Landslide forms and dynamics are very diverse and consequently many classifications can be considered. The main criteria, widely used to classify landslides are: the type of movement (e.g. fall, topple, slide, spread and flow); the nature of the slope material involved (e.g. rock, debris, earth); the form of the surface of rupture (e.g. curved or planar); the degree of disruption of the displaced mass; the rate of movement.
The latter criterion can be of highly practical importance since it indirectly expresses the chance that people have to escape at the onset of the phenomenon.
Rates of movement for different types of landslide are highly variable:
- Some landslides record only a few centimetres of movement a year, sustaining this rate for decades.
- Certain debris flows have recorded velocities of 100 km/h while large rock avalanches are capable of reaching velocities of 350 km/h. The Randa rock falls occurred on 18 April and 9 May 1991 in the Mattertal, near Randa, Switzerland. The events blocked the road, railway and river which have subsequently been diverted (Photo by B. Holl, 1995)
A simple way to classify landslides is by combining the type of movement with the type of material involved. This is the basic and commonly used classification proposed by Cruden & Varnes (1996), in its first 1978 version by Varnes, visible in the table below; in the following image, the block scheme of the Crude & Varnes’ landslide classification is used. The widely used landslide classification proposed by Cruden & Varnes (1996).
The block scheme of the Cruden & Varnes’ landslide classification. Source: British Geological Survey – www.bgs.ac.uk Go to 2.1 More information on different types of landslide)
Criteria generally used to distinguish different types of landslide include: the movement mechanism (e.g. fall, topple, slide, spread and flow), the nature of the slope material involved (rock, debris, earth), the form (curved or planar) of the surface of rupture, the degree of disruption of the displaced mass, the rate of movement. To distinguish different types of landslide we privilege here the rate of movement as it indirectly expresses the opportunity given to people to escape at the onset of the phenomenon.
Rates of movement for different types of landslide are highly variable:
- Some landslides record only a few centimetres of movement a year, sustaining this rate for decades.
- Certain debris flows have recorded velocities of 100 km/h while large rock avalanches are capable of reaching velocities of 350 km/h.
To keep things simple, we distinguish two main classes:
Slow movements The deformation is progressive and may be accompanied by rupture, but in principle with no sudden acceleration: it accelerates progressively and ends up in a sudden rupture after a phase of warning signs (cracks, deformations, subsidence), being not a direct threat if adequately monitored and controlled.
Go to 2.1.1 More details on slow movements.
Rapid movements
Movements may accelerate suddenly and they can be subdivided into two groups, depending on whether the material is propagated as a mass or reworked. These movements are more difficult to monitor and control and are consequently a major threat to personal safety.
Go to 2.1.2 More details on rapid movements
To be complete, at these two classes, we could add a third class which corresponding a spread. These are movements characterised by lateral spreading, typical of a jointed rock mass and often combined with subsidence occurring in softer, less competent underlying materials; sometimes it is not possible to recognise a basal slip surface nor well defined ductile deformation zones. Generally, the rock lateral spreading is a very slow movement but it can be very fast in the soil (debris) material: soil (debris)lateral spreading;
Go to 2.1.3 More details on lateral spreading
Another criterion used by the researchers in the identification of the landslide categories is the type of movement, whereas a further subdivision is made on the basis of the type of material.
Go to 2.1.4More information on classification
Slow movements may be distinguished by the following:
Sinking
Sinking is the process of soil consolidation under load. It affects compressible soils (peats, clays, etc). Sinking is a differential process and can cause damage to structures and buildings. Sinking is like a limited progressive subsidence.
Shrinking and swelling
Shrinking is a process caused by the desiccation of the soils due to intense and/or long periods of dryness. Shrinking produces slow, low amplitude, vertical deformations of the ground surface. Shrinking can be followed by progressive swelling processes when soil humidity is increased in the wet seasons. Clayey soils are particularly sensitive to shrinking and swelling because of the specific properties of clay minerals. Shrinking can damage buildings by partial and irregular sinking of the ground.
