A review of recent work on mass movements on slopes and on rock falls

Earth-Science Reviews, 21 (1984) 225-249

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Elsevier Science Publishers B.V., Amsterdam--Printed in The Netherlands

A Review of Recent Work on Mass Movements on Slopes and on Rock Falls A.E. SCHEIDEGGER Geophysics Section, Technical Uniuersity, Vienna (Austria)

ABSTRACT Scheidegger, A.E., 1984. A review of recent work on mass movements on slopes and on rock falls. Earth-Sci. Rev., 21: 225-249. This paper reviews the recent developments in the field of mass movements on slopes. The subjects treated include surface-slips, deep-seated soil creep, rock mass creep, surficial landslides, mud flows and Alpine debris flows. In this instance, the paper supplements the Chapter "Accidents on slopes" in the author's book "Physical Aspects of Natural Catastrophes (1975)" by new data published mainly between 1976 and 1983.

1. INTRODUCTION

Mass movements on slopes have been of great concern to mankind for a long time. They are the cause of much damage to roads, bridges or houses and, if they occur rapidly, they can even cause much loss of life. The movements are classified into slow and fast types, into creep slides and flows. A convenient classification explaining the corresponding nomenclature has been published by Bunza (1975b). Because of the social relevance of mass movements, much effort has been expended in the past decade in understanding, predicting and checking them. The early work on the subject of mass movements has been reviewed by Knoblich (1970, 1972) and in the writer's book (Scheidegger, 1975). It is the purpose of the present paper to bring these reviews up to date. In essence, it is an "update" of the writer's book, as far as mass movements are concerned; the divisions of the paper correspond to those of Chapter 4 ("Accidents on slopes"). 2. SURFICIAL PHENOMENA

2.1. General remarks

The "surficial" phenomena on a slope involve the uppermost few metres below its surface. In this instance, one generally distinguishes further be0012-8252/84/$08.00

© 1984 Elsevier Science Publishers B.V.

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tween movements of the vegetation layer (grass-sod!) and movements that involve up to a few metres more. These movements may be of a slow continuous creep-type, they may show some definite fluction-patterns or~ finally, they may affect a region that detaches itself from the lower substrata; the movements are then referred to as "slips". We shall discuss these possibilities in their turn. However, before embarking on this discussion, a remark may be made with regard to the kinematics on any slope. Gerber and Scheidegger (1979) have presented a systematic accounting for the possible positions of two slope surfaces relative to each other and relative to gravity. The resulting classification is fundamental to any discussion of the mass movement on such slopes and, particularly, in accounting for the continuity of the mass flux on the slopes and the valleys formed by them. The possible mass movements, then, must occur in conformity with the general constraints mentioned above.

Adrian E. Scheidegger was born in Switzerland in 1925. He received his Diploma (M.Sc.) in Physics at the Swiss Federal Institute of Technology in Zurich and his Ph.D. in Applied Mathematics at the University of Toronto. He was a Lecturer at Queen's University in Kingston, Canada from 1950 to 1952 and worked subsequently as a seismologist and research engineer in government and industry. He was an Associate Professor at the University of Alberta in Canada, a Professor in the College of Engineering at the University of Illinois in Urbana and Visiting Professor at the California Institute of Technology in Pasadena, the University of Sydney in Australia, the University of Mt~nster in Germany, the University of Alaska in Fairbanks, the University of Aarhus in Denmark, the Centre Universitaire Antilles-Guyane in Guadeloupe, the Universidad de los Andes in M6rida, Venezuela and the Universidad Nacional in Bogoth, Colombia. Since 1971 he has been Professor of Geophysics at the Technical University in Vienna, Austria (Gusshausstrasse 27-29, A-1040 Wien). He is a Fellow of the Geological Society of America and a Miembro de Honor of the Sociedad Colombiana de Geologia, as well as a member of many professional societies.

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2.2. Skin creep Skin creep involves the uppermost layer of a creeping slope. Mainly, it refers to a motion that involves essentially the vegetation cover. By changing the nature of the plant cover, man may be a cause of the initiation of such movements (DeGraff, 1979). Measurements of the movements may be done by geodetic means (Brunner et al., 1975) or by the positioning of filteringprobe inclinometers (Kalkani, 1980). The main morphological feature of a surficially creeping slope is its " h u m p e d " profile. The sequences of humps can have different "wave-length" scales. The smallest of the humps occur in the classic case of the movement of grass-sod on a slope (German: Rasengleiten). The resulting humps are then referred to as "terracettes" (Higgins, 1982). Their "wave-length" (in profile) is about 2 m (Brunner et al., 1975). Regarding the mechanical cause of the terracettes, Carson and Kirkby (1972, p. 174) favoured the theory that these features were caused by slides, but field investigations by digging appear to preclude this possibility. It is much more likely that the terracettes are the result of some instability in a flow process: if the flow velocity is affected by the thickness in such a fashion that the flow is impeded by the presence of material, then " h u m p s " will automatically develop. Somewhat larger-scale series of humps than terracettes occur in many drainage troughs which exhibit the morphology of steps with a wave-length of 20 m. Whilst the water in such a system undoubtedly plays a major role, it may be argued that the creep of trough-material itself is interacting with the trough. The motion is again dynamically unstable, it occurs in "squalls" since the material, when its thickness increases, loses water so that its velocity is decreased. This causes a deceleration or even a (temporary) termination of its motion. When the flow is stopped, the waterloss is all the greater so that a temporary consolidation takes place. During rapid creep, the width of the flow-trough is small; during stagnation, it is large. Thus the surface of the creeping material becomes stepped.

2.3. Surface creep We now look at larger-scale phenomena. Essentially, "surface creep" refers to the soil-layer, or some other surface-layer (e.g., debris, clay) on a slope. In this instance, it may be noted that the character of mass movements on a clay-slope is quite different from that on a slope covered with "ordinary soil". The two cases should therefore be treated somewhat separately. For the measurement of the rate of soil creep, optical instruments (Finlayson, 1981) or simple inserted rods (Young, 1978) have been used. In

228 temperate humid regions rates ranging from 0.04 to 50 mm per year (Young, 1974) have been found; similar values were also observed in tropical rain forest conditions (Lewis, 1974). Typically, there are two levels of maximum motion, the minimum occurring at the bottom of the root zone. In general it had been assumed that "normal" soil-creep follows an exponentially decreasing velocity profile. However, a close inspection of Young's (1978) results also shows that a second maximum occurs at some depth. Such a pattern was also postulated on theoretical grounds by Kirkby (1967). Young (1978) also ascribes a large influence to solution phenomena. As noted, surficial clay layers behave differently from surficial soil layers. A representative case of the latter type was investigated by Gerber and Scheidegger (1984): observations of a slide over a period of more than 18 years yielded the result that the main governing effect is a transition of solid to plastic and thence to fluid behaviour of the surface layer. Thus, the slide occurs intermittently: short periods of activity alternate with long periods of quiescence. The active phases are triggered by heavy precipitation in winter combined with melting of the snow: they occur only about once in a decade. A correlation study showed that the effect of the rain is felt progressively later as one proceeds from the head to the bottom of the slide: the effect is felt almost immediately at the top, at the bottom after 10-12 days. When the most active phase is reached, the motion involves actual liquefaction and rapid flowage of parts of the slide area.

