## 2 Scale and Scale Effects in Rock Engineering and the Use of GSI  
This section provides a brief summary that describes the importance of scale and scale effects as these are critical to understanding the reasons why attempts have been made to add quantitative measures to the GSI chart. Scale effects in rock engineering were first presented in the 1993 book called 'Scale Effects in Rock Masses 93' ([PERSON], 1993 [4]) where it was put forward that \"rock masses are basically inhomogeneous and discontinuous media\". The determination of rock, joint, and rock mass properties always involves uncertainties due to the variability of the rock material and the fracture networks. The results of laboratory and in situ tests are thus affected by both the chosen testing points and the volumes involved in the tests. The variation in the test results with the specimen size is often referred to as \"Scale Effects\". The authors of [4] also asserted that an understanding of \"scale effects\" is required when undertaking deformability, strength, and hydraulic properties estimates, and understanding the impact of internal stresses in rock masses. At the same time the concept of representative elementary volume (REV) was also introduced in [4] and further elucidated by [PERSON], 1993 [5], amongst others, and this is also a key concept that to this day forms the core of the characterization and strength and deformability assessment of rock masses. The REV concept in terms of engineering behaviour is shown in Figure 1.  
[PERSON] and [PERSON], 1981 [7]; [PERSON], 1992 [8]; [PERSON], 1994 [9]; [PERSON], 1996 [10]; [PERSON] and [PERSON], 1997 [11]; and [PERSON] and [PERSON], 1997 [12], (to name a few) reiterated that rock mass behaviour, and the appropriate choices of strength criteria, are scale-dependent, and this in turn can be attributed to the presence of flaws and fractures in a rock mass. In a geological framework, the authors of [10] provided the following guidance: relative scale in the context of rock masses can be defined as the \"ratio or the scale of observation to the scale of fracturing\" or the \"dimensions at which the scale of observation greatly exceeds the block sizes or fracture spacing\". A rock unit or structural domain can be described as a rock mass when these scale criteria are met. In terms of engineering response [12], this means distinguishing between rock properties at a point and rock properties over a volume. Point properties such as density, primary porosity, intact rock permeability, and point load strength may vary spatially for a particular rock mass, but these properties are not dependent on discontinuities. On the other hand, volume properties such as the modulus of deformation, rock mass shear and tensile strength, secondary porosity, permeability, and fracture intensities of a rock mass are dependent on the presence of discontinuities and their spatial distribution within a rock mass. In recognition of the importance of understanding geological structure in rock engineering, [PERSON] and [PERSON], 2000 [13], and [PERSON], 1997 [11], presented the graphical GSI chart that tried to capture geological structure (via caricatures and descriptions) and discontinuity condition descriptors and linked these to GSI ratings and to the H-B failure criteria equations. The aim of this paper is not to provide an extensive review of the history and development of the H-B equations or the GSI empirical system. Rather what is important to this paper, other than a reminder of the 40-year history of the development of H-B equations and 30-year history of GSI, is that GSI should not be used without understanding scale and scale effects. Initially [PERSON], 1992 [8], and then [PERSON], 1994 [9], followed by [PERSON] and [PERSON], 1997 [11], and many other authors subsequently recognized that the validity of the use of the H-B relationships and the use of GSI is scale- and failure mechanism-controlled.