Bull Eng Geol Environ (2013) 72:447–455
DOI 10.1007/s10064-013-0492-5

ORIGINAL PAPER

Analysis of rainfall infiltration effects on the stabilityof pyroclastic soil veneer affected by vertical drying
shrinkage fractures
A. Galeandro • J. Šimůnek • V. Simeone

Received: 9 March 2013 / Accepted: 4 July 2013 / Published online: 13 November 2013
Ó Springer-Verlag Berlin Heidelberg 2013

Abstract The paper presents a preliminary, simplified
evaluation of the effects of rainfall infiltration on the stability of slopes in layered pyroclastic soils affected byshrinkage vertical fractures. The analysis has been developed with a special reference to a stratigraphic sequenceobtained by an in situ survey at Pizzo d’Alvano (Southern
Italy). The analysis of rainfall infiltration is performed
using an original dual-permeability model. Results show
how fractures strongly condition infiltration depending on
rainfall intensity. Prolonged low-intensity rainfall may leadto a higher saturation of the surface soil layer than short,intense rainfall when water may flow quickly throughfractures into the underlying more permeable soil layers.
Calculated distributions of pore pressure are used for theslope stability analysis using the infinite slope approach.
Variations of the safety factor as a consequence of infiltration show that prolonged rainfall can induce a more
relevant decrease in the safety factor than intense
precipitations.
Keywords Dual-permeability model  Infiltration 
Layered soils  Pyroclastic soils  Rainfall-induced
landslides

A. Galeandro (&)  V. Simeone
Department of Civil Engineering and Architecture,
Technical University of Bari, via Orabona, n. 4, 70125 Bari, Italye-mail: annalisa.galeandro@poliba.it
J. Šimůnek
Department of Environmental Sciences, University of California
Riverside, Riverside, CA 92521, USA

Introduction
The stability of steep slopes in fine-grained soils is generally related to soil suction. Rainfall infiltration induces
variations of pressure heads and a decrease in soil suction
and consequently a decrease in shear strength and anincrease in soil weight, causing slope failure (Fredlung andRahardjo 1993; Wieczorek 1996; Iverson 2000). If a finegrained soil overlies a substratum of more permeable soils
or rocks, such as pyroclastics deposit over pumices, silt
upon fractured bedrock, or silt over sand or gravel, thelarge contrast between their soil hydraulic properties generates a capillary barrier for deep infiltration (Mancarella
et al. 2012) that may condition the infiltration process. The
capillary barrier can produce the accumulation of infiltrated
water in the less permeable surface soil, resulting in adecrease in the soil suction and a loss of stability.
An interesting example of slopes where a fine-textured
soil overlies a more permeable one is in Pizzo d’Alvano(Campania, Southern Italy) where a lot of landslides(debris flows and debris avalanches) were triggered byprolonged rainfalls in May 1998 (Del Prete et al. 1998;
Crosta and Dal Negro 2003; Guadagno et al. 2003, 2005;
Revellino et al. 2004; Cascini et al. 2008). Landslides were
triggered by the sliding of superficial pyroclastic debris
from the covering mantle and were subsequently channelized as debris flows (Fiorillo et al. 2001; Crosta and Dal
Negro 2003; Revellino et al. 2004). For these landslides,
Crosta and Dal Negro (2003), Fiorillo and Wilson (2004),
and Mancarella and Simeone (2012) reported that these
extreme instability phenomena took place after a period ofseveral days of not intense, but quite continuous rainfall,
which cannot be characterized by a really relevant return
time, to be considered hydrologically exceptional, butwhich were surely singular. Landslides happened in May,