2.4. Surface slips There remains a further type of surficial phenomenon to be discussed: up to now the mass movements considered have been continuous, now we have to consider the case when a surface layer actually detaches itself from the substratum. In such conditions, a tear scar is left behind (German: Blaike, Plaike) which is generally visible from afar in a landscape as an ugly bare patch without any vegetation. Why the vegetation cannot take root on such a scar is a bio-ecological problem which is beyond the scope of the present study (e.g., Kelch et al., 1977). Nevertheless, the widely visible scars can serve as geomorphological indicators of the occurrence of slips. A stability analysis purporting to determine the initiation of such slips has been published by Moser and Hohensinn (1983). Phenomenologically, surface slips show rectangular forms: 75-125 m 2 of vegetation cover are in motion to depths of h = 20-40 cm on slopes of a = 400-45 ° declivity (Engelen, 1967). Regarding a mechanical theory, it should be mentioned that Gerber and Scheidegger (1966) noted a superficial analogy of the phenomenon with certain types of snow avalanches (slabavalanches, German: "Schneebrettlawinen"). Based upon this idea, Brunner

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and Scheidegger (1975) set up a mechanical model of the phenomenon. For this purpose the equilibrium conditions for a square slab (edge l) of vegetation were considered:

2lhfs + lhfo + ~(pgh cos e~ - p )l 2 >~hl2pg sin a where ~ is the coefficient of friction at the lower side of the slab, and fs the shearing and f0 the tensile strength of the slab. Furthermore, p is the density of the slab and g the gravity acceleration. The expression in brackets is Terzaghi's "effective pressure" if p is the pore pressure. Division by hl 2 yields:

2 fs + l f o + ep( og cos ~ - P ' ) >~og sin ~ This equation explains all observed phenomena. An increase of the pore pressure (rain!) can produce instability. Furthermore, the stability limit will be exceeded if I becomes larger than a certain value, since the left-hand side decreases with increasing l. Thus, slabs of a certain size will detach themselves. The change in equilibrium owing to increasing pore pressure (rain!) can also be calculated and the maximum size of a stable slab can again be determined, using reasonable values for the shearing strength of the grass-sod. It turns out that the theoretical sizes of sliding grass-slabs are entirely in conformity with those observed in nature. Thus, the model is fully vindicated. 3. D E E P MASS M O V E M E N T S

3.1. Description (phenomenology) We turn our attention now to deep-seated mass movements. These may occur in a variety of materials ranging from soil to rock. Whether a material is regarded as "soil" or "rock" depends on whether its deformation can be treated by the methods of "soil mechanics" or of "rock mechanics". The first problem encountered in the analysis of deep mass movements is that of describing their phenomenology. The effects on landscapes and structures are often clear enough and are frequently reported even in the mass media. More exact are cartographic representations of the effects of mass movements; as examples, we cite such studies for the Alpine regions of Austria by Bunza (1976) and for the Alpine regions of Switzerland by Kyburz (1975). An extension of cartographic descriptions is obtained by paleogeomorphological studies. Thus, the technique of using dendromorphological analyses for the determination of ancient land-movements has been applied (Schroeder, 1978).

230 In order to obtain a quantitative description of the mass movements, geodetic surveys have to be made over periods of some years. The instruments useful for such purposes have been discussed by Otway and Wood (1975), the general methodology by Calculli et al. (1979) and by Ter-Stepanian (1975). The first problem is one of achieving sufficient accuracy in the surveys so as to obtain significant results: the movements are often only of a few mm per year; re-surveys can at best be taken a few years apart so that the demands on the surveys can readily be appreciated. However, even when accurate survey results are available, a second problem arises in their interpretation: since none of the survey points can be assumed a priori as stable or fixed, special methods have to be developed for the comparison of the point-coordinates of various epochs. Such methods have to be based, in principle, on adjustments of free nets and a calculation of " i n n e r " coordinates (Brunner and Hauswirth, 1976). This requires a substantial computational effort which is best carried out on a computer.

3.2. Deep-seated soil creep We shall now discuss some cases of deep-seated "soil" creep. Here, "soil" simply refers to a substance which can be treated by the methods of Terzaghi's "soil-mechanics". This implies a rheological equation of the Coulomb-type; a special case of the latter is " p u r e plasticity". In particular, the indicated behaviour is exhibited by various granular materials ("soils") and by clay. Features of the type under discussion occur particularly in flysh zones. The latter material gives rise to typically unstable landscapes with rounded hummocks whose "wave-length" is of the order of 100 and more metres (Brunner et al., 1975). A specific instance of the type of movements in question has occurred near Hallstatt where a layer of Triassic clays, rock-salt and anhydrite (so-called "Haselgebirge") shows a plastic behaviour upon leaching by water (Hauswirth and Scheidegger, 1976). Thus, although "soil mechanics" encompasses more than pure plasticity, it is the movement in purely plastic clay masses which is of the greatest interest. Detailed investigations by Boucek (1977) on a creeping slope in France by means of drilling, geoelectric and seismic studies, etc., yielded the result that friction of a sliding mass on a substratum was the determining creep mechanism. However, it is in m a n y cases not possible to discern or to identify a sliding surface at the bottom of the creeping mass: much rather, the clay is creeping internally. In such cases, the whole mass must be treated as a quasi-fluid, preferably of a viscous nature. U p o n this basis, Tschierske (1979) calculated (in plane-stress approximation) the displacement rate-field

231 in an a priori parabolic clay mountain induced by its own weight. The deduction of the dynamic equilibrium form of the clay mountain during its evolution (floating boundary problem!), however, has not yet been achieved. 3.3. R o c k mass creep

We now turn our attention to deep-seated creep phenomena in actual rocks. This includes such well-known phenomena as mountain fracture and valley closure. Descriptions of such phenomena, which are wide spread, have been given for instance in. the writer's book (Scheidegger, 1975). The actual mechanism of the deep rock mass creep is still the subject of some controversy. It is clear that an important role is played by the material. Thus, oolitic limonite deposits (Stoicovici and Muresan, 1968) tend to break up into unstable packets. This type of behaviour is exhibited to an even greater degree by various types of schists, such as B~ndnerschist (Huder, 1976). The schist breaks up into plate-like fragments and is subject to weathering so that the strength of the material is progressively reduced. Thus, creep movements are initiated. The materials, however, seldom begin to creep of their own accord. Usually, an essential contributing factor is water. The latter is usually supplied by precipitations, but may also be introduced by the damming of a lake (Albiker, 1977). As noted, the main cause of the supply of water and the destabilization of a slope caused thereby is the rainfall. Thus, studies of the correlation between creep and rainfall rate have been made. In quite general terms, the mass creep in rock in dependence on precipitation has been discussed by Kronfellner-Kraus (1974, 1980). Particular studies have been reported from various places; a notable one from the Gradenbach in Austria (Moser et al., 1980; Moser and Glumac, 1982a,b) in which the observed displacements clearly showed the dependence of the rock mass movement on water. A similar study was made by Carniel et al. (1975) in the Felber valley, also in Austria. Here, a correlation of the triggering of major movements by rainfall was found, but the basic design of the motion is of tectonic origin. The fact that tectonics plays a major role in the predesign of mass movements was also found elsewhere. The case of the mass movements in the Felber valley mentioned above (Carniel et al., 1975) was shown to fit together well with the general views on the tectonics of the Alps. The movements begin at the top of a ledge where they present the aspect of a mountain fracture. In the middle of the slope, they appear as rock mass creep and at the very bottom as valley closure. The orientation of the mountain fractures, of the joints bounding the slide area and of the features on the creeping slope all fit into one single stress pattern corresponding to

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the tectonics of the Alps. Corresponding observations were made at other locations in Austria, such as on the Lesach ridge near Kals (Hauswirth et al., 1979) and near WOrschach in the Enns valley (Hauswirth et al., 1982). Particularly at the Lesach ridge, the mass movements were shown to be predesigned by the neotectonic stress field: the mean displacement direction coincided closely with one of the principal stress directions of the neotectonic stress field as deduced from joint orientation measurements. A similar (although less definite) situation could be found in the Ennstal. In a corresponding fashion, Mialler-Salzburg and LOtgers (1974) have shown that a slide in the Peloponnesos (Greece) is directly connected with the type of graben-tectonics prevailing there. It is seen, thus, that deep mass creep is essentially predesigned by the regional neotectonic conditions. The sliding itself is influenced by the presence of water; the movement rates, therefore, are frequently correlated with the rate of rainfall. The slide mechanism itself is mainly one of blocks gliding and sliding on a lubricated substratum, although internal body-creep (some non-linear creep law) may also occur. 4. LANDSLIDES

4.1. General remarks

We now turn our attention to actual "slides". Under this term we understand rapid (as opposed to creep-type) phenomena without, however, the significant involvement of "external" agents such as water or ice. Thus, "landslides" are rapid mass movements. A convenient classification of such events is obtained by considering first (i) purely surficial effects, then (ii) slides in soil and clay, and finally (iii) rock slides of truly catastrophic dimensions. The subject of "landslides" has recently been reviewed in textbooks by Millies-Lacroix (1970), Veder (1979) and Zaruba and Mencl (1982) (new edition of an older book); these publications contain general presentations of various aspects of the landslide problem. Regarding particular regions, it may be noted that Abele (1974) has made a collection of the available information on landslides in the Alps. Much of this information concerns prehistoric slides. The publication is a valuable " u p d a t e " of the famous classic of Heim (1934). As a particularly valuable feature, it contains tables of volumes, areas and reaches of all known landslides in the Alps. A similar, though somewhat less extensive survey has been made of landslides in California by Nilsen et al. (1976). Landslides lead to specific morphological features which can be recognized in the topography of an area. Thus, some mounds in Lake Tahoe

