Proposed Ripon Bypass New Ure Bridge: Part 1. Geology, description and causes of subsidence in the vicinity

This report describes the bedrock geology and the superficial or drift geology of the proposed new River Ure Bridge to be built near the existing North Bridge at Ripon, North Yorkshire. The sequence of Permian rocks includes two thick units of gypsum, a rock which has partially dissolved, and continues to rapidly dissolve underground, to produce an expanding cave system beneath the site and most of the adjacent city of Ripon. Catastrophic collapse of caverns within the cave system frequently results in large subsidence hollows at the surface; these are commonly up to 30m across and sometimes up to 20m deep! Existing subsidence hollows are recognised in the vicinity of the proposed bridge, their cause and extent are described. Possible difficulties associated with some potential ground stabilisation works are also outlined.