Majority of experimental strength and deformational characteristics of rocks are obtained from uniaxial tests, mainly from compression tests, or through the particular triaxial experiments commonly known as Conventional Triaxial Compression (CTC). The procedure of CTC test usually consists of initial compression up to isotropic pressure imitating e.g. a component of geostatic stress at a given depth, and subsequent application of additional axial loading, which can be a model of anisotropic stress conditions or differential stresses originating from inhomogeneities or movement of surrounding rock masses. This CTC test, however, cannot always portray the in-situ conditions. For example such axial compression does not reflect the real stress paths during the rock burst. More appropriate experimental model for the phenomenon is axial stress reduction inducing axial extension in test named Reduced Triaxial Extension (RTE). Stress paths representing axial extension are also important in case of core disking. In addition to CTC and RTE paths, the next complementary paths can be considered: Conventional Triaxial Extension (CTE) or Reduced Triaxial Compression (RTC) using increasing or decreasing the lateral stress respectively. This work discusses the experimental methodology and technical solutions prepared for realization of several special loading paths in laboratory triaxial experiment. Besides the standard CTC path, the experiment includes the non-standard paths of RTE, CTE and RTC tests. Moreover, experimental results obtained from the new servo-controlled device at Institute of Geonics, CAS through above mentioned techniques will be presented. Specifically dependence of stiffness moduli of Brenna Sandstone on the above selected paths and initial conditions will be discussed.
Motivation for laboratory modeling of non-standard loading and unloading stress paths
Determination of in-situ stress conditions and investigation of its evolution are key activities of geoengineering projects and applications. Complementary laboratory testing gives a necessary experimental background for understanding the mechanical effects in the earth crust, particularly the deformational response of rocks to the in-situ stress. Role of these laboratory testing can be important in two points of view. First, a laboratory test determines the deformational response of samples exposed to a normalized condition usually suggested by technical standards. From the output a mean idealized intrinsic mechanical moduli of given rock can be determined through some theoretical assumptions and approaches. Later these moduli can be used in constitutive models of in-situ evolution of stress and strains. These models often meet with serious problems of reliable description of e.g. evolution of micro cracks. Therefore, the second attempt could be used, in which a mean local mechanical response of rock materials to various stress conditions is tested, and if necessary, appropriate effective moduli can be evaluated and used to estimate or model in-situ stress and strain field behavior.