The parameter depth of failure zone is used to identify behavior types 3 and 4 (shallow and deep shear failures). In cases where the stresses after excavation exceed the strength of the rock mass, a failure zone develops. The depth of failure zone (DFZ) is calculated with a closed form solution, or can also be determined with a FE or DE program. For this analysis the analytical model of Hoek was applied.
The DFZ- limit is expressed in terms of a fraction of the diameter, corrected by a scale factor (Figure 5).
The DFZ is also used to distinguish between BT 3 and BT 4. It may also serve as a criterion to further subdivide BT 3 and BT4.
For BT 1 and BT 2 the strength of the rock mass must not be exceeded, thus a failure zone depth of zero is a preconditions for those BTs.