May 24 – 26, 2017, Barcelona, Spain
JTC1 Workshop on Advances in Landslide Understanding
GEOMECHANICAL MODELS FOR SHALLOW RAINFALL-INDUCED
LANDSLIDES AT THE CATCHMENT SCALE BUILT IN FEEDBACKLOOP FROM GEOLOGICAL-GEOMORPHOLOGICAL INVESTIGATION
Marco Nicotera*, Alessandro Tarantino†, Antonio Santo*, Brunella Balzano†,
Melania De Falco*, Giovanni Forte*
* Università degli Studi di Napoli Federico II, Napoli, Italy
†
University of Strathclyde, Glasgow, United Kingdom
The 1997 winter season recorded prolonged and intense rainfalls all over the Campania region
in Italy. As a result, a large number of extremely rapid debris-earth flow-like landslides were
triggered. These phenomena occurred in the late Quaternary volcanoclastic deposits, mantling
the carbonate slopes of Campania region. The Sorrento Peninsula- Lattari Mts. was the most
affected area by this catastrophic event: some hundreds of shallow mass movements took
place during January 1997.
These phenomena have been studied and monitored for several years since then, and many
models have been proposed to explain the mechanisms of slope failures. Several field campaigns and investigations have been conducted in order to gain information about the stratigraphical setting of the areas affected by the slope instabilities and laboratory tests have been
performed in order to characterize the hydro-mechanical behavior of the soils involved in the
landslides.
The aim of this work is to build reliable physically-based numerical models for a number of
landslides events recognized from the past geological surveys. These models were built and
validated against well characterized slope failures and, therefore, used to investigate the geometrical factors making the slopes more prone to failure. Potential geo-physical methods to
characterize the slope geometry and their practical implementations are finally discussed.
Keywords: early-warning system, flow-like landslide, numerical models, debris-flow, slope
stability.
INTRODUCTION
In the last 20 years, several flowslides involving pyroclastic unsaturated soils affected the
carbonate ridges of the Campania region (South of Italy). They represent a main source of risk
for the community as already many victims and economic damages have been counted over
the years. Rainwater infiltration is considered the main mechanism that lead or predispose the
whole slope to failure by reducing matric suction in unsaturated soils or causing an increase in
pore water pressure in the saturated slope, thus reducing the shear strength (Pirone et al., 2015
and referenced papers). These landslides are characterized by high velocity and fluidity, they
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May 24 – 26, 2017, Barcelona, Spain
JTC1 Workshop on Advances in Landslide Understanding
were initially triggered by small falls or slides and subsequently evolved through an amplification phase, with an avalanche effect (Di Crescenzo and Santo 1999; 2005).
Winter 1996/97 is remembered for its high intense rainfalls. After a 4-month period of prolonged low intensity rainfall events, a very high intensity rainfall occurred on the 10th of January, triggering hundreds of landslides along the slopes of the Sorrento Peninsula – Lattari Mts.
This work aims to understand and model the failure mechanisms behind landslide events, involving geotechnical and geomorphological/geological studies interacting in feedback loop.
The results are adopted to validate the landslides occurrences at the catchment scale.
GEOLOGICAL SETTING AND TRIGGERING RAINFALL EVENTS
The study area is located on the Tyrrhenian coast of Campania where, during Plio-Quaternary
times, as consequence of several tectonic movements, many carbonate ridges were originated
in the area: the Sorrento Peninsula–Lattari, the Partenio, the Caserta, of Pizzo D’Alvano and
Maggiore Mts. They are made of more than 1500-m-thick Mesozoic dolomites and limestones. The most recent deposits on the limestone peaks are continental detritus and pyroclastic deposits; the latter are a few metres thick and linked to Late Pleistocene–Holocene eruptions of the Campi Flegrei and Somma-Vesuvio volcanic centers.
From 1997 to 1999, many landslides, often turning into extremely rapid flow-like phenomena,
repeatedly affected the carbonate slopes of Campania region. In particular, the Sorrento Peninsula was the most affected area of the 1997 regional slope-instability crisis. Some hundreds
of shallow mass movements took place during January 1997. The most important mass
movement occurred during the January 9–11, 1997, regional event. Infact days prior the
event, an intense precipitation occurred in Campania. Rainfall was especially heavy in the
western portions of the region, namely, the Sorrento Peninsula and the Lattari Mountains. A
maximum 3-day cumulative rainfall of about 280 mm was recorded in these locations, following a 4-month period of particularly high rainfall values. In the same days, many hundreds of
landslides were triggered in other areas of the region. Most of the landslides (about 400) occurred in the Sorrento Peninsula–Lattari Mountains.