2133 (Nevada, U.S.A.) are due to a series of massive, basin-wide prehistoric landslides (Hyne et al., 1973). This leads to the problem of registering and representing on maps all landslides that have occurred in a region. A cartographic procedure to do this has been presented by Rybar (1973). The geomorphological aspects of a landslide are connected with the geological setting. Such connections have been investigated in general terms by Furuya (1976) in Japan. A specific slide was studied from this point of view by Brown and Psutky (1980) in British Columbia. It seems that landslides in metamorphic rocks (such as schists) are potentially more violent than slides in Quaternary detrital deposits. The problem of tectonic predesign of large landslides has been studied by Ai and Scheidegger (1984) who showed that there is an exact correlation in the landslide risk between the slope orientation and the principal neotectonic stress direction. Of great importance would be knowledge of the internal structure of a (potential) landslide mass including the identification of a sliding surface, if one exists. Geophysical methods have been applied for this purpose. The method of choice is usually seismic (refraction) surveying. Unfortunately, the velocity contrasts involved are not very great (Furuya, 1976) so that it is difficult to obtain significant results (Kobayashi, 1981), although Stewart and Celis (1976) claim to have found a good correlation with borehole measurements in Colombia. In view of the difficulties generally encountered in seismic investigations, other geophysical methods have also been tried. Among these, various electrical methods have been used for the investigation of landslides, notably the resistivity (Stefanovic and Muzijevic, 1971) and the self-potential methods (Bogoslovsky and Ogilvy, 1977). Finally, in this section on general remarks on landslides, it remains to make a few remarks on the estimation of landslide risk. As is the case with risk estimates for other natural catastrophes, the approach to the problem is two-sided. First of all, general statistical studies on the correlation between various environmental factors and landslide incidence can be made (Okamoto, 1979; Carrara et al., 1982). Second, specific precursors indicating an impending landslide can be sought (Teufel, 1980).

4.2. Surficial slides Surficial slides are those which affect only the surface layer of a slope. It is, of course, to some extent arbitrary how thick a layer will be called "surficial". In essence, there are the layers of vegetation and, in addition, those whose thickness is relatively small (less than 10%) compared with the lateral extent of the slide area. As was pointed out by the writer (Scheidegger, 1975), such surface layers are mainly governed by the cohesion of the surface material (grass-sod!) and

234 the laws of friction during the motion. An interesting and different approach was suggested by Kotarba (1974; also Gil and Kotarba, 1977) who applied the queuing theory in systems to the problem. The mentioned study mainly envisages slow (creeping) motion, but it is conceivable that "shocks" build up which would lead to instability. In a study of superficial landslides in Calabria, Sorriso-Valvo (1979) found a connection between slow, deep gravitational creep and such slides: the superficial slides are, in effect, triggered by the slow build-up of deformation by the deep creep. It is the accumulating creep that causes the surficial slides. Finally, a prediction model for the occurrence of surficial slips was proposed by Neuland (1976). This is entirely based on a statistical correlation analysis which was performed on 250 objects from the Federal Republic of Germany. 4. 3. Slides in soil and clay

We now turn our attention to deep-seated slides in soil and clay. As usual, the initiation of the slides is determined by the stability conditions being obviated, the further progress by friction-type laws. In addition, there is the problem of clay turning to "quick-clay": the clay material becomes thixotropic under certain conditions and loses all cohesion. The general aspects of such slides have been recently reviewed by Schick (1978) and underlying stability analyses have been reported by Noble (1973), particularly as to how the latter can be based upon results of laboratory residual shear stress of the materials involved. Progressive failure as a slide triggering mechanism in loess has been studied by Lutton (1971). However, as indicated above, the biggest menace on clay slopes is the possibility of the material undergoing liquefaction: solid clay becoming quick-clay. Whilst studies of this problem have been made elsewhere also (e.g. in California by Kerr and Drew, 1971; in Sweden by Viberg, 1981) it is primarily in Norway where the phenomenon has been studied. The reason for this is that much of the bed of the North Sea adjacent to Norway is covered by clays. The development of oil fields and harbours in this material has necessitated the study of its properties. Similar marine clay deposits also cover large land areas in Norway so that the quick-clay problem is not only one for the offshore engineer. Among particular Norwegian slides studied we may mention those at Baastad (Gregersen and L0ken, 1979) and at Rissa (Gregersen, 1981). In the first of these the cause of the liquefaction was traced to the leaching of marine clay deposits by fresh water: this is a well known mechanism of destabilization of clay materials (Rosenqvist, 1975). In the second of the

2135 above slides (that at Rissa), the landslide was found to have been a two-stage process. The primary causes were excavations in the area, but the actual triggering of the slide occurred by retrogressive development of successive minor slides that took place over a relatively long period. When these had progressed far enough, a large flake-type slide started. As an interesting aside, it may be noted that sand- (rather than clay-) liquefaction has also been found to play a role in slide initiation (Katzikas and Wylie, 1982). Such liquefaction is entirely caused by an excess of pore pressure; leaching has no effect. The various results on ~lay liquefaction have been used for design purposes of engineering structures, such as excavations (Aas, 1976; Karlsrud and Myrvoll, 1976), offshore gravity structures (Lauritsen and Schjetne, 1976) and foundations for cyclically loaded platforms (Andersen, 1976). Actual protective measures for roads and highways were reviewed by Kollbrunner (1970).

4.4. Catastrophic landslides Large landslides can cause much damage and loss of life: the volumes involved may reach 10 l° m 3, the velocities may exceed 100 m/sec. They are an ecological force that has to be reckoned with in human development planning (Fukuoka, 1981). A review of recent literature on the subject matter has been given by Coates (1977). A bibliography has been published by the US government (Anonymous, 1981). Some notorious landslides have been investigated in great detail. One of the best studied events of this type is the slide that occurred near Vajont, Italy, on October 9, 1963. After some years of slowly creeping (up to 25 cm/week) and accelerating to a velocity of 80 c m / d a y , a mass of about 3 • 108 m 3 of rock and soil slid suddenly very rapidly (peak velocity about 30 m/sec) along a chair-shaped sliding surface from Mount Toc into the then recently constructed reservoir lake (Kiersch, 1964). It created a huge flood wave which spilled over the dam, causing great destruction and the death of almost 3000 people in its path. Preliminary descriptions and studies were made immediately after the occurrence of this disaster, in addition to Kiersch (1964) mentioned above, also by Schnitter (1964), Weiss (1964) and M~ller-Salzburg (1964). All these authors came essentially to the conclusion that the slide was triggered by an excess of groundwater due to the filling of the artificial lake; an increase in pore pressure is known to destabilize a slope. In addition, the rocks contained layers of marls and seams of clay which are susceptible to a reduction of strength as a result of quite modest displacements (Skempton, 1966). The unfortunate orientation of the geological layers (marls) then made the catastrophe inevitable.

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However, in contrast to the above opinions, Caloi (1966, also Caloi and Spadea, 1966) and also Migani (1968) seek the cause of the slide in geodynamic, mainly seismic, effects. These authors noted a continuing seismicity prior and subsequent to the disaster, possibly associated with the course of the pore pressure. Because of the potential significance of the Vajont disaster with regard to other hydroelectric undertakings, further studies of its possible causes continued to be made. Broili (1967) searched for clues for the disaster in additional studies of the geomorphology, geology and joint orientation patterns. He found again that most of the motion occurred in the marl layers; the slip surface was found to have been predetermined by the attitude of the strata. Similar conclusions were arrived at by Nonveiller (1967). In a comprehensive summing up of these early discussions of the disaster, Mi~ller-Salzburg (1968) again discussed all the possible causes, including progressive failure as a trigger and final clay-thixotropy as the feature that allowed the rapid, catastrophic motion to arise, without, however, committing himself to any one of these ideas. The matter did not rest there. About a decade later, new efforts at an elucidation of the true causes of the catastrophe were made. Thus, Chowdhury (1978) reexamined the idea that progressive failure was the cause of the triggering of the large-scale motion. This avoids the necessity of assuming unrealistically low values for the coefficient of friction in a purely frictional model to obtain the high observed velocities. The pore water would have played only a minor role in the failure process. The process envisaged was modelled numerically on a computer. Trollope (1980) subsequently also proposed a sequential failure mechanism, but considered that strain-softening characteristics of the material and a pore-water increase analogous to the phenomenon of aquaplaning contributed to the rate of movement. Finally (up to the present), Corbyn (1982) returned to the idea that the rock slide occurred as a result of the deepening of the reservoir-lake at the foot of the slope. Thus, in spite of much effort, the real cause of the occurrence of the Vajont slide is still somewhat a matter of a controversy. The problem has two distinct aspects: first the cause of the initiation of the slide (proposals: (1) destabilization by pore pressure increase, (2) progressive failure, (3) clay thixotropy, (4) small earthquakes), and second, the explanation of the observed, extremely rapid, movement ((1) pore pressure, (2) strain softening, (3) formation of a slide surface in clay, (4) aquaplaning). The last word on these problems has evidently not yet been spoken. Another notorious slide that has been the subject of extensive investigations is the one that occurred on Mount Huascar~n in Per/a on May 31, 1970: over two million m 3 of rock mixed with ice roared down the Llan-

