ABSTRACT
Natural catastrophic ground subsidence, caused by gypsum dissolution, occurs approximately every 3 years in the vicinity of the city of Ripon, North Yorkshire, England. Holes up to 80m across and 30m deep are present and holes up to 35m across and 20m deep have appeared without warning. In the past 150 years, 30 major collapses have occurred and about $1,500,000 worth of damage has been caused locally in the last ten years. The subsidence results from the partial collapse of a cave system in gypsum beneath the area. Gypsum dissolution subsidence affects many parts of the UK. It is especially prevalent in the Permian rocks of eastern England where it occurs in a belt about 3km wide and over 100km long. Similar subsidence related to gypsum dissolution is documented in central and southern Germany and around Paris, France. Because gypsum (CaSO4·2H2O) dissolves rapidly in flowing water the cave systems responsible for the subsidence are constantly enlarging, resulting in on-going subsidence problems. The joint system in the rock, and the hydrological pathways through the beds, are the main geological controls on the gypsum dissolution, and the resultant collapse. The Ripon cave system is phreatic with artesian water emanating from it; it probably compares with the maze cave systems described in Russia. Difficult ground conditions are associated with subsidence breccia pipes, crown holes and post-subsidence fill deposits. Site investigation techniques that have been successfully utilised, to define and examine the subsidence features, include microgravity and resistivity geophysical techniques, plus more conventional methods of drilling and probing. The engineering difficulties of dealing with such catastrophic subsidence are immense. Remedial measures such as grouting and deep piling are not generally practical, and the solution probably lies in the construction of low weight structures with spread foundation designs.
