A method for estimating the peak shear strength of a rock joint in situ is presented. The method is based on a new simple theoretical peak shear strength criterion, which uses a realistic mechanism of shearing and implies that peak shear strength at any normal stress is the result of two components: one purely frictional and one geometrical. For in-situ rock blocks the dilation is negligible and the peak shear strength can be estimated from the frictional component only.
Une methode pour estimer la maximum de resistance au cisaillement des joints de roche in situ est presente. La methode est basee a un nouvel et simple critere theorique de la maximum de resistance au cisaillement, que utilise un mechanisme realiste de fracture et entraine que la resistance maximum en fraction, quel que soit la tension normal, est Ie resultant de deux composants: un purement de frottement et un geometrique. La dilatation est negligeable pour les bloques des roches in situ, et la maximum de resistance au cisaillement peut etre estimer seulement par Ie composant de frottement.
Zur Abschatzung der In Situ-Spitzenscherfestikeit vor Ort wird ein Verfahren Vorgestellt, das auf einem neuen, einfachen theoretischen Kriterium basiert. Das Kriterium macht von einem realistischen Mechanismus der Sherkraft in Gesteinen Gebrauch, nach dem die Spitzenscherfestikeit einer Gesteinkluft unter Normaldruck aus zwei Komponenten besteht: einer Reibungskomponente und einer geometrischen Komponente. Bei Gesteinformationen ist die geometrische Komponente vernachlassigbar und die Spitzenscherfestikeit lasst sich alIein durch die Reibungskomponente abschatzen
A new simple theoretical peak shear strength for rock joints has been recently proposed by Papaliangas et al, (1995). According to the new criterion, the peak friction angle of a rock joint can be considered as a two-component quantity: a) a purely frictional (independent of normal stress) component øm due to shearing of the rock wall material and b) a dilational component µ, due to the surface roughness. friction angle arises from the shear strength of rock junctions, formed under normal stress sufficiently high to cause plastic deformation of the contacting asperities, as anticipated from the adhesion theory (Bowden & Tabor, 1950). This occurs when the true normal stress approaches the brittle-plastic transition stress and therefore, øm can be determined from triaxial tests at confining pressure of this magnitude (Figure 1). The brittle plastic transition stress may be higher or lower than the unconfined compressive strength. Some strong rocks, such as granites, may have a transition pressure as high as five times the unconfined compressive strength or higher, whereas limestones and marbles may have a transition pressure lowerthan the unconfined compressive strength (Mogi, 1966, Paterson, 1978). Byerlee (1978) found that mineralogy has little or no effect on the friction angle of rocks, but Mogi (1966) calculated friction angles for carbonate rocks which were markedly higher than those of silicate rocks. From an analysis of published experimental data, it appears that typical values for øm for natural, rough joints in fresh rock are about 39° for silicates and a few degrees higher for carbonates. Therefore, if the dilation is zero, the peak friction angle will be equal to øm which represents a lower bound. This lower bound, which is considerably higher than the friction angle obtained from saw-cut surfaces, is in agreement with most experimental results, where a lower bound of this magnitude is observed in most direct shear tests. Two examples of such results taken from Baldovin (1970) and Giani (1992) are shown in Figure 2. If the joint is filled with sandy or clayey material, both the dilational and frictional characteristics are altered (Papaliangas et aI., 1990 and 1993).