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ganuco valley. With the melting of the ice, the mass turned into a debris flow (see Section 5 of this paper), causing an estimated 20,000 casualties (Browning, 1970). In this case, however, it was clear that the slide was triggered by an earthquake. A similar, but smaller, event occurred in the same location on January, 10, 1962 (4,000 people killed). This latter (earlier) slide does not seem to have been triggered by an earthquake; in fact, the triggering mechanism is not known. A comprehensive study of the two catastrophes was published by Plafker and Ericksen (1978). Landslides, of course, occur in every region with high relief; they occur even on other planets, such as Mars (Lucchitta, 1978). They have been reported from the tropics (Brazil: De Ploey and Cruz, 1979; Costa Rica: Knoblich et al., 1977) and in the Arctic (Stauber, 1940). The classic country for landslide research, however, is Switzerland. The classic study of Heim (1934) has been mentioned before; meanwhile, special studies have been made of a slide near the Flimserstein in GR (Niederer, 1940), on the Wartenberg BL (Schmassmanm 1953), on the prehistoric slide near Tamins GR (Pavoni, 1968; Scheller, 1970), on a slide near Amden SG (Kovari et al., 1974), on a rock avalanche near Beichlen LU (Zollinger, 1980), and, finally, on a historic landslide (16th century) near Lake Geneva (Alexander, 1983). Another country in which landslides were extensively studied is Canada, because of the extensive settlements in the rugged country of the West. Studies started with a report on the famous landslide that occurred at Frank, Alberta on April 29, 1903 (McConnell and Brock, 1904). Other significant landslides in Alberta, mostly along river banks, have been described by Thomson and Morgenstern (1977) and by Cruden (1982). In the Territories to the north, slides were investigated by Mosley and Blakely (1977) and by Clague (1981). However, the main hazard from landslides in Canada exists in British Columbia. Such slides have occurred near Hope (Matthews and McTaggart, 1969), Rubble Creek (Moore and Matthews, 1978) Downie (Brown and Psutky, 1980), Vancouver (Eisbacher and Clague, 1981), Dusty Creek (Clague and Souther, 1982) and Drynoch (Van Dine, 1983). The above references to studies of specific landslides do not, of course, claim to be complete with regard to the entire world. References to slides in other countries may be found in the U.S. Government publication cited earlier (Anonymous, 1981). The studies of individual landslides have mainly been carried out on an entirely morphological basis: affected area, type of rock, possibly sliding velocity, etc. have been determined. With regard to the supposed mechanics of the slides, only guesses are available. The first problem concerns the initiation of the motion. Its cause must be sought in a breakdown of the stability of the slope. In most cases, this will be

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ultimately caused by plate-tectonic motions, uplift of mountain-regions, etc. Specifically, cases are known where earthquakes have triggered landslides (e.g. HuascarS_n: see above; Alaska: Shreve, 1966) or progressive failure (possibly Vajont: see above; Japan: Suemine, 1983). Such stability calculations are essentially the domain of the science of "soil mechanics". For the analysis of mass movements, it is the kinematics and dynamics of this motion which are of interest. The kinematics of the sliding motion can in some way be described by a "frictional model", using a fictitious coefficient of friction which is different for each slide (Scheidegger, 1975). Indeed, Banks and Strohm (1974) were able to reproduce the frictional coefficients observed in small slides by experiments with various materials in the laboratory. However, for large slides, the coefficients of friction f become exceedingly small. Scheller (1970) proposed a correlation of f with sliding velocity, but Scheidegger (1973) showed - - entirely empirically - - that a correlation with slide volume is much better borne out by the observational facts. KOrner (1976) improved on the purely frictional model by considering also the kinetic energy in the balance equation and estimated the instantaneous velocity of slides. The empirical correlations referred to above give no explanation for the observed low coefficients of "friction". Evidently some very significant processes occur within the body of the moving mass which cause a breakdown of cohesion or internal friction. The first proposal of such a process is some sort of fluidization mechanism involving air (either one assumes that a turbidity current arises, or the slide moves on a cushion of entrapped air). These proposals mainly go back to Shreve (1966) and others; they have been reviewed, e.g. in the writer's book (Scheidegger, 1975). The theory has recently lost much appeal because landslides similar to terrestrial ones have also been found on other planets such as Mars (see above). Air cannot be involved in such slides. A modification of the trapped-air hypothesis is one which assumes that instead of air, it is water vapour (pore fluid, vaporization through frictional heating) which provides for the gaseous pressure (Habib, 1975). Without actual vaporization occurring, frictional heat has also been thought to enhance the fluid pressure in the pores, reducing the strength in the slide region (Nonveiller, 1979; Voight and Faust, 1982). Thermal and hydrothermal alteration of the rock can even lead to liquefaction (Okunishi, 1982). The extreme case of the "heat-metamorphism-theory" is obtained if one assumes that actual fusion occurs by frictional heating. The discovery of fused rock in an old slide area near K6fels in the Otz valley (Tyrol) lends credence to such a hypothesis (Preuss, 1974; Erismann et al., 1977, Erismann, 1979; Masch et al., 1981). Calculations show that the envisaged mechanism is at least physically possible.

239 The occurrence of fusion may be a fact in some landslides, but would appear to be a rather extreme case. Since it is desirable to consider mechanisms which do not involve air or water (so the same process can be invoked for extraterrestrial landslides), a purely "dispersive" fluidization has been envisaged (HsiL 1975; Alexander, 1983). In such a process the particles are held in suspension solely by the mutual collisions. Sample calculations and scale model experiments again show that the proposed mechanism is at least physically possible. 4. 5. Landslide risk and protection

The knowledge of the causes and the mechanism of landslides has been used to make risk estimates and predictions of impending danger. Eisbacher (1979) has published zoning criteria that are mainly applicable to the Canadian Cordillera. A similar procedure has been developed for Japan by Okunishi and Iida (1981). Predictions of impending landslides have generally been based on the correlation analysis of suspected premonitory factors. Thus, Neuland (1976) has made a correlation study between nine different parameters. Most studies consider rainfall as the primary hazard factor (Gove and Sorzona, 1980); Okunishi and Okuda (1982) carried this type of prediction study to the point where it can be used for issuing orders for the evacuation of endangered areas by the civic authorities. Finally, the problem arises of designing protective measures against the effects of landslides. The problem can be attacked from two sides. An attempt can be made to stabilize a potentially sliding mass. Usually piles or anchors are used for this purpose (e.g., Kolinsky and Socha, 1974; Huder, 1978; 1983). Equally, cuts and fills have been used for slope stabilization (Hutchinson, 1977). However, attempts in the western United States at improving the management of watersheds by vegetative-type conversion (mostly bush to grassland) has been shown to increase the frequency of landslides (De Graft, 1979). Finally, protection from the effects of landslides can also be attempted by constructing protection walls etc. Such measures, of course, are only effective against very minor events; it is obvious that a major, catastrophic landslide can never be stopped by walls. 5. EXTERNAL FLOW PROBLEMS 5.1. General remarks

Attention will now be given to those mass movements which could be characterized as "external flows": the term implies the presence of a carrying

