Project Brief

Stonebridge Drain is located just North of the city of Boston, East Lincolnshire. The Drain is one of many main river flood water discharge water courses that serve the West Fens of Lincolnshire.
 

Exposed asbestos sheet piling photo.

The Stonebridge Drain generally comprises of earth banks which have a mixture of toe protection construction. This predominantly takes the form of stone on the northern lengths of the drain but changes to Asbestos Sheet piles on both banks of the lower reaches, for a length of approx 4km 
 
The 5m long Asbestos sheet piles were installed during the early 1980s and have since deteriorated and reached their asset life. The piles had broken down in size over the years and now posed a real threat to the stability and integrity of the earth banks. The right bank is frequented by the local Angling club for most of the 4km length, whereas the left bank is bounded by mainly farmland and the Boston Golf club
 

Exposed asbestos sheet piling creates a serious health hazard.

The existing Asbestos sheet piles were analysed by an Agency laboratory in Nottingham and tests found that the piles contained a degree of White Asbestos. Should the piles be disturbed, they would be regarded as Special Waste and would require controlled disposal off-site. The Primary Objective of this project was to ensure that the Agency satisfied its duty of care to manage its own controlled waste and thereby reduce the risk of exposure to Asbestos, to Agency Personnel and the Public. The proposal was to bury the existing piles and thereby remove any potential Hazard.
 
Various Options were considered by the Project team, including sheet piling, but the preferred option was to place rock rip rap stone, suitably profiled, in front, over and in some locations behind the existing defective Asbestos piles.
 
However, initial surveys undertaken, showed the presence of water voles along some parts of the reach, therefore, the use of stone was not ideal for these rare mammals. The Drain is also a Class 1 Fisheries. Salix were asked to come up with an alternative solution to stone that would conceal the asbestos but at the same time create habitat, look more natural and be far more sustainable than importing stone.

The Solution

Successful Salix 10m trial section sandwiched between unsightly stone option.

Initially a 10m trial section was constructed in 2007. Due to it's success, a further 500m section was constructed in Feb 2008. Salix were sub contractors for the main contractor, May Gurney. One of the biggest problems to overcome was the difference in seasonal water levels. In winter, the drain is kept 500mm below average summer water levels.

Coir rolls attached to 60kN loop geotextile during works.

The "Hybrid" design composed of a stone toe to average winter water level, with a "soft" vegetated system above this level to above average summer water level. The "soft" element, also referred to as the "bioengineering element" of the revetment would replace the alternative stone revetment from the average winter water level upwards.

The bioengineering detail can be broken down into two main elements:

1. Reinforced Soil
2. Pre-established Coir Rolls & NAG C125BN Matting