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1 INTRODUCTION
The North-Eastern part of Italy is subject to high hy-
dro-geological hazard due to its climatic, geological
and geomorphological conformation. An increasing
occurrence of sliding movements is usually observed
in consequence of long and intense rainfalls: for in-
stance, in November 2010, due to the exceptional
rainfall that hit the mountain portion of the Vicenza
Province, almost 500 new landslides have been ob-
served in 48 hours.
In the same way, the Southern part of Poland, lo-
cated in the Carpathians, is an area in which mass
movements lead many technical, economic and social
problems. The number of landslides insisting in the
Polish Carpathians and registered in the national pro-
ject SOPO (Preventive System against Mass-move-
ments) is now estimated at 60,000: they involve a to-
tal surface of 19,000 km2 with a frequency of 2-3
landslides per km2. In the two last decades, the land-
slide processes at global or local scale intensified in
some limited periods (1997, 2000, 2002, 2005 and
2010), generally connected with the occurrence of ex-
ceptional rainfalls. Moreover, at the beginning of XXI
century, it was estimated that one landslide is located
every 5 km of road line and 10 km of railway
(Poprawa & Rączkowski, 2003). According to the re-
port of the Ministry of Environment, only in 2010 in
the Polish Carpathians landslides destroyed about 560
buildings, causing damages to more than 2,260 build-
ings with relative total losses estimated at 2.9 billion
euros.
Therefore, the research for low-cost solutions for
risk mitigation and slope stabilization is fundamental
in both the regions.
A research program developed by the University
of Padova (Italy) in partnership with public agencies,
professional and businesses offices in Italy and Po-
land started in 2012. The research complains the de-
velopment and performance-cost evaluation of an in-
novative technique for landslide stabilization, named
composite anchors.
A brief summary of the general idea of this method
is thus presented, followed by its application in two
selected landslides in North-Eastern Italy and in Car-
pathians area. The first case, Cischele landslide (Re-
coaro Terme, Italy) is a slow-moving translational
landslide activated in 2010 that involves a road and
some houses. The second case study is located in
Ochojno (Poland), where a small rotational landslide
interrupts a provincial road.
2 THE COMPOSITE ANCHORS
2.1 Coupling a self-drilling bar with tendons: an
enhanced anchor bar
Self-drilling anchor bars are a good alternative to tra-
ditional nailing and anchor techniques. To expand
their application field and improve their mechanical
behavior, a special composite bar was developed
(Bisson et al., 2013), coupling a traditional carbon
steel bar with one or more harmonic steel tendons.
This is achieved by inserting the strands in the inner
cavity of the bar, then cementing with a special grout.
Passive composite anchors for landslide stabilization: an Italian-Polish
research program
A. Bisson & S. Cola
Dept. of Civil, Environmental & Architectural Engineering, University of Padova, Italy
P. Baran, T. Zydroń & A.T. Gruchot
University of Agriculture in Kraków, Poland
R. Murzyn
Przedsiębiorstwo Geologiczno-Inżynieryjne GEO-INŻ-BUD, Wiśniowa, Poland
ABSTRACT: The North-Eastern Italy and the Polish part of the Carpathians are subject to high hydro-geolog-
ical hazard due to its climatic, geological and geomorphological conformation, often inducing damages or de-
struction of residential development, economic and transport infrastructures. Thus, the search for cost-effective
solutions for risk mitigation and slope stabilization is fundamental in both the regions. The paper introduces a
research program that developed an innovative technique for landslide stabilization, named composite anchors,
improvement of the Soil Nailing, which consists of installing special passive sub-horizontal reinforcements in
unstable slopes. After a brief overview on the main technical aspects, two real applications of passive reinforce-
ments on slow-moving translational landslides are proposed.
The installation of a tendon locking head completes
the system (Fig. 1).
The composite anchor installation develops in the
following steps:
1) Installation of the self-drilling bar: it is obtained
like for a normal self-drilling bar, by roto-percus-
sion of the bar, provided of a lost drill bit, up to the
design depth; in the meanwhile, a cement grout is
injected from the drill bit and acts at the same time
as a purge fluid and hole support. The bar is fully
grouted outside, thus developing a frictional soil-
grout interface.
