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Taking The Strain

A water filled dam system and a cooperative canal manager can help to ease the pressure on replacing a railway bridge in Hampshire. Adrian Greeman reports.

A standard bridge replacement to upgrade the load capacity of an ageing crossing on the Pirbright to Ashvale rail line in Hampshire should have been straightforward. Instead it has become an interesting exercise in innovation and cooperation for contractor Edmund Nuttall and client Network Rail.

The bridge crosses a waterway, the restored Basingstoke Canal, owned by Surrey and Hampshire County Councils. This would not normally be a significant problem. Cutting a steel deck away and dropping in a new one should be possible from the side using a crane.

"Unfortunately though, the abutments must also be replaced," explains Gez Roberts, delivery manager for Nuttall in this region. The old Victorian bridge is supported on large cast iron columns he explains. These units are 1.2m in diameter, each with 25mm thick iron walls and a concrete filling, and should be capable. "But cast iron is brittle and they could be fractured somewhere along their length."

Geophysical tools cannot pinpoint their condition precisely and so it was decided to remake the abutments, using piled foundations; these would be done from the canal banks with a mini-piling rig.

The canal needs to be kept open," says Roberts, "because it has become a major local amenity." It is also a designated Site of Special Scientific Interest for most of its length, because of the abundance of rare dragonfly species in the area.

Access to the site was likely to be troublesome. Although there was a towpath on one side, the condition of some sheet piling forming the canal was uncertain."We also had very low headroom," says Roberts' colleague on site, site manager Rick Kopec. In the event, he says it could have proved dangerous since a suggested 6m length for the sheet piling from TDR site investigations proved over optimistic. "They were 2.1m and I think the canal bank might have given way under the weight."

A complication emerged with the trial piles into the local Bracklesham Beds, a type of sand and gravel mixture which, while firm enough undisturbed, loses structure when impacted.An alternative scheme had been mooted by designer Tony Gee and Partners for a concrete culvert. The structure would support a shallow embankment above to carry the tracks and eliminate the bridge. It was much cheaper at around £1.2M instead of over £2M, and required shorter possessions too.

But it meant closing and draining the canal. Nuttall and the client embarked on discussions with the Basingstoke Canal Authority and its director Ian Brown. He proved an enthusiastic supporter for engineering innovation, says Kopec, and willing to consider alternative solutions. "His help has been key on this project."The innovation was a new temporary dam system that Nuttall discovered after trawling its company knowledge base and then exploring the internet. The Aquadam, from the USA is an "inflatable" tube which is filled with water, unfolding across a waterway and blocking it. It works up to 2m depth.

Once filled and the watercourse dewatering is completed on one side, the Aquadam is subject to a "rolling" pressure from the full height of water on one side. It resists by using a U shape for the inflated tube. The tube doubles back on itself inside an outer sleeve and, as it fills, the two halves are pushed against each other forming a vertical central wall. Friction between the two tubes prevents the water head pressure from turning the tube.

Friction also prevents the tube sliding along the course, helped by the fact that the geotextile fabric tends to mould to the shape of the river or canal bed.

It is simple to erect," says Roberts. "The U-tube is like a skeleton as it is pumped full, stretching the dam across the canal. It takes only as long as the pumping time." An additional safety benefit is that no one has to be in the canal.

Such a non-disruptive method helped persuade the canal authority, the Environment Agency and various environmental watchdogs, to accept a short length of blocked canal.

"The canal has a tendency to lose level in the summer months which means that canal traffic is reduced anyway," says Roberts.

Ironically this was not the case last summer which saw disastrous flood level rainfall. Not only did this delay the construction work but closure had to wait longer because the canal trip season was extended to take advantage of the "Indian summer" that followed. Work finally began at the end of last year.

With a 35m length of canal now emptied out Nuttall has been able to press on with temporary works. Short sheet piles are going in to support the base of the rail embankment."After that, the work is conventional concreting essentially," says Kopec. The culvert will be formed in two halves because of headroom constraints under the railway above, which can remain live for the whole project.

"For the same reason we will use Lafarge self-levelling concrete for the soffit, which means we don't need to find ways to vibrate the concrete or get work crews to the track," says Kopec.

1. AQAUDAM DEPLOYED
Tube unfolded across the waterway

2. INFLATING TIMES
The tube is like a skeleton as it is pumped full

3. DEWATERING
Once the tube is inhaled the waterway is pumped dry

4. WORK BEGINS

With the waterway empty work can begin

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