Effect of the pre-peak loading condition on the post-peak behavior of the Brittle rocks

Arash Khosravi

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: FUNDAMENTALS - VII Soil & Rock Mechanics

ABSTRACT: The understanding of the post-peak behaviour of intact rocks can be useful to predict pillar instability or rockburst and to optimize the design phase of underground openings. It is always difficult to obtain the full post-peak curve of brittle rocks. In this paper, a new loading procedure with an incremental cyclic loading and axial displacement rate -called damage-controlled cyclic loading - was applied to hard rock samples. The results of uniaxial compression tests using the proposed damage-controlled method are presented for two brittle rock types. A discussion follows on some of the characteristics of the obtained stress-strain curves.

RÉSUMÉ: La compréhension du comportement post-pic des roches intactes peut être utile pour prédire l'instabilité de piliers ou coups de terrain et pour optimiser la phase de conception des excavations souterraines. Il est toujours difficile d'obtenir la courbe post-pic de roches fragiles. Dans cet article, une nouvelle procédure de chargement avec un taux cyclique incrémental de déplacement axial - appelé méthode de dommages contrôlés par chargement cyclique- a été appliquée aux échantillons de roches dures. Les résultats des essais de compression uniaxiale selon la méthode proposée sont présentés pour deux types de roches fragiles. Une discussion suit sur quelques-unes des caractéristiques des courbes de contrainte-déformation obtenues. 1 INTRODUCTOIN It is required to understand the mechanical behaviour of rocks in the design phase of an underground opening. Brittle fracturing occurs in the surrounding rock mass when the stress exceeds the ultimate strength. This phenomenon . One type of rockburst is a sudden brittle fracturing where the potential accumulated energy is released in a violent way. The discontinuities control the post-peak mechanical behaviour of intact rock. The dominant phenomenon, in rockbursts, is the movement of discontinuities A better understanding of the post-peak behavior of brittle rocks can help to characterize rockburst phenomena. The post-peak curve of an axial compression test is often quite difficult to obtain. Usually, samples will fail at peak load in a sudden and violent manner. This violent failure is not an intrinsic behavior of brittle rocks (Rummel and Fairhurst 1970; Wawersik and Fairhurst 1970) but is controlled by the stiffness of the loading system. Therefore, the post-peak behavior can be obtained in particular testing conditions. To obtain a complete stress-strain curve of a sample under compression, two conditions should be met: a) using a rigid testing machine to reduce the strain energy accumulation; and b) applying a particular loading procedure in slow strain rate (Cook 1965). By applying these conditions, the stored energy can be dissipated smoothly by converting the elastic energy into surface energy (Wawersik and Fairhurst 1970). Based on the work of Salamon (1974), the post-peak curve can be captured when the absolute value of the post-peak modulus of the rock sample was smaller than that of the testing machine. Cyclic loading was used to obtain the post-peak curve. To find the effect of cyclic loading on cracking process, an investigation was conducted at unstable crack propagation phase on the samples subjected to the triaxial and uniaxial compression test. (Martin 1993; Martin and Chandler 1994). Cyclic loading near peak strength can be used to avoid violent failure. Unloading of sample at this stress level helps to dissipate the elastic energy in a controlled manner. This method lets fracture to be formed homogeneously. This method is called damage-controlled. In this study, a similar approach was employed as the one proposed by Martin and Chandler (1994). However, partial unloading cycles were applied here with small axial displacement rate to obtain the complete post-peak curve instead of using complete unloading cycles and radial controlled strain rate. 2 UNIAXIAL COMPRESSION TEST There are some factors that influence the behavior of rocks subjected to compression including: loading boundary condition, sample anisotropy, size of sample, loading platen material and stiffness of the testing machine (Jaeger 1966). A few experimental procedures have been developed (Wawersik and Fairhurst, 1970; Martin and Chandler, 1994; Fairhurst and Hudson, 1999 among others) to obtain the post-peak curve of hard rocks from the laboratory tests. Two experimental methods were previously proposed, in the literature, to obtain the post-peak curve of medium to hard rocks and the proposed method in this study, are concisely described in the following:

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