Shear strength criteria for rock, rock joints, rockfill and rock masses: Problems and some solutions (original) (raw)
Related papers
The shear strength of rock joints in theory and practice
Rock Mechanics and Rock Engineering, 1977
The Shear Strength of Rock Joints in Theory and Practice The paper describes an empirical law of friction for rock joints which can be used both for extrapolating and predicting shear strength data. The equation is based on three index parameters; the joint roughness coefficientJRC, the joint wall compressive strengthJCS, and the residual friction angleφ r . All these index values can be measured in the laboratory. They can also be measured in the field. Index tests and subsequent shear box tests on more than 100 joint samples have demonstrated thatφ r can be estimated to within ± 1° for any one of the eight rock types investigated. The mean value of the peak shear strength angle (arctanτ/σ n ) for the same 100 joints was estimated to within 1/2°. The exceptionally close prediction of peak strength is made possible by performing self-weight (low stress) sliding tests on blocks with throughgoing joints. The total friction angle (arctanτ/σ n ) at which sliding occurs provides an estimate of the joint roughness coefficientJRC. The latter is constant over a range of effective normal stress of at least four orders of magnitude. However, it is found that bothJRC andJCS reduce with increasing joint length. Increasing the length of joint therefore reduces not only the peak shear strength, but also the peak dilation angle and the peak shear stiffness. These important scale effects can be predicted at a fraction of the cost of performing large scale in situ direct shear tests. Die Scherfestigkeit von Kluftflächen in Theorie und Praxis Zur Ermittlung der Reibungswerte in Kluftflächen wird ein empirisches Gesetz beschrieben, das sowohl das Extrapolieren als auch das Voraussagen von Scherfestigkeitszahlen ermöglicht. Die Gleichung ist auf drei Indexzahlen gegründet: Den Rauhigkeitskoeffizienten der KluftJRC (Joint Roughness Coeff.), die Druckfestigkeit des Felses der KluftwändeJCS (Joint Wall Compression Strength) und der residuelle Reibungswinkel der Trennflächeφ r . Die Indexzahlen können alle im Laboratorium bestimmt oder am Ort gemessen werden. Bestimmung von Indexzahlen mit nachfolgenden Prüfungen im Scherapparat von mehr als 100 Kluftproben haben erwiesen, daß für jede beliebige der acht untersuchten Gesteinsarten der Reibungswinkelφ r auf ± 1° genau geschätzt werden kann. Der Durchschnittswert des Reibungswinkels (arc tan (τ/σ n ) der Höchstscherfestigkeit wurde für dieselben 100 Klüfte auf ± 1/2° genau geschätzt. Die besonders genaue Vorausschätzung der Höchstscherfestigkeit ist durch Eigengewicht-Gleitversuche (niedrige Spannungen) auf Gesteinsblöcken mit durchgehenden Trennflächen ermöglicht. Der totale Reibungswinkel (arc tanτ/σ n ), bei dem das Gleiten eintrifft, ergibt eine Abschätzung des Rauhigkeitskoeffizienten der KluftJRC. Der Rauhigkeitskoeffizient bleibt über einen Normal-Spannungsbereich von mindestens vier Größenanordnungen konstant. Die IndexzahlenJRC (Rauhigkeitskoeffizient) undJCS (Druckfestigkeitskoeffizient) reduzieren sich aber bei zunehmenden Kluftlängen. Bei zunehmender Kluftflächengröße nehmen nicht nur die Höchstscherfestigkeit, sondern auch der zugehörige Dilatanzwinkel und die Schubsteifigkeit ab. Diese wichtigen Einflüsse der geometrischen Abmessungen können geschätzt und zahlenmäßig erfaßt werden, und zwar mit Kosten, die nur einen Bruchteil von denen betragen, die für große, direkte Scherversuche in situ erforderlich wären. La résistance au cisaillement des joints de roches en théorie et en pratique Le rapport traite d'une loi empirique du frottement dans les joints de roches, loi pouvant être utilisée tant pour l'extrapolation que pour la prédiction de données relatives à la résistance au cisaillement. L'équation est basée sur trois indices de paramètres: coefficient de rugosité du joint (joint roughness coefficient —JRC), résistance de la paroi à la compression (joint wall compressive strength —JCS), et l'angle de frottement résiduelφ r . Toutes ces valeurs d'indice peuvent être mesurées au laboratoire. Elles peuvent aussi l'être in situ. Des tests d'indice, avec ensuite des tests en boîte de