The importance of frictional properties of rock is described. The methods of determining friction in rocks are reviewed. The details of the experimental setup based on triaxial testing for determining friction are given. A number of Sicilian barble specimens were tested under various confining pressures and residual strengths were determined. Mohr's envelope was drawn for the specimens tested and the coefficient of friction was determined. The relation between shear stress corresponding to sliding friction and normal stress was found to be nearly linear and the coefficient of friction of sicilian marble was found to be 0.83.
L'importance du frottement interne dans les roches est mise en evidence et on passe en revue les differentes methodes pour determiner ses proprietes. Les details d'execution d'essais triaxiaux pour connaître ce frottement sont donnes. Des essais sous differentes etreintes laterales ont ete executes sur des eprouvettes de marbre sicilien et les resistances residuelles determinees. Le coefficient de frottement se calcule à partir de la courbe intrinsèque dans le diagramme de Mohr. La relation entre les contraintes tangentielles et normales sur la surface de glissement est pratiquement lineaire et le coefficient de frottement vaut 0,83.
Die Bedeutung des Reibungsverhaltens von Fels wird diskutiert. Die Methoden zur Bestimmung der Reibung im Fels werden besprochen. Die Details des Aufbaus fuer 3-axial-Versuche zur Bestimmung der Reibung werden angegeben. Proben aus sizilianischem Marmor wurden unter verschiedenen Seitendrucken geprueft und die Restfestigkeiten bestimmt. Die Mohr'sche Umhuellende wurde gezeichnet und der Reibungsbeiwert bestimmt. Das Verhaltnis zwischen Scherspannung entsprechend der Gleitreibung und der Normalspannung war nahezu linear, der Reibungsbeiwert von sizilianischem Marmor betrug 0,03.
The knowledge of friction along joints and fault surfaces in rock mass is essential for properly evaluating the stability of foundations, slopes, underground openings etc. It has also been considered very important in various theories of rock fracture such as the Coulomb criterion and Griffith theory (particularly the modifications suggested by McClintoek and Walsh, 1962). It also affects the physicomechanical properties of rocks, e.g. hardness, static and dynamic elastic constants etc. Inspite of the fact that friction plays a very important role in various rock mechanics problems, its mechanism has not been clearly understood so far. Some investigators have tried to apply the knowledge of friction of metals to rocks and minerals, but it is by no means certain that same fundamental processes of friction are operating in rock as well (Jaeger and Cook, 1969). It is difficult to carry out the study of frictional properties in situ as the fractures and the spacing between them are generally large. However, a systematic experimental study of friction can be done in laboratory. The above laws hold good at low normal pressures and velocities (Attewell & Farmers, 1976). The coefficient of friction depends on the nature of the materials and the state (finish etc.) of the surfaces in contact. Most rock and mineral surfaces are uneven so contact is invariably limited to asperities on their surface. The failure of asperities on a sliding surface occur through brittle rather than ductile processes (Brace & Byerlee, 1967).
Jaeger and Cook (1969) have described the various methods of determining friction. They have been depicted diagrametically in Fig. 1.
Fig. la depicts a simple system in which the surfaces are pressed together by a normal force W. Precautions should be taken to ensure that the normal force is applied uniformly over the surface. This technique has been used by Bowden and Tabor (1950), and Byerlee (1967 a; 1967b).
The method shown in Fig. 1b is a symmetrical system in which one block is pressed between two plans and parallel surfaces. It is suitable for large surfaces and for studying the effect of sliding (Hoskin et al., 1968; Rosengren & Jaeger, 1968). Though relatively a rigid system, the constraint is not perfect and there may be a tendency for the slide block to rotate slightly about a horizontal axis.
The rotating system (Fig. 1e) consists of a load W acting axially and the tangential force T causing the surfaces to rotate. This system has the advantage that the same surfaces are in contact throughout the experiment and so it is suitable for studying the large amount of sliding on these surfaces. It is easy to study the effect of water in this system by introducing it through the axial hole.
Fig. ld shows the system in which sliding occurs across an inclined plane in a cylindrical specimen subjected to triaxial stresses. The method has the disadvantage that the geometry changes during sliding and therefore, long continued experiments are not possible. However, it allows high normal and tangential stresses to be applied to the surface.