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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
331
DOI: 10.4408/IJEGE.2013-06.B-31
EFFICIENCY OF STABILIZATION TECHNIQUES
IN ACQUALORETO LANDSLIDE AREA (UMBRIA, ITALY)
L
ucio
DI MATTEO
(*)
, L
uciano
FARALLI
(**)
, n
eLLo
GASPARRI
(**)
, R
iccaRdo
PICCIONI
(**)
,
d
anieLa
VALIGI
(*)
& L
uca
d
omenico
VENANTI
(**)
(*)
University of Perugia, Department of Earth Sciences - Perugia, Italy
(**)
S.G.A. Studio Geologi Associati, Via XX Settembre 76, 06121 - Perugia, Italy (info@studiogeologiassociati.eu)
INTRODUCTION
Rises in groundwater level after stormwater events
or prolonged rainy periods is one of the main triggers
for landslides (e.g. V
an
a
sch
et alii, 1999; i
VeRson
,
2000). According to B
ishop
& m
oRgensteRn
(1960),
g
hiassian
& g
haReh
(2008) and d
i
m
atteo
et alii
(2013a), the most critical situation that a slope may
experience with respect to the mass stability is when
groundwater level is approaching to the slope profile.
As noted by h
utchinson
(1977), a proper drainage
system is the main method used to stabilize landslides,
followed by modification of slope geometry. Surface
water (runoff) is less difficult and expensive to control
than deep groundwater, especially in the case of deep
complex landslides. The efficiency of horizontal drains
is generally high, but long-term maintenance may suf-
fer due to fine particles which slowly clog drain pores
(B
Romhead
, 1992; s
anti
et alii, 2001; m
iningeR
et alii,
2011). The design and success of horizontal drains are
also often governed by local experience alone (R
aha
-
Rdjo
et alii, 2003). Although the ideal location for hori-
zontal drains is at the toe of the slope (s
antoso
et alii,
2010), this is often not feasible (costs, logistics, etc.).
Drains - together with flexible retaining walls for pro-
tecting dwellings - are often designed first at the higher
part of the main body of landslides, especially when
slope failure approaches buildings. Large-diameter
drainage wells linked to sub-horizontal drains are also
commonly used to lower the groundwater table and in-
ABSTRACT
This work, based on the results of a series of
geotechnical and monitoring studies, analyses two
landslides composed of heterogeneous altered marly
clayey arenaceous rock and eluvial-colluvial deposits,
located near the village of Acqualoreto (Central Italy).
The landslides (called here A and B) are characterized
by retrogressive movements approaching buildings
and roads near the village and by different sliding sur-
faces between 10 and 25 m b.g.l. Maximum displace-
ment rates were measured after the most significant
rainfall events in autumn months. Four years after the
installation of drainage systems in both landslides,
some considerations on the efficiency of stabilization
techniques (horizontal drains and drainage wells) are
presented, with analysis of inclinometric and piezo-
metric data, both pre- and post-installation. Results
indicate that single alignment of drainage wells in the
upper part of landslide A cannot be considered resol-
utive in reducing the landslide hazard: therefore, new
drainage works in the central part of landslide mass
and in the accumulation zone should be considered.
The horizontal drains in the central/upper part of land-
slide B did greatly reduce movement, although new
drains should be installed along the toe of the land-
slide for definitive control the groundwater rise.
K
ey
words
: Acqualoreto landslide, horizontal drains, draina-
ge wells, drain efficiency
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L. DI MATTEO, L. FARALLI, N. GASPARRI, R. PICCIONI, D. VALIGI & L.D. VENANTI
332
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
eral geotechnical surveys to determine the geometry
of both landslides were carried out between October
2002 and July 2008: site investigations included bore-
holes; nucleus destruction boreholes; seismic refrac-
tion profiles and DPSH/SPT dynamic penetration
tests; pressuremeter tests (DRT) and Lugeon and Le-
franc permeability tests. Figure 3 shows a lithological
sketch oriented roughly NW-SE (longitudinal to the
body of landslide B). Several samples of undisturbed
soils were also collected for geotechnical laboratory
testing. The results were extensively discussed by
F
aRaLLi
et alii (2004). For information on landslide
kinematics, displacement measures and groundwater
observations were collected. According to data from
about 16 inclinometers, landslide movements occur
between 10 and 25 m below ground level (b.g.l.) (Fig.