240 agent, primarily water; in some extreme cases it could also be air. Essentially, there are two types of such flows which are significant: the motion of a mud slide, and the flow of a debris current. General discussions of the external flow problem have been given by Bunza and Karl (1975) in connection with a morphological map of Bavaria and by Kronfellner-Kraus (1974) in connection with debris flows. The two aspects of external flow phenomena will now be discussed separately. 5.2. Earth and mud flows Earth and m u d flows are characterized by the fact that their motion depends on the internal mobility of the material - - i n contrast to debris flows where bodies are carried in suspension by some transporting agent. Some recent flows of this type have occurred in the Chicoutimi area of Quebec (Dionne, 1972), in Switzerland (Furrer, 1972), in California (Kelsey, 1978; Weber and Treiman, 1979), in the Carpathians (Ayzenberg et al., 1978), and particularly in New Zealand (Crozier, 1973; Wasson and Hall, 1981, 1982). The morphological studies made on these mud flows have borne out the fact that there is a correlation between ground moisture and slide activity. This fact was also found in a study made by Gerber and Scheidegger (1984) on a clay slide in Switzerland (cf. also Section 2 of this paper). The slow, creeping motion in such slides will, under extreme conditions, result in an actual liquefaction. Basically, the liquefaction of the material evident in mud slides is tied up with the properties of clays. Such substances have the ability, to swell by electrokinetic adsorption upon contact with water (Philip, 1970); this process leads to an increase of the internal pressure on a breakdown of the cohesion (De Ploey, 1971). A mud flow needs not become completely liquefied; clay humps or plugs (Craig, 1981) may subsist in the mass (Vallejo, 1979). The general mechanics of clay materials is commonly treated in textbooks on soil science (e.g. Scheffer and Schachtschabel, 1970). From the equations of state of clay for various moisture contents it is possible to investigate the dynamics of the flowage. Felix (1980) has published a bibliography of the literature on the subject. The possible equations of motion have been collected by him; mostly these are laws involving the shearing strain rate j, and the shearing stress ~- in some nonlinear fashion such as

241

where 77, ~~, To and 8 are empirical parameters. Attempts at solving particular boundary value problems with such a law of motion are listed. In addition, Felix reviewed some experimental Russian work on the subject in which the pertinent parameters for the flow equations have been determined. 5.3. Debris flows

Finally, we consider debris flows. These are suspensions of rocks, tree trunks, soil etc. in water which often occur in the wake of a landslide and move downhill with great speed, possibly leading to much destruction. • Debris flows occur in all mountainous countries. Thus, they have been reported, e.g., from Tibet (Tang, 1981), Japan (Okuda, 1978), Australia (Wasson, 1978), New Zealand (Blong, 1973; Pierson, 1980), and Scandinavia (Rapp and Nyberg, 1981). Many studies, naturally, have been made in the Alps. Hohensinn (1979) published a general description mostly with a view to elucidating the triggering mechanism of such events (stability analysis). Of the many studies of individual events we mention only that of the Kaiserbach in East Tyrol (Bunza, 1975a) and the Enterbach in Tyrol (Hopf and Wanner, 1974). The motion of a debris current is evidently made possible by some dispersive pressure as was postulated for the first time by Bagnold (1954); his theory was applied to Alpine debris flows by Breitfuss and Scheidegger (1974). Other investigators have tackled the problem of the mechanics of debris flows from an entirely phenomenological basis. Thus, Rodine (1974) defined an empirical "mobility index" which he attempted to determine phenomenologically, and Hampton (1979) introduced a buoyancy mechanism. A more significant set of studies has been made by Takahashi (1978, 1980, 1981) which, however, are primarily concerned with the condition for the initiation of debris flows, not with their motion after they have been started. Thus, it is again phenomenological parameters (effective cohesion, depth of flow, slope angle) which have been correlated. Such studies are, of course, important for risk estimates. It is of some interest to determine where (in which creeks) and under what conditions debris flows are likely to occur. Here it is the initiation, not the mechanics of the motion, which is a primary interest. The general ecological problem has been discussed, for instance, by Eisbacher (1982) in Canada and by Kronfellner-Kraus (1975) in Austria. To diminish the risks, the construction of check dams in the catchment areas of the creeks that are potentially dangerous, has been extremely effective. For instance, a summary of such works as those constructed in South Tyrol has "been presented by Stacul (1979). Similar efforts, of course, have been made in many parts of the world.

242 REFERENCES Aas, G., 1976. Stability of slurry trench evaluations in soft clay. Proc. 6th Europ. Conf. Soil Mech. Found. Eng., 1(1): 103-110. Abele, G., 1974. Bergstiarze in den Alpen. Wissenschaftliche Alpenvereinshefte No. 25, Mianchen, 230 pp. Ai, N.S. and Scheidegger, A.E., 1984. On the connection between the neotectonic stress field and catastrophic landslides. Proc. 24th Int. Geol. Congr., Moscow, C06: 180-189. Albiker, B., 1977. Stabilit~t einer Rutschscholle unter Einstau. Rock Mech., 9:173 181. Alexander, D., 1983. "God's handy-worke in wonders" - - landslide dynamics and natural hazard implications of a sixteenth century disaster. Profess. Geogr., 35(3): 314 323. Andersen, K., 1976. Behaviour of clay subjected to undrained cyclic loading. Publ. Nor. Geotek. Inst., 114: 33-44. Anonymous, 1981. Landslides 1970-December 1981. Citations from the NTIS data base. U.S. Nat. Tech. Inf. Serv., Springfield, VA, 178 pp. Ayzenberg, M.M., Ludin, S.M., Semenikhina, A.S. and Yablonskiy, V.V., 1978. Trudy Ukrain. Nauchn. Issl. Gidrometeor, In-ta, 162: 64-70. Bagnold, R.A., 1954. Experiments on gravity free dispersion of large solid spheres in a Newtonian fluid under shear. Proc. R. Soc. Lond., A, 225: 49-63. Banks, D.C. and Strohm, W.E., 1974. Calculation of rock slide velocities. Rept. 3rd Int. Symp. Rock Mech. Denver, 2B: 839-847. Blong, R.J., 1973. A numerical classification of selected landslides of the debris slideavalanche-flow type. Eng. Geol., 7(2): 99-114. Bogoslovsky, V.A. and Ogilvy, A.A., 1977. Geophysical methods for the investigation of landslides. Geophysics, 42(3): 562-571. Boucek, B., 1977. Gleitreibungskoeffizient w~hrend einer Bodenrutschung. VerOff. Inst. Grundbau, Bodenmech. etc. Rhein-Westf. Tech. Hochsch., Aachen, 4: 170-190. Breitfuss, G. and Scheidegger, A.E., 1974. On a possible mechanism of Alpine debris flows. Ann. Geofis. Roma, 27(1): 47-57. Broili, L., 1967. New knowledges on the geomorphology of the Vajont slide slip surfaces. Rock. Mech. Eng. Geol., 5(1): 2-9. Brown, R.L. and Psutky, J.F., 1980. Structural and stratigraphic setting of the Downie slide, Columbia River valley, British Columbia Can. J. Earth Sci., 17(6): 698-709. Browning, J.M., 1970. Catastrophic rock slide, Mount Huascar~n, northcentral Peril, May 31, 1970. Bull. Am. Assoc. Petrol. Geol., 57(7): 1335-1341. Brunner, F.K., Gerber, E. and Scheidegger, A.E., 1972. On the mechanism of surficial creep in hilly regions. Riv. Ital. Geofis. Sci. Aft., 2(1): 53-57. Brunner, F.K., Gerber, E. and Scheidegger, A.E., 1975. On the mechanism of surficial creep in hilly regions. Riv. Ital. Geofis. Sci. Aft., 2(1): 53-57. Brunner, F.K. and Hauswirth, E.K., 1976. Geod~tische Untersuchungen einer rezenten Grosshangbewegung bei Hallstatt/OberOsterreich. Osterr. Z. Vermessungswes., 64(1): 1-17. Brunner, F.K. and Scheidegger, A.E., 1975. Zur Dynamik des Rasengleitens. Interpraevent, 1975, 1: 25-32. Bunza, G., 1975a. Gefahrenherde im Gebiet des Kalserbaches (Osttirol) und ihre Kartierung. Interpraevent, 1975: 329-343. Bunza, G., 1975b. Klassifizierung alpiner Massenbewegungen als Beitrag zur Wildbachkunde. Interpraevent, 1975, 1: 9-24. Bunza, G., 1976. Analyse und Kartierung von Bodenbewegungen und Erosionsvorgangen an