Fig.1 Inventory map of the January 1997 landslides in the Sorrento Peninsula. (1) Debris or earth fall; (2) rock
fall; (3) rotational landslide; (4) translational slide; (5) flow; (6) complex landslide.(Calcaterra and Santo, 2004)
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JTC1 Workshop on Advances in Landslide Understanding
GEOLOGICAL AND GEOTECHNICAL CHARACTERIZATION
Although most of the landslides activated across the peninsula involved moderate volumes of
soil, a number of catastrophic events occurred, which were associated with landslides evolving in debris flows.
The catastrophic nature of these events led the local authorities to commission studies to assess landslide hazard and develop early warning systems. As a result, several field campaigns
took place after the landslide events, all over the region, in order to acquire information about
stratigraphy, geomorphology, landslides geometry and collect soil samples for the geotechnical characterization.
Geological surveying campaigns allowed characterizing the geometry of the landslides bodies, estimating their volumes and locating the possible failure surfaces.
Soil samples were taken from the pyroclastic deposits to characterize the hydro-mechanic
behaviour of the materials forming the different pyroclastic layers. The surveying has shown
that all the areas affected by shallow landslides are characterized by the presence of 3 main
soil layers:
 A) an upper layer of pyroclastic soils, made of ash and reworked pumices (A2). Its
upper part have been modified by pedogenetic processes (A1). It comes from the 79
AD Vesuvius eruption. The action of vegetation and micro-organisms has affected the
hydraulic and mechanical behavior of this layer;
 B) an intermediate layer of white pumices, of 1-6 cm of diameter, related to the 79 AD
Vesuvius eruption;
 C) a bottom layer of ashes deposits coming from an older eruption, with significant
clay fraction.
Figure 2 shows a schematic stratigraphy for the area under study. It is worth noticing that the
layer of white pumices or compacted ashes may be missing in some areas.
Fig. 2 Typical stratigraphy of the areas affected by landslide events.
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JTC1 Workshop on Advances in Landslide Understanding
Laboratory tests have been carried out on these soils. The main average hydraulic and
mechanical properties are reported in the Table 1.
Tab. 1
Property/ Soil type
[kN/m3]
Dry density γd
Void Ratio e
[-]
Friction Angle φ’
[°]
Saturated hydraulic conductivity ksat
[m/s]
Soils properties
Ashes(C)
Pumices(B)
Pyroclastic
cover(A)
9.012
5
9.056
1.8
2.8
2.5
36
37
38
4.07 e-9
1.3 e-4
5.5e-7
Volumetric Water Content at saturation θs
0.672
0.8
0.653
Residual Volumetric Water Content θr
0.196
0.1
0.001
Due to the presence of vegetation the pedogenized upper layer is assumed to have shear
strength higher than unrooted soil due to the root reinforcement.
CONCLUSIONS
This work has focused on 2 landslides case studies where the effect of the stratigraphy appears to be predominant. The soil profile has been characterized based on the geological survey after the landlslides event. The hydro-mechanical models for the chosen case studies have
been validated with respect to their ability to reproduce failure conditions when subjected to
the same antecedent and triggering rainfall events. The hydro-mechanical models were then
exploited to investigate the geometrical factors that are making the slopes more likely to fail
upon a rainfall event. These factors need to be measured to quantify the susceptibility to failure of given area. We finally propose a range of possible technologies for diffuse measurements of soil profiles that might be practically deployed in the field.
REFERENCES
Di Crescenzo G., Santo A. (1999) Analisi geomorfologca delle frane da scorrimento- colata rapida in depositi
piroclastici della penisola Sorrentina. Geografia Fisica e Dinamica Quaternaria 22, 57-72.
Di Crescenzo G., Santo A. (2005) Debris slides-rapid earth flows in the carbonate massifs of the Campania region (Southern Italy): morphological and morphometric data for evaluating triggering susceptibility. Geomorphology 66, 255-276.
Calcaterra D., Santo A. (2004) The January 10, 1997 Pozzano Landslide, Sorrento Peninsula, Italy. Engineering
Geology 75 (2004), 181-200.
Pirone M., Papa R., Nicotera M.V., Urciuoli G. (2015) In situ monitoring of the groundwater field in an unsaturated pyroclastic slope for slope stability evaluation. Landslides, 12, 259–276
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