Why might a small change in applied force result in a considerable change in factor of safety?

It has been observed that in some cases a small change in applied force (seismic, external loads, water pressure), results in considerable changes in factor of safety.  The reason has to do with the mode of failure of the wedge. 

In Swedge there are two modes of failure, 1) sliding on one face or 2) sliding on two faces (along the line of intersection).  Note that the mode of failure for a wedge can be seen in the sidebar.  If the mode of failure is sliding along one joint, then the shear strength is only due to the shear strength along that one plane on which sliding occurs.  If the mode of failure is sliding along 2 joints, then the shear strength is due to the strength of both planes.  The shear strength from 2 planes can be considerably higher than just one plane, especially if there is cohesion.  So if a small change in force causes a change in mode of failure, then you can get large changes in factor of safety.  The factor of safety is usually higher if sliding is on two planes over one.  Look for a change in mode of failure when you see this type of behaviour.

 

For information on how mode of failure is determined, see the Swedge Theory Manual.