## 5 Example of GSI Assessment: Graphical GSI Chart and V-GSI Chart (Without and with Scale)  
To demonstrate the potential impact not explicitly understanding scale can have on a GSI assessment, this section is dedicated to a comparison of a purely qualitative (graphic) assessment of GSI (Figure 3) versus an assessment of V-GSI (Figure 6) that considers the scale of the problem. Figures 7 and 8 show that the visual assessment of a rock mass's structural condition using the Marinos Hoek [13] graphical chart without scale can lead to GSI rating estimates that may not be correct. Figure 7 shows a 2D DFN representation of a vertical rock face (LHS -Figure 7). A graphical GSI chart assessment without any scale is shown Figure 7 (RHS). The DFN simulation shown is from a real rock mass project ([PERSON], [PERSON], [PERSON], 2018 [27]). A photograph of the rock mass in question is also shown adjacent to the DFN block in Figure 7 to help with the visualization of the DFN model. Without knowledge of scale (the size of the blocks, the number of joint sets, the spacing of the joint sets, or the persistence  
of the joint sets) and by visual comparison with the graphical representation of the rock mass structure divisions, it is a reasonable assumption that the rock mass appears to be somewhere between \"Very Blocky\" to \"Blocky\" on the vertical axis as shown in Figure 7. It is known (from extensive geotechnical mapping of this outcrop) that for this rock mass the joint surface conditions are generally \"Very Good\" as plotted on the horizontal axis in Figure 7. This translates to a GSI rating of around 60-70 and a mean value of say 65, also shown on the GSI chart. In Figure 8 the scale of the DFN block is shown along with the same photograph as the DFN block, as shown in Figure 7. The DFN block shown is in fact 40 m in width and  
Figure 8: V-GSI assessment with scale and measurement for the same rock mass shown in Figure 7.  
Figure 7: Graphical GSI assessment [13] without scale. 71.5 m high. The detailed mapping and statistical characterization of the measured data provided the number of joint sets along with the mean true joint spacings per set (Sti), and these are shown in Figure 8. It is also known from field mapping that sets Jo through J4 generally had measured persistences greater than 10-20 m. An 'orientation' bias-free VFC parameter of close to 1.9 fractures / m\({}^{3}\) was estimated using Equations (1), (2) and (5), and this translates to a VFC scale rating of around 46 (Figure 8--RHS). Using Equation (3) it is also a reasonable estimate that P32 is likely to be just greater than 2 m\({}^{2}\)/m\({}^{3}\) for the same rating of 46. The same joint surface conditions apply as described for Figure 7, and this is plotted on the horizontal axis in Figure 8 and translates to a horizontal axis rating of around 40, resulting in combined V-GSI rating estimates of around 85, approximately. Equation (7) gives the same answer. This is a scale-driven (on the vertical axis) assessment that is different to the graphical chart assessment without true knowledge of scale. Without knowledge of the scale of assessment and the spacing and persistence of individual joint sets, it would not have been possible to understand that the rock mass in question is 'Blocky at the Scale of Interest' as plotted on Figure 8. Figure 8 only shows the mean VFC but, with the use of Equation (4), VFC limits can be estimated and plotted on the chart. Figure 9 shows the difference in the rock mass strength and deformability estimates for these two assessments (graphic GSI chart, lower curve, and V-GSI chart, upper curve) generated with RSData by Roccscience. Clearly the graphical assessment of GSI without scale can be misconstrued, and this misjudgment can be made by either a geologist or an engineer when scale is not fully appreciated.