Route location studies involve the identification of a study corridor within which alternative routes are evaluated on economic and environmental grounds. Four stages are generally adopted: inception; comparative assessment; finalize preferred route; design. The scope of the geotechnical input at these stages is illustrated by reference to the Castleton to Coryton Section of the M4 Motorway north of Cardiff. Geology exercises a significant control because of its influence on the earthworks, upon which construction costs are dependent.
Geological issues were raised at the Public Inquiry and these are described.