This paper concerns the behavior of joints sliding first in one direction and then in the opposite direction. This problem proved to be fundamental to achieving realistic numerical duplication of joint behavior in many applications, e.g., excavations which induce shear stresses on joints, opposite to the original stresses. A series of direct shear tests with rough joints cast in plaster yielded a consistent picture and permitted us to propose an empirical model of behavior. Some features of this model are also supported theoretically.
On etudie dans cet article le comportement des joints qui glissent suivant le sens contraire à celui suivi precedemment. L'objectif fondamental est d'obtenir un modèle numerique realiste pour le comportement des diaclases dans quelques applications pratiques comrne, par exemple, le cas des excavations capables de produire des efforts de cisaillement contraires aux efforts originaux. Une serie d'essais de cisaillement direct sur joints visiblement rugueux formes avec du platre ont donne des resultats consistants qui permis d'obtenir un modèle empirique de comportement. On presente aussi la theorie qui explique des phenomènes.
Dieser Bericht behandelt das Verhalten von Trennflachen im Fels, die zuerst in die eine und dann in die entgegengesetzte Richtunq gleiten. Es wurd nachgewiessen daβ dieses Problem wesentlich ist, um das Verhalten von Trennflachen in vielen Anwendungen, z.B. Erdarbeiten bewirken Scherspannungen in Trennflachen entgegengesetzt zu den urspruenglichen, in numerisch realistischer Weise wiederzugeben. Eine Serie von direkten Scherversuchen mit rauhen Trennflachen von Gipsmodellen erbrachte uebereinstimmende Ergebnisse und erlaubte uns, ein empirisches Hodell dieses Verhaltens aufzustellen. Einige Erscheinnungsformen des Modells wurden auch theoretisch bestatigt.
The behavior of rock discontinuities is usually treated with parameters independent of the loading path. However, the strength and deformability properties of joints change continuously during shear, because due to wear in previous loading, a new geometry of asperities always confronts a new load. If the direction of the shear load is reversed, even greater changes in behavior should be expected since the modifications of the roughness geometry become dependent on the direction of load. Classical methods of analysis used by engineers do not handle continuous change of parameters. Numerical methods in current use, however, can be employed to analyze general load path situations if a realistic and also general joint behavior law is adopted. An improved understanding of the behavior of joints applies to more than modelling, however, as it adds to the confidence with which joint constitutive theory can be used in all other applications. There are a number of practical situations with loading in reversed direction or with repeated cycles of loading in the same direction. For example, under seismic loads, depending on the intensity of shaking and the initial shear stress, joints may slide back and forth. Reversed shearing can also occur following excavations of rock slopes or tunnels which cause new shear stresses on discontinuities with sign opposite to the initial shear stresses. If those initial stresses have caused previous shear movements and depending on the intensity of the new loads, peaks of stresses in opposite directions can be reached consecutively. In laboratory or field multi-stage residual sliding tests, due to the elevated costs for reaching and trimming blocks "in situ," several direct shear tests with different normal stresses are usually performed on the same block. Since only the first test is "virginal," analyses should include path dependency. The properties of joints subjected to reversible shear load will be discussed in the light of tests performed on discontinuities having a constant geometry. An empirical model of behavior is then proposed as well as a theoretical model capable of explaining some of the features of the empirical model.
The first attempts to give rock joints a theoretical treatment adopted a purely frictional behavior. However, varying properties of joints within a single rock type called attention to the influence of roughness. For the evaluation of the peak shear strength, Patton (1966) concluded that at low normal stresses, the surface asperities are simply overridden; In this case, the apparent friction angle can be obtained by adding the angle of inclination of the asperities i to the friction angle on the sliding surfaces μ. At high normal stresses, less work is required to shear off the asperities than to override them and the peak shear strength of the joint depends on the strength of the intact material. A formula for the transition normal stress at which the change in the mode of failure occurs was not given. According to this theory, the ratio between the displacements Δv, normal to and Δu, parallel to the direction of the average shear plane is constant and independent of the normal stress (provided it does not exceed the transition value mentioned above). This ratio should be equal to the tangent of the inclination angle i.