3). The landslide movements are mainly roto-trans-
crease slope stability (c
encetti
et alii, 2005; p
opescu
&
s
asahaRa
, 2008; R
onchetti
et alii, 2009).
The present work, based on the results of a series
of geotechnical and monitoring studies, illustrates the
stabilization techniques used in two landslides (lo-
cated near the village of Acqualoreto, Central Italy)
and examines their efficiency. The site of Acqualoreto,
between the towns of Todi and Orvieto, is taken as a
reference, being classified as high landslide risk area
(R3). According to risk zoning for landslide in Italy, R3
indicates an area with high risk characterised by vic-
tims, functional damage to buildings and infrastructure,
as well as partial interruption of economic activities are
possible (c
ascini
et alii, 2010). Four years after the in-
stallation of drainage systems in the two landslides, the
efficiency of stabilization techniques (horizontal drains
and drainage wells) was studied, following monitoring
data recorded since 2002.
SETTING AND DESCRIPTION OF STUDY
AREA
In the Acqualoreto area (42° 44' 0", 12° 20' 0"),
a large well-known landslide occupies the left bank
of the river Tiber (Fig. 1): the village of Acqualoreto
has been one of the areas to be stabilized in the Um-
bria region since 1966. The study area is character-
ized by rocks belonging to the Tuscan Domain (marly
clayey arenaceous rock and calcarenite of the “Scaglia
Toscana Formation - Calcareniti di Dudda”) and
Umbria-Marche Domain (marly limestone). Eluvial-
colluvial deposits and talus extend over the body of
the landslide, which has a maximum width of 3.5 km.
The landslide is composed by heterogeneous altered
flysch rocks and heterogeneous eluvial-colluvial de-
posits and talus. Figure 1 shows the location of the
study area, with the main lithological and geomopho-
logical features. The extension and geometry of the
Acqualoreto landslide were studied by multiple and
multi-scale observation of aerial photos, together with
extensive geotechnical investigations and monitoring
(F
aRaLLi
et alii, 2004). The present work takes as its
reference part of two landslides where some stabiliza-
tion techniques have been designed: both are located
in the main body of the larger landslide and are char-
acterized by retrogressive movements approaching
buildings and roads near the village (Fig. 1). They are
called here “landslide A” and “landslide B” (Fig. 2):
the latter is located downstream of Acqualoreto. Sev-
Fig. 1 - Lithological and geomorphological features of
Acqualoreto area (modified from B
aLDucci
et alii,
2013). Red rectangle: location of landslides stud-
ied here. LEGEND: 1a) active complex landslide;
1b) active slide landslide; 2a) suspended complex
landslide; 2b) suspended slide landslide; 3a) dor-
mant complex landslide; 3b) dormant slide land-
slide 4a) stabilised and relict complex landslide;
4b) stabilised and relict slide landslide; 5) eluvial
and colluvial deposits; 6) alluvional deposits; 7)
arenaceous bedrock; 8) marly bedrock; 9) calcar-
eous bedrock
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EFFICIENCY OF STABILIZATION TECHNIQUES IN ACQUALORETO LANDSLIDE AREA (UMBRIA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
333
STABILIZATION TECHNIQUES
A series of flexible retaining walls was con-
structed to protect the village of Acqualoreto (Fig. 2).
Drainage systems were built into the main body of
both landslides. As shown in Fig. 2, a group of drain-
age wells connected to a Horizontal Directional Drill-
ing (HDD) pipe outlet were drilled in the upper part of
landslide A. Two sets of HDD drains were constructed
in the central/upper part of landslide B.
lational sometimes associated with flow-type motion
(B
aLducci
et alii, 2013). These kind of kinematics is
widespread in flyschoid complexes in the Umbria re-
gion (g
uzzetti
et alii, 1996).