Progressive subsidence
Progressive subsidence is the process of slow surface deformation with or without fractures due to the evolution of natural or artificial caves (subterranean quarry or mines -iron, salt, coal, etc.-), circular or oval cavities or depressions appeared in surface. Go to 2.1.1.1 More information on progressive subsidence
Slide and Mudslide (or slump-earthflow):
The movement takes place by means of a shear displacement along one or more surfaces or along thin zones of intense shear strain. Either rotational slides (slumps), in which the movement occurs along a curved and concave failure surface, or translational slides, in which the mass moves along a plane or undulating surface, can be found. Go to 2.1.1.2 More information on slide
In some cases, a slide can change into a mudslide or slump-earthflow, especially on steep slopes, in highly tectonized clays or silty formations (Picarelli, 2001; Maquaire et al., 2003). As shown in figure 13, three morphological units can be distinguished: a primary source area with the main scarp and cracks, a track zone thinly covered by a flow (the paleotopography is still clearly discernible), and an accumulation zone, sometimes with several successive lobes. Go to 2.1.1.2 More information on slide>> << Return to More information on different types of landslide
Progressive subsidence is the process of slow surface deformation with or without fractures due to the evolution of natural or artificial caves (subterranean quarry or mines -iron, salt, coal, etc.-), circular or oval cavities or depressions appeared in surface,
What is a progressive subsidence phenomenon?
Subsidence, like sinking, is a very slow process of vertical deformation of large extent. The deformations are triggered by:
- The natural dissolution of soluble materials (karst phenomena),
- The extraction of materials (calcareous, etc.) or extraction of ores (iron, salt, coal, etc.).
Subsidence is a phenomenon relatively slow which can last of many years. It generally occurs in ground terrains with flexible behaviour, or in the case of artificial cavities (subterranean quarry or mines), when the depth of exploitation is important compared to the thickness of the size. Subsidence can be the precursor sign of an evolution by rapid subsidence. Subsidence (with vertical component of the movement) results in low depth of vast topographic depressions, generating relative deformations prejudicial to the structures, or, on a large scale, a disorganization of the system of drainage. In limit of these depressions (called sink hole in case of a karst dissolution), zones in extension (with shearing and traction, yielding stresses) can lead to the appearance of cracks.
Scheme of the progressive subsidence process above a subterranean quarry by slow and progressive deformation of the ground terrain with flexible behaviour. With these indications: below, the extension of the subsidence zone, and. above, the extension of the influenced zone (from BRGM, website BDcavité.net, developed by BRGM).
References:
COTE, PH., FAUCHARD, C., POTHERAT, P. (2005). Méthodes géophysiques pour la localisation de cavités souterraines : potentialités et limites. In Evaluation et gestion es risques liés aux carrières souterraines abandonnées. Actes des journées scientifiques du LCPC, pp. 8-17.
EMBLETON, C., AND EMBLETON C. (EDS.) (1997), Geomorphological Hazards of Europe. Developments in Earth Surface Processes 5. Amsterdam : Elsevier, 524p.
FLAGEOLLET, J. C. (1988), Les mouvements de terrain et leur prévention, Paris : Masson, 224p. LCPC (2000). Guide technique pour la caractérisation et cartographie de l’aléa dû aux mouvements de terrain. Collection ‘les risques naturels’. Laboratoire Central des Ponts et Chaussées, 91 p.
MAQUAIRE, O., (2005). Geomorphic hazards and natural risks, In: Koster, E., A. (ed.), The Physical Geography of Western Europe, Oxford Regional Environments, Oxford University Press, Chapter 18, 354-377.
MINISTERE DE L’ENVIRONNEMENT, (1997), Plans de prévention des risques naturels (PPR) : guide général.. La Documentation Française, Paris, 76p.
MINISTERE DE L’ENVIRONNEMENT, (1999), Plans de prévention des risques naturels (PPR) : risques de mouvements de terrain. Guide méthodologique.. La Documentation Française, Paris, 71p.
MINISTERE DE L’ECOLOGIE ET DU DEVELOPPEMENT DURABLE, (2004). Dossier d’information sur le risque Mouvement de terrains, 20 p. (à télécharger sur site du MEDD).
POTHERAT, P. (2005). L’opération de recherche « Carrières souterraines abandonnées ». Localisation, diagnostic de stabilité, gestion. Rapport de synthèse. Géotechnique et risques naturels, GT 77. LCPC, 132 p.
