A series of in-situ and laboratory triaxial compression tests were conducted for a wide range of specimen size, the diameters of specimens, d = 20 − 400 mm. Based on the tests results, this study focuses on scale effects on pre-failure mechanical characteristics and post-failure deformations. Importance of large-scale in-situ triaxial compression tests is emphasized for validation of sophisticated models of numerical analyses, which can incorporate the influence of discontinuities and/or heterogeneities on the mechanical behaviors of rock masses.
Recently some field rock test methods of element testing were developed to investigate the mechanical properties of rock masses necessary for designs of various rock structures. These field test methods are in-situ uniaxial/triaxial compression/extension tests which are similar to the corresponding laboratory tests. In 2017, Japanese Geotechnical Society set a working group to investigate the necessity of standardization for these test methods. After one year of discussion, this working group concluded that two tests methods, i.e. in-situ triaxial compression test and in-situ uniaxial extension test, should be standardized. The reason for this is that, these new test methods of element testing were considered technologically advanced compared with the conventional test methods of non-element testing, i.e. plate load test, rock/block shear test and pressuremeter test (Tani 2018).
However, as rock masses are generally discontinuous and/or heterogeneous in nature, it is impossible to achieve an ideal element testing assuring uniformly distributed strains and stresses within a specimen. Even in an apparently continuous and homogeneous rock mass, such large cylindrical specimens of diameters, d, greater than 0.4 m used for the in-situ tests inevitably deform non-uniformly due to potential discontinuities and/or possible inclusion of imperfections. Precisely speaking, the large-scale in-situ tests on rock masses are deemed to have an aspect of non-element or pseudo-element testing.