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IRKSOME PROBLEM

GEOTECHNICS OF TRANSPORT

Geosynthetics are being used to repair an historic canal embankment that last year failed for the second time. Damon Schünmann reports.

Rochdale Canal north of Manchester has a past that has come back to haunt nearby residents.

In 1927 the wing walls of an aqueduct that crosses the River Irk, at Chadderton in Oldham, failed during ooding that locals say claimed four lives. Last year this section of the waterway failed again.

'In April 2005 we got an emergency call from a resident saying the embankment at the bottom of their garden no longer existed, ' says British Waterways project manager Neil D'Arcy.

'The fist issue was to make sure none of our customers - boat users - were in the area, which they weren't. The next was to preserve the ecology at what is both a site of special scientic interest (SSSI) and a candidate special area of conservation (cSAC).

'The third issue was to control flow into the area as soon as possible because the canal to the south is only fed water from the north. This meant the water level to the south was down, so we needed to protect against a fall from height risk to the public, ' D'Arcy says.

'Also, any water coming from the north would ow through the hole in the embankment next to the aqueduct.'

The stricken section had to be dammed and the canal was closed between locks 54 and 83. Pumps were installed to transfer water from the north end, past reinstatement works and on to the south. 'The pumps handle 18Ml a day to maintain the feed to the southern area and keep the canal water in Manchester at the correct height, ' D'Arcy says.

Jarrod Parkin, project manager for main contractor Galliford Try Construction, says: 'We put the bunds in last May and began reinstatement works this January.

The over-pumping allows navigation to the north and south of the site.'

D'Arcy adds: 'But the overpumping is mainly for health and safety reasons as we've got almost 20km of canal between here and Manchester.'

The contractor is employed under a four-year framework contract that sees it doing about 90% of British Waterways' works in the north of England.

An extensive ground investigation concluded that the embankment failure was due to sandy fill used in the original construction becoming saturated from leakage and eventually liquefying.

'The problem with the old construction was the puddle clay [used to waterproof the bed], ' Parkin says. 'Over-dredging can eventually go through it to the underlying material.'

D'Arcy says: 'We believe the aqueduct was built of ne of the original river and once complete the river would have been diverted under it, meaning the embankment was built over the old river bed.

'So there is a tendency for the river to want to go the way of the old alluvial deposits [causing instability to the embankment] and I think this is an inherent weakness.

'When it failed in 1927 it pushed over the south-west wing wall which was replaced by brick instead of the original stone, and the repair did not have the engineering techniques we have today.'

In last year's collapse, a considerable volume of water swept through the breach. This was because the aqueduct sits within a 2.4km long pound (between two locks), a lengthy stretch by Rochdale Canal standards.

'That is why the scour was so great, ' D'Arcy says.

The failure not only undermined the bed of the canal but, as the support gave way, it brought down the end of the south-east stone wing wall of the Irk Aqueduct. However, there was no structural damage to the aqueduct itself.

At this point the embankment is 6.4m high and the canal bed here is being replaced by a concrete channel over a foundation of well graded, compacted granular fill. Although this channel will extend the 170m length of the repair, it will only contain reinforcement in the 45m long central aqueduct area.

Beneath the embankment, at both ends of the aqueduct, a drainage layer will dissipate any water ingress and discharge it to the river below.

D'Arcy explains that a bottom carpet layer will protect an impermeable liner from puncture from the formation below. This safeguard will be needed once repairs are nished and the canal's water load returns.

A top layer of carpet will protect the 3mm thick Alkorplan thermoplastic impermeable liner against puncture from above.

D'Arcy says: 'Because of the dams at either end, the original puddle clay liner has been exposed to the elements and that may have had a detrimental effect. We couldn't guarantee the integrity of this material so we replaced it with the Alkorplan. The technology for the impermeable liner comes from landfill sites where it is used for leachate management.'

Parkin adds: 'The concrete canal channel above is there purely to protect the [underlying] impermeable liner from over-dredging and future waterway users.'

Reinforced concrete, without the membrane liner, is being used in the aqueduct. It will consist of four 10m bays with water bars in between to allow for movement and expansion.

Wash walls are being rebuilt over the sides of the concrete channel to retain the previous appearance of the canal. But they are no longer needed for erosion control as the membrane will protect the underlying sands and gravels that now form the embankment.

'Apart from the aqueduct, where you might see a little band of concrete, the rest will look as it did, so it's a meeting of the old with the new, ' Parkin says.

Galliford Try is rebuilding the failed section of the aqueduct's 6m high embankment using a Tensar geogrid for soil reinforcement. This will be for the full height to intersect and resist potential shear surfaces.

About 8m wide by 30m long, the area of the geogrid stabilised slope has a general angle of 45º and abuts the canal channel. It also supports the towpath at the top of the slope.

Galliford Try Construction foreman John Milton says: 'The slope is being constructed with well compacted 6I/6J graded granular fill reinforced with horizontal layers of geogrid. Embankment construction like this is extremely easy as the slope remains very stable as its height grows.'

The embankment is being built in 1m high sections with the main Tensar reinforcement used at the bottom and top. Secondary reinforcement is also used within each 1m lift at one-third and twothird height intervals.

Once the bulk earthworks are complete, the face of the reinforced soil slope is covered with topsoil and Tensar Mat, a flexible matrix of polymer filaments. The mat, which is anchored at the crest and toe, retains the topsoil in the short term and provides long-term erosion protection by reinforcing the root system of the vegetation.

D'Arcy says: 'Conventional techniques for reconstructing this embankment and stabilising the slope, like piling and the use of concrete reinforcement, would have been very costly and potentially taken much longer to complete.

'These techniques could also have had detrimental effects on the stability of the adjacent aqueduct's masonry wing wall, which was exposed by the embankment failure.

In May, when Galliford Try finish, we should be back in water and have traffic, ' he says.

The project is worth about £750,000 to the main contractor with an additional £450,000 for the pumping scheme, design costs and site access trackway.

The Rochdale Canal The 200-year-old Rochdale Canal connects central Manchester to the Bridgewater Canal at its southern end, and 53km further on, links to the Calder and Hebble Navigation which continues to the east of the Pennines.

It was the first of the three trans-Pennine canal routes to be completed (the others being the Leeds and Liverpool and the Huddersfield Narrow) and transported craft carrying coal, salt, grain or textiles in payloads up to 70t.

Built between 1794 and 1804 from both ends simultaneously, the canal rises as high as 183m at its summit (requiring extensive reservoir feeding) and craft must negotiate 92 locks, an average of nearly three per mile. Between 2000 and 2002 the canal was restored by British Waterways and The Waterways Trust.

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