Extract from Maquaire and Malet, 2006)
A slide is a mass movement of material throughout the length of a ‘rupture’ or ‘sliding’ surface. Slide could be rotational (the sliding surface is curved) or translational (the sliding surface is more or less straight). It depends on the materials but also on the shape and length of the slope. The 1:10 ratio between depth and length is a criterion for the classification of a landslide rotational or translational. Many slides are composite and the movement takes place over the length of a sliding surface which is concave upstream and flat downstream. Many slides also occur over an irregular surface (Flageollet, 1988), and they vary considerably because of the nature and size of the materials (fragments of coherent rock, loose rock, soil) and the velocity.
Rotational slides
A single rotational slide is a ‘more or less rotational movement, about an axis that is parallel to the slope contours, involving shear displacement (sliding) along a concavely upward-curving failure surface, which is visible or may reasonably be inferred’ (Varnes, 1978). The morphology of the rotational slide is typical (Figure): upstream a main scarp with a steep slope, which is the visible part of the sliding surface, and tilted blocks (counterslopes) curtailed by scars along which slide striations are sometimes visible.
Figure 1: Typical block diagram of a rotational slide (from Dikau et al., 1996)
They may be single, multiple, with several movements of the same type close to each other, or successive, i.e. interlocking (Figure). Rotational slides can vary from terracettes with an area of only a few square meters (in this case, they are considered as shallow landslides) to large slides of several hectares.
Translational slides
In translational slides, the material displaces along a planar or undulating surface of rupture, sliding out over the original ground surface. Translational slides often follow discontinuities more or less parallel to the slope, and are often superficial (such as contact between the rock and residual soils). Deeper, they occur along structural faults, joints, or bedding planes.
Translational slides on single discontinuities in rock masses have been called rock block slides (Figure 7g) or planar slides (Cruden and Varnes, 1996). Sometimes the surface of rupture may be formed by two discontinuities that cause the contained rock mass to displace down the line of intersection of the discontinuities, forming a wedge slide (Figure 7h). A stepped slide may result if two or more sets of discontinuities, such as bedding surfaces and some joint sets, penetrate the rock masses (Figure 7i).This type of slide is usually very rapid. Smooth discontinuities or thin clay levels may act as a lubricant; infiltration water reduces friction, triggers excess pore pressures and provokes the sliding of one rigid block on another.
Translational slides can occur along soil-bedrock discontinuities or permeable/impermeable soil junctions in slopes formed by coherent, fine soils or coarser debris. In this case, translational slides are termed soil slides, debris slides (Figure j) or slab slides. Debris slides and slab slides are normally shallow according to their length and width. Their velocities are linked to seasonal variations in groundwater levels and in the saturated conditions.
Figure 9: Oblique (on the left) and vertical (on the right) aerial photograph of the Bindon translational slide Devon, UK (from Dikau et al. 1996)
Figure 10: Translational earth slide in predominantly fine material at Deva County, Romania (photos by D. Castaldini)
Figure 11: Translational rock slides occurred in November 1994 in Piemonte Apennines, Italy (Photos Archivio CNR IRPI, Torino)
In some cases, a slide can change into a mudslide or slump-earthflow, especially on steep slopes, in highly tectonized clays or silty formations (Picarelli, 2001; Maquaire et al., 2003). As shown in figure 13, three morphological units can be distinguished: a primary source area with the main scarp and cracks, a track zone thinly covered by a flow (the paleotopography is still clearly discernible), and an accumulation zone, sometimes with several successive lobes.
Figure 14: Mudslide triggered near the town of Corps in the Spring of 2001 (Trièves, South Alps, France, May 2001).
Figure 17: Boschi di Valoria earth flow, northern Apennines, Italy in 1996 (on the left) and in 2001 after its reactivation (on the rigth) (photos by A. Corsini)
References:
For the references herein and for knowing sources of didactic material go to 1.3 Selected references.
Movements may accelerate suddenly and they can be subdivided into two groups, depending on whether the material is propagated as a mass or reworked. These movements are more difficult to monitor and control and are consequently a major threat to personal safety.
