Stability Graph Method
Empirical databases such as the Q and RMR systems were develop as a tool to help guide engineers when designing excavations underground, these databases are primarily based on civil engineering tunnel cases at low to moderate depth. These tunnels were designed as permanent openings with high traffic. The Q and RMR systems are very important to tunnel design work, however can be over conservative when applied to temporary or non-entry excavations. Mathews (1981) developed an empirical method to dimension stopes based on a stability number, N which defines the rock mass's ability to stand up to given ground conditions, and shape factor, S which is the stope face hydraulic radius that accounts for the geometry of the stope surface. The method dimensions each active stope face based on, N and S. The initial stability graph developed by Mathews is based on 50 case histories.
Potvin (1988) further expanded the original stability graph with an additional 175 case histories and introduced the modified stability number, N' to replace Mathews stability number. The modified stability number is similar to the N value developed by Mathews, but has different factor weightings.
The database assembled for the modified stability graph reflects Canadian practice, and is bias towards Canadian ground conditions