2) Installation of the tendons: before the inner grout
hardening, the harmonic steel strands are manually
inserted into the cavity of the bar. The injection of
water from a washing tube permits to clean the
cavity in the external portion, thus dividing the ten-
dons in two parts, the foundation length and the
free length.
3) Eventual pre-tensioning of the tendons: after the
tendons foundation has been created, and after
grout hardening, the strands can be tensioned and
connected to the external anchoring structure by a
special locking head. Thus, different configura-
tions can be obtained: an “active” composite an-
chor whether the tendons are pre-stressed, a “pas-
sive” composite anchor whether the tendons are
just connected to the bar without pre-stressing.
Assuming congruence and equilibrium of the cou-
pled system, i.e. considering the strains of the bar and
the strands equal, the composite bar may be charac-
terized by an equivalent axial stiffness defined as:
ttbb
eq AEAEEA
(1)
where Eb and Et are the Young moduli of bar and ten-
don respectively, and Ab and At are their net steel sec-
tions.
With respect to the traditional anchor bars, compo-
site anchor has many geotechnical and technological
advantages:
- minor cost at constant mechanical properties;
- high ultimate tensile strength and low elongation
(serviceability);
- better durability (minor cracking, better protection
from corrosion);
- easy transport and quick installation;
- anchorage length adaptable to different geological
and geotechnical in situ conditions;
- higher flexural inertia and good continuity given
by the strands to the full reinforcement (improved
if compared to simple coupling sleeve).
Composite anchors can be used in consolidation of
excavations, soil and rock slope reinforcements,
foundations, and landslide stabilization too.
2.2 General idea for slow-moving landslide
stabilization
Passive reinforcements represent a cost-effective
technique for slow-moving landslide stabilization
(Cola et al., 2012). Since in these applications the ax-
ial and bending forces developed in the reinforce-
ments may be huge, composite anchors are a good al-
ternative to reach high strength reducing installation
time and costs, as in the recently proposed “floating
anchor” technique (Bisson, 2015). The technique, de-
veloped as an improvement of the Soil Nailing, con-
sists of installing passive sub-horizontal reinforce-
ments in unstable slopes, to increase the resistant
forces contrasting the sliding. The reinforcements are
passive self-drilling or composite bars grouted along
the entire profile, with a sufficient foundation in the
bedrock, and coupled with individual external con-
crete slabs (i.e. the “floating” element). The slabs,
having appropriate shape and size, are not in connec-
tion among them.
If some slope movements occur, axial forces in the
passive reinforcements develop because of the shear
stresses growing at the soil-grout interface in the
slow-moving mass: consequently, the axial forces
contrast part of the forces inducing the instability,
thus reducing the landslide evolution process until it
completely stops. Since the axial head force at the ex-
ternal slab is small, the system does not require a con-
tinuous facing, and, whether the slope deforms, the
slabs may be englobed inside (Fig. 2).
As in the Soil Nailing, the design capacity of a
floating anchor depends on the available frictional
strength at the soil-grout interface, as well as the size
and the tensile strength of the bar itself. It must there-
fore ensure that:
1) The tensile strength of the steel bars is sufficient to
withstand the maximum developed axial stress; the
total stabilizing force Qa generated by each ele-
ment is the sum of the head force absorbed by the
floating plate Qp and the integral of the friction
stresses activated along the soil-grout interface in
the active zone of the slope:
a
L
upa dxxDQQ
0
)(
(2)
where D is the diameter of the bar with the cement
grout, La the length of the anchor in the active zone
Figure 1. Installation scheme of a composite anchor.
and τu(x) the shear strength at the soil-grout inter-
face at the abscissa x. The calculation of τu(x) can
be performed by extending the two methods pro-
posed by Bustamante & Doix (1985).
2) The length of each bar within the passive zone, that
is, the portion of the bars that extends beyond the
potential or actual slip surface, is sufficient to pro-
vide a pull-out resistance equal to the total stabi-
lizing force Qa generated in the bar (Geoguide 7,
2008).
