Interaction between landslides and man-made works

Landslides and Engineered Slopes – Chen et al. (eds)
© 2008 Taylor & Francis Group, London, ISBN 978-0-415-41196-7
Interaction between landslides and man-made works
G. Urciuoli
Università di Napoli Federico II, Naples, Italy
L. Picarelli
Seconda Università di Napoli, Aversa, Italy
ABSTRACT: The interaction between landslides and man-made works is a major issue in risk assessment.
This paper reviews data and general approaches generally adopted, seeking to summarize and rationally classify
experience. Case studies from Italy are reviewed, concerning both slow slope movements and rapid catastrophic
landslides.
1
FOREWORD
In recent decades the growth of population, structures,
infrastructures and lifelines has been responsible for
an enormous increase in landslide risk in many parts
of the world. As a consequence, land management
and planning of new settlements and activities require
careful and reliable risk assessment. Unfortunately,
current approaches to analysing the consequences of
landslides on exposed property are inadequate. In
addition, experience in such matters has not yet been
summarised or organised.
This paper seeks to make a contribution on the
above issue and deals with the subsequent topics:
– the cinematic nature of landslides, (including the
range of velocities, run-out and duration of movement) as well as their relationship with their mechanisms;
– the impact of landslides on man-made works, as a
function of the cinematic features of the landslide;
– the damage induced on exposed property as a
function of the type of landslide;
– the management of areas at risk and the control
works to be adopted for risk mitigation according
to the type of landslide.
Italian sentiment and experience of landslides and
of their effects on man-made works is thoroughly
reviewed, summarised and classified by some tables
and figures.
2
INTERATION BETWEEN LANDSLIDES
AND MAN-MADE WORKS
Landslide velocity and run-out are major factors in
risk assessment, thus modern landslide classifications
include an indication of likely ranges of peak velocity
(Varnes, 1978). Nowadays, the Cruden and Varnes
(1994) classification (tab. 1) is the most widely used
in the literature.
This classification allows for the variety of the geomaterials involved and the mechanisms of rupture and
of post-rupture movement: there are landslides, such
as rock-falls, flow-like landslides and lateral spreads
provoked by liquefaction of the basal formation, which
display high velocity and run-out, and landslides, such
as reactivated slides and mudslides and lateral spreads
in clay, which display an extremely low displacement
rate (tab. 2). In addition, several intermediate situations exist. For example, there are slope movements,
classified by Varnes (1978) in the special category
of complex landslides, whose features change during
movement: falls which turn into debris flows, slides
which turn into mudslides, and so on. A rather special case is that of mudslides whose velocity slowly
changes from rapid or moderate to extremely slow
while its style turns from flow to slide. In fig. 1, cases
Table 1. Classification of landslides based on velocity
(from Cruden and Varnes, 1994).
Velocity
Class
Description
m/s
1
2
3
4
5
6
7
Extremely rapid
Very rapid
Rapid
Moderate
Slow
Very slow
Extremely slow
5 m/s
3 m/minute
1.8 m/hour
13 m/month
1.6 m/year
16 mm/year
1301
5
5 · 10−2
5 · 10−4
5 · 10−6
5 · 10−8
5 · 10−10
Table 2.
Landslide types and typical cinematic characters.
Class Description
Typical length of
run-out
Landslide type and materials
Typical slope morphology
1
Extremely
rapid
Up to kilometres Rock avalanches, flowslides (saturated loose
sands and non-plastic silts),debris flows
2
Very rapid
3
Rapid
4
Moderate
5
Slow
From hundred of
metres to a few
kilometres
Hundreds of
metres
Up to hundreds
of metres
Ten of metres
6
7
Very slow
Metres
Extremely slow
Cliffs and very steep slopes, often
characterized by deep drainage
network
Rockfalls, debris flows, rock slides,
mudslides (after collapse)
Topplings, mudslides (after collapse)
Fractured rock cliffs and steep to
relatively gentle slopes
Mudslides, first-failure slides in brittle sands
and OC clays
Mudslides (long-term), undrained first-time
Gentle slopes
slides in clays, reactivated slides in clay
Active rotational and translational slides in clay
Active translational slides, lateral spreads in
Various geomorphologic
clay and in rock
conditions
15,0 cm/g
A
MARINO
(SITE 1)
DE NICOLA
(SITE 2)
ACQUA DI LUCA (SITE 3)
measured
5,0 cm/g
likely
B
0,4 cm/g
10 year
1 year
C
D
1900
1908 1916 1924 1932
1941 1949 1957 1965 1973 1982 1990
?
