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