This paper gives details of the foundation problems at a housing development on a 4–7° slope of Blue Lias limestone, overlying Rhaetic and Keuper Marl.
Cambering in the past has produced extension gulls with cavities up to 1 m wide and zones of disturbance over 5 m wide. These structures were not visible on the surface or on the aerial photographs and presented unforeseen problems when discovered in foundation trenches. Extensive site checks have been undertaken, the foundations for each building plot being inspected by one of the authors.
A simple classification of the extension gulls on this site, based on the mode of opening along joint and bedding surfaces and any subsequent collapse or infilling, is presented. The overall stability of the hillside is discussed with reference to the residual strength of the Rhaetic, which here is a highly plastic, montmorillonitic horizon containing well marked shear planes.
The presence of the gulls necessitated several changes of the site layout in an attempt to avoid the most highly disturbed ground. The developer suffered a 150ss in the proposed number of units.