I expect it is a true rotational failure
and therefore complete and based on a simple complex of shear plane in a basal system, rather than a lot of interactive system failures, or a false rotation which may be actually a cause from anoterh source
sorry running out of web load time but that is probably it
ask Structural Soils Ltd Bedminster Bristol they do a lot of foundation site in 'orrible materials and are very good at it especially wet, silty ground
MikeHydroPhys
mdshydroplane