3.1.2 More information on landslides causes

By Alessandro Pasuto1, Mauro Soldati2, & Doriano Castaldini2
1 CNR – IRPI Padova (Italy)
2 Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia (Italy)

As regards the causes of mass movements, they can be divided into two large categories:

  • internal causes: they lead to the failure of the slope without the intervention of perceptible changes on the surface; they are in fact causes which reduce the material’s shear strength;
  • external causes: they determine an increase of the shear stresses by means of more or less evident modifications of slope morphology.

Naturally, the factors belonging to these two categories can be combined to varying extents so they do not fit in with any particular classification.

Let us now consider more in detail the internal causes which condition the stability of a slope.

  • Geological factors.
  • Morphological features.
  • Climatic conditions.

As regards external causes which result in an increase of shear stresses, those processes giving rise to changes in slope geometry, mainly with an increase of the gradient, should first be considered. Among these processes the following are particularly important:

  • Undercutting due to the erosion of a watercourse or to the excavation for road construction,
  • Exploitation of quarries or open pits
  • Overloading
  • emoval of lateral support
  • Lateral pressure
  • Tectonic activity
  • Effect of vegetation

External causes are related to driving forces (Geometrical changes, Unloading, Loading, Shocks and vibrations). They determine an increase of the shear stresses by means of more or less evident modifications of slope morphology. Internal causes are related to resisting forces (Progressive failure, Weathering, Seepage erosion, Water regime change). They lead to the failure of the slope without the intervention of perceptible changes on the surface; they are in fact causes which reduce the material’s shear strength

Geological factors.
They are given by all the primary characteristics that make up a slope, first of all lithology which includes both the rock’s mineralogical and textural composition and its structure. Among rock types more easily subject to disarrangement phenomena, clays are to be quoted, both those of primary origin and those derived from the meteoric weathering of volcanic materials. Argillaceous soils are particularly subject to instability phenomena such as flows.

Fig. 1: Superficial earth slide (Soil slip) in weathered volcanic materials at S. Cassiano, Dolomites, E Alps, Italy (photo by M. Marchetti)

Fig. 2: Earth flows in clay materials in the Torrent Tiepido valley, Northern Apennines Italy (photos by D. Castaldini)

Also the structure of the formation involved is very important. For example, formations made up of alternating layers, such as flysch, where rock types showing ductile mechanical behaviour overlie brittle rocks, are usually subject to widespread and vast landslides. Also discontinuities, both primary (bedding planes) and secondary (schistosity and foliation planes), play an important role in the materials’ geomechanical behaviour especially with regard to mass movements such as the attitude of layers (low-angle dip-downstream beds will generally be more prone to sliding than dip-upstream levels).

Fig. 3: Rock block slide in formation made up of alternating layers with low-angle dip-downstream beds, Northern Apennines, Italy (photo Archivio CNR-IRPI, Torino)

Of course, all these intrinsic characteristics of the materials can undergo changes of varying intensity and rapidity during the stages of a slope’s evolution. In fact, certain kinds of processes which to some extent modify the materials’ chemico-physical characteristics can take place, causing a consequent decrease of shear strength. Among geological factors, also the tectonic history of the slope should be considered. The presence of important structural discontinuities such as faults, fractures, cleavage, joints, slickenside or even folds, fault-folds etc. induce an intrinsic weakness of the internal bonds thus making the slope particularly prone to landsliding.

Fig. 4: Rock fall (below) in fractured volcanic formation (above) at Bletterbach Gorge, Northern Italy (photos by D. Castaldini)

Morphological features.
They include first of all the slope geometry, i.e. its height, length, shape, slope angle and aspect. The influence of the slope gradient is quite evident with respect to stability; indeed, very vast and steep slopes are more predisposed to disarrangement; on the other hand, even low-angle slopes can be subject to particular types of landsliding such as, for example, flows.

Fig. 5: Incipient earth flow in low-angle slope made up of predominantly fine material, La Villa, Dolomites, E Italian Alps (photo by M. Marchetti)

Climatic conditions.

Although there are several climatic parameters playing some role in the starting of landslides, the most important one is without doubt rainfall. Rainfall should be considered not only as the triggering pluviometric event (considering therefore only its intensity) but also as the amount of rain falling and accumulating during a certain period (considering also the length of time). If the percolation velocity is low, the rainfall duration assumes predominant importance for the triggering of the movement; on the contrary, in soils characterised by high hydraulic conductivity the function played by rainfall intensity becomes of paramount importance. The role of water mainly consists of an increase in porewater pressures caused by variations of the groundwater level. The cyclicity of these changes is a determining factor in weakening the internal bonds of the materials making up the slope. The influence of the various types of rainfall will of course be different according to the type of landslide. (go to 3.1.1 More details on the role of water)

Therefore, intense and time-concentrated rainfall will be particularly effective in triggering spatially limited movements such as debris flows, soil slips, mud flows, whilst prolonged precipitations could mainly give rise to slides and complex phenomena.