123

448

at the beginning of the warm season, so that fine-textured
soils were easily affected by sub-vertical shrinkage cracks,
which may have further conditioned the infiltration process. Fractures constitute preferential flow paths for rainfall
water and may favor infiltration toward deeper permeable
layers, especially for high-intensity rainfall.
This paper presents and discusses the results, withrespect to slope stability, obtained by analyzing infiltration
phenomena into a fractured pyroclastic soil layer overlying
pumiceous layers for different rainfall scenarios. Stratigraphy, slope inclination, and hydraulic and geotechnicalsoil properties have been obtained by an in situ survey at
Pizzo d’Alvano (Campania, Italy) (Mancarella et al. 2012)and by laboratory tests (Mancarella and Simeone 2012). In
these hill slopes, fine shrinkage fractures may affect thefine-textured pyroclastic surface soil overlying a more
permeable layer during the warm season.
Infiltration processes in a fractured pyroclastic soil layer
overlying a more permeable pumiceous layer have been
analyzed by considering a horizontal soil surface and different rainfall scenarios, and by evaluating water content
and pressure head distributions at the end of each of them.
The analysis has been performed by means of a dual-permeability model (Galeandro and Simeone 2010, 2012;
Galeandro et al. 2011, 2013b). The results obtained bymeans of this analysis have been extended for a sloping soil
surface, in order to evaluate the effects of infiltration processes on slope stability (Galeandro et al. 2013a). Evaluations of slope stability have been developed according to
the infinite slope analysis (Skempton and De Lory 1957;
Doglioni et al. 2013a). Even though the model so far
simulates only infiltration into horizontal layers, the results
have been used to perform stability evaluations of sloping
covers. Although this approximation is not formally correct, the main purpose of this work is to show how thefractures may affect rainfall infiltration and slope stability
conditions. It is thus our opinion that this analysis can beconsidered as an acceptable first-level approximation of aqualitative evaluation of the influence of rainfall scenarios
on slope stability of the modeled situations.

Methods
Infiltration processes in a horizontally stratified and vertically fractured soil layer of a fine-grained pyroclastic soil
overlying a more permeable pumiceous layer have beenmodeled for different rainfall scenarios by a finite elements
dual-permeability model (Galeandro et al. 2011, 2013b).
The water flow infiltration in the soil matrix is describedusing the two-dimensional Richards equation, while soil
hydraulic properties are described using the van Genuchten(1980) and Mualem (1976) relationships. When the rainfall

123

A. Galeandro et al.

intensity exceeds the infiltration capacity of the matrix, acertain amount of water is not able to infiltrate into thematrix and starts flowing into fractures. Infiltration flow in
fractures is modeled using the continuity equation. The
flow rate in the fractures is obtained, at different depths, as
the difference between the fracture inflow rate and theamount of water laterally adsorbed by the matrix, which
depends on the amount of water available in the fracturesand on the saturated hydraulic conductivity of the soil
matrix (Fig. 1a).
Matrix swelling is evaluated assuming a linear relationship between the matrix volumetric water content andthe soil volume, so that the fractures completely close
when complete saturation is reached in the matrix. The
model additionally implements a capillary barrier below
the surficial fractured soil. Water accumulates in cracks up
to a maximum capillary height that depends the width of acrack opening and capillary tension, while in the matrix, it
is stored above the interface between layers until the critical pressure head is reached. As output, the model provides the distribution of pressure heads in the soil matrix
during the considered meteorological event, the variationof crack openings, and the time required to break thecapillary barrier.
Distributions of the degree of saturation and pressure
heads were then used in the stability analysis of a hill slopewith the above-described stratigraphic sequence. The
adoption of these results for the stability evaluations ofsteep hill slopes is not formally correct, since the infiltration model works, so far, only for horizontal soil layers.
However, the main purpose of this work was to underline
the potential influence of the fractures and the capillarybarrier on the infiltration process and on the slope stability.
Depending on different rainfall scenarios, this approximation was considered to be acceptable as a first-level qualitative evaluation of the problem.
Evaluations of the hill slope stability have been performed according to the infinite slope model (Fig. 1b). An
incipient failure of infinite slopes is described using anequation that balances the downslope component of gravitational driving stress against the resisting stress due tobasal Coulomb friction (mediated by the pore water pressure). Failure occurs at a depth z if at that depth (Iverson
2000):FS ¼

tan /0 hðz; tÞcw tan /0
c0

þ
¼1
tan b
cz sin b cos b
cz sin b cos b

ð1Þ

where z is the depth, at which the factor of safety isevaluated, c0 is the cohesion (kPa), /0 is the friction angle, cis the soil unit weight (kN/m3), cw is the unit weight ofgroundwater (kN/m3), b is the inclination of the slope andh(z, t) is the average pressure head (cm) at depth z and timet, evaluated by the infiltration model.

