The stability of rock openings, especially those with wide spans, is largely determined by the state of initial stresses in the rock in combination with the joint properties. In order to produce correct designs and especially to be able to chose between sites of different suitability, it has to be possible to determine the state of the stresses in the rock mass during the project stage, in the boreholes made during the geological survey. For this purpose, a procedure has been developed at the Swedish State Power Board by which rock stresses can be measured by the method of Leeman and Hayes in water-filled boreholes several hundred metres deep.
La stabilite des cavites souterraines, en particulier de celles de grande portee, est determinee dans une grande mesure par l'etat des contraintes dans la roche en combinaison avec les caracteristiques de fissuration de la roche. Pour projeter une cavite correct et surtout pour pouvoir choisir entre des emplacements qui conviennent differemment, il faut done determiner l'etat des contraintes dejà au cours de l'ètablissement du projet, dans les trous de forage de la prospection geologique. Dans ce but, la Direction Nationale de l'Energie Electrique en Suède a developpe un procede permettant la mesure des contraintes dans la roche selon la methode de Leeman et Hayes dans les trousde forage remplis d'eau de plusieurs centaines de mètres de profondeur.
Die Stabilitat von Untertageraumen, besonders von solchen grosser Spannweite, ist weitgehend durch den initialen Spannungszustand des Gebirges in Verbindung mit dessen Klufteigenschaften bestimmt. Man muss also, urn richtig konstruieren zu können und vor allem, um eventuell zwischen verschieden guenstigen Anlageplatzen wàhlen zu können, den Gebirgsspannungszustand schon im Projektstadium, also in den Bohrlöchem der geologischen Untersuchung bestimmen. Zu diesem Zweck wurde bei der Schwedischen Staatlichen Kraftwerksverwaltung ein Verfahren entwickelt, das gestattet, die Gebirgsspannungen nach der Methode von Leeman und Hayes in mehrere hundert Meter tiefen, wassergefuellten Bohrlöchern zu messen.
The bedrock in Sweden is generally of good quality. Accordingly, the demands made on it are high and it is usually expected that even large underground openings can be constructed with minimal supports or even without any supports at all (Hiltscher 1972). Because of this good rock quality, it is also comparatively easy to obtain accurate measurements of the initial rock stresses, i.e. tectonic and gravitational stresses. Since the stresses in a mass of homogeneous rock with comparatively few discontinuities can be assumed to diverge only moderately from the ideal state of stress, it is also possible to discuss the stability of rock openings on the basis of the distribution of initial stresses obtained by measurement, considering the deviations introduced by the physical properties of the rock mass, the engineering-geological situation and the practical demands as regards construction and safety. During the past 15 years, the Swedish State Power Board has carried out a number of rock stress measurements on its construction sites and the stress behaviour of the rock has been observed, inter alia, in several long tunnels, notably the Suorva-Vietas tunnel (7,5 km), the Ritsem tunnels (18 km) and the Juktan tunnels (25 km). It is evident from these measurements and, observations that, as a general rule, it is not possible to predict the magnitude and direction of the initial stresses, since they may change considerably from place to place, even in a seemingly homogeneous rock mass. Comprehensive measurements are necessary for the knowledge of the stresses in a sizable rock mass. Usually, the measurements have been made because of difficulties in excavation and it has often been a question of dimensioning the necessary supports and of measures for safety purposes. If the stress situation in a rock mass is to be used in the layout and design of rock openings, the rocks tresses must be measured before the start of the construction work, i.e. before driving adits or sinking shafts. Thus, the measurements have to be carried out in deep boreholes. A number of structures, such as underground power plants, storage chambers for oil, air-raid shelters, etc., are often situated at depths of, say, 30 to 150 m below the surface. For other structures, for instance, storage tunnels for nuclear waste and underground, pumped- storage, power plants, measurements to a depth of about 500 m are needed. Two main problems had to be solved in this connection: To develop a remote controlled, automatic, measuring device and, because of the chosen method, to master the technique of cementing strain gauges under water, since deep boreholes normally contain water.
For the measurement of rock stresses, the well-known method of Leeman and Hayes (1966) was chosen. In this method, three strain-gauge rosettes, arranged as shown in Fig. 1, are cemented to the wall of a small bore in the bottom of a larger borehole (Fig. 3).