243 Alpinen Gebieten. Jb. Verein z. Schutze der Alpenpflanzen und -Tiere e.V. Mi~nchen, 41 : 119-143. Bunza, G. and Karl, J., 1975. Erl~uterungen zur hydrographisch-morphologischen Karte der Bayerischen Alpen. Bayer. Arnt f. Wasserwirtschaft, Mianchen, 68 pp. Calculli, S., Caretti, A. and Galetto, R., 1979. Determinazione dei movimenti dei corpi franosi. Geol. Appl. e Idrogeol. (Bail), 14: 389-404. Caloi, P., 1966. L'evento del Vajont nei suoi aspetti geodinamice. Ann. Geofis. Roma, 19(1): 1-74. Caloi, P. and Spadea, M.C., 1966. Principali risultati conseguiti durante l'osservazione geodinamica, opportunamente estesa nel tempo, di grandi dighe di sbarramento, e loro giustificazioni teoriche. Ann. Geofis. Roma, 19(3): 261-278. Carniel, P., Hauswirth, E.K., Roch, K.H. and Scheidegger, A.E., 1975. Geomechanische Untersuchungen in einem Rutschungsgebiet irn Felbertal in 0sterreich. Verh. Geol. B.-A., 1975(4): 305-330. Carrara, A., Sorilso-Valvo, M. and Reali, C., 1982. Analysis of landslide form and incidence by statistical techniques, Southern Italy. Catena (Braunschweig), 9: 35-62. Carson, M.A. and Kirkby, M.J., 1972. Hillslope Form and Process. University Press, Cambridge, 475 pp. Chowdhury, R., 1978. Analysis of the Vajont slide--new approach. Rock Mech., 11: 29-38. Clague, J.J., 1981. Landslides at the south end of Kluane Lake, Yukon Territory. Can. J. Earth Sci., 18(5): 959-971. Clague, J.J. and Souther, J.G., 1982. The Dusty Creek landslide on Mount Cayley, British Columbia. Can. J. Earth Sci., 19(3): 524-539. Coates, D.R., 1977. Landslides. Rev. Eng. Geol., Geol. Soc. Am., 3: 1-278. Corbyn, J.A., 1982. Failure of a partially submerged rock slope with particular references to the Vajont rock slide. Int. J. Rock Mech. Min. Sci., 19: 99-102. Craig, D., 1981. Mudslide plug flow within channels. Eng. Geol., 17(4): 273-281. Crozier, M.J., 1973. Techniques for the morphometric analysis of landslips. Z. Geomorphol., 17(1): 78-101. Cruden, D.M., 1982. The Brazeau Lake slide, Jasper National Park, Alberta. Can. J. Earth Sci., 19(5): 975-981. De Graft, J.V., 1979. Initiation of shallow mass movement by vegetative-type conversion. Geology, 7(9): 426-429. De Ploey, J., 1971. Liquefaction and rainwash erosion. Z. Geomorphol., 15(4): 491-496. De Ploey, J. and Cruz, O., 1979. Landslides in the Serra do Mar, Brazil. Catena (Braunschweig), 6: 111-122. Dionne, J.C., 1972. Les basses terrasses de la r6gion de Chicoutimi, Qu6bec. Rev. G6ogr. Montr6al, 26(4): 407-420. Eisbacher, G.H., 1979. First-order regionalization of landslide characteristics in the Canadian Cordillera. Geosci. Can., 6(2): 69-79. Eisbacher, G.H., 1982. Mountain torrents and debris flows. Episodes, 4: 12-17. Eisbacher, G.H. and Clague, J.J., 1981. Urban landslides in the vicinity of Vancouver, British Columbia, with special reference to the December 1979 rainstorm. Can. Geotech. J., 18(2): 205-216. Engelen, G.B., 1967. Landslides in the metamorphic northern border of the Dolomites (North Italy). Eng. Geol., 2: 135-147. Erismann, T., 1979. Mechanism of large landslides. Rock Mech., 12: 15-46. Erismann, T., Heuberger, H. and Preuss, E., 1977. Der Bimstein von K6fels (Tirol), ein Bergsturz - - "Filktionit". Tschermaks Mineral. Petrogr. Mitt., 24: 67-119.

244 Felix, B., 1980. Le fluage des sols argileux, l~tude bibliographique. Rapp. Rech. Lab. Central, Ponts et Chauss6es Paris, 93: 1-231. Finlayson, B., 1981. Field measurements of soil creep. Earth Surf. Proc. and Landforms, 6(1): 35-48. Fukuoka, M., 1981. Landslides - - a force to be reckoned with. Nature Resour., 17(4): 24-28. Furrer, G., 1972. Bewegungsmessungen auf Solifluktionsdecken. Z. Geomorphol. Suppl., 13: 87-101. Furuya, T., 1976. Geological and geomorphological factors at some landslide areas in Shikoku. Bull. Disas. Prev. Res. Inst. Kyoto Univ., 26(2): 101-118. Gerber, E. and Scheidegger, A.E., 1966. Bewegungen in Schuttmantelh~ngen. Geogr. Helv., 21: 20-31. Gerber, E. and Scheidegger, A.E., 1979. Systematics of geomorphic surfaces and kinematics of movements thereon. Z. Geomorphol., 23(1): 1-12. Gerber, E. and Scheidegger, A.E., 1984. Eine chronische Rutschung in tonigem Material. Vierteljahrsschr. Naturforsch. Ges. Zi~rich, 129(3): 294-315. Gil, E. and Kotarba, A., 1977. Model of slide slope evolution in flysch mountains (an example drawn from Polish Carpathians). Catena (Giessen), 4: 233-248. Gove, M. and Sorzana, P.F., 1980. Landslide susceptibility as a function of critical rainfall amount in Piedmont basins (north-western Italy). Stud. Geomorphol. Carpatho-Calcanica, 14: 43-61. Gregersen, O., 1981. The quick clay landslide in Rissa, Norway. Pub. Nor. Geotek. Inst., 135(2): 1-6. Gregersen, O. and L6ken, T., 1979. The quick-clay slide at Baastad, Norway, 1974. Eng. Geol., 14: 183-196. Habib, P., 1975. Production of gaseous pore pressure during rock slides. Rock Mech., 7: 193-197. Hampton, M.A., 1979. Buoyancy in debris flows. J. Sediment. Petrol., 49(3): 753-758. Hauswirth, E.K. and Scheidegger, A.E., 1976. Geomechanische Untersuchungen der Grosshangbewegung Hallstatt-Plassen (Osterreich). Riv. Ital. Geofis. Sci. Aft., 3(1): 85-90. Hauswirth, E.K., Pirkl, H., Roch, K.H., Scheidegger, A.E., 1979. Untersuchungen eines Talzuschubes bei Lesach (Kals, Osttirol). Verh. Geol. B.-A. Wien, 1979(2): 51-76. Hauswirth, E.K., Lahodynsky, R., Roch, K.H. and Scheidegger, A.E., 1982. Geophysikalische Untersuchungen an der Grosshangbewegung W6rschachwald (Ennstal, Steiermark). Mitt. Naturwiss. Ver. Steiermark Graz, 112: 75-90. Heim, A., 1934. Bergsturz und Menschenleben. Fretz and Wasmuth, Ziarich, 218 pp. Higgins, C.G., 1982. Grazing-step terracettes and their significance. Z. Geopmorphol., 26(4): 459-472. Hohensinn, F., 1979. Bodenmechanische Analyse von Gel~indebriachen bei Murenbildung in kristallinem Verwitterungsboden. Mitt. Inst. Bodenmech., Felsmech. Grundbau Univ. Innsbruck, 2: 1-91. Hopf, J. and Wanner, J., 1974. Probleme eines murstossf~_igen Baches im Inntal: Enterbach, Gemeinde Inzing. In: Festschr. 100 Jahre Forstliche Bundesversuchsanstalt Wien, pp. 215-226. Hsi~, K.J., 1975. Catastrophic debris streams (sturzstroms) generated by rock falls. Bull. Geol. Soc. Am., 86: 129-140. Huder, J., 1976. Creep in Biandner schist. Laurits Bjerrum Memorial Volume, Norweg. Geotech. Inst., Oslo, pp. 125-153. Huder, J., 1978. Boden- und Felsanker: Anforderungen, Pri~fung und Bemessung. Schweiz. Bauztg., 96(40): 1-9.