Analysis of rainfall infiltration effects

449

Fig. 1 a Schematic
representation of the model(after Galeandro et al. 2013b)
and b schematic representationof the hill slope

The unit weight c (kN/m3) for pyroclastic soils maychange significantly as a function of the water content,
affecting the evaluation of the slope stability. The value
used for the slope stability analysis has been calculated as afunction of the average degree of saturation S reached at aparticular depth, the specific gravity Gs, and the void ratioe, which is assumed to be constant in the entire surface
fine-textured soil layer:cðSÞ ¼

ðGs þ SeÞcw
1þe

ð2Þ

The case study
The proposed infiltration model is applied here to the soil
stratigraphy described by Mancarella and Simeone (2012),
based on the in situ surveys in the area of Pizzo d’Alvano(Campania, Italy, Fig. 2a). It is a pyroclastic veneer consisting of alternating ashy and pumiceous layers (Fig. 2b),with very different grain size distributions and permeabilities. Geotechnical and hydraulic properties of this pyroclastic soil outcropping in that area were extensively
studied by several authors by both in situ and laboratory
measurements (Esposito and Guadagno 1998; Basile et al.
2003; Crosta and Dal Negro 2003; Olivares and Picarelli
2003; Cascini and Sorbino 2003; Bilotta et al. 2005;
Mancarella and Simeone 2012; Esposito et al. 2013 andothers).
The hydraulic properties of both soils have been selectedbased on in situ permeability measurements and laboratory
hydraulic conductivity tests on pyroclastic soil columns,
carried out by Mancarella and Simeone (2012) andMancarella et al. (2012). The saturated hydraulic conductivity Ksat of the upper layer is equal to 10-4 cm/s, the van
Genuchten parameters a and n for the pyroclastic fine soil

are equal to 1 m-1 and 1.8, respectively, and the residual
and saturated water contents are equal to 0.1 cm3cm-3 and0.4 cm3cm-3, respectively. The water-entry pressure headof the pumiceous layer can be assumed to be approximately
equal to the water-entry pressure head of a gravel and equal
to 2 cm (Stormont and Morris 1998), due to the similarity
between their grain sizes (a grain size of about 1 cm) andsaturated hydraulic conductivities (10-2 to 10-1 cm/s).
The analysis of the infiltration process has been performed only for the surface soil layer of fine-grained
pyroclastic with a thickness of 40 cm, overlying a coarser
pumiceous layer. The upper layer is characterized by aregular system of fractures spaced 80 cm apart in bothhorizontal directions with a fracture thickness of 2 cm.
Table 1 summarizes hydraulic properties and geometricproperties of the soil system. The upper layer has been
spatially discretized into soil elements of small dimensions(2 cm 9 2 cm). Simulations have been performed using atime step of 10 s. The infiltration process has been analyzed for a meteorological event characterized by a total
rainfall height of 70 mm, with two different rainfall scenarios: a low-intensity rain of 5 mm/h (event A) and a quite
intense rainfall of 50 mm/h (event B). The analysis has
been performed assuming two different values of an initialdegree of saturation of the surface soil layer: 25 % inscenario IS1 and 40 % in scenario IS2 (Table 3).
For the slope stability analysis, according to Mancarella
et al. (2012), the slope inclination has been assumed to beequal to 45° and the thickness of the surface fine-textured
soil layer to 40 cm (Fig. 1b). The parameters of the shear
strength have been chosen according to previous geotechnical studies (Bilotta et al. 2005): the cohesion c0 and thefriction angle /0 are set equal to 2 kPa and 37°, respectively (Table 2). The unit soil weight c(kN/m3) has been
evaluated according to the indication of Esposito and

123

450

A. Galeandro et al.

Fig. 2 a Location of the study
area and b schematic
stratigraphy, indicating
thicknesses (m) and saturated
hydraulic conductivities Ksat ofeach soil layer (after Mancarella
and Simeone 2012, modified)

Table 1 Hydraulic properties of the fine-textured soils and geometricfeatures of the system
Hydraulic properties (pyroclastic soil)
Residual water content hr (m3m-3)0.1

Saturated water content hs (m3m-3)

0.4

Saturated hydraulic conductivity Ksat (cm/s)

10-4

-1

a (van Genuchten 1980) (cm )0.01

n (van Genuchten 1980)

1.8

Geometry
Spacing L (cm)80

Half opening d (cm)1

Thickness S (cm)40

Table 2 Geotechnical, physical, and geometric properties
Geotechnical, physical, and geometric properties
c0 (kPa)2/0 (degrees)37b (degrees)

45c (kN/m3)

7.5

Guadagno (1998). A specific gravity Gs of 2.62 g/m3 and aunit dry weight cd of 7.5 kN/m3, corresponding to a void
ratio e of 2.45, have been used for the entire surface soil
layer. Figure 3 shows the initial safety factor versus depth
evaluated using Eq. 1. The initial safety factor is in bothcases (two initial saturations) much higher than 1, indicating that the stability of the hill slope is clearly guaranteed for both considered initial degrees of saturation(Table 3).

