The paper describes a series of direct shear tests performed on concrete samples which were reinforced by untensioned passive steel bolts put perpendicularly to the shearing direction. The surface of the joints was formed by regular teeth inclined at an angle of 17°. In these tests, the diameter of the bolts varied from 2.5 mm to 5 mm and their number between 1 and 2. The shear tests were carried out in a special device without applying external normal forces, simulating thus conditions prevailing for slope stabilization in hard stratified rock. Besides the tests, a theoretical expression for the prediction of the "bolt effect" was developed and successfully verified.


Cette communication decrit une serie d"essais de cisaillement direct realises sur des echantillons en beton renforces par des boulons passifs mis en place perpendiculairement à la direction de cisaillement. La surface des discontinuites etait caracterisee par des dents regulièrement disposees et faisant un angle de 17° avec la direction de cisaillement Dans ces essais, le diamètre des boulons variait de 2.5 mm à 5 mm et leur nombre entre 1 et 2. Les essais de cisaillement ont ete entrepris dans un cadre special, sans application d"effort exterieur normal afin de simuler des conditions rencontrees pour la stabilisation de versants dans des roches dures stratifiees, De plus, une expression theorique pour la prevision de l"effet du boulonnage a ete developpee et verifiee avec succès sur les resultats des essais.


Es wird eine Reihe direkter Scherversuche an Betonprobekörpern beschrieben, die normal zur Scherrichtung durch schlaffe passive Stahlbolzen bewehrt waren. Die Trennflachen wiesen regelmaßig angeordnete, unter 17° zur Scherrichtung geneigte Zahne auf. In den beschriebenen Versuchen wurde der Stahldurchmesser von 2.5 mm bis 5 mm und die Anzahl der Anker zwischen 1 und 2 variiert. Die Versuche wurden in einem speziellen Scherrahmen, ohne Aufbringung außerer Nonnalkrafte durchgefuehrt, um die Verhaltnisse bei der Stabilisierung von Böschungen in hartem, geklueftetem Fels zu simulieren. Zusatzlich wurde ein analytischer Ausdruck zur Vorhersage des "Dubeleffekts" entwickelt und mit Erfolg an Hand der Versuchsergebnisse verifiziert.


Although passive untensioned rock bolts have for many years been widely used in engineering practice, and moreover for many years numerous researches have been carried out with the aim to explain the strengthening effect of bolts in the rock mass, the understanding and knowledge of this subject is still incomplete. The point is not the traditional understanding of bolt support as binding and suspending rock blocks, but the increase in shear strength of the jointed rock mass due to bolting. It is generally understood that bolts work as an additional resistance against shear failure along joints, hence the entire rock mass becomes stronger and deforms less.

In recent years, much experimental research work has been carried out to find quantitative relations between the properties of joints and the surrounding intact rock, the parameters of the bolting system and the increase of the rock mass strength (e.g.Azuar, 1977; Bjurström, 1974; Blondeau et al., 1984; Egger & Spang, 1987; Spang, 1988; Spang & Egger 1990). Nevertheless, despite some serious advances, there exists no general theory explaining thoroughly the bolting effect. This results mainly from the high number of parameters involved, such as: joint geometry.

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