245 Huder, J., 1983. Stabilisierung von Rutschungen mittels Ankern und Pf~hlen. Schweiz., Ingen. Architekt, 1983 (16). Hutchinson, J.N., 1977. Assessment of the effectiveness of corrective measures in relation to geological conditions and types of slope movement. Bull. Int. Assoc. Eng. Geol., 16: 131-155. Hyne, N.J., Goldman, C.R. and Court, J.E., 1973. Mounds in Lake Tahoe, California-Nevada: a model for landslide topography in the subaqueous environment. J. Geol., 81(2/: 176-188. Kalkani, E.C., 1980. Filtering probe inclinometer data to identify characteristics of slope movements. Rock Mech., 13(2): 57-69. Karlsrud, K. and Myrvoll, F., 1976. Performance of strutted excavation in quick clay, Proc. 6th Europ. Conf. Soil. Mech. Found. Eng., 1(1): 157-164. Katzikas, C.A. and Wylie, E.B.,' 1982. Sand liquefaction: inelastic effective stress model..1. Geotech. Div. ASCE, 108(GT1): 63-81. Kelch, G., Drexler, O. and Zech, W., 1977. fiber den Bodenabtrag im Kampenwandgebiet. Z. Geomorphol. Suppl., 28: 134-147. Kelsey, H.M., 1978. Earthflows in Franciscan mrlange, Van Duzen River Basin, California, Geology, 6: 361-364. Kerr, P.F. and Drew, I.M., 1971. Clay mobility in ridge route landslides, Castaic, California. Bull. Am. Assoc. Petrol. Geol., 56(11): 2168-2184. Kiersch, G.A., 1964. Vajont Reservoir disaster. Civil Eng., 34(3): 32-47. Kirkby, K.J., 1967. Measurement and theory of soil creep J. Geol., 75: 359-378. Knoblich, K., 1970. Massenbewegungen. Zbl. Geol. Palaontol., 1972, 1(7/8): 955-977. Knoblich, K., 1972. Massenbewegungen, 2. Bericht. Zbl. Geol. Palaontol., 1972, 1(9/10): 538-551. Knoblich, K., Zirfas, J. and Torres, C., 1977. Rutschungen im Bereich der Cordillera Central und der Cordillera de Talamanca von Costa Rica. Giessener Geol. Schriften, 12: 175-198. Kobayashi, Y., 1981. The seismic refraction survey in landslide areas. Bull. Disas. Prev. Res. Inst. Kyoto Univ., 31(1): 1-15. Kolinsky, J. and Socha, K., 1974. Charakteristik und Analyse einer Felsrutschung im Kluftk6rper von Tonschiefer. Rock Mech. Suppl., 3: 47-52. Kollbrunner, C.F., 1970. Schutz der Strassen vor Rutschungen und Felssttirzen. Pub. Inst. f. Bauwissensch. Forsch. Ziarich, 11: 3-42. Krrner, H.J., 1976. Reichweite und Geschwindigkeit von Bergsti~rzen und Fliessschneelawinen. Rock Mech., 8: 225-256. Kotarba, A., 1974. Modelling of flysch slopes by landslips illustrated by examples chosen from the Polish Carpathians. Abh. Akad. Wiss. Grttingen, Math.-Phys. KI., (3) 29: 102-110. Kovari, K., Amstad, C. and Grob, H., 1974. Messung von Verschiebungen und Deformationen an Bauwerken mit dem Distometer ISETH. Schweiz. Bauztg., 92(36): 1-7. Kronfellner-Kraus, G., 1974. Die Wilbacherosion im allgemeinen und der Talzuschub im besonderen. In: 100 Jahre Forstliche Bundesversuchsanstalt Wien, pp. 309-342. Kronfellner-Kraus, G., 1975. Ober die Einsch~tzung von Wildbachen; Einft~hrung in die Problemstellung. Mitt. Forstl. Bundes.-Vers.-Anst., Wien, 112: 7-24. Kronfellner-Kraus, G., 1980. Neue Untersuchungsergebnisse in Wildb~ichen: Der Talzuschub in Abh~,ngigkeit von Niederschl~igen. Interpraevent, 1980, 1: 179-192. Kyburz, W., 1975. Erdbewegungen in der Schweiz. Z. Wirtschaftsgeogr., 19(5): 149-150. Lauritsen, R. and Schjetne, K., 1976. Stability calculations for offshore gravity structures. Publ. Nor. Geotek. Inst., 113: 17-22.

246 Lewis, L.A., 1974. Slow movement of earth under tropical rain forest conditions. Geology, 2(1): 9-10. Lucchitta, B.K., 1978. A large landslide on Mars. Bull. Geol. Soc. Am., 89: 1601-1609. Lutton, R.J., 1971. A mechanism for progressive rock mass failure as revealed in loess slumps. Int. J. Rock Mech. Min. Sci., 8(2): 143-151. Masch, L., Erismann, T., Heuberger, H., Preuss, E. and Schrrcker, A., 1981. Frictional fusion on the gliding planes of the large landslides. Bull. Liaison Labo. Centr. Ponts et Chaussres, Paris, 10: 11-14. Matthews, W.H. and McTaggart, K.C., 1969. The Hope landslide, British Columbia. Proc. Geol. Assoc. Can., 20: 65-75. McConnell, R.G. and Brock, R.W., 1904. Report on the Great Landslide at Frank, Alberta. Appendix to Report of the Superintendent of Mines, Canada Department of Interior, Annual Report, 1902-1903, Part 8, 17 pp. Migani, M., 1968. Microsimicita e variazione della pressione sul fondo di un bacino idrico. Ann. Geofis. Roma, 21(2): 219-233. Millies-Lacroix, A., 1970. Landslides in civil engineering. Centre de Doc. Univ. et Ste. d'Editions d'Enseignement Suprrieur, Paris, 406 pp. (in French). Moore, D.P. and Matthews, W.H., 1978. The Rubble Creek landslide, British Columbia. Carl. J. Earth Sci., 15(7): 1039-1052. Moser, M. and Glumac, S., 1982a. Geotechnische Untersuchungen zum Massenkriechen in Fels am Beispiel des Talzuschubes Gradenbach (K~rnten). Verh. Geol. B.-A., 1982(3): 209-241. Moser, M. and Glumac, S., 1982b. Zur Kinematik von Talzuschiaben, dargestellt am Beispiel des Talzuschubes Gradenbach/K~nten. Allg. Vermess.-Nachr., 89(5): 174-193. Moser, M. and Hohensinn, F., 1983. Geotechnical aspects of soil slips in Alpine regions. Eng. Geol., 19: 185-211. Moser, M., Glumac, S., Natau, O. and Leichnitz, W., 1980. First results of investigations to stabilize a 1000 m high creeping slope in phyllitical rock masses. Proc. Int. Syrup. Landslides, New Dehli, 6: 349-353. Mosley, M.P. and Blakely, R.J., 1977. The Coppermine landslide, southeastern Ruahine Range. Soil Water, 13(6): 16-17. Mialler-Salzburg, L., 1964. The rock slide in the Vajont valley. Rock Mech. Eng. Geol., 2(3): 148-212. Mialler-Salzburg, L., 1968. New considerations of the Vajont slide, Rock Mech. Eng. Geol., 6(1): 1-91. Mi~ller-Salzburg, L. and L6tgers, G., 1974. Eine Rutschung am Rand eines geologischen Orabens. Rock Mech. Suppl., 3: 53-67. Neuland, H., 1976. A prediction model of landslips. Catena, 3(3): 215-230. Niederer, H., 1940. Der Felssturz am Flimserstein. Jahresber. Naturforsch. Oes. Oraubi~nden, 77: 3-27. Nilsen, T.H., Taylor, F.A. and Brabb, E.E., 1976. Recent landslides in Alameda County, California (1940-1971): an estimate of economic losses and correlations with slope, rainfall and ancient landslide deposits. Bull. U.S. Geol. Surv., 1398: 1-21. Noble, H.L., 1973. Residual strength and landslides in clay and shale. J. Soil Mech. Found. Div. ASCE, 99: 705-719. Nonveiller, E., 1967. Zur Frage der Felsrutschung im Vajont-Tal. Felsmech. Ing. Geol., 5(1): 2-9. Nonveiller, E., 1979. Dynamic development of slides. Rad. Gradev. Inst. Zagreb, 12: 334-346.

