Britpave: The British In-Situ Concrete Paving Association

Flood Alleviation Scheme

In March 2006, Beach Soil Stabilisation Ltd commenced the excavation and filling of the 75,500 m3 of the earth bund section of this Environment Agency Scheme together with Mowlem Civil Engineering (now Carillion).

The EA had secured a large borrow pit area and had tested the soils for suitability for use as construction fill in the bund. They had found that the moisture content of the grey clay was slightly too high and had to be reduced before use. The material was tested by excavating trial pits and then laboratory tests were carried out, the results of which revealed the need to mix in 2% of lime. This would reduce the moisture content to within the optimum limits.

The first section of bund was completed by excavating the clay and spreading it out in layers on the bund area. The wheeled spreader and Wirtgen mixer were able to travel over this material and carry out a normal ‘mix in place’ operation. However, as the excavation went deeper into the borrow pit, the material changed from firm clay to very soft clay. In addition, the north end of the pit changed into running silt with water pouring into the excavations.

A major re-think had to be carried out and while the Environment Agency pursued an imported clay option, Beach Soil Stabilisation Ltd carried out more tests on the materials. It was found that by increasing the amount of lime, the material could still change so that it could be used as embankment fill. A method was then devised of extracting the soft clay and silt by using long reach excavators and then loading the dumptrucks, which would be fitted with tailboards. A specially purchased tracked dumper was then converted into a lime spreader so that it could traverse the softest of material.

In the meantime, the EA found out the problems associated with importing good quality clay. Firstly, there were none available. Secondly, the access track leading to the site would not take the amount of heavy transport equired to complete the works and finally, the cost would greatly exceed their budget.

Beach Soil Stabilisation Ltd was instructed to go ahead with their suggestion and the EA were amazed that it worked perfectly. The bank was completed in time before the winter rains and the material that was trimmed off the sides of the batters was used in the final topping up of the bund’s crest to allow for any settlement that might occur over the lifetime of the flood defence.

For more information contact Alistair McDermid Jnr (