Program Assumptions

There are some important limitations and assumptions of the UnWedge program that should be considered when interpreting the results:

• UnWedge should be used to analyze wedge failure around excavations constructed in hard rock where discontinuities are persistent and where stress-induced failure does not occur. It is assumed that displacements take place at the discontinuities and that the wedges move as rigid bodies with no internal deformation or cracking.
• The wedges are tetrahedral in nature and defined by three intersecting discontinuities. A maximum of three structural planes can be analyzed at one time. If more than three major planes are identified for the analysis of the structural data, all combinations of these planes should be considered.
• All of the discontinuity surfaces are assumed to be perfectly planar.
• Discontinuity surfaces are assumed to be persistent and to extend through the volume of interest Therefore the discontinuities defining the wedge do not terminate within the region where the wedges are formed. The implication is that no new cracking is required in the analysis of wedge movement.
• The discontinuities are considered to be ubiquitous: in other words, they can occur at any location in the rock mass.
• The underground excavation is assumed to have a constant cross-section along its axis.
• The default analysis is based upon the assumption that the wedges are subjected to gravitational loading only. By default, the stress field in the rock mass surrounding the excavation is not taken into account. While this assumption leads to some inaccuracy in the analysis, the error is generally conservative, leading to a lower Factor of Safety. However, you may include the effect of in-situ stress on the wedges with the Field Stress option.
• UnWedge always initially calculates the maximum-sized wedges that can form around the excavation. To scale down the size of the wedges according to actual field observations (e.g., observed joint trace lengths), use the Scaling Wedges options.