Our project involved the construction of a tunnel approximately five kilometres in length and four metres in diameter.? The tunnelling process produced some 200,000 tonnes of material (known as “arisings”), that had to be taken off site.
As the tunnel progressed beneath the Humber estuary it passed through varying geology, beginning with a clay-like band of ground at Goxhill, known as glacial till.? It then moved into soft chalk, then through into a harder, denser chalk, before finally passing through a similar clay-like band of ground at Paull, on the north bank of the Humber.
As the tunnel boring machine (TBM) made its way underground, the excavated material was mixed with water and transported back to the surface at Goxhill as a slurry.? Here the slurry was pumped through a series of sieves and screens to remove as much of the coarse material as possible, including large pieces of chalk and flint.? The extracted material was then transported by a conveyor belt to a storage bay where it was stored temporarily before being taken off site by lorry.
The remaining slurry, which was still carrying material in suspension, was then passed through a filter press which squeezes out excess water.? This produced a “filter cake” which is much drier and could be transported to the storage bay before being removed from site.? The remaining slurry was pumped back to the TBM and the process was repeated.
Chalk ‘filter cakes’
The arisings were taken to several sites near Scunthorpe.? There were many hundreds of lorry movements required to take all of the material off site, and? to ensure there is minimal impact to the local community, all of the lorries? followed a agreed traffic management route.