One major objective of rock mechanics testing in a laboratory is to characterize strength and deformational behaviours under in-situ stress state. It is a well-known fact that in-situ rock masses are under a stress state with three principal stresses i.e. triaxial stress state. Knowledge of the mechanical behavior of rocks under triaxial stress conditions comes mainly from the Karman-type triaxial tests (conventional triaxial test) where a cylindrical specimen is axially loaded keeping the lateral load constant. However, the stress path that is specific to these tests is certainly not unique in practical situation. Loading direction in-situ rock masses subjected to engineering or tectonic processes can be arbitrary orientation where principal stress axes can have arbitrary orientation with respect to vertical and horizontal direction. It is difficult to carry out these types of tests because of the difficulties in controlling the stress in those arbitrary orientations and also due to the fact that no detailed guideline on the methodologies of these tests is available in literature. Therefore, with due need, starting from the background of the triaxial testing, this paper will discuss the technical aspects of developed experimental methodologies for these tests in our servo controlled rock mechanics system from MTS corporation and are verified with the case study of deformational behavior in selected special stress regimes. This article will also address the difficulties that one can face during the testing along with their possible solutions.

1. Introduction

Determination of in-situ stress conditions along with the deformational behaviors are the key activities of geo-engineering projects. It is well established that in-situ rock masses are under a stress state with three principal stresses i.e. triaxial stress state. Laboratory tests corresponding to the in-situ stress condition can give necessary experimental background for understanding the mechanical effects in the earth crust. The triaxial test, which closely corresponds to the in-situ state of stress, is one of the most worthwhile and widely performed geotechnical laboratory tests which allows the strength and stiffness of rock to be determined for their use in engineering design. In a typical triaxial test, a cylindrical rock specimen ranging from as small as 38 mm to around 100 mm diameter is placed into a cell that can be pressurized [1]. Height-to-diameter ratio of these test specimens have an approximate ratio of 2:1, and are wrapped within a rubber membrane. After this preparation, specimen is loaded, allowing the material response to be observed under conditions that may approximate the in-situ state of stress.

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