Figure 1.
2004).
1991
1992
1993
1995
1996
1997
1999
2000
2001
2003
Displacement rate in cm/day against time for three mudslides observed in the Basento Valley (Pellegrino et al.,
of long-lasting observed mudslides in the Basento
Valley are reported: strong variations of velocity are
shown (Pellegrino et al., 2004).
An attempt to relate velocity and run-out to the
mechanism of movement is made in tab. 2. As shown,
in general, there is some relationship between the
displacement rate and the other factors listed above.
Typically, rapid landslides involve brittle materials, such as rock, OC clay or loose silts and sands
when subjected to undrained shearing. Indeed, the fall
in strength after a peak causes an unbalanced force
to rise, which is responsible for acceleration of the
landslide body.
In fig. 2 the cinematic evolution through the collapse of the S. Barbara open pit (Bomba site), in OC
clays, and Allori open pit, in ductile fissured clay, are
reported (D’Elia, 1984). Prior to collapse, the measured velocity was 0.3 metre/minute in Bomba site and
5 · 10−5 metre/minute in Allori site, as a consequence
of different stress-strain relationships of soils.
Because their high peak strength, brittle materials
typically form steep slopes. In the case of long slopes
Figure 2. Displacements versus time, measured before the
collapse, of two cuts: in brittle clayey soil (Bomba) and in
ductile soil (Allori) (D’Elia, 1984).
rupture leads to a sharp increase in the post-failure
velocity. Due to the high kinetic energy and inertia
possessed by the landslide body, this can cover a large
distance, even along a very gentle slope, in a very
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Figure 3. Fosso S. Martino landslide: a) slope cross-section, b) water level measured versus time, c) displacement velocity
versus time (Bertini et al., 1986).
short time. Due to the rather high ultimate strength
and the gentle slope of the accumulation area, arrest
of granular soils is not followed by further movement.
The same does not hold for clay which presents a low
residual strength.
Slow landslides involve ductile materials, such as
slightly OC clay or highly OC clays at residual, which
dissipate into friction all the energy possessed. Due
to low soil strength, the morphology of the slope is
necessarily gentle and the distance covered by the soil
mass very short. For the same reason, any small change
in boundary conditions can reactivate the landslide; in
addition, creep can play some role. Usually movement
is strictly correlated to pore pressure fluctuations as is
clearly shown by data concerning the Fosso S. Martino
landslide (fig. 3). Data from the Italian literature on
velocity of active landslides in clay are reported in
tab. 3.
In general, man-made works can be located only
on gentle slopes. This is the case of: i) infrastructures and lifelines which often must cross unstable zones, because of their continuity, ii) villages
built in zones where risk of landslide was not
detected. Because of slope morphology and, often,
nature of soil (typically, gentle slopes consist of
clay), gentle slopes may be affected by moderate to extremely slow active landslides or by dormant landslides which can be reactivated by heavy
rains, as in the case of the Fosso S. Martino landslide.
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Table 3.
Italian data on landslide displacement rates in ductile soils (Urciuoli, 1990).
Velocity
Site
Type of
movement
Yearly average
Maximum
References
(in Urciuoli, 1990)
Brindisi di Montagna
S. Agata di Esaro
M. Marino
M. De Nicola
M. Acqua diLuca
S. Barbara
S. Barbara
S. Barbara
Valle del Sinni
Vallone Fossate
Vallone Fossate
Assisi
Campegli
Contrada Musa
Fosso S. Antonio
Fosso S. Martino
Motta S. Anastasia
S. Pietro in Guarano
Vagli
Voltaggio
Mudslide
Mudslide
Mudslide
Mudslide
Mudslide
Slide
Slide
Mudslide
Mudslide
Mudslide
Mudslide
Slide
Slide
Slide
Slide
Slide
Slide
Slide
Slide
Slide
0.6–3.6 m/year
1–1.5 m/year
1–2 m/year
0.2 m/year
–
9 cm/month
–
–
–
0.1 m/year
7 m/year
3.5 cm/year
0.5 cm/month
10 cm/month
2–3 cm/year
1 cm/year
3 cm/year
1–1.5 cm/year
4 cm/month
0.33 m/year
0.6 m/day
–
12 cm/day
2 m/month
5 cm/month
0.2 mm/day
2 mm/day
10 ÷ 15 mm/day
1 ÷ 20 m/year
1 cm/day
1 m/mouth
–
1 cm/month
–
–
0.35 mm/day
–
–
1.5 m/day
–
Cotecchia et al., 1986
Allevato et al., 1980
Pellegrino et al., 2004
’’
’’
D’Elia et al., 1984
D’Elia et al., 1984
Esu et al., 1986
Manfredini et al., 1981
D’Elia, Tancredi, 1979
’’
Canuti et al., 1986
Formigoni et al., 1986
Giusti et al., 1986
Bertini et al. 1986
’’
Maugeri Motta, 1986
Cascini, 1986
Baschierì, Gulì, 1956
Cancelli Olcese, 1984
If the landslide is very slow, damage can be tolerated
by the structures whose conditions can be periodically
improved by means of structural maintenance.
