Failure mechanism of overhanging slopes in sedimentary rocks with dissimilar mechanical properties

The paper discusses a slope failure which occurred in December 2009 adjacent to a road in northern Greece. Thick Miocene limestone horizons are interbedded with relatively thin weak horizons of silty to sandy clays and loams which are more easily eroded, resulting in the formation of notches. The weak horizon acts as an aquitard, inhibiting downward percolation from both rainfall and house discharges. The two main areas of failure occurred where the notches were particularly deep (hence the overhang was greatest) at a time when heavy rainfall created a hydrostatic head within the bedrock to the west of the slope face. The stability of the notched benches is controlled by the tensile strength of rock bridges in the limestone’s discontinuities. The analysis showed that the limestone’s in situ tensile strength and cohesion are on average three times lower than those obtained from laboratory tests, hence their application in the failure analysis would lead to an overestimation of the slope stability.