Karst phenomena with gas emanations in Early Oligocene conglomerates: risks within a highway context (Jura,Switzerland)

During the construction of the A16 motorway, four cavities were discovered in the Early Oligocene carbonate conglomerates of the Jura mountains, northwest Switzerland. One cavity was particularly interesting because of its size and gas emanations. Previous geological works carried out in this key area are summarized, notably the effects of tectonic evolution on the formation of karstic networks. The methods used for the exploration and study of the principal cavity are described. The tectonic and lithological influence on the spelaeogenesis of the network; the natural and/or anthropic origin of the gas emanations and the implications of the cavities for the highway construction are discussed.