Unstable slopes often show time dependent movements which may be highly variable. In general these variations are often explained with changing water levels behind and in the slope resulting from rainfall. In engineering practice this dependency is commonly accepted. But experience shows, that in low permeable soils heavy rainfall will more likely run off at the surface rather than penetrating into the subsoil at a substantial rate. Thus the influence of rainfall on the piezometric line in low permeable soil should be expected to be minor and delayed in time. Because variations in pore water pressure control effective shear stress (Terzaghi and Peck, 1948) this accepted soil mechanical principle has been enhanced by including time dependent effects of external pressure variation and its influence on the soil below the piezometric line. So far accepted external pressure variation may originate in changes of water level (e. g. tidal effects in estuaries, water level draw down effects in reservoirs or waterways). In this paper it is proposed, that variations of barometric pressure acting on a slope may also influence pore water pressure conditions. Under certain conditions this effect may be decisive in triggering landslides. Extensive field measurements have been performed in unstable clay slopes. In this paper recent results will be presented of these measurements in progress as well as results of back calculations.
For a navigable canal (Stichkanal Hildesheim -SKH-) a cut has been constructed in Lias clay close to Lühnde (Germany) in the 1920s. After major failures in the beginning due to extremely steep slopes (1: 1.5) the slopes still remain unstable at slope inclinations presently ranging at about 1: 2.5 to 1: 3.5. In 1996 inclinometer casings have been installed and periodically read.