High Speed 1, UK: a silty sustainable earthworks case study

In the UK, the High Speed 1 project formally pioneered on a large scale the earthworks design philosophy of maximising reuse of excavated materials and minimising waste. Now, in 2011, it is considered best practice for sustainable earthworks on all major infrastructure projects. This philosophy was a requirement of the Channel Tunnel Rail Link Act 1996, and a waste minimisation hierarchy was therefore included within construction specifications. This paper presents two different solutions found on one contract to enable two difficult silty soils, which would historically have gone to mitigation or taken off site, to be successfully retained and used as engineered fill. The first soil was quarry overburden material of Thanet Sand with considerably varying optimum moisture contents. A ‘family of curves' approach was developed to enable rapid single-point compaction testing to predict optimum moisture contents. The second was an artificial soil called cement kiln dust, generated as a by-product of cement manufacture. This material had a high silt proportion, and very high natural and optimum moisture contents, and was in a condition much wetter than optimum. A strength-based end-product acceptance criterion was developed using a CBR MEXE probe.