Results and discussion
Rainfall infiltration analysis
The results of the analysis performed for the two rainfall
scenarios (A and B) for the two initial degrees of saturation

123(IS1 and IS2) are shown in Fig. 4. With regards to the lowintensity rainfall scenario A (5 mm/h for 14 h) and bothconsidered initial degrees of saturation, water flows relatively slowly into the fractures, reaching a maximum depthof only 16 cm (Fig. 4a, b). In both cases, the surface soil
layer is quite close to full saturation and the fracture isalmost completely closed at the end of the rain event. In
case A1 (Fig. 4a), water flows into the matrix and reaches
the bottom of the soil layer, resulting in an increase in the
degree of saturation from its initial value of 25–57 %. The
volumetric water content increases from its initial vale of
0.175–0.27 cm3cm-3. Similarly, in case A2 (Fig. 4b), thesaturation degree at the interface between the surface soil
layer and the underlying layer reaches a value of 70 % at
the end of the rainfall event. The water content increases
from the initial value of 0.22–0.31 cm3cm-3.
In the rainfall scenario B (50 mm/h for 1.4 h) and for bothconsidered initial degrees of saturation, water reaches thebottom of the fractures quite quickly (after only 2 min),allowing accumulation of water in the fractures until the
breakdown of the capillarity barrier. Lateral infiltration ofwater involves only few centimetres of the soil close to thefracture walls (Fig. 4c, d), as a consequence of vertical
fracture flow. Even when water reaches the bottom of theupper soil layer, only a very few centimetres of the topsoil
are really affected by significant changes in water content(6 cm in case B1, Fig. 4c, and 8 cm in case B2, Fig. 4d).
Apart from the first few centimetres of the soil near thefracture interface (which is saturated due to lateral adsorption), the rest of the soil matrix does not show significant
changes in the water content: from 0.175–0.177 cm3cm-3 incase B1 and from 0.22–0.23 cm3cm-3 in case B2.
In order to emphasize the effects of the rainfall intensityand duration on the saturation of the pyroclastic cover, theaverage saturation degree of the first 20 cm of the soil
matrix has been evaluated for each evaluated scenario. The
average degree of saturation of the first 20 cm of the soil
reaches the value of 95.5 % in case A1 and 99.8 % in caseA2, while for the rainfall scenario B, the degree ofAnalysis of rainfall infiltration effects

451

Fig. 3 An initial safety factor
for both considered initialconditions (IS1 and IS2)

Table 3 Summary of the analyzed combinations
Cases

Rainfall intensity
i (mm/h)DurationT (h)Initial saturation degree
IS (%)

A1

5

14

25

1.4

B1

50

A2

5

14

B2

50

1.4

40

saturation is much lower, equal to 38 % in case B1 and to
53 % in case B2.
The fractures (cracks) start closing due to swelling of thesoil matrix at the soil surface, and their closure propagates
toward the bottom, essentially depending on lateral water
adsorption from the fractures into the soil matrix. It isnoteworthy that for the low-intensity but prolonged rain,
the swelling induces the complete closure of cracks about
10 h after the beginning of the rainfall (corresponding to arainfall height of 50 mm) in case A1 (Fig. 5a) and after
about 7.3 h (corresponding to a rainfall height of about
37 mm) in case A2, preventing more water flowing into thefractures.
In scenario B with intense precipitation, the swelling isquite irregular and does not induce the complete closure ofcracks. The soil initially swells at the surface, inducing
cracks to narrow, while deeper parts of the soil are not yet
reached by infiltrating water. In case B1 (Fig. 5b), theswelling process causes a crack closure of 73 % of the initial
half-width value, while in case B2, the crack closure is moresignificant and equal to 80 % of the initial half-width value.