Fig. 6: Examples of debris flow and soil slip, following an intense rainy period in autumn 1994, which caused roads interruptions Piemonte Region, NW Italy) (photos by F. Luino)

Fig. 7: Episcopio, Campania, Southern Italy: aerial views of the mud flows which occured on 5th and 6th May 1998, following a period of prolonged rainfall (photo by Commissariato di Governo per l’Emergenza Idrogeologica in Campania, http://www.commissario2994.it)

As regards external causes which result in an increase of shear stresses, those processes giving rise to changes in slope geometry, mainly with an increase of the gradient, should first be considered. Among these processes the following are particularly important:

  • Undercutting due to the erosion of a watercourse or to the excavation for road construction,
  • Exploitation of quarries or open pits
  • Overloading
  • Removal of lateral support
  • Lateral pressure
  • Tectonic activity
  • Effect of vegetation
Fig. 8: Rotational earth slide in clay formation due to the erosion of the River Panaro at the foot of the slope. Northern Apennines, Italy (photo by D. Castaldini)
Fig. 9: Earth slides in clay formation due to exploitation of a quarry located in the same area of Fig.7 River Panaro valley, Northern Apennines, Italy (photo by D. Castaldini)
Fig. 10: Rock fall due to the erosion of the Atlantic Ocean at the foot of the cliff. Ericeira, Central coast of Portugal (photo by D. Castaldini)

Slope geometry can change also because of phenomena taking place underground: for example, the formation of large cavities owing to mineral chemical solution or squeezing out of underlying ductile materials.

Overloading can be another phenomenon inducing an increase of applied shear stresses. This can happen owing to both natural causes, such as soil imbibition by rain or snow melt, accumulation of materials from volcanic eruptions, avalanches, debris flow or other movements, types of vegetation etc., or human activities as, for example, in embankment construction, buildings or other structures, in accumulation of tailings from mines and pits or in lacustrine basins or artificial reservoirs.

Also the removal of lateral support exerting a counterthrust on the slope could be a cause of unbalance. For example, a glacier’s withdrawal is one of the most frequent causes which determined vast post-glacial landslides in the Alps. To this category also tectonic phenomena such as activities along fault planes and subsidence should be ascribed, besides gravitational movements themselves. It is, in fact, obvious that the detachment of a portion of slope induces in the in-place material a redistribution of the stresses which can eventually cause a new movement.

Another factor which can favour the development of mass movements is lateral pressure. It can be exerted by water percolating into joints or by ice; also swellings due to hydratation could induce considerable lateral pressures as well as the mobilisation of residual tectonic stresses.

Tectonic activity can be a direct cause of slope movements in the case of regional tilting or uplift. Also in this case the effect of the tectonic process takes place essentially by means of an increase of the slope gradient. Finally, superficial deformations induced by volcanic processes can increase the shear stress within a slope also locally.

A particular discussion should be reserved for seismic shocks and other transitory stresses such as vibrations induced by operating machines, explosions, rock falls etc. These events can cause both a reduction of the materials’ shear strength and an increase, although temporary, of shear stresses. The former can be exemplified by phenomena such as liquefaction induced by sudden, cyclic increases of porewater pressures with a consequent instantaneous decline of the soil’s resistance and the collapse of the slope. This happens most frequently in saturated cohesionless medium-fine materials such as sands, silts, clays etc. The increase of shear stresses is on the contrary linked to both the accelerations to which the materials are submitted during vibrations and the temporary increment of the gradient angle in unstable slopes.

One of the first earthquake-triggered landslide was documented as early as 372-373 B.C. when Helice, a Greek city on the northern coast of the Peloponnese, slid into the sea after having been razed to the ground.

Thousands other examples occurred in historical and recent times. For instance the earthquake that occurred in Friuli (eastern Italy) on 6th May, 1976, with a main shock of magnitude 6.4 and intensity IX-X (Mercalli scale), produced numerous landslides activated; they mainly consisted of rock falls and, to a lesser extent, block-slides.

Fig. 11: Landslides induced by the earthquake which occurred in Friuli (North- Eastern Italy) on 6th May, 1976, magnitude 6.4, intensity IX-X (photos by M. Panizza)

Fig. 12: Landslides induced by the earthquake which occurred in Campania and Basilicata (Southern Italy) on 23rd November 1980, magnitude 6.8, intensity X (photos by M. Panizza)

Finally, the effect of vegetation on slope stability may be twofold: sometimes it promotes stability by reducing the action of climatic agents on the slopes, protecting the soil from precipitations, wind and sunshine; it retains a considerable amount of rain water by means of leaves, branches etc. and eliminates water through evapotranspiration; runoff and erosion decrease and there is an increase in shear resistance of the slope thanks to the roots. On the other hand, the presence of vegetation may originate negative effects by increasing the load on slopes, enhancing the action of wind forces on the trees and disjointing the soil through the action of tree roots which widen cracks and favour infiltration.

References:

PASUTO S. and SOLDATI M. Landslide Hazard in PANIZZA M (1996). Environmental Geomorphology. Developments in Earth Surface Processes 4. Elsevier Science B.V., 180 -187, Amsterdam.