3) If the slope movements do not completely arrest,
the axial forces developing in the reinforcements
may vary in function of the viscous effects. To take
account of this, a model of long-term soil-rein-
forcements interaction, similar to those developed
by Gudehus & Schwarz (1984) and Puzrin &
Schmid (2012) for dowels or retaining walls on
slopes, has been proposed (Bisson, 2015) assum-
ing a viscous behaviour of the soil. This model re-
lates the displacement rate and the strength mobi-
lized on the slip surface, assessing the reduction of
the landslide velocity as a function of the number
of anchors installed and some geometrical param-
eters (gradient of slope and reinforcements) and
the viscosity index of the soil.
The main advantage of floating anchors is their
flexibility, since anchors can support the slope defor-
mations without losing effectiveness, finding their
equilibrium condition without cracks or structural
failure, unlike traditional rigid works such as gravity
or micropile walls. The intervention can also be cali-
brated in progress by adding other reinforcements or
coupling with other techniques, according to the ob-
servational method. The reachable large installation
depths (up to 55 m), the low environmental impact
and the easy and quick installation make this tech-
nique cost-effective and interesting.
The composite anchor applied to landslide stabili-
zation has been analysed both in physical and numer-
ical models, and the real-scale behaviour was also an-
alysed by installing and monitoring them in some test
sites in Italy and Poland, such as Cischele and
Ochojno landslides.
3 FIRST CASE: CISCHELE LANDSLIDE
(RECOARO, ITALY)
3.1 Site description
The exceptional rainfall that affected the Venetian
Prealps in Vicenza province in November 2010 acti-
vated the movement of some houses and of a portion
Figure 2. Working scheme of the “floating anchor” technique
with composite bars and external slab.
Figure 3. Cischele site localization (maps.google.com).
Figure 4. Cischele landslide: surveys and stabilization works.
of the road that connects the Cischele hamlet with Re-
coaro Terme (Fig. 3 and 4). The area, approximately
120 m wide and 180 m long, is located at an altitude
between 550 and 600 m a.s.l. and has a mean inclina-
tion of 24°.
The landslide is a slow-moving translational phe-
nomenon with a strong correlation between displace-
ments and change in pore water pressure. Many visi-
ble signs of slow and continuous movements have
been observed in the structural elements of the houses
and fractures in the walls.
3.2 Geology
Considering the geological stratigraphy, the area is
based on an ancient crystalline basement consisting
of quartz-phyllite of the Recoaro subalpine area.
Above it, there is a layer of Val Gardena sandstone, a
sedimentary rock composed of clastic deposits, such
as quartz and feldspars sands and silts. At the top,
there is the Bellerophon formation, consisting mainly
of limestone, often minutely decayed, with frequent
interbedded silty clays in the lower part. A major tec-
tonic action affects the area, due to compression time
of the Alpine orogeny. This makes the Schists crys-
talline basement visible somewhere on the surface.
The unstable area is located between two major verti-
cal faults with a prevailing backing horizontal move-
ment.
In order to characterize the stratigraphic profile of
the area, three continuous core surveys were carried
out in 2011 (S1, S2, S3), two in 2012 (S4, S5) and
four in 2014 (A, B, C, D) (Fig. 4). In the central part
of the slope, a 10-12 m thick cover, constituted by
completely weathered Bellerophon Limestone, lies
above a clayey silt and sandy clay layer originated by
the alteration of Val Gardena Sandstone. Up to a
depth of 20-30 m from the ground surface, a bedrock
of low to medium altered phyllite was found. At the
landslide toe, below the first 5 m of backfill clay, a
strongly altered phyllite in a reddish silty clay matrix
exists, but Sandstone or Bellerophon layers are not
noticed (Fig. 5).
