The Bosco Piccolo snow-melt triggered-landslide (southern Italy): a
natural laboratory to apply integrated techniques to mapping, monitoring
and damage assessment
M. Lazzari
National Research Council of Italy, IBAM, Potenza, Italy
ABSTRACT: The paper focuses on a landslide case history occurred in Basilicata region (southern Italy) on
February–March 2005 at Bosco Piccolo village 5 km far from Potenza main town, when an important landslide event has been triggered after continuous snowfalls and a rapid snowmelt occurred during short periods
of high temperatures. This landslide, inducing damages and collapses of about 80% of the buildings in the village and affecting 4 ha of surface and a maximum depth of 20 m, represents a natural study lab, where an integrated multidisciplinary approach has been adopted to mapping and monitoring, with high level of accuracy, the geometry of the investigated landslide body. Furthermore, the structural survey carried out on each
involved building permitted to propose a Landslide Damage Scale as well useful for the risk assessment.
1 INTRODUCTION
Basilicata region (southern Italy) is characterized by
high density of landslides with more than 27 landslide areas every 100 km2 (Guzzetti 2000) and can be
therefore considered a natural outdoor laboratory to
mapping and study landslide phenomena. Most of
Basilicata region, located in the southern Apennines,
is characterized by landslides often developing in
clayey–marly formations. This high landslide density
is related to predisposing conditions such as prevailing clayey materials as well as morphological setting
of the slopes, and to determining conditions such as
extreme rainfall events (Piccareta et al. 2004) or human activity, such as cave excavation (Lazzari et al.
2006), deforestation (Boenzi & Giura Longo 1994)
and intense urbanization and industrialization.
Besides, many landslide events have been historically triggered by extreme rainfall or snowmelt occurrences.
The most important event happened (on February–
March 2005) at Bosco Piccolo village (Fig. 1) 5 km
far from Potenza (Naudet et al. 2008), subsequently
to rapid snowmelt occurred during alternating short
periods of high temperatures and intense and continuous snowfalls. This complex landslide affected 4
ha of surface and reached a maximum depth of 20 m
inducing damages and collapses of about 80% of the
buildings in the village.
Figure 1. Geographical location of the study area and Bosco
Piccolo landslide.
2 GEOLOGICAL AND GEOMORPHOLOGICAL
SETTING
The Bosco Piccolo landslide is located along the
southern border of the Cozzo Staccata–Piano Grande
ridge that divides the hydrographic basin of Tiera
river from that of Arvo river. The slope is mainly
characterized by the oldest Apennine formational
units (Cretaceous–mid-lower Miocene) mainly represented by clayey–marly–arenaceous deposits and by
marly limestones of the Corleto Perticara Formation
(CPF lower Miocene–Oligocene, (Pescatore et al.
1988), both particularly susceptible to landsliding.
Besides, the Corleto Perticara Fm defines the local
morphology with calcareous ridges less erodible than
the surrounding clayey deposits.
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Figure 2. Multitemporal landslide map showing the events occurred during last fifty years (1955–2005) in the study area.
During the last fifty years, more extreme events
characterized by an increase in the rainfall-snowfall
intensity and a progressive decrease in rainy days,
have been recorded (Piccareta et al. 2004). The
lithology of the substratum (clayey–marly–
arenaceous) and the extreme meteorological events
are the two main factors responsible for the slope
evolution and landslides triggering of Bosco Piccolo
countryside.
3 HISTORY AND CAUSES OF THE
LANDSLIDE TRIGGERING
The Bosco Piccolo landslide developed progressively between the 24 and 25 February 2005 (first
detachment) along the neo-formation shear-surface,
showing the first collapse-zones below the main
road, and then retrogressed upward with secondary
detachment during the following week.
After the first and most important sliding, classified according to Cruden & Varnes (1996) as complex type (rotational slide and mudflow), the upslope areas have been involved in the mass movement with a retrogressive evolution (Fig. 3). Besides, along the involved slope a lot of counterslopes
and compressive structures have been generated by
the rotational movement of the landslide body with
concavo-convex soil deformation and tension
cracks, inducing the progressive formation of a wide
landslide lake 3 m deep. The 4ha of surface with a
total soil volume of 350,000 m3, involved in the
mass movement showed a progressive enlarging to-
wards the flanks, as testified by several trenches at
the margins of the landslide body (Figs. 3, 4).
The historical landslide hazard scenario was
mapped during the last sixteen years through an integrated approach of geomorphological field surveys
and the interpretation of multiple sets of stereoscopic aerial photographs (1954–55, 1989, 1990,
1999, 2001 and 2004). The results show that the
whole slope, on which the village is located, has
been repeatedly affected by landslide movements
during the last fifty years (Fig. 2).
The total soil volume has been determined on the
basis of geometrical reconstruction started from the
outcropping first detachment shear surface and
borehole stratigraphic logs (Figs. 3, 4).
The landslide area extends towards SE from 814 m
above s.l. to 720 m above s.l. and is approximately
700m long and 200m wide.
Three boreholes between 10 and 20m depth have
been performed after the event between May and
June 2005. They permitted to deduce the stratigraphy and gave direct information on the depth of the
landslide sliding surface. The main shear surface has
been depicted at a maximum depth of 19m in boreholes B2 and at 15m in B3. Moreover, a landslide
monitoring has been carried out with an inclinometer installed in B2. It showed a total displacement of
18.5 cm along a secondary shear surface located between 6 and 8 m depth during three weeks (June
2005) of observation. Due to these high displacements, the inclinometer was broken after 3 months.
