An example of the influence of lithology on slope stability

Abstract

The right bank of the Tagus river, near Santarém, 70 km northeast of Lisbon, is a slope about 2 km long and 80 m high. The geological formations are nearly horizontal, interbedded Miocene sand and silty clay layers of estuarine character. The slope dips about 30 to 40 degrees throughout a large part of its extent. Thus, small surface slides occur every year due to loss of cohesion within the formations, leading to a normal geomorphological evolution of the slope.

An important landslide which involved about 200,000 m3 of soil occurred recently at one end

of lhe slope. ln this zone, the dip of the slope was only about 20 degrees; consequently, no landslide was expected. The study carried out and described showed that the accident was due to the presence of an extraordinarily plastic organic clay layer, interbedded in one of the silty clay complexes at mid-height of the slope, where a creep procéss, initiated long ago, led to rupture. This clay layer was limited to the sliding zone.