cisaillement, sur plus de 100 échantillons de joints, ont permis de constater que, pour n'importe quel des huit types de roche étudiés, l'angle de frottementφ r peut être évalué avec une précision de ± 1°. La valeur moyenne de l'angle maximum de la résistance au cisaillement (arctanτ/σ n ) pour les mêmes 100 joints fut évaluée avec une précision de 1/2°. La prédiction particulièrement précise de la résistance maximum au cisaillement est rendue possible par la réalisation d'essais de glissement dits “de poids propre” (faible contrainte) sur des blocs à joints passant de part en part. L'angle de frottement total (arctanτ/σ n ) auquel le glissement se manifeste, fournite une estimation du coefficient de rugosité du joint,JRC. Ce dernier est constant à l'intérieur d'une plage de tensions normales effectives d'au moins quatre ordres de grandeur. Cependant, on a trouvé que les indicesJRC (rugosité) comme ceuxJCS (compression) diminuent quand la longueur du joint augmente. Ainsi, si la langueur du joint augmente, cela réduira non seulement la résistance maximum au cisaillement, mais aussi l'angle maximum de dilatation et la rigidité maximum de cisaillement. Ces importants effets d'échelle peuvent être prédits, et ce à des coûts ne représentant qu'une fraction de ceux liés à des tests directs de cisaillement effectués in situ.
Shear Behaviour of Rock Joints
2000
This title covers the fundamental properties of rock joints, the method of laboratory testing of rock joints, and shear strength assessment under different loading conditions. This work is intended as a reference text for students and practitioners in mining and rock engineering.
Characterization of the parameters that govern the peak shear strength of rock joints
In Switzerland, there is concem that sliding along joints under dams could lead to stability problems. As part of a research project funded by the Swiss Federal Office for Water and Geology, more than fifty constant-normal-load direct-shear tests have been performed on induced tensile fractures for seven rock types. Damage zones are evident on all of the sheared surfaces. There is evidence of both crushing and breaking of surface asperities. Damage is relatively sparse, and the location of the damaged zones is strongly related to geometrical features. However, the relationships between surface roughness, stress distribution, and damage are complicated and difficult to study, in part, because the boundary conditions goreming the mechanical behavior change continuously during shearing. One of the primary objectives of this work is to better understand the micromechanical behavior of joints under shear loads, including the creation of damage zones. This requires understanding the relationships between material properties, surface geometry, contact area, stress distribution, and the creation of damage during shearing. A methodology for predicting damage during shearing has been developed based on analysis of maps of the joint surfaces obtained before and after shearing using a three-dimensional optical system. The surface data is analyzed to identify the areas on the joint surfaces most likely to be in contact during shearing; i.e. areas with positive slope with respect to the shear direction. Ix)cal gradients are also taken into account in predicting those areas of the joint surfaces most likely to be damaged during shearing. The damage predicted is compared to the damage mapped on laboratory test specimens.
Shear strength of rock joints under constant normal loading conditions
2019
The variation of shear strength of rock joints under constant normal loading conditions was studied. Three dimensional printing technology was incorporated to produce moulds of rock joints. Rock joints samples with three different roughness values were cast using concrete with uniaxial compressive strength of 20 MPa. Samples were sheared using a direct shear testing machine for normal stress values ranging from 0.25 to 0.7 MPa. In addition, effects of shear rate on shear strength properties of rock joints were experimentally investigated. It was found that the shear strength of rock joints is a function of normal stress, joint roughness and shear rate values. In addition, it was shown that three dimensional printing technology is a useful tool to replicate real rock joints.
