Client : May Gurney Rail working for Network Rail
Date : March 2006 – April 2006
Scope of Work : 3 km of bank stabilisation works using pre-established coir rolls, pallets and brushwood revetments
Value : £205,000
Background:
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The Foss Dyke is believed to be the first artificial navigation canal in the UK and was probably dug by the Romans around 120AD to link the nearby Lincoln (Lindum Colonia) to the navigable River Witham. With the decline in commercial boating and the recent rise in pleasure boating on the Foss resulted in larger pleasure boats and greater boat wash. The rail embankment that follows the Foss began to erode in areas where sheet piling was not present or was starting to age and fail. Remedial works were planned for 2007-8, but close monitoring revealed that the rate of bank erosion was far more rapid than first thought (up to 2.5 metres in a year over a 2km stretch). Hard revetments, such as sheet piling or gabion baskets were considered but dismissed because of the ecologically sensitive nature of the site. King Fishers and Water Voles were recorded on the stretch of bank and therefore a solution was required that could meet the vital engineering criteria but also enhance the marginal habitats for the Water Voles.
Design:
The basis of the design was to use pre-established coir rolls to absorb the wave action caused by the increasing boat wash. The coir rolls needed to be set on deadwood faggots in order for the coir roll to sit at the correct level. Initial plans looked at importing approximately 2000 m3 of stone backfill to fill the scour hole. However, when Salix were appointed as specialist sub contractor to main contractor May Gurney an alternative and innovated backfill solution was introduced as part of a value engineering exercise. As part of the works Salix were due to clear over 3km of shrubs and trees that were growing on the railway embankment. This material was due to be chipped on site but it was clear that the material was ideal for use in a “brushwood” revetment used to fill the scour holes. The brushwood was staked into place using chestnut fixing stakes and a 1m wide pre-established coir pallet was placed on top of the brush revetment at water level. This now meant that a 1.3 metre wide vegetative revetment, ideal for absorbing wave action and excellent water vole habitat, could be created for lower cost and with greater sustainability than the proposed imported stone backfill.
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Construction:
Works were started in early March 2006 with the initial program of 22 weeks being greatly reduced by the speed of construction using the brushwood revetment to only 13 weeks. Salix workforce included 8 men and a specialist long reach excavator sat on a floating pontoon.
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Results:
The coir rolls have been effective from the day of installation at absorbing wave action and preventing further erosion to the rail embankments. As time goes on the system will get stronger and will self sustain itself with whilst providing over 2km of enhanced water vole habitat.
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The coir rolls and pallets used were pre-established at Salix’s two nurseries in South Wales and Norfolk. Each plant was rooted right through the organic revetments so that there was no risk of the planting being washed out by the wave action. Such was the maturity of the coir rolls that within a month of installation (April 2006) the plants in the rolls were producing new multiple stems from rhizomes runners within the coir Thus the rate of vegetation establishment has been very rapid.
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PHOTOS BELOW: 5 MONTHS AFTER WORKS WERE COMPLETED
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