Alternatively, the problem can be definitively
solved by control works (tab. 4). In the case of slow
landslides, damage is related to static interaction
between structures and the landslide body that may
induce differential displacements or increase of the
thrust against retaining works (tab. 4). The response
of structures depends on their global deformability. Embankments, for example, can tolerate large
displacements keeping working with frequent maintenance to the pavement. Masonry and reinforced
concrete structures are more vulnerable than embankments and suffer damage, depending on the abundance
of constraints (degree of redundancy) that characterize
the structure (tab. 5).
Urban settlements and most infrastructures and
lifelines cannot be built on steep slopes: man-made
works are typically sited at the foot of slopes. In
these geological contexts, typical slope movements
are rapid landslides, such as rock falls, debris flows
and flowslides. The landslide body possesses a high
kinetic energy which is transferred to the obstacles: the interaction is hence dynamic (tab. 4). The
structures are destroyed and people cannot evacuate
because the velocity of the landslide body is higher
than that of running people. In addition, no stabilizing works can be carried out due to lack of time.
The only way to protect the population is by passive
works built in advance or by efficient early warning
systems capable of predicting in real time landslide
occurrence.
In the case of the well-known catastrophe of Sarno
(1998), flowslides some 100, 000 m3 in volume
reached the foot of the slopes at high velocity (up
10 m/s and more), destroying many buildings.
Victims can also be caused by relatively small
soil masses invading basements of buildings or open
spaces (tab. 4).
Besides landslide velocity and mass, the risk
depends also on the vulnerability of the elements at
risk. For example people are highly vulnerable to
rapid movements. By contrast, slow movements give
the population the possibility to perceive the risk and
evacuate in time (Table 5).
3
RISK MANAGEMENT AND MITIGATION
Management of unstable slopes is a very complex
challenge, both due to the intrinsic difficulties of
the problem and the chronic shortage of financial resources to allocate to environmental problems.
Therefore, the most common solution consists of
non-structural counter-measures (limitations in land
use, town-planning regulations, emergency plans)
that allow activities to continue without slopes being
stabilised (tab. 6).
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Table 4.
Interaction between landslides and man-made works and expected damage.
Types of man-made
works interacting
Class Description with landslides
1
Extremely
rapid
2
Very rapid
3
Rapid
4
Moderate
5
Slow
6
Very slow
7
Extremely
slow
Table 5.
Expected damage and population reaction
(modified, after Cruden and Varnes, 1996)
Mode of interaction
Structures,
Impact, invasion of open
infrastructures
spaces and filling of
and lifelines located
basements
along the landslide
path or at the
foot of the slope (I)
Catastrophic event. Buildings and structures
destroyed by the impact. Many victims.
Evacuation is impossible
As above; some victims. Evacuation is
impossible
(I) + (II)
Buildings and other structures are destroyed or
severely damaged; infrastructures out of
action; lifelines damaged. Evacuation is
possible
Invasion of open spaces and
infrastructures (roads,
railways), increase of
thrust against retaining
structures
Some special structures can be kept working
with maintenance (highway embankments,
banks. . .)
Structures,
Invasion of open spaces and
Stabilization works can be carried out even
infrastructures
infrastructures, increase in
during landslide movement. Some structures
and lifelines located
thrust against retaining
can be working by means of frequent
on the landslide
structures. Deformation of
maintenance works
body (II)
buildings and of other
structures
Increase in thrust on retaining Damage is not destructive. Structures can be
structures. Deformation of
kept working, despite secondary damage
buildings and of other
Movements cannot be detected without
structures
instrumentation. Buildings and structures
are not seriously damaged
Vulnerability of property.