With regard to the capillary barrier phenomenon, for lowintensity rainfall (rainfall scenario A), the capillary barrier
remains active in the matrix and fractures during all rainfall
events, since the pressure head at the interface between the
pyroclastic cover and the pumiceous layer does not reach thecritical breaking value (2 cm) and water flow does not reachthe bottom of the fractures. For the intense rainfall event(event B), it is possible to observe a really fast breakthroughof the barrier in the fractures, since water flow reaches thebottom immediately. The maximum capillary height isreached in both cases after 120 s (a corresponding rainfall
height of 1.67 mm) after the beginning of the rainfall.
Implication on slope stability
At the beginning of the rainfall events and for both considered initial saturation degrees, the initial safety factors at
all depths of the potentially unstable soil layer are significantly higher than 10 (Fig. 3), as a consequence of highsuction values. Rainfall infiltration induces a decrease in
the soil suction and consequently also in the safety factor.
Spatial and temporal distributions of water contents andsoil suctions obtained by the rainfall infiltration analysis
have been used for the evaluation of safety factor variations
induced by rainfall infiltration. In each simulation, thesafety factors at different depths have been evaluatedconsidering water content, saturation degree, and soil
suction distributions in the pyroclastic soil layer at times
when a rainfall height reached 10-mm increments (i.e., 10,
20, 30, 40 mm, etc.). Results are presented in Fig. 6.

123

452

A. Galeandro et al.

Fig. 4 Pressure head distributions for rainfall scenarios: a A1 (rainfall A and initial condition IS1). b A2 (rainfall A and initial condition IS2).
c B1 (rainfall B and initial condition IS1) and d B2 (rainfall B and initial condition IS2)

Fig. 5 Crack closure dynamics for rainfall scenarios: a A1 (rainfall A and initial condition IS1) and b B1 (rainfall B and initial condition IS1)123

Analysis of rainfall infiltration effects

453

Fig. 6 Safety factor variations during the rainfall for rainfall scenarios: a A1 (rainfall A and initial condition IS1) and b B1 (rainfall B and initialcondition IS1)

In case A1 (r = 5 mm/h, T = 14 h, IS1 = 25 %), after
2 h from the beginning of the rainfall (corresponding to arainfall height of 10 mm), the factor of safety is \5 at all
depths, with a minimum value of 2 at the bottom of thefine-grained pyroclastic layer (Fig. 6a). Subsequently, thefurther decrease in the safety factor is more gradual andfollows the S-shaped trend shown in Fig. 6a. The safety
factor reaches the failure condition (FS = 1) at a depth ofabout 7 cm after little more than 8 h (corresponding to arainfall height of 40 mm), while at the bottom of the soil
profile, the safety factor reaches the unit value after 10 h(corresponding to a rainfall height equal to 50 mm). It isnoteworthy that the upper part of the cover reaches thefailure condition before the rest of the profile. In case A2(r = 5 mm/h, T = 14 h, IS2 = 40 %), as consequence ofthe higher initial water content, slope failure occurs more
quickly, after about 6 h from the rain initiation (corresponding to a rainfall height of 30 mm), and simultaneously at a depth of about 7 cm and at the interface
between the pyroclastic cover and the pumice.
In both B1 (r = 50 mm/h, T = 1.4 h, IS1 = 25 %,Fig. 6b) and B2 (r = 50 mm/h, T = 1.4 h, IS2 = 40 %)cases, it is possible to observe an initial relevant decreasein the safety factors, especially during the first 12 min(corresponding to a rainfall height of 10 mm). However,the decrease in the safety factor affects only the surface
layer (about 10 cm), while only small variations can beobserved in the deeper parts of the pyroclastic cover, withthe factor of safety is always being higher than 1. This isdue to the almost immediate breakthrough of the capillarybarrier in the fractures, which allows rainfall water toflow downward into the pumiceous layer. The amount ofwater which is absorbed by the matrix is not high.
Absorbed water is stored in the first few centimeters of
the soil matrix close to the soil surface and to the fracture

