Large Shear Strength Parameters for Landslide Analyses on Highly Weathered Flysch

Many significant landslide movements are often observed in the upper weathering zone of flysch, which constitutes the most critical landslide-prone geological formation in Western Greece. In this article, a laboratory approach is adopted to investigate the behavior of highly weathered and tectonically decomposed flysch for slope stability analyses with the performance of large shear testing in reconstituted soil specimens. The testing program included several reconstituted flysch specimens derived from three representative landslides. Tests under large direct shearing (300 × 300 × 120 mm) were conducted in moisture- and density-controlled conditions and ring shear tests were conducted in the finer material. The test results revealed that the values of the effective angle of friction in the flysch material decrease with the increasing water content. Moreover, dense specimens showed curved failure envelopes due to dilatancy, especially in dry conditions. A comparison of laboratory test results with those obtained by performing back-analyses under saturated conditions has shown that the sliding of the weathered and decomposed flysch mainly depends on its residual angle of friction which was found to be 1°–6° lower than the ultimate angle of friction as it was estimated by the large shear tests