Tow groups of rapid movements can be distinguished, depending on whether the materials are propagated in a mass or reworked: The first comprises:
- rapid subsidence by rapid spontaneous collapse producing sinkholes or shafts caused by the rapid rupture of natural or artificial subterranean cave vaults,
- fall: rock fall, stone fall, pebble fall, boulder fall, debris fall, soil fall arising from the mechanical development of cliffs or extensively fractured rocky escarpments,
- rockfall avalanche, topple, rockslide, of slabs from cliffs or rocky escarpments, depending on pre-existing discontinuity slabs,
Rapid subsidence
Rapid subsidence is the process of brutal spontaneous collapse producing sinkholes or shafts caused by the rapid rupture of natural or artificial subterranean cave vaults (subterranean quarry or mines -iron, salt, coal, etc.-). Go to 2.1.2.1 More information on rapid subsidence
Fall
Generally the mass is detached from a very steep slope along a surface on which little or no shear displacement takes place. It occurs mainly in the air and the phenomenon includes the material’s free fall, saltation, bouncing and rolling. Go to 2.1.2.2 More information on fall>>
Rockfall avalanche
The movement is due to stresses which cause a toppling momentum around a rotation point situated below the centre of gravity of the rock mass affected. The phenomenon can evolve into either a fall (rapid movement) or slide (slow deformation).
Go to 2.1.2.4 More information on topple
The second group corresponds to the flow phenomenon. Flow is characterised by continuous movements in space fall within this class; in them the shear surfaces (or thin zones of distributed shear) are short-lived, closely spaced and not usually preserved. From the kinematic standpoint the movement could be compared to that of a viscous fluid.
This second group comprises:
- debris flow arising from the transport of material in viscous or fluid flows in mountain stream beds or on hillslopes for the debris avalanche,
- mudslide or slump-earthflow, mudflow, soil flow, generally due to the evolution of the front of landslips. Their propagation mode is somewhere between mass displacement and viscous or fluid transport.
Debris flow and debris avalanche:
Debris flow is a very rapid to extremely rapid flow (> 1 m.s-1) of saturated non-plastic debris in a steep channel (Figure 1.m). The key characteristic of a debris flow is the presence of an established channel or regular confined path, unlike debris avalanches which are thin, partly or totally saturated, and which occur on hillslopes.
Go to 2.1.2.5. More information on flow (debris flow and debris avalanche)
Mudflow (soil flow)
Small mudflow (from Dikau et al., 1996)
Soil flows occur in wet sands or in silty-clays which are so reworked with water or so liquefied by structural collapse that they adopt a flow mode (Locat and Leroueil, 1997; Hight et al., 1998). A common term used for these conditions is mudflow. These are similar in form and behaviour to debris flows.
Go to 2.1.2.5 More information on flow (mudlow or soil flow)
Rapid subsidence is the process of brutal spontaneous collapse producing sinkholes or shafts caused by the rapid rupture of natural or artificial subterranean cave vaults (subterranean quarry or mines -iron, salt, coal, etc.-).
What is a rapid subsidence process?
Rapid subsidence is a brutal spontaneous collapse producing sinkholes or shafts of a more or less large extent (diameter of a few meters to several hectares) and a variable depth (a few meters to many hundreds of meters). Rapid subsidence can occur after a progressive subsidence. Commonly, rapid subsidence occurs above man-made voids, such as tunnels, wells and covered subterranean quarries or mines (iron, salt, coal, etc.) It is also frequent in karst terrains, where dissolution of limestone by fluid flow in the subsurface causes the creation of voids (i.e. caves). If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface.
In uban zones, rapid subsidence can cause numerous victims. A well known disaster occurred in 1958 in Roosburg (Belgium) when 18 people were killed by a tunnel collapse of an ancient subterranean quarry. In Clamart and Issy-les-Moulineaux (France), 21 people died as an ancient subterranean quarry collapsed in 1961.
Why and how does the collapsing happen?
The Process of roof collapsing
When a funnel or a shallow hole of a few meters of diameter and a few meters of depth suddenly occurs at the surface it is scientifically termed roof collapsing. Its dimensions depend on the size of the void and the nature of the bedrock which separates it from the surface.
How does this happen? For example, in the beginning there may be a gallery consisting of several vaults. These vaults may increase little by little until they finally reach the surface. Still, the roof collapsing will not occur if the gallery is sufficiently deep, because the expansion of the blocks of the roof comes to fill the void before it does not reach surface. The hazard/threat of roof collapsing is reduced if a thick and resistant bench stops progressive degradation
Subterranean carrier in limestone (Tours, France): progressive degradation of the pillar (diminution of its width) and roof fall (Photos: O. Maquaire, CERG).