walls. There is a noticeable decrease in suction only inthese parts of the soil matrix, while in the rest of thematrix, the pressure heads do not change significantly.
Consequently, the average suction at a certain depth isstill quite high, and the corresponding decrease in theshear strength is not sufficient to trigger the slope failure.
On the other hand, in rainfall scenarios A, almost all
rainfall water enters the soil matrix by vertical infiltration,
resulting in almost full saturation of the soil matrix, which
produces a significant decrease in suction and triggers theslope’s failure.
Results are in good agreement with the failure mechanisms observed by many authors for the analyzed slope.
According to Crosta and Dal Negro (2003), Fiorillo andWilson (2004), Mancarella et al. (2012), and Mancarella
and Simeone (2012), instability phenomena affecting Pizzo
d’Alvano slopes took place after a period of several days of
not intense, but quite continuous rainfall, which cannot be
considered hydrologically exceptional. Fiorillo and Wilson(2004) observed that the total amount of rainfall was significantly less than during other major precipitation events
recorded during wet periods which did not trigger slides.
While rainfall was not remarkable with respect to its statistical return time, it produced severe consequences. The
constancy and temporal extent are the most remarkablecharacteristics of the May 1998 precipitation. Rainfall
lasted from April 28 to May 6 with very few and shortinterruptions.
In summary, results show how prolonged low-intensity
rainfalls could easily lead to the saturation of the surface
soil layers and could trigger instability phenomena. Suchtypes of rainfall are more dangerous with respect to slope
failure in surface fine-grained soil covers than intense andshort precipitations. The schematic representation of these
phenomena is shown in Fig. 7.

123

454

A. Galeandro et al.

Fig. 7 Schematic
representation of infiltration
scenarios at the end of (a) a lowintensity and prolonged rain and(b) an intense and shortprecipitation. The red line
indicates the potential failure
plane

Conclusion
The analysis of the effects of the presence of vertical cracks
due to drying and shrinkage phenomena on the evaluationof the hill slope stability of layered pyroclastic deposits fordifferent rainfall scenarios has been presented. The analysis
has been performed with special reference to a stratigraphic
sequence obtained by an in situ survey at Pizzo d’Alvano(Campania, Southern Italy). The analysis of the infiltration
process in the fractured soil overlying a coarse soil has
been performed using a specifically developed dual-permeability model, which simulates infiltration into the surface soil in the presence of fractures and a capillary barrier
below this layer. The results obtained for a horizontal soil
surface have been used to evaluate the stability of a sloping
soil.
The results show that for a low-intensity rainfall event,fracture flow is limited only to the top of the soil surface
layer, while for an intense precipitation event, waterreaches the bottom of the fractures quite quickly and flows
quite easily downward in the underlying more permeable
layer. Distributions of the soil suction obtained from theinfiltration analysis have allowed us to evaluate the hillslope stability. The results show that the hill slope failure ismore likely for long, low-intensity rainfall than for short,intense precipitation. In the latter case, water is drained
quickly by the fractures downward toward the capillarybarrier, which is quickly broken, allowing water to flow
downward in the pumiceous layer.

123

Results are consistent with the failure mechanisms
observed at Pizzo d’Alvano as a consequence of theevents in May 1998. Extreme instability phenomena took
place after a time period of several days of low-intensity,
but quite continuous rainfall, during which the total
amount of rainfall was significantly less than during othermajor precipitation events recorded during wet periods
which did not trigger landslides. Thus, our results show
how prolonged low-intensity rainfall could easily result in
the high saturation of the top surface soil layers and could
trigger instability phenomena in these surface layers. Suchrainfall events are potentially more dangerous for surface
fine-grained soil covers than intense and shortprecipitation.

References
Basile A, Mele G, Terribile F (2003) Soil Hydraulic behaviour of a
selected benchmark soil involved in the landslide of Sarno 1998.
Geoderma 117:331–346
Bilotta E, Cascini L, Foresta V, Sorbino G (2005) Geotechnicalcharacterization of pyroclastic soils involved in huge flowslides.
Geotech Geol Eng 23:365–402
Cascini L, Sorbino G (2003) The contribution of soil suction
measurements to the analysis of flowslide triggering. Workshop‘‘Flows 2003—occurrence and mechanisms of flows in naturalsoils and earthfills’’. May 2003, 1:77–86
Cascini L, Cuomo S, Guida D (2008) Typical source areas of May
1998 flow-like mass movements in the Campania region,
Southern Italy. Eng Geol 96:107–125

