In some designs, the rock at the face of an unlined storage cavity may be stressed beyond its maximum strength, and the article presents how an analytical model can be used to describe the conditions of failure in the rock and their effects on stability. It is henceforth possible to use underground storage techniques at moderate depths in soft rocks or at great depths in hard, brittle rocks.
Underground storage first developed by making use of very favourable geological conditions like those available in salt domes or large expanses of crystalline rock. The design of facilities to store oil, and liquified or gaseous gas is a relatively simple matter in this kind of context. Over the last decade or so, however, because of the move to capitalize on the unusually high standard of environmental protection and safety associated with underground storage, attention has been focused on extending it to a much wider range of stored products in much less favourable geological conditions and there are even cases of such projects now in the design stage. The new products to be stored produce very different stress conditions in the rock. Liquified natural gas and ethylene involve low-temperature stresses. Residual fuel oil, compressed air and nuclear wastes apply thermal stresses. The alternating and sudden pressure changes associated with the operation of compressed air storage facilities are a severe test of stability. The advantages of underground storage have induced engineers to examine its application in much less favourable geologies, and Geostock is currently engaged in conceptual and project design studies for facilities in weak to very weak soft rock, even down to almost unconsolidated clays. Such formations are very commonly found in the large alluvial plains where the world's major cities are built, so that safety is an obvious criterion. In such rock, it is quite possible that the cavities will not be inherently stable if rock strength is ina dequate to withstand the applied loads. Conventional design rules derived from theoretical rock me chanics models do not enable us to estimate the permissible limits in such design, or to optimize support. In order to provide a means of examining the feasibility of such projects, Geostock has been running a programme of research jointly with the Laboratoire de Mecanique des Solides of the Ecole Polytechnique in Paris since 1974, to investigate materials behaviour when stresses are equal to or in excess of their maximum strength as obtained from crushing tests. A set of experimental data has also been analysed to calibrate the results of this theoretical research, and check the accuracy of the findings. We shall first briefly review the shortcomings of conventional models before going on to describe the main features of the new model and compare findings with a variety of case histories. The article concludes with comments on the new opportunities offered by this model for designing underground storage facilities
in poor engineering rock
where stress conditions are complex,
in strong rock where stresses are expected to exceed rock strength, or
where support optimization is a prime consideration.