Vulnerability level Fast landslides
Slow landslides
High
Average
Viaducts, tunnels, dams, masonry buildings
Low
Very Low
Human lives, lakes, building basements
Cuttings, masonry buildings, reinforced
concrete buildings, steel buildings
Embankments, viaducts
Embankments, cuttings, aqueducts, electric
pylons, reinforced concrete buildings, steel buildings
Tunnels, aqueducts, gas ducts, electric pylons, Human life, gas ducts, lakes,
dams
When an infrastructure crosses a landslide, one
of the most common counter-measures is to reduce
its performance to reduce risk. In the case of motorways and railways, a reduction in traffic velocity can
be imposed; moreover parts of the road must be closed
to traffic, if necessary. In the case of aqueducts, water
flow can be reduced, to lower the consequences of possible pipe cutting, caused by landslide movement. As
regards artificial lakes, if a fast landslide is recognized
as likely on the slope, lake water level must be lowered, to mitigate the risk associate to formation of high
waves (due to the slide of the landslide into the lake)
which could over top the dam.
Drawing up emergency plans allows the population to continue living in areas exposed to landslide
risk. The main technical part of an emergency plan
is prediction of landslide collapse, for which specialized geotechnical knowledge is required. Prediction
can be based on monitoring of indicators (Damiano et
al., 2008), as displacements, when the movement is
slow enough to allow controls in the phase preceding
collapse, or on monitoring of precursors (such as rainfall) when the pre-failure phase is expected to be very
short.
In the first case, management of slopes should
include a comparison of predicted displacement rates
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Table 6.
Risk management and mitigation.
Class
Description
Risk management
Counter-measures
1
3
4
Rapid
Moderate
5
Slow
6
Very slow
7
Extremely slow
Limitations in land use, town-planning regulations,
reduction in executing man-made works,
emergency plans (strategies for collapse forecasting
and prevention), early warning systems
Limitations in land use, town-planning regulations,
reduction in executing man-made works,
emergency plans
Limitations in land use, town-planning regulations,
downgrading of service level of man-made works,
stabilization works, maintenance of structures
Downgrading of service level of man-made works,
stabilization works, maintenance of structures
Maintenance of structures
Active and/or passive stabilization works
2
Extremely
rapid
Very rapid
Figure 4.
Active stabilization works
Acceleration of monitored landslide, against time to failure (Pellegrino and Urciuoli, 1996).
or acceleration with thresholds for different alert levels. With respect to this aim, Pellegrino and Urciuoli
(1996) analysed data from a large number of slopes
which had been carefully monitored until failure. Interpretation of these data is presented in figure 4 where
the relationship between slope acceleration and time
to failure is shown. The dotted line represents the best
regression of data, whereas the solid line bounds 95%
of them. Three threshold values of the slope acceleration were selected: the first threshold, equal to
1 mm/day2 , corresponds to an average time to failure
of 27 days (when using the lower, more conservative
envelope, the time to failure is 2.2 days); the second
threshold, corresponding to 10 mm/day2 , leads to a
time to failure of 5 days (from the lower envelope the
time to failure is 10 hours); finally, the third threshold,
equal to 100 mm/day2 corresponds to a time to failure
of 1 day (from the lower envelope, the time to failure
is 2 hours). As this paper is devoted to the cinematic
behaviour of landslides, alert systems based on rainfall
control have not been dealt with.
Alert systems can be used as transient countermeasures while structural control works are being
designed and built or, when financial resources are
lacking, they can be assumed as the definitive solution
to the problem.
In table 6 physical counter-measures, such as active
or passive stabilization works, are indicated as suitable
1306
for the different cases examined. Active measures lead
to an increase of the safety factor producing a decrease
in landslide hazard. Passive measures reduce the
risk in some areas, protecting man-made works from
impacts.
4
CONCLUSION
The interaction between landslides and man-made
works mainly depends on the velocity and mass of
landslides. To schematize the problem it is useful
to classify landslides into two groups: fast and slow
movements. The first are extremely hazardous for
human lives and for all structures located along the
run-out of the landslide. In contrast, slow movements
cause damage mainly to structures and infrastructures
built on the slope because of differential soil movements. In all cases vulnerability of man-made works
plays an important role in risk assessment. Classification of landslides into the two described groups is also
useful to establish a proper strategy to protect areas at
risk and for the choice of stabilizing works.
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Picarelli L. 2008. Advanced monitoring criteria for precocious alerting of rainfall-induced flowslides. Proc. X Int.
Symp. on Landslides, Xian, submitted for publication.
D’Elia B. 1984. Comparison between processes of internal
deformation in two cut slopes. Proc. IV Int. Symp. On
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