Analysis of rainfall infiltration effects
Crosta GB, Dal Negro P (2003) Observations and modelling of soil
slip-debris flow initiation processes in pyroclastic deposits: theSarno event. Nat Hazard Earth Sys 3:53–69
Del Prete M, Guadagno FM, Hawkins AB (1998) Preliminary reporton the landslides of 5 May 1998, Campania, southern Italy. Bull
Eng Geol Environ 57(2):113–129
Doglioni A, Galeandro A, Simeone V (2013) Lateral strength andcritical depth in infinite slope stability analysis. Int J Numer Anal
Methods Geomech. doi:10.1002/nag.2190 (in press)
Esposito L, Guadagno FM (1998) Some special geotechnicalproperties of pumice deposits. Bull Eng Geol Environ 57:41–50
Esposito L, Esposito AW, Pasculli A, Sciara N (2013) Particularfeatures of the physical and mechanical characteristics of certain
Phlegraean pyroclastic soils. Catena 104:186–194
Fiorillo F, Wilson RC (2004) Rainfall induced debris flows inpyroclastic deposits, Campania (southern Italy). Eng Geol
75:263–289
Fiorillo F, Guadagno FM, Aquino S, De Blasio A (2001) The
December 1999 Cervinara landslides: further debris flows in the
pyroclastic deposits of Campania (southern Italy). Bull Eng Geol
Environ 60(3):171–184
Fredlung DG, Rahardjo H (1993) Soil mechanics for unsaturated
soils. Wiley, New York
Galeandro A, Simeone V (2010) A dual porosity model for infiltration
processes in fractured porous swelling soils. In: Proceedings ofthe 11th IAEG congress, geologically Active, Auckland NZ,
5–10 Sept 2010. pp 683–689
Galeandro A, Simeone V (2012) Infiltration processes in fractured
and swelling soils and their influence on the stability of surficial
covers. Rend Online Soc Geol It 21:518–520
Galeandro A, Šimůnek J, Simeone V (2011) Simulating infiltration
processes into fractured and swelling soils as triggering factorsof landslides. In: Margottini C, Canuti P, Sassa K (eds)
Landslide science in practice, vol 3 Spatial analysis andmodelling; chapter 1—Advances in slope modeling. (World
Landslide Forum 2, Rome, October 2011)
Galeandro A, Doglioni A, Simeone V, Šimůnek J (2013a) Analysis of
infiltration processes into fractured and swelling soils as
triggering factors of landslides. Environ Earth Sci. doi:10.
1007/s12665-013-2666-7 (in press)
Galeandro A, Simeone V, Šimůnek J (2013b) A dual-permeability
model for simulating water infiltration into unsaturated, fractured
and swelling soils. Rend Online Soc Geol It 24:152–154

455
Guadagno FM, Scarascia Mugnozza G, Martino S (2003) Influence of
man-made cuts on the stability of pyroclastic covers (Campania,
Southern Italy): a numerical modelling approach. Environ Geol
43:371–384
Guadagno FM, Forte R, Revellino P, Fiorillo F, Focaresta M (2005)
Some aspects of the initiation of debris avalanches in theCampania region: the role of morphological slope discontinuities
and the development of failures. Geomorphology 66:237–254
Iverson RM (2000) Landslide triggering by rain infiltration. Water
Resour Res 36(7):1897–1910
Mancarella D, Simeone V (2012) Capillary barrier effects inunsaturated layered soils with special reference to the pyroclastic
veneer of the Pizzo d’Alvano, Campania, Italy. Bull Eng Geol
Environ 71:791–801. doi:10.1007/s10064-012-0419-6
Mancarella D, Doglioni A, Simeone V (2012) On capillary barrier
effects and debris slide triggering in unsaturated layered covers.
Eng Geol 147–148:14–27. doi:10.1016/j.enggeo.2012.07.003
Mualem Y (1976) A new model for predicting the hydraulicconductivity of unsaturated porous media. Water Resour Res
12:513–515
Olivares L, Picarelli L (2003) Shallow flowslides triggered by intense
rainfalls on natural slopes covered by loose unsaturated pyroclastic soils. Geotechnique 53(2):283–288
Revellino P, Hungr O, Guadagno FM, Evans SG (2004) Velocity andrunout simulation of destructive debris flows and debris
avalanches in pyroclastic deposits, Campania region, Italy.
Environ Geol 45:295–311
Skempton AW, De Lory FA (1957) Stability of natural slopes in
London clay. Proc 4th Int Conf Soil Mech 2:378–381
Stormont JC, Morris CE (1998) Method to estimate water storage
capacity of capillary barriers. J Geotech Geoenviron Eng
124(4):297–302
van Genuchten MTh (1980) A closed form equation for predicting thehydraulic conductivity of unsaturated soils. Soil Sci Soc Am J
44:892–898
Wieczorek GF (1996) Landslide triggering mechanisms. In: Turner
AK, Shuster RL (eds) Landslides: investigation and mitigation.
Transportation Research Board-National Research Council,
Special report, vol 247, pp 76–90

123