E3S Web of Conferences
Current standard direct shear test methods for rock joints do not account for damage to the specimens' asperity profiles; tests require shearing of a single specimen to large displacements under successive normal stresses (the multistage test), or the use of similar specimens in multiple tests. Due to the inherently unique nature of rock joints and corresponding difficulty in obtaining specimens with identical or even similar geometries, multistage tests are more common. A major issue with the multistage test is that successive shearing of the specimen damages the surface asperities and changes its overall roughness profile, reducing the peak shear stress and consequently resulting in underestimation of the friction angle and overestimation of the joint shear intercept (cohesion). The limited displacement multistage direct shear (LDMDS) test method minimizes these testing imperfections by allowing shearing of a single specimen without extensive asperity damage, accomplished by i...
Solid Earth Discussions, 2015
Darley Dale and Pennant sandstones were tested under conditions of both axisymmetric shortening and extension normal to bedding. These are the two extremes of loading under polyaxial stress conditions. Failure under generalized stress conditions can be predicted from the Mohr–Coulomb failure criterion under axisymmetric compression conditions provided the best form of polyaxial failure criterion is known. The sandstone data are best reconciled using the Mogi (1967) empirical criterion. Fault plane orientations produced vary greatly with respect to the maximum compression direction in the two loading configurations. The normals to the Mohr–Coulomb failure envelopes do not predict the orientations of the fault planes eventually produced. Frictional sliding on variously inclined sawcuts and failure surfaces produced in intact rock samples was also investigated. Friction coefficient is not affected by fault plane orientation in a given loading configuration, but friction coefficients in...
Assessing the Shear Behavior of Oriented Rock Joints under Constant Normal Loading Conditions
Geo-Congress 2014 Technical Papers, 2014
This paper presents results from a series of constant normal load direct shear tests on artificially created rock joints, in an attempt to quantify the effects of different factors that contribute to the shear strength along "ideal" rock joints. Particular focus is given to the effect of asperity orientation with respect to the direction of shearing. In many historical rock joint shear strength criteria, the effect of a joint's roughness on the joint shear strength is described based on analysis of only a single profile in the direction of shearing. More recent studies have observed that the distribution of sheared area on a joint surface during shear is significantly affected by the location and distribution of the three-dimensional contact area of the joint surfaces, and results can vary with changes in the asperity inclination angle, the direction of shear, and the applied normal stresses. Results from the current study indicate that the asperity orientation angle has a significant influence on the relative contribution of the contact surface area to the shear strength of a rock joint. In joints with oriented asperities, a combination of vertical displacement (dilatancy) and lateral displacement during shearing was observed. The lateral displacement resulted in a reduction in the magnitude of observed shear-induced dilation of the joint, as well as a reduction in the strength of the joints against the induced shear load. Finally, an alternate approach for describing the shear behavior of oriented rock joints was used to characterize the impact of asperity orientation with respect to the direction of shear on the shear behavior of the rock joint.
Rock Mechanics and Rock Engineering, 2014
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EVALUATION OF SHEAR STRENGTH OF MODEL ROCK JOINTS BY EXPERIMENTAL STUDY
In this paper, variation of the shear strength of artificial rock joints under constant normal loading condition is studied. Idealised joint surfaces were prepared using a developed molding method with special mortar and shear tests were performed on these samples under CNL conditions. Different levels of normal load and shear displacement were applied on the samples to study joint behaviour before and during considerable relative shear displacement. Nine types of saw-tooth joints have been selected for simplicity of modelling to quantify the effect of CNL conditions on joint shear behaviour. It was found that the shear strength of joints is related to rate of shear displacement, joint roughness (varying joint asperity angles) and applied normal stress condition. Finally, based on the experimental results and observations made of sheared joint samples, a new peak shear strength envelope is proposed to model sawtooth type joints tested under CNL conditions.