Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
ABSTRACT: The mechanical behaviour of rock bridges is analysed on simple crack configurations. The results show that direct and induced tensile crack propagation occurs in either stable or unstable conditions depending On crack spacing and applied confinement stresses. RESUME: On analyse Ie comportement mecanique des ponts rocheux à partir de quelques simples configurations de fractures. Les resultats montrent que la rupture dans les ponts rocheux peut etre du type stable ou instable selon I'epaisseur des ponts rocheux et les contraintes de confinement appliquees. KURZFASSUNG: Das mechanistic Verhalten von Gesteinsbruecken wurde anhand einfacher Rißverlaufe untersucht. Die Ergebnisse Eigen, day seine lurch Zugkrafte verursachte direkte und induzierte Verbreitung der Risse in enema stabile odder night stabile Muffled auftritt und von der Entfernung der Risse und von den Einsehlußkraften abhangt. 1 INTRODUCTION The presence of rock bridges in not fully persistent natural discontinuity sets is a significant factor affecting the stability of rock structures (e.g. rock slopes). The behaviour of rock bridges has been studied by several authors, both in itself and in relation to rock slope stability (see for instance: Jennings, 1970; Einstein et al., 1983; Stimpson, 1978), according to the traditional concepts of the resistance of materials. More recent studies have analysed these phenomena on the basis of fracture mechanics concepts and through the Displacement Discontinuity Method (DDM) (e.g., Sheen, 1993; Scavia, 1989, 1995). The approach adopted by Scavia, in particular, makes it possible to study the degree of stability of rock slopes on the basis of the propagation of natural discontinuities in the rock bridges located in the rock mass. For application purposes, the fundamental limitation of this approach is that, like all discontinue methods, it presupposes a knowledge of the location of the discontinuities in the rock mass. In the authors' opinion, this difficulty can be overcome only by resorting to a probabilistic approach (see for instance Einstein at air., 1983; Scavia et al. 1990), and by analysing several geometric configurations obtained from the statistical distributions of the geometrical characteristics of the discontinuities (orientation. spacing, persistence). However, when using the DDM to deal with a large number of complex geometric configurations, the calculation process proves exceedingly time-consuming. This paper illustrates a process, based on the method proposed by Scavia (1995), aimed at identifying an elementary fracture set which might be deemed to be representative of the phenomena taking place in more complex fracture sets. This approach would make it possible to study the behaviour of rock structures by conceiving them as the sum of several elementary fracture sets. 2 BASIC ASSUMPTIONS AND NUMERICAL TECHNIQUE In relation to the boundary conditions of the system being examined, the analysis of crack propagation in rock bridges calls for the use of a numerical technique. Account taken of the states of stress present in rock Structures, this technique must be able to simulate the propagation of closed and open cracks in tensile and compressive stress fields. If we assume the behaviour of the material making up the rock bridge to be linear elastic, and that non linear effects are limited to the crack surfaces. it is advantageous to refer to the BEM technique of the DDM (Crouch and Star field, 1983) suitably modified in order to take into account the singular state of stress at the crack tips and the propagation process (see for instance, Sheen, 1993: Napier, 1995). In particular, following the method proposed by Scavia, by means of a special computation code (BEMCOM), each crack delimiting a rock bridge is subdivided into a certain number of elements (DD elements) characterized by a constant distribution of the displacement discontinuity and two square root elements located at the crack tips which make it possible to determine the stress intensity factors (Fig. I).
- Europe > Austria (0.28)
- Africa > Cameroon > Gulf of Guinea (0.24)
- North America > United States > Alabama > Lamar County (0.24)
ABSTRACT: A series of Short Rod tests has been carried out on rock specimens of different diameters in order to evaluate rock fracture energy. The results, showing strong dependence on specimen diameter, were interpreted through a scaling law based on fractal concepts. In order to assess the applicability of this law to rock, fracture surface profiles were produced and the relative degree of fractality was evaluated. RESUME: Une serie de tests de laboratoire a ete etfectuee afin de determiner I'energic de fracture d'echantill de gres. Les resultats montrent que l'energie de fracture augmente avec les dimensions des echantillons. On a utilise une lois d'effet d'echelle pour l'interpretation des resultats. Afin d'etablir l'applicabilite de cite lois aux materiaux ruche on a relive des profiles de surfaces de rupture et on a value leers dimensions fractals. KURZFASSUNG: An Gesteinsproben mitt unterschiedlichen Durchmessern warden cine Rehire von Short Rod Tests durchgefuehrt, um die Bruchenergie des Gesteins zu bewerten. Die Ergebnisse, die stark vom Durchmesser der Proben abhingen, warden mitt Hilfe eines Gesetzes ausgelegt, das auf fraktalen Konzepten basiert. Um die Anwendbarkeit dieses Gesetzes fuer Felsgestein zu beurteilen, warden Profile der Bruchflachen angefertigt und deren Grad der Fraktalitat bewertet. 1. INTRODUCTION Size effect is a crucial factor in the determination of the values of rock fracture energy to be used in the analysis of the propagation of natural discontinuities in full size rock structures (Matsuki et al., 1991; Scavia, 1993, 1995). Numerous size effect laws have been proposed (see for instance: Bazant, 1991). A recent approach (Carpinteri, 1992, 1994) explains size effects in concrete on the basis of the fractal nature of the specimens' fracture surfaces. Aim of this investigation is to assess the applicability of this approach to rock. To this end, the results obtained from a series of Short Rod tests (ISRM, 1988) performed on sandstone specimens were interpreted through the Multifractal Scaling Law (Carpinteri, 1994). To be able to evaluate the applicability to rocks of the basic hypothesis underlying this theory, the fractal nature of the fracture surfaces of sandstone was investigated by determining the fracture profiles of the materials with the aid of a laser beam and analysing the profiles obtained through three different methods. 2. EXPERIMENTAL EVALUATION OF FRACTURE ENERGY Fracture energy values for Sarnico sandstone (average grain size = 0.10 mm, average uni-axial compressive strength, Co = I08 MPa; average secant elastic modulus E,= 17700 MPa) were determined through the Short Rod testing method proposed for rock materials by the ISRM (International Society for Rock Mechanics) in 1988, that specifies the use of cylinders obtained from core sections in which special V shaped notches are cut. Specimens of different diameters (80, 50,33 mm) were used to study scale effects. It proved impossible to go below 33 mm on account of the disturbance problems arising during specimen drilling and preparation. The fracture energy (Gf) values obtained are shown in Table 1: it can be seen that fracture energy tends to increase with increasing specimen size. 3. INTERPRETATION OF SCALE EFFECTS ON FRACTURE ENERGY The scale effects observed in the sandstone specimens have been studied on the basis of fractal theory concepts following the approach proposed for concrete by Carpinteri (1992,1994). According to this approach, if a specimen's fracture surface turns out to be a statistically self-similar entity it has a fractal dimension between two and three, as it occupies more space than a smooth plane, without, however, reaching the consistence of a three dimensional element. This consideration must be taken into account to determine fracture energy, which is strictly correlated with the presence of voids, micro and macro defects in the material that are reflected in the roughness of the fracture surfaces.
- Geology > Geological Subdiscipline > Geomechanics (0.87)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.86)