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Figure 3. Geomorphological features of the landslide (a) deduced from the aerial view (b) and in-field geomorphological analyses
with the location of boreholes. Legend: 1) main rotational scarp, 2) secondary rotational scarp; 3) earth and mudflow scarp; 4)
counterslopes; 5) tension cracks; 6) longitudinal fractures zones; 7) mudflows; 8) shallow shear zones; 9) flow direction; 10) boreholes. Boreholes stratigraphy is also indicated with: a) related to the weathered Varicoloured Clays (AVF), b) the Varicoloured
Clays with calcareous lithofacies, and c) the Varicoloured Clays with clay–marly lithofacies.
Figure 4. Detail of geomorphological map of Bosco piccolo landslide. 1) Main detachment scarp; 2) Secondary scarps;
3) Mudflow; 4) Counterslope; 5) Cracks; 6) Scarps; 7) Landslide body; 8) Shear zone; 9) Flow direction; 10) Landslide lake and
aerial view; 11) Borehole location.
One of the causes of the Bosco Piccolo landslide
triggering was the intense and continuous snowfall
occurred between the end of January and the middle
of March 2005, where continuous snowfall of 72h
have been also recorded (Fig. 5a). Moreover, these
events have been alternated to brief warmer periods
during which a quick snowmelt occurred and a great
amount of water permeated into the clayey–marly
deposits of the slope.This process determined a
quick soil saturation, inducing an increase in weight
and in pore-water pressures, due to the addition of
water, and the overloaded of the slopes which,
reaching their limit equilibrium, collapsed with progressive downslope breaking (Fig. 5b).
Moreover, information coming from boreholes
and electrical resistivity and self-potential surveys
had permitted to depict the main shear at a maximum depth of 19 m (Naudet et al. 2008).
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Figure 5. Daily variation of the height of snow–mantle, between January and March 2005, when intense snowfalls were alternated
by brief warmer periods (a). Photos represent the progressive structural damages south of the Bosco Piccolo and illustrate the landslide main scarp evolution (b).
Figure 6. Electrical resistivity tomographies obtained after inversion with topographical correction (after Naudet et al. 2008). Profile E1 (AA′) carried out parallel to the landslide body axis with stratigraphical data comparison (boreholes B1, B2 and B3).
Dashed lines represent the supposed slipping surfaces, obtained by combining resistivity, stratigraphical and in-field geomorphological information.
The high velocity of mass movement creates a
weak dislocation of the sliding mass as there is no
observable resistivity contrast.
In May 2005, after the landslide event, a drainage
network was designed on the site to reduce the
groundwater level and hydraulic pressures along the
slope, which are determining parameters in the instability of clayey materials. The electrical resistivity tomographies (Fig. 6) allowed to characterize the
clayey deposits involved in the old mass movement.
Less easy was the identification of the recent sliding
surfaces. Only the integration between geoelectrical
and stratigraphical data allowed to draw the hypothetic sliding surface, because the sliding is occurring inside the clayey formation and not between
two lithological interfaces.
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4 LANDSLIDE DAMAGE SCALE
In order to evaluate the damage due to landslide
event, a Damage Landslide Scale (DLS) has been
adopted on the basis of field observation on each involved building, focusing the observations on nonstructural and structural damages.
The DLS considers five main classes:
D1 non-structural damage, slight non-structural
damage. Fall of small pieces of plaster only and
hair-line cracks in very few walls.
D2 moderate non-structural damage, slight structural
damage. No damage is visible from outside the
building, while inside it can be seen that cracks
have occurred in slot-wall joints.
D3 moderate structural damage, heavy nonstructural damage with large and extensive
cracks in most walls.
D4 very heavy structural damage; buildings have
suffered partial collapses with serious failure of
walls. The loss of connection between external
walls is also visible.
D5 - This is very heavy structural damage and part
of this buildings are collapsed completely.
The figure 7 shows the landslide damage classes
distribution in Bosco piccolo village, where a high
concentration of heavy structural damages is observable close to the main detachment scarp, while nonstructural damage or slight structural damage are located in the middle and northern part of the village.
These last are due to the retrogressive upward landslide movement with secondary detachments.
structural survey carried out on each involved building permitted to propose a Landslide Damage Scale
as well useful for the risk assessment.
To investigate and mapping, with high level of accuracy the geometry of the investigated landslide
body, combined electrical resistivity tomography
and self-potential measurements have been carried
out and calibrated with boreholes stratigraphy. The
electrical resistivity tomography allowed to characterize the clayey deposits involved in the old mass
movement.
Historical accounts and geological evidence show
that similar landsliding types and of different scales
have been occurring at and near Bosco piccolo village during the last sixteen years and, on a relatively
frequent basis, up until the present. There is no reason to believe this pattern of landsliding will stop.
In the future, subsequently to significant rainfall
or snowfall, some landslide scenarios are possible:
a) the remainder of the 2005 landslide could
remobilize as a deep complex slide similar to
that in 2005. This mode of movement would
most likely be relatively slow (compared to
2005) but still could pose serious hazards to
houses, routes and life.
b) The 2005 landslide body could mobilize into
a mudflow such as occurred in the past.
c) Subsidiary landslides could be triggered
from part of the 2005 landslide deposits or
scarps.
d) Mudflow and/or rotational slides on adjacent
hillsides could mobilize
e) Intense rainfall or snowfall could trigger
rapid mudflows and debris flows from various nearby slopes.
The landslide scenarios prospected above could
potentially impact on other urban settlements close
to Bosco piccolo and built on the similar geological
and morphological conditions, inducing heavy structural damages, such as the Landslide Damage Map
of Bosco piccolo village showed in figure 7.
REFERENCES
Figure 7. Landslide Damage Map of Bosco piccolo village.
5 FINAL REMARKS
An integrated multidisciplinary geomorphologic and
geophysical approach has been adopted to study the
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