To aid in a better understanding of the ground, a 3D model of the tunnel route was produced using the computer package VULCAN, originally designed for modelling opencast coal mining sites. Over the last five years, the BeWT (Bechtel Water Technology) geotechnical section has developed its use for civil engineering schemes.
The site investigation data were provided in digital format from the site investigation data handling system (SID) which connects directly to VULCAN. The advantage of the 3D model over the long section was that it enabled an appreciation of the lateral distribution of the materials at any point across the site to be gained relatively quickly.
Its usefulness was particularly demonstrated at the location of the main drive shaft located to the north of the site area. The shaft was to be constructed using diaphragm walling techniques passing through waterbearing gravels into a thin horizon of glacial clay before keying into rockhead. The tunnel break out level would ideally take place within the glacial clay to ensure a dry excavation which, hopefully would extend more than 100m in front of the shaft to allow the tunnelling and ancillary equipment to be constructed. This would avoid expensive rock excavation or the need to treat gravels adjacent to the River Derwent.
Confirmation that the clay was sufficiently extensive was achieved by adding information to the model gained from the construction of the diaphragm wall launch shaft.
