The friction angle mobilized by shallow earthflows in tectonized clay shales

Earthflows are typical landslides in tectonized clay shales. In the long-term stage after triggering and rapid downslope spreading, the mass gradually slows down attaining a lower and lower displacement rate while taking a slide style. In this stage the displacement rate is governed by pore water pressure fluctuations thus a simple back-analysis can allow to calculate the mobilized friction angle that is often even much higher than the value measured by laboratory tests. Accurate analyses of the pore pressure regime show that this apparent inconsistency can be explained by the peculiar effective stress field that establishes in the landslide body.