Anisotropy of deformational and strength responses of four schistose rocks have been critically examined. These rocks were obtained from the foundations of two underground power house sites in the Himalayas. Specimens at different orientations (ß) varying from 0° to 900with respect to axial stress (б1) in unconfined and confined state up to 100 MPa of confining pressure were prepared and tested to evaluate the applicabilty of the non-linear criterion for prediction of triaxial compressive st rength. A methodology has been developed to predict the modulus and stress-strain curves with minimum pre-evaluation experimental data.


L'anisotropie des reponses de la resistance et de la deformation des quatre roches schisteux, ont ete attentivement examinees. Ces roches ont ere obtenus des fondations des deux centrals electriques sousrcrrains siiuees en Himalaya. Des specimens ont eres prepares et examines dans les orientations (/3) variants de 0° a 90° par rapport a la co n t ra in t e axiale (al) dans les conditions de compression sans et avec et re inte jusqu'au 100 MPa de la pression de confinement pour I'evaluation de la mise en application du cruere non -Lineair e propose et pour I'estimation de la resistance ala compression triaxiale. Une met hodotogie a ere developpec pour I'estimation de l a deformation du moc.Jule et les courbes etreinte - deformation avec un minimum de pre - evaluation des donnees exprimentales.


Die Reaktion von vier Schiefergesteinen wurde b ezu gl ich der Vorformung - und Festigkeitsanisotropie genau untersucht. Die betreffenden Gesteine stammen von Fundamenten weir unteschiedlichen Kraftwerke in Himalaya. Die Proben wurden mit verschiedenen Orientierungen (fJ) von DObis 900beziiglich der Axialstrcss (al) mit u nd ohne allseitiger Druck bis zu 100 MPa prepariert und gerest et, urn Anwendbarkeit von nicht-linearem kriterium von kompressiven triaxialen Festigkeit vorauszusagen. Es wurde cine Methode entwickelt, mil der das modulus und Spannung-verformung-kurve unter Anwendung der minimalen experimentellen Dalen bes rimmt wurden.

I Introduction

Rational and realistic approach to the analysis and design of the on ground or underground engineering structures on anisotropic rocks necessiates indepth evaluation of anisotropy in the measurment of strength (tensile and compressive) and deformation. Inherent anisotropy acquired by metamorphic rocks make them weaker and dictate its overall behaviour. Anisotropic behaviour of schists is due to the process of metamorphic differentiation (segregation of constituents). In response to high pressure and temperature gradiants associated with mountain building activities, rocks flow and recrystallize under new tectionic conditions and develop layers of contrasting mineralogical facies. Such weak foliation planes or schistosity will affect the strength and deformational responses of anisotropic rocks when orientation of stresses with respect to these weak planes changes. Prediction of the anisotropic respenses of strength and deformation of anisotropic rocks involves preparation of specimens at different orientation angles, ß (the angle between major principal stress direction and the foliation planes). The measurement of the strength anisotropy for various anisotropic rock types have been carried out by many investigators e. g. Donath (1964), Chenevert and Gatline (1965), McLamore and Gray (1967), Hoek (1968), Attewell and Sandford (1974), and Brown et al. (1977), on shales and slates, Deklotz et al. (1966), Akai et al. (1970), McCabeand Koerner (1975), Nasseri (1992), on gneisses and schists, phyllite by Ramamurthy et al, (1988), Horine and Ellicksone (1970), and Rao et. al, (1986) on sandstones, Pomeroy et al, (1971), on coal and Allirote and Boehler (1979) on diatomite. An overall analysis and review of their works exhibit that maximum failure strength is either at ß = 0° or 90° and it is minimum usually arround ß = 30°, The Shape of the curve between the uniaxial compressive strength (бc) and the orientation angle ß designated as the "type of anisotropy" and is found to be generally of three types such as "U - shape", "shoulder" and "wavey" (Fig. 1).

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