Landslides and residual strength in marl profiles in Israel

Many areas in northern Israel show evidence of old landslides in marly profiles, and new slides are often activated in these regions. Such a slide occurred in the town of Rechasim, near Haifa, during an exceptionally wet winter in 1992. The present paper analyzes the causes for this slide, and suggests that it was due to a combination of downslope flow in the overlying, severely cracked chalk–limestone layer, and mobilization of residual strength at the marl–chalk interface. The residual strength of Israeli marly soils has been studied, and it is shown that while many of these consist predominantly of clay-sized (< 2 μm) particles, and may, therefore, expect to have a very low residual friction angle, the amount of clay sized carbonate in the soil is of major influence on the residual friction angle. When the clay-sized material includes less than about 11% carbonate, the clay particles dominate the shearing mechanism and result in a residual friction angle of the order of 12°; this was the case at Rechasim. However, when more than about 30% of the clay-sized material is carbonate, the carbonates control the shearing mechanism, and residual friction angles can be as high as 30°. Results of a preliminary study of the change in residual strength with time are also presented.