A damage mechanics theory for discontinuous rock mass is proposed and applied to an excavation problems of underground cavern of three different shapes for a hydro-electric power station. Distributed joints in rock mass are characterized by a second-order symmetric tensor, called the damage tensor which is evaluated by a technique of three-dimensional photographic surveying with a micro-computer system. Mechanical effects of joints are represented by an additional force vector by introducing the net stress which is derived from the stress vector acting on the effective part of the material body. Results of the damage analysis are compared with those by a conventional analysis, and the influences of joints on the stability of each shaped cavern is checked.
Une theorie de mecanique de dommage d'une mass rocheuse discontinue est proposee et appliquee à un problème d'excavation d'une caverne souterraine de trois formes differentes pour une centrale hydro-electrique. Les diaclases distribue dans la masse roche sont caracterisees par un tenseur symetrique de second-degre, designe comme le tenseur de dommage qui est ici mesure par releve photographie trois-dimensional assiste par ordinateur, Les effects de mecanique des diaclases peuvent etre representes par un vecteur additionel en introduisant 1'effect de contrainte reel derive du vecteur de contrainte qui agit sur le section effectif de corps materiau. Les resultats de 1'analyse des dommages sont ensuite compares avec ceux obtenus par analyse conventionnelle et l'influence des diaclase sur la stabilite de chaque forme de caverne est evaluee.
Eine Theorie fuer Schadensmechanik von diskontinuierlichen Felsmassen wird vorgeschlagen und auf Abtragungs-probleme bei drei unterschiedlich geformten Untergrundkavernen eines hydroelektrischen Kraftwerkes angewendet. Verteilte kluefte in Felsmassen werden durch einen symmetrischen Tensor zweiter Ordnung, den Schadenstensor, beschrieben, der ueber dreidimensionales fotografisches Vermassen mit Hilfe eines Mikro-Computers ausgewertet werden kann. Mechanische Effekte von klueften werden durich einen zusatzlichen Kraftvektor dargestellt, indem die resultierende Spannung eingefuehrt wird, die aus dem Spannungsvektor, der auf den effektiven Teil des Materials wirkt, abgeleitet wird. Resultate der Schadensanalyse werden mit denen einer konventionellen Analyse verglichen, und dar Einfluss von klueften auf die Stabilitat dar unterschiedlich geformten kavernen wird untersucht.
The stability of engineering structures constructed in/on rock mass is mainly concerned with the mechanical effects of discontinuities such as fault and joint. Several numerical models have been proposed to analyse the effects of discontinuities (e.g. Goodman, Taylor and Brekke 1968, Zienkiewicz, Best, Dullage and Stagg 1970, Ghaboussi, Wilson and Isenberg 1973), however, they. are effective only for discontinuities of relatively large scale since they all attempt to model the geometrical and mechanical characteristics of distinct discontinuities. Distributed discontinuities, such as joints, are complicated in their geometry, so that it is difficult to model each of them explicitly. There is neither excellent theory nor powerful numerical method to analyse the effects of the distributed discontinuities
We have introduced the damage concept for the distributed discontinuities, and have proposed a theory to treat the the mechanical behaviours of discontinuous rock mass (Kyoya, Ichikawa and Kawamoto 1985a,b, 1986). In that theory, distributed discontinuities are characterized by the damage tensor which is determined from in-situ' data of joint sets. Then, their mechanical effects are accounted by the net stress which is a transformation of the Cauchy stress by using the damage tensor. The components of the net stress have a meaning of the traction acting on the effective resisting part in the damaged material body. The constitutive law is given between the net stress and the strain, and it is determined only from the usual laboratory tests of intact rocks. By introducing the damage concept, mechanical behaviours of discontinuous rock mass are reasonably treated in a framework of continuum mechanics.
In the previous work (Kyoya, Ichikawa, Kusabuka and Kawamoto 1986), the damage mechanics theory was applied to several laboratory tests on cracked specimens and an excavation problem of a small sized underground cavern for a hydroelectric power station. Numerical results gave good agreement with measured data, and the efficiency of the theory was confirmed.In this paper, an excavation problem of an underground power station is solved for three different types of cavern, that is, the traditional mushroom shape, the horse-shoe shape (Wardech II type) and the bullet shape. The damage tensor of the rock mass is evaluated by a three-dimensional photographic surveying with a micro-computer system proposed by Kondoh et al.(1986). Results of the damage analysis are compared with those by the conventional finite element analysis, and the influences of distributed joints on the mechanical behaviours of each shape of the caverns are discussed.