Data from 12 open standpipe and Casagrande
piezometers indicate that an unconfined aquifer is
hosted in the dismembered rock slide body. Perme-
ability data from 15 Lugeon and Lefranc tests show
that the permeability coefficient varies from 10
-6
to
10
-9
m/s (F
aRaLLi
et alii, 2004): high variability of
the permeability coefficient, typical of landslide ar-
eas characterized by altered flysch rocks (R
onchetti
et alii, 2009), was also observed, in both vertical and
horizontal directions.
According to data from the Corbara raingauges
(140 m a.s.l.), located about 10 km SW of village
of Acqualoreto, the mean annual precipitation in the
area is about 730 mm (1963-2012 period). Analysis
of monthly data indicated that the main precipitation
peak is recorded in autumn (October-December), with
a mean value of about 270 mm (Fig. 4).
Fig. 2 - Boundaries of landslides A and B. LEGEND: 1) landslide crowns; 2) landslide boundaries; 3) inclinometer; 4)
piezometer; 5) HHD pipe outlet; 6) drainage wells; 7) first-order HDD drains; 8) second-order HDD drains; 9)
superficial drainage network; 10) retaining walls; 11) line of lithological section
Fig. 3 - Lithological sketch oriented longitudinally to body of
landslide B (line of section shown in Fig. 2). LEG-
END: 1) altered marly-clayey arenaceous rock
and calcarenite, eluvial-colluvial deposits and
talus; 2) bedrock (marly-clayey arenaceous rock
and calcarenite); 3) sliding surface; 4) hypotheti-
cal sliding surface; 5) inclinometer; 6) piezometer
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L. DI MATTEO, L. FARALLI, N. GASPARRI, R. PICCIONI, D. VALIGI & L.D. VENANTI
334
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
lower the groundwater level and to control pore wa-
ter pressure along the slope.
ANALYSIS OF EFFICIENCY OF DRAINA-
GE SYSTEMS
To study the efficiency of the drainage systems
constructed in both landslides, displacement rates and
DRAINAGE WELLS (LANDSLIDE A)
Due to the size of landslide A (up to 500 m long
and 150 m wide) and considering that the main
crown is very close to buildings and roads (Fig. 2), a
group of 28 drainage wells 1.20 m in diameter and at
a distance of 6 m from each other was built in the up-
per part of the landslide at altitudes between 380 and
390 m a.s.l. Each well reached a maximum depth of
about 15 m (Fig. 5). In order to redirect groundwater
outside the landslide mass, the wells were linked to
form an interconnected subsurface and surface drain-
age network (Figs. 2 and 5).
HORIZONTAL DIRECTIONAL DRILLING
(LANDSLIDE B)
For efficient drainage of the body of landslide
B, HDD technology was applied (Fig. 6), with the
implementation of controlled Paratrack and Vector
Magnetics devices, which allowed the pilot well to
be targeted with an error of less than 5 cm. A system
of drainage pipes up to 140 m long was distributed
throughout the central/upper part of landslide body,
for increased drainage efficiency. Planimetric and
altimetric locations of drains and their interactions
were designed according to well-known methods
(h
utchinson
, 1977; d
i
m
aio
et alii, 1988). HDD
technology was applied to install two orders of drain-
age pipes on two pitches at 323 and 343 m a.s.l., at
about 100 m from each other. For maximum effi-
ciency, the tubes were placed in the portions of the
sliding surface where soil permeability was higher.
The drains were placed parallel to the landslide
axis, at 10 m from each other. Two orders of drains
were overlapped, to limit groundwater rising in the
transition area between the two groups of drains, to
Fig. 4 - Mean monthly precipitation at Corbara rain-
gauge for 1963-2012 period
Fig. 5 - Detail of installation of drainage wells in landslide
A. Scheme of well groups from B
aLDucci
et alii (2013)
Fig. 6 - Sketch of HDD Techniques (modified from B
aLD
-
ucci
et alii, 2013) and detail of drainage works
in landslide B
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EFFICIENCY OF STABILIZATION TECHNIQUES IN ACQUALORETO LANDSLIDE AREA (UMBRIA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
335
movements are concentrated along the main slip sur-
face (Fig. 3), although secondary surface movements
are produced by superficial splay. Most of the casings
of the inclinometers were broken, so that only a few
instruments remained to monitor displacement rates be-
fore and after the installation of the drainage systems. In
order to update the monitoring system, new inclinom-
eters were then set up near the damaged instruments.