References
Côte, PH., fauchard, C., Pothérat, P. (2005). Méthodes géophysiques pour la localisation de cavités souterraines : potentialités et limites. In Evaluation et gestion des risques liés aux carrières souterraines abandonnées. Actes des journées scientifiques du LCPC, pp. 8-17.
Embleton, C., and Embleton C. (eds.) (1997), Geomorphological Hazards of Europe. Developments in Earth Surface Processes 5. Amsterdam : Elsevier, 524p.
Flageollet, J. C. (1988), Les mouvements de terrain et leur prévention, Paris : Masson, 224p. LCPC (2000). Guide technique pour la caractérisation et cartographie de l’aléa dû aux mouvements de terrain. Collection ‘les risques naturels’.
Laboratoire Central des Ponts et Chaussées, 91 p.
Maquaire, O., (2005). Geomorphic hazards and natural risks, In: Koster, E., A. (ed.), The Physical Geography of Western Europe, Oxford Regional Environments, Oxford University Press, Chapter 18, 354-377.
Ministère de l’Environnement, 1997, Plans de prévention des risques naturels (PPR) : guide général.. La Documentation Française, Paris, 76p.
Ministère de l’Environnement, 1999, Plans de prévention des risques naturels (PPR) : risques de mouvements de terrain. Guide méthodologique.. La Documentation Française, Paris, 71p.
Ministère de l’Ecologie et du Développement Durable, 2004. Dossier d’information sur le risque Mouvement de terrains, 20 p. (à télécharger sur site du MEDD).
Pothérat, P. (2005). L’opération de recherche « Carrières souterraines abandonnées ». Localisation, dignostic de stabilité, gestion. Rapport de synthèse. Géotechnique et risques naturels, GT 77. LCPC, 132 p.
Internet Links:
http://fr.wikipedia.org/wiki/Subsidence
http://www.cgm.org/themes/soussol/mines/
http://www.prim.net/professionnel/documentation/dossiers_info/nat/low/mouvtTerr.pdf
Fall
Generally the mass is detached from a very steep slope along a surface on which little or no shear displacement takes place. It occurs mainly in the air and the phenomenon includes the material’s free fall, saltation, bouncing and rolling.
What is a fall phenomenon?
(Extract from Maquaire and Malet, 2006)
Falls (and also topples) comprise a free movement of material from steep slopes or cliffs. A topple is very similar to a fall in many aspects, but normally involves a pivoting action rather than a complete separation at the base of the failure.
Their general characteristics are as follows: the shape of the rupture surface is usually smooth and vertical; the material falls suddenly from a main scarp following a preparation phase during which a slice of material is separated, damaging the intact mass; the volume and size of the fallen material are extremely variable, depending on the morpho-structural and lithological conditions of the slope. These phenomena occur on cliffs when the base is eroded by the action of the sea or of rivers. The falls are always sudden and very quick, while topples vary in speed from extremely slow to extremely quick, with acceleration and deceleration phases (Maquaire and Malet, 2006).
A fall starts with the detachment of rock or soil from a steep slope along a surface on which little or no shear displacement takes place. The trajectory of the fallen material is rectilinear and vertical for slopes with an inclination of more than 70° (Cruden et Varnes, 1996).
For a rock fall, material on slopes with an inclination of between 45 and 70° moves by successive rebounds, depending on the size of the material, the restitution coefficient and the angle between the slope and the trajectory of the falling mass. Throughout the length of slopes of less than 45° the falling material may roll. In the latter two cases, the material, which is very mobile, may move considerable distances from the source zone. Debris falls and soil falls are typically shallow landslides. Debris and soil falls are triggered in loose materials and their volume varies from a few cubic metres to tens of cubic metres. The accumulated material, which is not consolidated, may be easily mobilized and transported.
