Locating dissolution features in the Chalk

Dissolution features are common in the Chalk and may result in differential or collapse settlement of foundations if undetected. Dissolution pipes and cavities may be easily missed by conventional drilling methods. Probing an d. geophysical methods of investigation offer an attractive alternative due to their ability to cover large areas rapidly and thus minimize cost. The success of geophysical methods depends on many factors, principally the siz e of the feature in relation to the depth of burial and the cover material. This paper describes a study of dynamic probing and a number of geophysica l methods used to locate dissolution features at two sites with contrasting ground conditions. The first site contained a bowl-shaped doline over a cl ay-filled dissolution pipe beneath a relatively thin soil cover. At the sec ond site there was a thick, predominantly granular cover material that cont ained cavities which had migrated from dissolution pipes in the chalk below . Ground truth data from trenching was obtained to provide a basis for eval uating the investigation methods used. The ability of both dynamic probing and geophysical methods to locate and map dissolution features is discussed .