The site investigation allowed the route of the 14 km long water supply tunnel for the reservoir to be chosen to avoid tunnelling in ground affected by faults and and valley bulging, and the tunnel depth to be selected within the highly over-consolidated Upper Lias Clay to avoid the overlying water-bearing deposits of the Inferior Oolite Series while keeping well above the underlying Marlstone Rock Bed which contained water under high pressure. Special attention was given to investigation of the water regime. Construction employed expanding wedge block lining, and tunnelling machines which were successfully modified to deal with bands of hard calcareous clay with limestone modules that were encountered during the work. A shaft was flooeded when water gained access from the Marlstone Rock Bed below, due to fracture of the strata by valley bulging, and dewatering wells were employed. Site investigation and construction work for the 1.3 km long reservoir outlet tunnel and shaft is also described. Flexible management of the site investigations allowed additional ground information to be readily obtained whenever required.