## 4 Discussion  
In the finite element example provided in Section 3 above, a single V-GSI rating value (80.2) was used in the modelling. In the examples provided in Section 2, the best practice method of quantifying the V-GSI rating is to include a range or confidence limits for the mean V-GSI rating where possible. This would require running the model three times using the upper bound value, mean value and lower bound values. This approach would provide a sensitivity check on the range of deformation for comparison with the monitoring data. As can be seen in Figures 11 and 12, the movement in the crown of the tunnel ranges from 1 mm of upward movement to a maximum of 5 mm of downward movement over a period of time. The support is installed before monitoring starts. The prism installation accuracy levels are typically in the 1 mm or less range, and therefore the monitoring data are indicative rather than absolute values. The finite element modelling predicts just greater than 2 mm in the crown. This level of deformation is also indicative rather than absolute, and depends on the degree of confidence of the input parameters when setting up the models. As can be seen in Figure 10, for example, deformation in the crown is quite sensitive to horizontal stress inputs, with deformation increasing up to 3.75 mm for \(\sigma_{v}=\sigma_{H}=\sigma_{h}\) conditions. Despite the indicative nature of the monitoring data and modelling results, there is a reasonable level of correlation between the two data sets, and they both approximate the real deformation at this location in the tunnel. It was standard practice to determine V-GSI rating for each heading during the M8 tunnel excavation, and on occasions where monitoring results indicated deformation threshold exceedances (shown in Figures 11 and 12), it was possible to undertake modeling to determine if revised or additional ground support was required. The V-GSI rating provided key input data to this type of modeling. While only one example of 3-D finite element modeling is shown in this paper, it demonstrates that the V-GSI ratings established via the method presented in Section 2 can be used with some confidence to determine tunnel rock mass conditions, and hence performance. Figure 12: Comparison of measured displacement over time from stations to left (green) and right (blue) of centerline of tunnel crown with modelled displacement from tunnel crown (pink). The calculation of a _VFC_ for a rock mass, and its application to the V-GSI, does have limitations that need to be acknowledged. For example, discontinuity sets that are sub-parallel to the working heading should be sampled in the crown and sidewalls where possible, along with corrected true spacing measurements, and these data should be included in the VFC estimates to avoid underestimating this parameter. The _VFC_ value can also be easily overestimated if intensely fractured areas of rock are exposed in a heading associated with a joint swarm and the spacing within the swarm are used to determine the _VFC_ contribution of a particular set without considering areas of rock mass with less fracturing between swarms. Persistence of discontinuities (trace lengths), if not adequately understood, can also result in overestimating _VFCs_. This can be partially overcome by sampling where joint terminations are noted and larger spacings are present between joints that are farther apart. Bedding-parallel discontinuities are common within the Hawkesbury Sandstone, and as tunnelling progresses, these may occasionally be encountered (but not seen or mapped) a short distance above the tunnel crown. The _VFC_ contribution from bedding will accommodate this--as explained in Section 2--because 'greater than' (>) spacing measurements are included on the assumption that there is a bedding plane located just above the crown.