On a world scale, the dissolution of limestone and gypsum by natural waters creates extensive karst landforms that can be very difficult ground for civil engineers. Caves threaten foundation integrity, notably where their width is greater than their roof thickness. Sinkholes pose many problems, and are classified into six types, including subsidence sinkholes formed in soil cover within karst terrains. Rockhead morphology varies from uniform to pinnacled, also creating difficult ground to excavate or found upon. A proposed engineering classification of karst defines various complexities of ground conditions by the geohazards that they provide, mainly the caves, sinkholes and rockhead relief. Ground investigation techniques and foundation design philosophies are considered so that they are appropriate to the ground conditions provided by the different classes of karst.

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The occurrences of natural and artificial cavities are reviewed and their causes are assessed. Natural cavities are found principally in carbonate rocks and the processes of sinkhole formation are described. Solution cavities in non-carbonate rocks and cavities in insoluble rocks are also considered. Extraction methods for coal, metalliferous minerals and salts are described in relation to the creation of underground cavities. An outline procedure for locating cavities emphasizes the importance of the desk study in this type of investigation and the difficulty of proving the absence of cavities beneath a site.

In the afternoon of Wednesday 23 April 1997, a large subsidence crater opened up in front of a house on Ure Bank Terrace, on the northern outskirts of Ripon in North Yorkshire. Overnight its sides collapsed inwards, so that the hole had doubled in size by the next morning (Fig. 1). The subsidence crater was then 10 m in diam- eter, and 5.5 m deep to a choke of debris overlain by water 1 m deep. Its sudden appearance was the cause of considerable concern to the occupants of the adjacent house, and the event was widely reported in the national press and media. A subsidence hollow was mapped at this site by the 1856 Ordnance Survey and documented by Cooper (1986). More subsidence had occurred at the Ure Bank site in previous years, but this latest collapse had rather more impact. Creeping movement of the soil towards the new hole meant that the adjacent house was destined for demolition. The event was the latest of a series of ground collapses that have occurred, at an average rate of about one per year, in and around the city of Ripon. While they are little more than an inconvenience in farmland, they have the potential to cause serious damage when they occur in built-up areas. The immediate cause of the Ure Bank subsidence was the downward movement of soil, drift and recent fill into actively expanding voids within the ground. Ultimately, it was caused by the partial collapse of a cave