Fig. 2: Maè Valley (NE Italian Alps): January 1985 rock fall (Archivio CNR – IRPI, Padova)
Fig. 3: Rock fall in progress at Piz Sompluf peak, Dolomites, NE italian Alps (photos Summer 2006, from Archivio Provincia di Bolzano)
Fig. 5: Randa rockfalls, Switzerland, occurred on 18th April, 9th May 1991. The events blocked the road, rail way and river which have subsequently been diverted (from Dikau et al., 1996)
Fig. 6: Rockfall at the west coast of Zakynthos Island Greece (Photo by M. Soldati)
References:
For the references herein and for knowing sources of didactic material go to1.3 Selected references
A rock avalanche is a large bulk of mostly dry rock debris deriving from the collapse of a slope or cliff and moving at a high velocity and for a long distance, even on a gentle slope.
What is a rockfall avalanche?
(Extract from Dikau et al., 1996)
A rock avalanche is a large bulk of mostly dry rock debris deriving from the collapse of a slope or cliff and moving at a high velocity and for a long distance, even on a gentle slope. Its speed can be in the order of tens of meters per second, the travel distance in the order of kilometres. In the area of accumulation, its volume can exceed 1 x 106 m3, covering a total surface of over 0.1 km². Owing to its velocity and dimensions, this kind of landslide can be extremely costly in terms of human lives.
The rock avalanche can develop in two ways: first, by the fall or slide of a rock body which during movement progressively looses its cohesion by turning into dry debris and thus continues its advancement as a debris avalanche; secondly, by the sudden mobilisation of a debris avalanche, debris flow, either because of the fall of an overhanging rock mass or because of a seismic shock.
The Valpola rockfall avalanche (Italy): 28 July 1987
For the latter it is important to describe the phases that occurred on 28 July 1987 which led to be sliding of a portion of Mont Zandila in Valtellina (Italian Central Alp). At 7:25 a.m., after a preliminary phase lasting less than four days and without any sign of preceding seismic events, the detachment of a large rock volume of about 34 million cubic meters suddenly occurred. Although the collapse took place at a great velocity, its main phases of development can nevertheless be reconstructed thanks to direct witnesses and morphological evidence recorded after the event:
- The first displacements, which were relatively limited in volume, happened because of a progressive uphill widening of the crown generated by the falls occurring an hour earlier;
- After a few seconds, the entire rock mass started to slide along two main shear panes: the first was known to have 45° dip to the east, the second was a neo-formation plane dipping 35° to the north;
- Along the latter plane, the translational movements to the north, that is, toward the deep valley-floor of Valpola, took place initially with a series of short successive impulses and progressively increasing accelerations until it eventually came to a stop after the impact with a rock bluff which bordered the unstable slope. The thickness of the rock body that came into collision was estimated to be in excess of 70 m;
- Following the impact, the displaced rock, which up to that moment had remained fairly compact, was subdivided into several fragments of various dimensions, falling to the valley-floor in an easterly direction from altitudes ranging from 600 to 850 m. Therefore during this phase the gravitational event which had started as a slide, rapidly turned into rock avalanche, involving in its movement also the wood cover and the debris deposits distributed along the underlying slope. The fragmented collapsed rocks, after having obstructed a large area of the valley-floor, went up the opposite slope to a height of about 300 m, preceded by a cloud of dust which raised an altitude of 200 m.
A portion of the collapsed material fell into a previously formed barrier-lake, causing a mud wave which destroyed the village of Aquilone, located a few kilometres upstream, and claimed several lives. The volume of the accumulation deposit was estimated as 40 million m3, with a maximum thickness of 90 m. On the surface of the landslide deposit the finer fraction was composed of millimetre to decimetre fragments, while the coarser material was concentrated along the flanks of the mass movement route.
Fig. 1: The Valpola rock avalanche occurred on 28 July 1987 in the Italian Central Alps: left, aerial view to the west; right, general view to the south (from Dikau et al., 1996)
Topple:
The movement is due to stresses which cause a toppling momentum around a rotation point situated below the centre of gravity of the rock mass affected. The phenomenon can evolve into either a fall or slide.
What is a topple phenomenon?
(Extract from Maquaire and Malet, 2006)
Topples (and also falls) comprise a free movement of material from steep slopes or cliffs. A topple is very similar to a fall in many aspects, but normally involves a pivoting action rather than a complete separation at the base of the failure.