In order to monitor the landslide movement over
time, two inclinometers were installed within holes
S1 and S3 and displacement were registered in the pe-
riod January-May 2011 (Fig. 6). The acquired data
confirm the absence of movements in S1, located in
the stable zone over the landslide crown, while in S3
the total displacement significantly increased from 5
to 13 mm in occurrence of the heavy rains of March
2011, thus indicating a close dependence of gravita-
tional movements to the groundwater level inside the
landslide body. The trigger occurs along with partic-
ularly intense rainfall events and the slip surface is
individuated at the contact between the Bellerophon
formation and the Val Gardena Sandstone.
The rains generate a very rapid rise in the water
table, with also the appearance on the slope of a num-
ber of temporary springs. For instance, during the rain
event of 13-17 March 2011 characterized by a cumu-
lative rainfall equal to 231 mm with a peak of 141 mm
on 16 March, the piezometer installed near inclinom-
eter S2 showed a water table rising 3 m in some hours
(Fig. 7) with a trend in delay of some hours respect
the rain intensity. The discharge of water from the
body of the landslide is not extended in time being
completed within 2-3 days after the event (Provincia
di Vicenza, 2013), fact that evidences a relatively
high permeability of the slope mass.
3.3 Soil properties and basic stability analysis
To obtain the geotechnical parameters of the involved
soils, the values obtained from direct shear tests on
samples of siltstone and sandstone, taken at depths of
Figure 5. Cischele landslide: geological cross section (Bisson,
2015).
Figure 6. Inclinometers S1 and S3 (Darteni, 2013).
Figure 7. Correlation between groundwater rising and daily
rainfall during the 13-17 March 2011 event (Darteni, 2013).
11 to 18.5 m, were considered. Additional geotech-
nical parameters for the deeper materials were ob-
tained by standard penetration tests (SPT).
A back-analysis of slope stability was done to as-
sign the geotechnical materials, within the range iden-
tified by the test results. The parameters thus obtained
are reported in Table 1. Stability calculations showed
a current value of safety factor equal to 1.01.
Table 1. Cischele landslide: selected soil parameters.
Layer
Parameters
γ
[kN/m3]
φ
[]
c
[kPa]
Bellerophon limestone
18.0
21.5
5.2
Val Gardena sandstone
20.0
38.0
100
Phyllites
27.0
55.0
100
Legend: γ=self-weight, φ=friction angle; c=cohesion.
3.4 Stabilization works
In order to consolidate the unstable slope and secure
the housing and the main road, it was initially planned
a meteoric water control system in the upstream side
of the road, coupled with a large diameter draining
well to lower the water table. However, the geological
surveys highlighted a particularly critical stratigraphy
at the suitable position for the well. This would make
it very expensive. Therefore, the contracting authority
opted for a slope consolidation with drains and float-
ing anchors with composite bars. Given the complex-
ity of the geological context and the many uncertain-
ties that affect the slope behavior, this solution
allowed to carry out the work in successive stages,
applying the observational method, as provided in the
Italian Technical Law (NTC, 2008). The intervention
may also be integrated in progress with more other
anchors if it will be proved not sufficient.
From June to December 2014 stabilization works
were carried out (Fig. 8). The intervention consisted
in 33 composite anchors, 40-50 m long. Due to the
considerable depth, special enhanced composite bars
were installed. They consist of a self-drilling hollow
steel bar with a diameter of 76 mm, coupled with
7x0.6” pretensioned tendons cemented inside.
Thanks to the presence of inner tendons, these an-
chors have an ultimate tensile strength increased from
1400 to 3000 kN with an increment of cost less than
20% with respect to the self-drilling bar of origin. At
the head of each anchor, a prefabricated reinforced
concrete slab with frustoconical shape and outer di-
ameter of 1.5 m was connected (Fig. 9), to retain and
contrast the slope thrust. The reinforcements were ar-
ranged in a single row with horizontal spacing of 5 m,
one series in the Northern portion of the landslide, be-
low the road, and one series in the Southern portion,
downstream of the housing (Fig. 4).
Further stability analysis computed a factor of
safety of 1.05.
Figure 8. Installation of composite anchors at Cischele landslide
(2014).
Figure 9. Installed floating plates at Cischele landslide (2015).
