Job Description

Client: Environment Agency
Quantity Stabilised: 75,500m³

 

Project Overview

We commenced the excavation and filling of earth bund section of this Environment Agency Scheme together with Mowlem Civil Engineering (now Carillion). The EA had secured a large borrow pit area. They then tested the soils for suitability for use as construction fill in the bund. They found that the moisture content of the grey clay was slightly too high and had to be reduced before use. We also tested the material by excavating trial pits and then carried out laboratory tests. This told us that we needed to mix in 2% of lime. This would reduce the moisture content to within the optimum limits.

 

Scope of Works

We completed the first section of the bund by excavating the clay and spreading it out in layers on the bund area. Our wheeled spreader and Wirtgen mixer were able to travel over the material and carry out a normal ‘mix in place’ operation. However, as we excavated deeper into the burrow 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 excavations.

We had to carry out a major re-think. While the Environment Agency pursued an imported clay option, we carried out more tests on the materials. By increasing the amount of lime, we could still change the material so that it could be used as embankment fill. Then we devised a method of extracting the soft clay and silt. This was achieved by using long reach excavators. We would then load the dumptrucks, which would be fitted with tailboards. Also, we specially purchased a tracked dumper and converted it into a lime spreader. This enabled it to 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 required to complete the works and finally the cost would greatly exceed their budget.

Therefore, they instructed us to try out our suggestion and were amazed that it worked perfectly. The bank was completed in time before winter rains. Also the material that was trimmed off the sides of the batters was used in the final topping up of the bund’s crest. This allowed for any settlement that might occur over the lifetime of the flood defence.