Large-scale partial dissolution of the Chalk of the Devon coast and its engineering significance

Numerous site investigations have described ‘reconstituted’, ‘putty’, ‘soft’ and ‘rubbly’ chalks which have generally been grouped together for geotechnical description purposes as ‘structureless’, to distinguish them from ‘structured’ (bedded and jointed) in situ chalks. Structureless chalks have mostly been assumed to be redeposited materials or materials that have been intensely mechanically reworked in place. They include sludge (Head), partially water-sorted (Coombe), fluvial (Dry Valley and Nailborne) and cryoturbation (Head) deposits. Recent surveys of the east Devon coast have shown that an extensive, thick chalk deposit, which in small exposures or in cored boreholes could be mistaken for redeposited material, has formed in situ by the partial dissolution of a particular type of chalk. The best exposures, in the cliffs at and adjacent to Beer, Devon [SY 230 890], show a 15 m- to 30 m-thick layer of partially decalcified, nodular chalk that is underlain and overlain by intact clay-rich chalks. The boundaries of the decalcified unit are stratigraphically controlled and sharply defined. It contains angular to rounded, granule to boulder sized litho-relics in a degraded, chalk-fines matrix. Blocks of intact, largely unweathered chalk up to tens of metres thick occur as detached masses ‘floating’ in the partially decalcified unit, and even larger masses have settled down into it. Some of these contain the youngest Chalk preserved in Devon.