Displacement data from inclinometers S6, S6b,
S34 and S22 were used for landslide A (see Fig. 2
for location of instruments). Figure 8a,b shows the
cumulate displacement of inclinometers S6, S6b
and S34 at two deformation depths (around 11 and
16 m b.g.l.): data was collected at 1- and 2-month
intervals. During the study period, maximum cu-
mulate deformations were recorded at a depth of 11
m for all inclinometers. As Fig. 8a shows, the mean
monthly displacement rates before the drainage wells
were drilled (between 1.5 and 1.9 mm/month over 18
months) were much higher than those observed after
installation (around 0.1 mm/month over 23 months).
In particular, the decrease recorded by S34 occurred
after a prolonged rainy period (2008-2010 in Fig. 7),
indicating that the drainage system contributed to re-
ducing landslide motion, at least in the highest part
of the main body. This was confirmed by the recent
piezometric responses to precipitation were analysed
during both pre- and post-installation periods.
PIEZOMETRIC DATA
Before HDD and drainage wells were installed (be-
fore September 2008 for landslide A and before January
2009 for landslide B), the response to precipitation of
some piezometers, measured at approximately monthly
intervals, was studied. Field data (collected from Feb-
ruary 2004) indicated that the seasonal groundwater
response to precipitation before installation of the
drainage systems was different between the foot and
the lowest part of the main body of the landslides, with
maximum fluctuation in the accumulation zone (up to
10 m after heavy autumn rain). Taking as reference the
lowest part of the main body of landslide B (piezometer
S5D; Fig. 7), during the study period there was a 5-m
increase in groundwater level, due to heavy autumn
rain in 2004 and 2005 (400-500 mm, corresponding to
about two-thirds of the average annual value). Piezom-
eter S5D was chosen because it had the longer data set.
During the study period, 1-month responses of
groundwater level to precipitation were identified for
piezometers located in the main body of the landslides,
and 2-month responses for those in the accumulation
zone. Although piezometric data before February 2004
were not available, the piezometric heads in the body
of the landslides were expected to be lower than those
of the 2004-2009 period: during 2000-2003, a pro-
longed drought affected the study area and the central
Apennines in general (d
i
m
atteo
et alii, 2013b). After
the installation of the first-order HDD in the main body
of landslide B (September 2008-January 2009), the re-
sponse of groundwater level to precipitation changed
considerably. As shown in Fig. 7, autumn rain was
heavy in 2008 (about 500 mm, similar to autumn 2004
and 2005), but produced a rise in the water table of
only 1.30 m. In the period September 2008-July 2011,
the groundwater level fluctuated at around 322-324 m
a.s.l.: these values correspond to the lowest and highest
elevations of the first-order HDD. Due to the lack of
piezometric data prior the installation of drainage wells
in landslide A, the analysis of the effect of drainage
system on groundwater was not carried out.
INCLINOMETRIC DATA
Analysis of displacement rates of the inclinometers
located in both landslides indicates that most of the
Fig. 7 - Effect of precipitation on groundwater level (pi-
ezometer S5D), February 2004 - July 2011, for
lowest part of main body of landslide B (data
measured at approximately monthly intervals).
Data recorded at Corbara raingauge
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L. DI MATTEO, L. FARALLI, N. GASPARRI, R. PICCIONI, D. VALIGI & L.D. VENANTI
336
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
heavy rainfall data for autumn 2012 (about 500 mm),
which did not produce any significant movement in
landslide A. The efficiency of the drainage system
was not observed at inclinometers located about 120
m below it (e.g., inclinometer S22 in Fig. 8c).
