Scheidegger (1973) presented an observation that the net friction is lower in larger landslides than in smaller ones. This implies a certain mechanism to exist that reduces slide resistance only in larger slides. As a possible dynamic cause for the phenomenon, it is proposed that crushing of slide mass is caused by unstable oscillation in the bottom of the slide mass which in turn reduces the friction angle only when the slide speed exceeds a threshold value. This postulate is consistent to an empirical relation presented by Davies (1982) that attributes the apparent reduction of resistance in larger landslides to the spreading of fluidized slide mass.
Scheidegger (1973) hat eine Observation prasentiert, daβ der Netto-reibungskoeffizient in gröβeren Rutschungen niedriger aIs der in kleineren ist. Das bedeutet irgendeiner Mechanismus zu geben der die Gleit-widerstiinde nur fuer grôûere Rutschungen niedriger macht. Als eine mögliche dynamische Ursache dafuer, es wird vorgeschlagen daβ Fragmentation der Rutchungsmasse verursacht wird durch die unstabile Schwingung in der Sohle der Rutscungsmasse, die in die Abnahme der Gleitreibung fuehrt, erst wenn die Gleitgeschwindigkeit einen Grenzwert ueberschreitet. Diese Annahme stimmt mit der von Davies (1982) ein, der der Verbreitung der in die Flueβigkeit-veranderten Rutchungsmasse fuer die scheinbare niedrigere Widerstande in gröβeren Rutchungen verantwortlich hielt.
Scheidegger (1973) a presente une observation que coefficients net de frottement sont plus reduis pour glissements de terrain grands que pour ceux petits. Cela signifie que il existe quelqu'un mecanisme qui reduit les resistances seulement pour glissements de terrain grands. Comme une cause dynamique possible pour le phenomène, on propose que la fragmentation de masse rocheuse des glissements de terrain est effectue, qui contribuera à la reduction de coefficients de frottement, par oscillations instabiles dans le fond de masse glissante qui soit excite quand la vitesse de glissement excede une certaine valeur limitante. La supposition est concordante à la relation empirique de Davies (1982) qui a attribue resistances reduies apparentes dans glissements de terrain grands à la etendue de la masse glissante fluidifiee.
Scheidegger (1973) pointed out an observed fact that the net friction (the tangent of the slope connecting the top of slide scarp and the toe of a slide mass) is smaller in larger landslides than in smaller ones (Fig.1). This implies that there is some mechanism that reduces slide resistance in larger landslides. Many hypotheses have been proposed for explaining the mechanism for this reduced net friction in large-scale landslides. Some of the hypotheses are either those which assume some lubrication media such as air (Kent, 1966; Shreve, 1968), porewater (Ashida and Egashira, 1986; Sassa, 1988), etc. (Habib, 1975), or those which assume effects of some dynamic phenomena such as acoustic fluidization (M elosh, 1979) which counts on temporal stress relieving by acoustic waves in the sliding debris, or a basal pressure wave (Foda, 1990) which arises in the basal layer of a landslide when the slide speed exceeds a certain threshold value. In view of landslides in various conditions which span dry to wet environments including those on the Planets where neither air nor water can be expected (Melosh, 1979), there should be such kind of landslides that do not need lubricating materials for reducing slide resistance. The hypothesis assuming acoustic fluidization by M elosh (1979) is one of such candidates and that assuming a basal pressure wave by Foda (1990) is another. The former is however unsatisfactory since it lacks the generating mechanism of acoustic waves, while in the latter the mechanism for generating unstable vibration is the central problem.
