Completion of the towpath side retaining wall on Friday 14th October has allowed the length of canal between Wallbridge Upper and Lower Locks to be refilled with water.The technical explanation for this project is a 65metres long by 3metres high ‘Porcupine block’ sloping revetment wall with a 7½° batter, has been installed.
Definition & explanations
- Revetment — A sloped facing in stone or concrete to retain an earth embankment particularly to deal with flowing water
- Batter — In construction terms ‘batter’ describes a receding slope of a wall, structure, or earthwork when a wall is intentionally built with an inward slope
- Porcupine block wall — A revolutionary and versatile concrete block retaining wall. The Porcupine wall is a gravity wall system that uses its mass, times the angle of repose, to retain earth. The system needs no mortar and requires the minimum of plant and machinery for installation. As the height of the block wall, and the angle of the batter, is increased, then the retaining mass must increase. The height and steepness of the batter angle of the Wallbridge revetment called for a retaining mass of over 100 tonnes of concrete, reinforced with steel mesh, to be installed behind the blocks in order to retain the earth embankment
- Porcupine block — The individual Porcupine block element is an interlocking curved concrete block which achieves stability by the spines on the top surface of one block matching those on the bottom of another. These details and the graphic below were kindly provided by the supplier, Ruthin Precast Concrete (RPC) of Denbighshire.
Paul Weller is pictured here with a Porcupine block
Volunteers and professionals
The work has been carried out by a collaborative team of volunteers and professionals.
Dave Marshall, Stroud District Council’s Canal Manager, commented, “It has been a seven day a week operation to get this completed in order to comply with the Environment Agency fish spawning directives.
“This has been achieved by a diverse group of people working together. The teams have come from Stroud District Council’s Tuesday & Thursday Group, led by Paul Weller and Jon Pontefract, Cotswold Canals Trust Western Depot volunteers and members of the Waterway Recovery Group. We even had a group of Network Rail’s administrative staff on a team building exercise. They have all pitched in.
“We mustn’t forget the professionals too of course. Gardiners Construction based at Chalford installed the concrete base at the start of the project and then came in again at the conclusion.”
Paul Williams, (pictured below right) a groundwork machine operator with Gardiners (and a Minchinhampton resident) commented, “It has been really good working on this job. We all love the fact that the canal is coming back into action through Stroud. I have been really impressed by the volunteers and the way Paul Weller leads them. People of all ages and different abilities and skills all getting stuck in. And look what we’ve done!”
Utilisation of available resources
Paul Weller applied his characteristic ingenuity and practicality when he and Waterway Recovery Group volunteer Martin Thompson together with Dr. David Lyle, a regular Tuesday & Thursday Team volunteer, worked long shifts over a weekend to construct a comfortable and safe working facility for the volunteer ‘block layers’.
The ingenious trio utilised the wooden pallets on which the Porcupine blocks had been delivered to construct the timber boardwalk illustrated in the photo below.
Martin Thompson is seen relaxing on that boardwalk. It was his birthday by the way!
- 45 people, mainly volunteers, installed 5,200 porcupine blocks
- Those blocks weigh 19.5kg each
- Which makes a total of just over 101 tonnes
- And that is just the blocks! Before, and during, block installation a great deal of steel reinforced concrete was poured to create the base of the wall and its retaining mass
- The result will be a better defined and wider water channel with a hard edged towpath alongside
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