Data from S27, S4, S4b and S30 were analysed for
landslide B (Fig. 9a,b). Maximum deformation from
inclinometer S27, located close to the first-order HDD
drains, was also recorded at a depth of 11 m. As shown
in Fig. 9a, the displacement rates reached a maximum
of 2.6 mm/month, due to the rain of September-No-
vember 2008 (250 mm): the installation of the first-
order HDD (September 2008) followed by that of the
second (January 2009), practically halted landslide
motion. By using observations from S4, S4b and S30,
two deformation depths were recorded along the toe of
landslide B at 12 an 25 m b.g.l. respectively (Fig. 3).
As shown in Fig. 9b, the displacement rates have not
decreased after the installation of HDD drains.
CONCLUSIONS
The present work contributes to understanding
of the efficiency of drainage systems, with particu-
lar reference to drainage wells and horizontal drains:
although the latter are widely used to stabilize land-
slides, there are very few reliable studies reporting
their efficiency (e.g., a
hmed
et alii, 2011). Both land-
slides investigated here are characterized by different
sliding surfaces, the main of which occurs between 10
and 25 m below ground level. By correlating defor-
mations recorded by inclinometers with precipitation,
maximum displacement rates were measured after the
most significant rainfalls during autumn months. Ob-
servations from instrumental site monitoring after the
installation of drainage systems allow the following
considerations on displacement rates:
- Landslide A: drainage wells installed about four
Fig. 8 - Cumulate displacement rates in 4 inclinometers in
landslide A (S6, S6b, S34, and S22). Inclinometers
S6, S6b, S34 are located close to drainage system,
while the inclinometer S22 is located about 120 m
below the drainage system (the location of incli-
nometers is shown in Fig. 3). a) data registered at
inclinometers S6, S6b, S34 at a deformation depth
of 11 m b.g.l.; b) data registered at inclinometers
S6, S6b, S34 at a deformation depth of 16 m b.g.l.;
c) data registered at inclinometer S22 at a defor-
mation depth of 11 m b.g.l.
Fig. 9 - Cumulate displacement rates in 4 inclinometers in
landslide B (S27, S4, S4b, and S30). Inclinometer
S27 is located close to HDD drains while incli-
nometers S4, S4b, and S30 are located in the low-
est part of landslide (the location of inclinometers
is shown in Fig. 3). a) data registered at inclinom-
eter S27 at a deformation depth of 11 m b.g.l.; b)
data registered at inclinometers S4, S6b,and S30
at a deformation depth of 12 m b.g.l.
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EFFICIENCY OF STABILIZATION TECHNIQUES IN ACQUALORETO LANDSLIDE AREA (UMBRIA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
337
substantial reduction in displacement rates in the
accumulation body, far from the drainage system.
In conclusion, single alignment of drainage wells
in the upper part of landslide A cannot be considered
resolutive in reducing the landslide hazard: therefore,
new drainage works in the central part of landslide
mass and in the accumulation zone should be con-
sidered. The horizontal drains in the central/upper
part of landslide B did greatly reduce movement, but
their long-term maintenance, together with new HDD
drains to be installed along the toe of the landslide,
must be pursued in the next few years.
ACKNOWLEDGEMENTS
The authors wish to thank the Servizio Difesa del
Suolo, Cave, Miniere ed Acque Minerali 2a Sezione - Pi-
ani e programmi per la difesa del suolo - Idrografico Re-
gionale Umbria, who provided the meteorological data.
years ago contributed to reducing landslide move-
ment, at least in the highest part of the main body.
In particular, the prolonged rainy period of 2008-
2010 and recent heavy autumn rainfall in 2012 did
not cause significant movement.
- Landslide B: two orders of HDD drains construc-
ted between September 2008 and January 2009
in the central/upper part of the landslide body did
not allow the rises of groundwater table, impro-
ving slope stability in the short and medium term.
Of note is the fact that the recent heavy autumn
rain in 2012 only produced a displacement rate of
0.17 mm/month. Taking autumn 2004 as a refe-
rence period with rainfall similar to that of autumn
2012, most of the inclinometers in the central part
of landslide body were damaged. This comparison
is interesting, because both seasons considered
were preceded by well known very dry periods.
The effects of HDD drains did not produce any
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