3.5 Monitoring system
In order to analyze the real-scale soil-reinforcements
interaction and assess the effects of interventions with
composite bars in floating anchors, a monitoring sys-
tem was installed. Two load cells record the axial
force applied to two floating plates, while, in order to
monitor the tension along the passive bars, a specifi-
cally designed strain gauge strip was connected inside
two anchor bars (Fig. 10), thus reconstructing the en-
tire stress field of the reinforcements. The tensional
data are crossed with the observations of the incli-
nometers and piezometers placed in the landslide
body, thus evaluating the correlation between varia-
tions in groundwater and superficial/deep move-
ments. A topographic survey at regular time intervals
complete the monitoring, thus recognizing the move-
ments of the floating plates and some fixed points in
the landslide body in time.
Since the monitoring period has been short, the ob-
served data are not sufficient to make full general
conclusions. Even so, the preliminary results give no
significant post-work displacements, even if in spring
2014 the Vicenza Province was interested by another
exceptional rainy period that activated many other
mass movements. This fact foreshadowed the viabil-
ity and technical efficiency of the method.
Figure 10. Cischele landslide: composite anchor (a) and strain
gauge strip scheme for axial force monitoring (b).
4 SECOND CASE: OCHOJNO LANDSLIDE
(CRACOW, POLAND)
4.1 Site description
The analysed landslide area, 4.8 hectares extended, is
located near Ochojno village in the Malopolska prov-
ince (Poland). The map in Figure 11 shows the gen-
eral location of the area. The mass movement encom-
passes a part of the county road No. 2167K, some
residential buildings, cultivated and recreation fields,
as well as technical networks: water, gas and electric
power lines are located within landslide borders.
Because of mass movements, the road was seri-
ously damaged and visible cracks appearing on the
wall of one residential building, located above the
slope.
4.2 Geology
For the diagnosis of geological conditions, 6 bore-
holes were made with a depth of 15 to 30 m in the
landslide area. In this area, there are layers of the Cre-
taceous Flysch, covered by Quaternary strata. Flysch
appeared in the form of shale and schist, delaminated
occasionally by thin sandstone inserts. Shale are
green-red, green, gray and gray-green and rugged-
plastic state. Schist are gray, brittle and cracked. The
sandstone inserts are fine and middle grained, light
gray and cracked in some places. Hillside sediments
compose the Quaternary cover, also known as resid-
ual soil cover. They have the typical features of resid-
ual soil profile, because they present a fine matrix var-
ying from pure clays to clayey-sandy silt, with frag-
ments of parent rock: with the increasing depth, a
number and size of rock fragments are increasing as
well, until a typical rock strata are reaching. A thick-
ness of quaternary layers is up to several meters.
Figure 11. Ochojno site localization (maps.google.com).
Figure 12. General view of Ochojno landslide area with incli-
nometers mark I-1, I-2 and I-3 (main stabilized site is marked
by rectangle) (Kos et al., 2015).
Figure 13. Results of inclinometers investigation from 2013 to
2015 (Kos et al., 2015).
In order to investigate the landslide movement
over time, three inclinometers were installed where
the largest mass movements were expected (Fig. 12).
The inclinometer data (Fig. 13) show that the
Ochojno landslide is still active and subjected to
movement with significant scale and dynamics. The
largest movements are registered in inclinometer I-1,
located in the southern part of the landslide at the top
of the active zone, the smallest in inclinometer I-3,
located in periodically active zone of the landslide.
4.3 Soil properties and basic stability analysis
The surface geological layers was separated into 4
layers: anthropogenic soil (I), quaternary colluvium
(II), schist, shales and sandstones in the colluvium
(III), flysch (IV). In Figure 14 one of geological cross
sections is shown.
Laboratory tests were performed to determine the
basic geotechnical parameters: grain size distribution,
density, moisture content and Atterberg limits, or-
ganic content and shear strength parameters. Table 2
shows the selected geotechnical properties of the
soils.
The stability analysis was performed using the soil
parameters reported in Table 2, and the “large block
method”, the latter being based on the Morgenstern-
Price limit equilibrium method (Zhu et al. 2005). Sta-
bility calculations revealed that the value of safety
factor is 0.94. In order to bring the safety factor above
the minimum value required from the Polish code
(Wysokiński, 1991), i.e. at least equal to 1.5, the anal-
ysis showed a need for an additional horizontal com-
ponent of the stability forces of 1100 kN/m.