Their general characteristics are as follows: the shape of the rupture surface is usually smooth and vertical; the material falls suddenly from a main scarp following a preparation phase during which a slice of material is separated, damaging the intact mass; the volume and size of the fallen material are extremely variable, depending on the morpho-structural and lithological conditions of the slope. These phenomena occur on cliffs when the base is eroded by the action of the sea or of rivers. The falls are always sudden and very quick, while topples vary in speed from extremely slow to extremely quick, with acceleration and deceleration phases (Maquaire and Malet, 2006).
A topple is the forward rotation of a mass of rock or soil about an axis located below the centre of gravity of the displaced mass (Figure 1.c). Topples may lead to falls or slides of the displaced mass, depending on the geometry of the rupture surface and the orientation and extent of the kinematically active discontinuities. Cruden and Varnes (1996) state that the way in which topples occur may be very varied: flexural toppling occurs in rocks with one preferred discontinuity system, oriented to present a rock slope with semi-continuous cantilever beams which may develop into retrogressive complex rock topple-rock fall (Figure 1.d); block toppling occurs where the individual columns are divided by widely-spaced joints; chevron toppling occurs along complex structural configuration, where the change of dip is concentrated at the surface of rupture to give a complex rock topple-rock slide (Figure 1.e).
References:
For the references herein and for knowing sources of didactic material go to 1.3 Selected references
What is a flow phenomenon?
(Extract from Maquaire and Malet, 2006)
A flow is a landslide in which the individual particles travel separately within a moving mass. Unlike slides, occurring along more or less well-defined shear zones, flow-like landslides are characterised by internal differential movements that are distributed throughout the mass (Picarelli, 2001). Their flow-like morphologies are much longer than they are wide and their uneven topography shows successive lobes. They have a considerable erosive capacity and can carry material eroded from slopes or banks over considerable distances and cover sizeable surfaces with varying thicknesses. They contribute to positive erosion balances, as the material can be carried outside the slope basin. Coussot (1993) suggests a rheological classification of these flows. Hungr et al. (2001) distinguish several types of shallow flow-like landslides.
Debris flow and debris avalanche:
Debris flow is a very rapid to extremely rapid flow (> 1 m.s-1) of saturated non-plastic debris in a steep channel. The key characteristic of a debris flow is the presence of an established channel or regular confined path, unlike debris avalanches which are thin, partly or totally saturated, and which occur on hillslopes. Debris flows and debris avalanches are complex movements. They consist of a mixture of coarse material (gravel and boulders) embedded in a sandy-silty matrix, with a variable quantity of water (Costa and Wieczorek, 1987; Iverson et al., 1997). In spite of varying velocities and total solid fractions debris flows and debris avalanches show many similarities, particularly in the way they are triggered. Debris flows and debris avalanches are commonly triggered by an excess of water: intense rainfall, rapid snowmelt, and more rarely, glacier or lake overflows which mobilize unconsolidated material in their path. Rainfall intensity and duration, along with antecedent rainfall conditions, are strong controls for debris flow triggering (Dikau et al., 1996).
Mudflow (soil flow)
Mudflows (or Soil flows) are similar in form and behaviour to debris flows. They can be very slow to very mobile and can flow downslope quite quickly. They tend to follow gullies or shallow depressions to spread out into a flat, bulbous fan or even a thin sheet.
References:
For the references herein and for knowing sources of didactic material go to 1.3 Selected references
What is a lateral spreading phenomenon?
(Extract from Maquaire and Malet, 2006)
Lateral spreading are lateral displacements situated at a certain depth and resting on stable formations. When they occur in coherent rock, whether or not resting on clay deposits or on plastic marl, they may affect a rock massif over considerable thicknesses and are therefore considered as deep-seated landslides. For further information the reader should refer to the following: Jahn, 1964; Bentley and Smalley, 1984.
Soil (debris) lateral spreading
(from Maquaire and Malet, 2006)
We will confine our discussion to shallow lateral spreading occurring in non-cohesive heterogeneous materials such as moraine deposits (debris spreading) or fine clay or sand formations (soil spreading). Fine formations particularly sensitive to this type of movement include varved clays deposited on the banks of the great Pleistocene glaciers in Scandinavia or over smaller areas in the old pro-glacier lakes of the Swiss Alpine border or in the Alpine valleys (Trièves in France; Nieuwenhuis, 1991). Their sensitivity is due to their very low plasticity. Sands may easily liquefy and give rise to this type of movement. Lateral spreading may also develop in coarse and heterogeneous moraine formations (Noverraz et al., 2001).
