the meaning of shear ratio (srat)

Hi, everybody!

I have some problem with understanding of shear ratio history (srat) in UDEC.

Here is the description of this function in UDEC manual: "srat - ratio of shear stress to shear strength at contact".

If I take the value 1/srat then I should normally get the value of factor of safety (FS) for shear condition, since this value will be the ratio of shear strength to shear stress at contact. But shear displacement history (sdis) shows that there still can be some shear displacement in case of FS=1/srat more than 1.