The fatigue failure characteristics under low frequency (0.1 Hz) cyclic loading were determined for rocksalt with reference to the operations of underground storage caverus. Results show that rocksalt is significantly weakened by cyclic loading and under uniaxial compression the fatigue limit is 60% of the static strength. The effect of partial unloading and rest intervals as part of the operational procedures of storage caverns was also examined.
Die Brucheigenscheften unter Wechsellast des Steinsalzes wurden unter Niederfrequenz (0.1 Hz) an Dauerfestigkeitsproben im Hinblick auf Untergrundgasspeicherkavernen gemessen. Resultate zeigen dass Steinsalz beizyklischer Belastung bedeutsam geschwacht oder ermuedet wird. Bei uniaxialer Kompression is die Dauerfestigkeit 60% del'' statischen Festigkei ''t, Die Wirkung del''partiellen Entlastungen und del''vorriibergehenden ruhezustiinde in Hinblick auf Speicherkavernen wurde geprueft.
Les caracteristiques de la rupture du sel gemme ont ete determines par des essais de fatigue a basse frequence (0.1 Hz). Les resultats montrent que Ie sel gemme diminue son comportement sous les effets de fatigue. Pour les essais en compression uniaxiale la contrainte de la rupture après fatigue n''est que de 60% de la contrainte de la rupture originale. Les effets du dechargement parcial et des intervales du repos en liaison avec les operations de cavites du stockage sont aussi examines.
The growth in the use of salt caverns for the storage of gas is becoming more and more impressive and research on this topic assumed a position of increased importance. In determining the stability of any underground storage facility consideration is given to the effects of gravitational, and possibly tectonic, loading and of loading due to the pressure exerted by the stored gas. The latter loading is expected to vary in magnitude according to whether gas is being injected into or withdrawn from the salt cavern, thus subjecting the rock mass surrounding the opening to a cyclic stress field. This type of cyclic loading often causes a material to fail at a stress level lower than its determined ultimate strength, a phenomenon called fatigue. Cyclic fatigue on materials such as metals, polymers, concrete and soil has been investigated in the last decades in an extensive manner. In contrast the fatigue characteristics of rock materials have received a rather limited attention and research in the fatigue of rocksalt in particular is still in its infant stage. The present paper reports the results of the second phase of a comprehensive experimental study of the fatigue characteristics of rocksalt in connection with the loading conditions related to the operation of gas storage caverns. Fatigue studies are customarily conducted by subjecting the material under investigation to an application of stress conditions of lower magnitude than the equivalent ultimate strength determined from static tests. Experimental results from fatigue tests are commonly presented in the form of S-N curves where S is a percentage of the ultimate static strength and N is the number of cycles to cause failure at that stress level. The main objective of the investigation is to provide a better understanding of the fatigue strength of rocksalt by establishing the S-N curves for various loading conditions corresponding to operating pressures of storage caverns. In addition the investigation aims in assessing the extent of any cumm:lative damage that might be exerted upon a salt storage cavern subjected to a cyclic storage operation. During the working life of a storage cavern the cyclic changes of internal pressure are followed by corresponding variations of temperature on the cavern surface caused by the adiabatic compression and expansion of the stored gas. The effect of such temperature changes on the fatigue of rocksalt was not investigated during this phase of the experimental programme and all reported tests were conducted under constant ambient room temperature.
Salt Specimens All tests were conducted on Triassic rocksalt from the Cheshire basin originating from the Meadow Bank Rock Salt Mine operated by Imperial Chemical Industries. Rocksalt in that area is generally restricted to two horizons within the Keuper marls (Poole & Whitman 1966) and shows a considerable variation in composition, colour and texture. The individual halite grains range in size from 2mm to 20~n being more typically large and they vary in shape from being anhedral in the holocrystalline rock to being subhedral or euhedral where mudstone content is higher. Right-cylindrical specimens of 75mm diameter were cored out of a large salt block by diamond drilling in one direction only, thus reducing substantially the effect of sample variability. Each specimen was initially trimmed to 152~n length using a dry carborundum wheel and subsequently the ends were ground to better than G.008mm of parallel, and to the required length of 150~n. 2.2 Apparatus The salt specimens were cyclic loaded in an electro-hydraulic Avery universal testing machine which is servo-controlled by an R.D.P. Servo con unit. The testing machine has a capacity of 250 kN which was considered suitable for the testing programme since the highest load required was 140 kN.
