Canal channel contract complete
The fifteen strong team of Abergavenny based contractors Alun Griffiths Construction set off for home at 4pm on Sunday 2nd June, leaving behind a very impressive piece of 21st century canal engineering.
A look back in time
The Thames & Severn Canal was constructed alongside the Frome in the 1780’s.
It passed through the site of a woollen mill that had used the power of the river since the 1400's.
Railways, roads & rubbish
In 1844 a railway viaduct spanned the valley and the canal.
In the 1970’s the area was used as a landfill site.
In the 1980’s Stroud’s bypass, Dr Newton’s Way, was built on the line of the canal at this point.
The new channel forms a ‘dog leg’ route as it winds through the railway viaduct and Stroud’s former domestic waste tip.
Impressive facts & figures
The topography means that the channel width ranges from 6 metres at its narrowest point through the viaduct to 26 metres at the widest point where it will re-join the line of the original canal.
The total length of the new route is around 325 metres, of which 110 metres is edged with sheet piling.
The bottom of the canal to the west of the railway viaduct is lined with a 600mm layer of blue clay. This retains water to a depth of 1.5 metres.
18 metres of new water main now passes safely 3.5metres beneath the canal.
183 metres of channel to the east of the viaduct has been constructed in contiguously bored concrete piles to provide a retaining wall 35 ft. high at its central point.
The piles range in depth from 9 metres to 15 metres.
Well over 200 tonnes of steel reinforcing bar and timber formwork provided the structure to allow the pouring of 3,600 tonnes of concrete to create the channel.
The retaining wall of piles has been capped and strengthened by 53 multi strand ground anchors.
The installation of ground anchors to secure the wall involved drilling 35 metre deep holes, down into the bedrock, at an angle of 35 degrees.
The concrete lined channel is supported by 208 bottom driven piles to depth of 3 metres.
Public interest in open days
Over 1,800 people took the unique opportunity to walk along the bottom of the new canal channel on Friday & Saturday 10th & 11th May.
That channel has now been filled with five million litres of water to a depth of 1.5metres.
Post and rail fencing, made more secure by the application of wire mesh panels, has been installed by local landscaping company Gardiners.
Key to the pictures
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Rubbish to remain on site – but out of sight
Many tonnes of 1960's & 70's domestic refuse were excavated to make way for the concrete channel. All that waste has remained on site.
It has been capped by a 600mm layer of crushed recycled concrete ballast. This layer has been topped with a huge carpet of Bentomat. This is a double layered textile sandwich that encapsulates granulated Betonite.
Halcrow's principal Environmental Scientist Clive Williams was on site recently to supervise its installation. When asked to explain the Betonite filled 'Bentomat' and its purpose he replied, "It is a natural mineral that soaks up water and expands. It is best understood if you think 'cat litter'."
The matting has, in turn, been covered with layers of subsoil and top soil all sourced from the site. The Griffiths earth moving artists have sculpted this top layer into a smooth profile ready for the next stage.
Gardiners will soon return to site to carry out the hydro seeding process. This is the system whereby a water based mulch of grass seed and fertilizer is sprayed over the whole surface of what the men from Abergavenny have dubbed 'Mount Griffiths'. This will very quickly create a green sward over the former rubbish tip. The site will also be seeded with a wild flower mix.
The nature of the substrate of recently relocated and dampened domestic waste may mean that some methane could be created. The tubular vents currently projecting from the surface will vent this away safely.
See the result
The site is now open to the public who are advised to follow the paths through the site. It is well worth a visit to view what will become a valuable waterside open space asset to the people of Stroud.