4.4 Construction structure and monitoring
The following technical treatments were made to re-
build the damaged road infrastructure in the analyzed
landslide area:
a) Installation of 5 floating anchors with composite
bars (Figg. 15, 16) with a length of 33 m each;
b) Build of a French drainage system to drain ground-
water outside of the landslide area;
c) Build of road barrier with reinforced concrete
foundation and top beam for additional road sup-
porting system;
d) Construction of ground collector to transport water
from slope surface outside of the landslide area.
The composite floating anchors constitute the most
important element for the stabilization of the slope.
They were composed by cylindrical concrete plates,
1.8 m large and 0.5 m thick, connected to reinforce-
ment 66 mm steel bars inserted into slope using self-
drilling technique for a length of 33 m and with an
inclination angle of 15°. Inside the bars, 4 tendons
with a cemented part 22 m long were installed in or-
der to reach a total bearing capacity of 3000 kN for
each composite anchor.
Two lines of anchors were built up at 5 m and 2.5
m respectively below the road line. A special steel net
was additionally installed among the anchors and
filled with cobbles.
Further stability analysis revealed the factor of
safety was increased to value of 1.55, so this stabili-
zation technique seems to be suitable.
Stabilization of the Ochojno landslide is still under
construction. Despite the intervention does not allow
the complete stabilization of the whole landslide area,
it enable to use both the two lines of the road, so it is
very useful for people living there. Additionally, the
surface and ground water can be now collected and
directed out of the area. During lifetime, the behavior
Table 2. Ochojno landslide: selected soil parameters for the sta-
bility analysis (Kos et al., 2013).
Layer
Parameter
w
[%]
[g/cm3]
IL
[-]
φ
[]
c
[kPa]
IIa
23.0
2.01
0.10
8
32
IIb
30.0
2.02
0.30
7
39
III
23.0
2.02
-0.01
10
45
IV
17.0
2.03
-0.30
11
55
Legend: w=moisture content; =density, IL=consistency index;
φ=friction angle; c=cohesion.
Figure 14. Cross section through the Ochojno landslide (Kos et al., 2013).
of the intervention will be observed using inclinome-
ter holes. Additionally there are plans for precise dis-
placements measuring of the reinforcement elements.
5 CONCLUSIONS
Composite bars are a special development of self-
drilling bars used as passive nailing technique. Due to
their high strength and ease of installation, they are a
cost-effective system for slope consolidation. A full
theoretical and experimental study has been done on
the interaction of these reinforcements with soil (Bis-
son, 2015). The application of the composite bars in
some stabilization works with floating anchors in real
landslides activated in North-Eastern Italy and South-
ern Poland complete the research. The monitoring of
the stabilized slopes is still in progress, but the pre-
liminary results foreshadow the viability and tech-
nical efficiency of the method.
6 ACKNOWLEDGEMENTS
The Sirive® Special Composite Anchor is an Italian
Patent of Dalla Gassa company (Cornedo Vicentino,
Italy). The European Patent application (EP
20130157515.1) is still pending (2015). The struc-
tural and geotechnical behaviour of these composite
anchors in landslide stabilization (Sirive® Floating
Anchor, EP 2354323 B1, European Patent granted in
2015) has been analysed within a PhD program at the
University of Padova, Italy, thanks to a fund supplied
by Dalla Gassa itself. We thank all those who contrib-
uted to this study, in particular Dalla Gassa company,
Geosoluzioni Engineering, Giara Engineering, Geo-
Inz-Bud professional companies, the Soil Protection
Division of the Vicenza Province and the University
of Agriculture in Cracow.
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Figure 15. Slope stabilization on Ochojno landslide using
float-ing anchors with composite bars (design draw by
courtesy of Draft Spółka Inżynierska S.C.).
Figure 16. Slope stabilization on Ochojno landslide using
floating anchors with composite bars.
