During the diversion of 3.8kms of the River Nith, at the House of Water, Ayrshire, to gain access to one million tonnes of underlying coal deposits, Scottish Coal needed to reinforce the new clay riverbed base to prevent the water leaking away. The solution adopted was the installation of Tensar SS40 geogrid as a strain control to stop fissures from forming in the clay beds with subsequent loss of water, arising from any settlements within the underlying deep backfill to a former opencast pit.
SS40 grid is the stiffest biaxial grid Tensar manufacture and so picks up the strain very quickly, to control differential movement in any direction. Around 120,000m˛ of geogrid was installed in one or two layers within the clay bed.
The clay liner and geogrid layers form the base of the riverbed and are laid on top of 80m of thick mudstone, shale and sandstone overburden used to backfill the underlying worked out pit. The clay liner is up to 50m wide in places, and a 4-5m wide channel has been dug through it to accommodate the river. The channel is lined with 500mm of graded gravels and topped with a 250mm layer of large gravels and cobbles. Coir matting is secured to the banks to prevent erosion and allow vegetation to become established.
The River Nith is one of Scotland’s largest and most important salmon rivers and, having been re-routed once previously, this move returns the river to its approximate original course. Whilst the actual diversion of the river is a major task in itself, the success of the whole project depends on creating a habitat for the fish to return. Scottish Coal’s David Booth commented, “Twenty percent of the job is the river diversion and the rest is creating the habitat which is what we’re measured on.” Liaising with the Technical Working Group, consisting of local organisations involved with the management of the river including SEPA and the River Nith Fishery Board, the diversion was designed to replicate the existing River Nith and allow the salmon to successfully return to breed.
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