247 Okamoto, M.B., 1979. Prediction method for risk rate of slope failure caused by heavy rainfall. Natural Disaster Sci., 1(2): 25-40. Okuda, S., 1978. Observation on the motion of debris flows and its geomorphological effects. Proc. Int. Geograph. Union. Comm. Field Experiments in Geomorphology, Paris Symp., pp. 1-24. Okunishi, K., 1982. Kinematics of large-scale landslides. Trans. Jpn. Geomorphol. Union, 3(1): 41-56. Okunishi, K. and Iida, T., 1981. Evolution of hillslopes including landslides. Trans. Jpn. Geomorphol. Union, 2(2): 291-300. Okunishi, K. and Okuda, S., 1982. Refuge from large landslides - - a case study: Nishiyoshino landslide, Nara Prefecture, Japan. J. Disaster Sci., 4(2): 79-85. Otway, P.M., Wood, P.R., 1975. Surveying equipment and procedures for earth deformation studies. N.Z. Geol. Surv. Earth Deformation Section Rept., 28: 3-20. Pavoni, N., 1968. Uber die Entstehung der Kiesmassen im Bergsturzgebiet von BonaduzReichenau (Graubi~nden). Eclogae Geol. Helv., 6(2): 494-500. Philip, J.R., 1970. Hydrostatics and hydrodynamics of swelling soils. Water Resour. Res., 5(5): 1070-1076. Pierson, T.C., 1980. Erosion and deposition by debris flows at Mt. Thomas, North Canterbury, New Zealand. Earth Surface Processes, 5(3): 227-247. Plafker, G. and Ericksen, G.E., 1978. Nevados HuascarS_q avalanches, Per/a. In: B. Voight (Editor), Rockslides and Avalanches. Elsevier, Amsterdam, pp. 277-314. Preuss, E., 1974. Der Bimsstein von KOfels im Otztal/Tirol. Jb. Ver. Schutze der Alpenpflanzen und -Tiere e.V., 39: 85-95. Rapp, A. and Nyberg, R., 1981. Alpine debris flows in northern Scandinavia - - morphology and dating by lichenometry. Geograph. Ann., A, 63(3-4): 181-196. Rodine, J.D., 1974. Analysis of the mobilization of debris flows. Rept. No. DA-ARO-D-31124-71-G158, Dept. Geology Stanford Univ., 226 pp. Rosenqvist, I.T., 1975. Clay mineralogy applied to mechanics of landslides. Geol. Appl. Idrolgeol. (Bari), 10(2): 21-32. Rybar, J., 1973. Representation of landslides in engineering-geological maps. Landslide, 1(l): 15-21. Scheffer, F. and Schachtschabel, P., 1970. Lehrbuch der Bodenkunde, Enke, Stuttgart, 448 pp. Scheidegger, A.E., 1973. On the prediction of the reach of velocity of catastrophic landslides. Rock Mech., 5: 231-236. Scheidegger, A.E., 1975. Physical Aspects of Natural Catastrophes. Elsevier, Amsterdam, 289 PP. Scheller, E., 1970. Geophysikalische Untersuchungen zum Problem des Taminser Bergsturzes. Diss. ETH Zi)rich, No. 4560, 91 pp. Schick, R., 1978. Hangrutsche: Lawinen aus Erde und Stein. Bild Wiss., 15(9): 98-113. Schmassmann, H., 1953. Die Rutschung am Si)dwestabhang des Wartenberges. T~t.-Ber. Naturforsch. Ges. Baselland, 19: 29-128. Schnitter, G., 1964. Die Katastrophe von Vajont in Oberitalien. Wasser- und Energiewirtschaft, 56(2/3): 61-69. Schroeder, J.F., 1978. Dendrogeomorphological analysis of mass movement on Table Cliffs Plateau, Utah. Quat. Res., 9: 168-165. Schreve, R.L., 1966. Sherman landslide, Alaska. Science, 154(3757): 1639-1643. Skempton, A.N., 1966. Bedding-plane slip residual strength and the Vajont landslide. Gdotechnique, 16: 82-84. Slosson, J.F. and Krohn, J.P., 1979. Mudflow/debris flow damage. February 1978 Storm-Los Angeles Area. Calif. Geol., 32(1): 8-11.

248 Sorriso-Valvo, M., 1979. Trench features on steep-sided ridges, Aspromonte, Calabria (Italy). In: Trans. Polish-Italian Seminar on "Superficial Mass Movements in Mountain Regions", Warsaw, pp. 98-109. Stacul, P., 1979. Wildbachverbauung in Siadtirol gestern und heute. Presel, Bozen, 117 pp. Stauber, H., 1940. Uber eine Massengleitung grrssten Ausmasses von Sedimenten im Alttertiiir von Ostgr~Snland. Eclogae Geol. Helv., 33(2): 194-200. Stefanovic, D. and Muzijevic, R., 1971. Geophysical investigations of landslides. Trans. Yugoslav. Symposfum on Hydrogeology and Eng. Geol. (lst) Hercegnovi, May 1971, Eng. Geol. Geoph. and Seism. 2, Proc. Belgrade, pp. 205-213. Stewart, M. and Celis, A., 1976. The use of geophysics in landslide studies. Rept. Transport Road Res. Lab., Dept. Environ., 703: 1-19. Stoicovici, E. and Muresan, I., 1968. Contributions to the investigations of the sliding phenomena from Satra-Capus of the oolithic limestone deposits. Studia Uni. Babes-Bolyai. Ser. Geol.-Geogr., 13(2): 3-14. Suemine, A., 1983. Observational study on landslide mechanisms in the area of crystalline schist. Bull. Disast. Prev. Res. Inst. Kyoto Univ., 33(3): 105-127. Takahashi, T., 1978. Mechanical characteristics of debris flow. J. Hydrauh Div. ASCE, 104: 1153-1169. Takahashi, T., 1980. Evaluation of the factors relevant to the initiation of debris flows. Proc. Int. Symp. Landslides, New Dehli, 3: 136-140. Takahashi, T., 1981. Estimation of potential debris flows and their hazardous zones: soft contermeasures for a disaster. J. Natural Disaster Sci., 3(1), 57-89. Tang, Pang-Hsing, 1981. Basic features of debris flows on the Xizang (Tibetan) Plateau. In: Ma and Noble (Editors), The Environment, Chinese and American Views. Methuen, London, pp. 197-208. Ter-Stepanian, G.I., 1975. Geod~tische Methoden zur Untersuchung der Dynamik von Rutschungen. Ubersetzt aus dem Russischen von P. Bilz. VEB Deutscher Verlag f~r Grundstoffindustrie, Leipzig, 202 pp. Teufel, L.W., 1980. Precursive pore pressure changes associated with premonitory slip during stick-slip sliding. Tectonophysics, 69: 189-199. Thomson, S. and Morgenstern, N.R., 1977. Factors affecting distribution of landslides along rivers in southern Alberta. Can. Geotech. J., 14(4): 508-523. Trollope, D.H., 1980. The Vajont slope failure. Rock. Mech., 13: 71-88. Tschierske, N., 1979. 12Iber die m/Sgliche Fliessbewegung ganzer Tonberge. Rock. Mech., 12: 99-113. Vallejo, L.E., 1979. An explanation for mudflows. Gdotechnique, 29(3): 351-354. Van Dine, D.F., 1983. Drynoch landslide, British Columbia - - A history. Can. Geotechn. 2f., 20(1): 82-103. Veder, C., 1979. Landslides and Their Stabilization. Springer Verlag, Wien, 231 pp. Viberg, L., 1981. Mapping and classification of landslide conditions, Proc. Int. Conf. Soil Mech. Found. Eng., 10th, Stockholm, 3: 549-554. Voight, B. and Faust, C., 1982. Frictional heat and strength loss in some rapid landslides. Grotechnique, 32(1): 43-54. Wasson, R.J., 1978. Leveed debris flows. Aust. Geograph., 14(2): 125-126. Wasson, R.J. and Hall, G., 1981. Mudslide reactivation: Waerenga-o-Kuri, New Zealand. Aust. Geogr. Stud., 19(2): 217-223. Wasson, R.J. and Hall, G., 1982. A long record of mudslide movement at Waerenga-o-Kuri, New Zealand. Z. Geomorphol., 26(1): 73-85. Weber, F.H. and Treiman, J.A., 1979. Slope instability and debris flows: Los Angeles area. Calif. Geol., 32(1): 3-5.

249 Weiss, E.H., 1964. Vajont - - Geologische Betrachtungen zur Felsgleitung in den Stausee. Steir. Beitr. Hydrol., 15/16: 11-36. Young, A., 1974. The rate of slope retreat. Inst. Brit. Geogr. Spec. Pub., 7:65 78. Young, A., 1978. A twelve year record of soil movement on a slope. Z. Geomorphol. Suppl., 29: 104-110. Zaruba, Q. and Mencl, V., 1982. Landslides and Their Control. Second completely revised ed. Elsevier, Amsterdam, 324 pp. Zech, W., Kelch, G. and Drexler, O., 1977. Uber den Bodenabtrag im Kampenwandgebiet. Z. Geomorphol. Suppl., 28:134 147. Zollinger, F., 1980. Der Bergrutsch von Fliihli: vom Bergsturz zum Murgang. Mensuration, Photogr., Genie Rural, 10(80):. 401-405. [Received February 17, 1984; accepted after revision August 20, 1984]