More and more people are discussing what constitutes a deep mine. Logically, many focus on depth below ground surface. But depth is just one aspect to consider and, as a surprise to many, not the dominant factor. Two of the more important aspects to consider are: (1) in situ stress state which varies globally ranging from conditions where the major principal stress is vertical to where the major principal stress is horizontal and multiples in magnitude of the vertical stress. For example, stresses in and around Pennsylvania can have horizontal to vertical stress ratios >4. a. Example 4000m depth where the major principal stress is vertical (e.g., deepest UG mine in South Africa is ~4000m) would have a Sigma 1 ~108MPa. b. Example 2000m depth where the major principal stress is horizontal and 2 times the vertical would also have a Sigma 1 ~108MPa. Both cases assuming 0.027MPa/m for vertical stress gradient. (2) If you couple the in situ stress state (for which depth is a variable) with intact rock strength, you have the makings of a parameter (the Stress Level Index, e.g., Kaiser et al 2000) that is valuable for defining deep mining conditions. The stress Level Index (SLI) relates the induced maximum stress (σmax) around an equivalent circular excavation in elastic rock to the rock’s intact compressive strength (σc), where σmax = 3σ1 – σ3 (with σ1 and σ3 representing the major and minor principal stress in the vicinity of the tunnel in the plane perpendicular to the tunnel axis). By benchmarking the SLI to real conditions (from these and more…Wilson 1971, Barton et al. 1974, Wiseman 1978; 1979, Hoek and Brown 1980a, Stacey and Page 1984, Jager et al. 1990, Kaiser et al. 1996, Board and Brummer 1997, Brummer 1998, and Martin et al. 1999, amongst others) you find: - Spalling initiates at σmax/σc= 0.3-0.5 (which corresponds with the unconfined rock mass strength near the excavation wall) - Notch formation with deep spalling (>20% of tunnel radius) and potentially minor strainbursting (spitting and popping) is to be expected for σmax/σc>0.5 to 0.6 - Moderate strainbursting and-or deep spalling at σmax/σc>0.6 to 0.8 - Major strainbursting and-or deep spalling at σmax/σc>0.8 This allows for a meaningful relative assessment of stressed ground challenges. In Figure 1, I have plotted my operating mine and project experience for cases available in the public domain to show the value of looking at SLI vs depth. One can quickly see that depth is not the driving factor and that it is a combination of in situ stress and intact rock strength. These plots are for the in situ stress before mining but induced stress at different times of mining can be used to assess SLI as well and see how it changes over a mine’s life. Furthermore, in the past, Sigma 1 / UCS was used as a stressed ground indicator. I argue that SLI is more valuable. When the same data is plotted with S1/UCS (Figure 2), there is limited range in the results reducing decision making potential. One of the reasons for this is the importance of the other principal stresses in assessing when deep mining challenges could be expected.