6.1. More information on human behaviour which can influence the landslides causes

Man transforms, corrects and modifies natural processes by increasing or decreasing their rate of action and by causing the rupture of certain equilibria which Nature will try to reconstitute in different ways. With the passing of time this modifying action has assumed increasingly widespread and intense patterns.

Actually, besides high-magnitude mass movements which occur quite seldom, there is a huge number of medium to small sized landslides which are so widespread that the related cost for human society is even higher than that of catastrophic events. Increasing losses due to low-magnitude, high-frequency events are fostered by human activity which tends to increase landslide hazard and favours vulnerability situations.

Man-made processes, can influence the triggering of mass movements acting on the landslides external causes.

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 be considered. Among these processes the following due to the human activity are particularly important: excavation, road construction, mining and quarrying, exploitation of materials or water tables overloading, deforestation, terracing etc.

In particular, human activity in the vicinity of a slope may increase the forces tending to cause failure.
Human induced modifications thay may adversely affect external loads are: i) grading of the existing slope or of adjacent slopes; ii) adjacent construction; iii) construction blast damage; iv) vibrations of passing vehicles.

Slope re-garding may result in a condition similar to that caused by natural erosion and deposition in which the toe become s oversteep or material is accumulated on the crest. Costruction adjacent to the crest of a slope can surcharge a slope. Grading adjacent to a slope can consist of excavation or filling; excavation would create oversteepened slopes, whereas filling would create a surcharge. These conditions should be anticipated or prevented so that an appropriate stability analysis can be made.

Excavation blasting can cause excessive fracturing of otherwise intact rock material. Improper blasting loosens the rocks, decreases the density and strength, and increases the permeabilità. These conditions should be recognized in the field so that slope remediation can be carried out durino or immediately after construction.

Most vibrations of passing vehicles impose small loads on adjacent slopes, but some vibrations can influente slope stability. For instance, a freight train moving on tracks a shortb distance from a steep slope with with shallow groundwater may induce vibrations at a frequency and amplitude that could cause permanent deformation or failure of the slope.

Some examples of human activity, which influenced the triggering of landslides are found in the River Panaro valley (Modena Apennines).

Fig. 1: Earth slides in clay formation due to exploitation of the Roncobotto quarry (photo by D. Castaldini)
Fig. 2: The cutting of the slope for the construction of the “Al Boschetto Restaurant” (on the left), reactived a dormant landslide. The construction of a retaining wall was not sufficient as protection measure. Therefore a new retaining wall with an anchor system has been built in 2007 (photo by D. Castaldini)

The Ca’ Bonettini landslide body (Fig. 3 and 4) resumed movement on 15th September 2003, just a few hours after a M 5.0 seismic shock .

Considering the distance of the landslide from the epicentre (35 km away in the Bologna Apennines) and the fact that locally the quake was not felt by the population but was recorded only at an instrument level, it is unlikely that a low-energy shock might be considered as the main, intrinsic cause of landslide reactivation.

Field observations, subsurface investigations and laboratory tests seem to indicate that the predisposing causes of the Ca’ Bonettini landslide could be found in the deep shrinkage fissures that dismembered the whole clayey slope as a consequence of a 3.5-month long summer drought, with a progressive decline of shear strength parameters. In addition, another important factor in further reducing stability was identified in major construction works at the foot of the landslide body, with the removal of large amounts of earth. These works were carried out without considering that the area chosen for industrial development corresponded to the foot of a dormant landslide. Therefore, the 14th September 2003 low-intensity quake was only the triggering cause of a slope movement which would have probably started all the same a few days or weeks later, as the removal of soil from the landslide foot continued as planned.

Fig. 3 Ca’ Bonettini landslide. The industrial settlement is located at the foot of a landslide (photo by D. Castaldini)
Fig. 4. Detail of the crown of the Ca’ Bonettini landslide (photo by D. Castaldini)

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.

Fig. 5 Roof collapsing above subterranean caves in limestone, Pays d’Auge, Normandy, France (Photo : O. Maquaire, Cerg)

In urban 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.

A large sinkhole was produced by a major rock collapse on 10 february 1989 at Utsunomiya City (Japan)(Sassa1999). The plan shape of the initial sinkhole was elliptical with a length of 70 m and with of 60 m; the original ground surface (Fig.6). A factory of a building stone company collapsed into the sinkhole without casualties, fortunately, as the factory was unoccupied at that time.

Fig.6. The first subsidence at Utsunomiya City on 10 february 1989 (Photo by Sassa, 1999)

A second collapse, which enlarged the the first sinkhole, took place on 5 march 1989 (Fig.7). Its plane shape become lenticular (140 m in the NS direction and 90 m in the EW direction) and its area reached 10,000 m2. The depth of the sinkhole was about 30 m and the hole volume was estimated at 300,000 m3. A third failure occurred on 18 march 1989. It was 30 m long and 15 m wide along the western rim of the 2nd collapse.

The movement of the first collapse was largely a vertical fall or rapid subsidence. That of the second one was a combination of sinking and sliding in the northern part and sinking in the eastern part. The third one looks like a slump.
The sinkhole area is one of several important areas of stone production for building material in the Kanto region. Stone is cut in underground quarries from a Miocene Tuff formation. The tuff has been used as building material since the 8th century. The typical mining method utilizes rooms up to 10 m wide. 100 m long and 15 m high. The rooms are separated by pillars. It is not known if the the initial collapse involved a pillar failure or roof caving. People living near the sinkhole had heard sounds of low frequency for two months before the first collapse. However no cracks or deformation were recognized the ground surface. After the first collapse, many cracks were observed at the ground surface near the hole.
Important factors causing the collapse could be: I) shallow depth of the quarries (less than 40 m); ii) presence of two levels of underground workings at shallow depth over a wide area.

Since 1959, similar sinkholes due to rock collapse have occurred at least twenty time in that area as result of the underground quarrying (3 persons have been killed).

Fig.7. The second collapse at Utsunomiya City on 5 march 1989 (Photo by Sassa, 1999)


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