This paper describes the practical use of rock stress and deformation measurements in Norwegian mines. The methods used are mentioned, and accuracy and reliability of the methods are discussed. Practical arrangement of measurements with case histories is presented.
Cet article decrit l'utilisation de la contrainte de roche et des mesures des deformations dans les mines norvegiennes. Les methodes utilisees sont exposees, et on examine la precision et la sûrete de ces methodes. On presente ensuite l'organisation pratique de ces mesures avec des etudes de cas.
In dieser Publikation wird die praktische Anwehdung von Gebirgsdrucks, und Deformationsmessungen in norwegischen Gruben beschreiben. Die benutzten Methoden sind erwahnt und die Genauigkeit und Zuverlassigkeit derselben erörtert. Die praktische Durchfuehrung der Messungen sowie "Case histories" werden vorgestellt.
Rock stress and deformation measurements have been carried out regularly by the Mining Department. The Norwegian Institute of Technology (NTH) since 1964. Before that time rock stress measurements were carried out by professor Hast a few locations as early as 1958. From the very beginning the NTH engagement in the rock mechanical field has been very practically aimed, and the Rock Mechanics Laboratory has all the time enjoyed a very good cooperation with the Norwegian mining industry. At a very early stage the industry realized that proper measurements with proper equipment could be a very valuable practical tool. Hence the industry also was willing to grant considerable funds for the education of experts and development of measuring techniques. Since 1964 measurements have been carried out in the great majority of Norwegian, mines. During the last 7–8 years a large number of measurements have also been carried out in connection with hydroelectric power plants, road tunnels etc. This paper will, however, emphasize on the mining engineering side of the matter.
During the years several measuring techniques have been used, including borehole deformation gages and photo elastic discs. In 1965 the doorstopper method was introduced, and during the years 1965–68 numerous tests with different versions of the doorstopper were carried out. Even three dimensional measurements by means of measurements in three orthogonal holes were tried with mixed success. The drawbacks of the doorstopper i.e. the necessary corrections for the stress concentrations at the bottom of the borehole were, however, always a problem to be kept in mind. Especially the correction for the influence of the axial stress was difficult to handle. Therefore a self developed version of the C.S.I.R. triaxial cell was introduced in 1968. Except from some starting up problems, the experiences with this method have been very good. The technique has been under continous development until this date and is at present our main technique. The doorstopper is, however, still widely used, but mainly in cases where it may be assumed that the stress component acting parallell to the measuring hole is zero or very small. Typical cases where the doorstopper is used is by measurements in mine pillars and in the immediate mine roof close to the surface, where the above mentioned conditions are satisfied. In practically all other cases the triaxial cell is used.
The deformational behaviour underground openings and rock slopes may be controlled through deformation measurements. For this purpose the Rock Mechanics Laboratory, NTH, has used and is using different methods:
On some occations simple geodetic leveling techniques have been used to measure deformations in rock structures. The leveling telescope is then situated in an assumed stable location, from which movements in unstable areas may be recorded. Different systems have been used to visualize the movements. Fig. 1 shows some examples.
For measurement of the deformations of drifts and tunnels, telescopic invar steel extensometers with built in dial gages are (Soiltest Inc.). The measurement set up is ally as indicated on fig. 2, making it possible to record deformations relative to different distances from the drift surface.
For the measurements of movements and deformations of larger rock masses single position or multiple position borehole extensometers are used. These may be of the string type, which transfer movements between anchors at different depths and the surface through stainless steel strings (Terrametrics Inc.), or the rod type where movements between anchors at different depths and the surface are transferred through stainless steel rods (Kurt Stitz). Even if the latter type is more expensive, it has been preferred during the last years due to its better stability and easier installation. The readings may either be manually using dial gages, or electronically using inductive transducers (L.V.D.T.'s). Fig. 3 shows a borehole extensometer installation.
During the years the question of accuracy and reliability of rock stress measurements has been very thoroughly discussed throughout the world.
