Long term soil strength for slope analysis is akin to residual strength that we see with direct shear tests. Beyond a specific shear displacement, the shear strength drops to some residual value and remains at that level. This reduced level has a phi angle, as can be seen in the unit force vs unit perpendicular loading across the shear surface. Note that there is no such thing as shear strain, there is no distance to use for nominalization.
Slope failures start the rupture surface at the bottom toe, and the shear strength of the soil drops to residual once the displacement is adequate. The individual slices of simple shear analysis assumes that there is uniform distribution of the shear stress along the circle of analysis, which is oversimplification. The rupture surface starts at the toe, and later at the scarp, and proceed to where the dramatic happens. This may take years or days.
Flows, where excessively wet materials, loess, or liquafication occur, may happen seemingly instantly, with no warning. The hill is sitting there, and then it fails. Slopes and life around loess and liquifying soils is dangerous. As is life over carst formations, liquid mining (Fort Saskatchewan), and old coal mine workings. Ever geotechnical investigation should check these parameters.
All silts should be checked for loess condition. The first simple test is a "rolled down from natural moisture" plastic limit is conducted, and the results compared to a dried then plastic limit. Even a small difference is in question. Sensitivity is the second sign. Often in drilling, the material is observed as wet and soft, yet cone penetration shows a firm of hard condition. The data does not match, sensitivity is high, now what?