These movements may occur on very gentle slopes of 5° or less. They display a characteristic morphology (Figure): upstream at the source, a scarp with multiple lobes dominates the displaced part where horsts or rift valleys appear; downstream these shapes are elongated by perpendicular folds in the direction of the movement. The space affected by these movements is often as wide as it is long and they often occur over a very short period of time.
Rock lateral spreading
(From Dikau et al., 1996)
Rock spreading consists of lateral extensions of rock masses either in homogeneous rock or in cohesive rock overlying ductile materials. The latter occur along shears with tensile fractures occurring on overlying coherent rock. Though often decribed as part of complex slope movements, in certain geological conditions they give rise to such peculiar morphological features that they are considered as a separate type of movement.
Schematic block diagram showing lateral spreading in homogeneous (a) and non-homogeneous (b) rocks (from Dikau et al. 1996)
Lateral spreading at Simoncello and Sasso di Simone, northern Apennines, Italy. In the right photos a detail of cracks and trenches on the Sasso di Simone. (from Dikau et al. 1996
Panoramic view (left) and trench details of the lateral spreading at Popeye Village, NW coast of Malta: (photos by M. Soldati)
References:
BENTLEY SP, SMALLEY IJ.. 1984. Landslips in sensitive clays. In Slope Instability, BRUNSDEN D, PRIOR DB. (eds.). Wiley: Chichester; 457-490.
JAHN A. 1964. Slope morphological features resulting from gravitation. Zeitschrift für Geomorphologie, Supplement Band 5: 59-72.
NIEUWENHUIS JD. 1991. Variations in the stability and displacements of a shallow seasonal landslide in varved clays. Balkema: Rotterdam.
NOVERRAZ F., BONNARD C, DUPRAZ H, HUGUENIN L. 2001. Versinclim: comportement passé, présent et futur des grands versants instables en fonction de l’évolution climatique. Hochschulverlag AG: Zürich.
For the other references herein and for knowing sources of didactic material go to 1.3 Selected references.
By Alessandro Pasuto1, Mauro Soldati2, & Doriano Castaldini
21 CNR – IRPI Padova (Italy)
2 Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia (Italy)
When a study of mass movements is enterprised, which also aims to define landslide hazard, it is of paramount importance to use clear terminology which can be universally understood also by non-specialists (IAEG, 1990). It is therefore important for these phenomena to be immediately identified and subdivided according to the characteristics directly observable. In scientific literature, there are several “classifications” regarding to slope movements which have been proposed since the beginning of the 20th century. These often complied with the needs of various authors but more recently an effort has been made to use the least ambiguous terminology.
Most recent classifications try to emphasize both the processes leading to the development of a landslide and the materials involved; others are based on morphology, landslide mechanism, deformation velocity, causes of movement, geometry of the failure area and the deposits, age, etc. This section does not aim to describe the numerous “classifications” proposed but particular attention will be given to the most commonly accepted, that is illustrated and subsequently modified by Varnes (Varnes, 1978; Cruden & Varnes, 1996) Varnes proposed his first subdivision in 1958; subsequent elaborations were carried out in 1978 and 1996 and at the moment this subdivision is universally accepted.
The main criterion used by the author in the identification of the landslide categories is the type of movement, whereas a further subdivision is made on the basis of the type of material. As for the type of movement, five categories are represented: falls, topples, slides, spreads and flows. On the other hand, the materials are divided into two types: rock and engineering soil; the latter is further subdivided into debris and earth. In this way a landslide can be described by means of two words: the first one describing the material and the second one the type of movement (Table 1).
Complex: is the result of the combination of two or more of the five classes above illustrated. In the Cruden & Varnes (1996) classification the term «complex» has been retained as a description of the style of activity of a landslide. Moreover, other attributes are introduced so that the definition becomes more elaborate but more information is thus available on the gravitational movement. The type of landslide is described by a series of adjectives placed in sequence which provide information about the activity, the rate of movement, the moisture content, the material and type of movement (e.g. active rapid wet rock slide).
For the references herein and for knowing sources of didactic material go to 1.3 Selected references.