey for both men and materials is not the canal excavation itself but the rail tunnels that lie alongside and slightly above it.

Three rail tunnels were bored along the same route but only one remains in use today to carry the main transpennine route. The earlier two tunnels - the first completed in 1849 and its neighbour 22 years later - have long been abandoned and their tracks removed (see box).

But they were built using the canal tunnel itself for access via 36 short cross adits dug between the tunnels. The old rail bores have been well preserved, and Railtrack is considering reopening one as a dedicated new freight line. But, at present, BW's refurbishment team can gain some access from them to the adjacent canal tunnel.

Ironically, it is the railway - the very medium that led to the canal's early death last century as a commercial transport route - that today contributes significantly to its rebirth for the 21st.

Why read this

5.2km long tunnel to beunblocked

10,700t of silt being pumpedout

3,000t of rockfalls to reprofileor remove

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