Document Actions

ijege-13_bs-zampieri-adami.pdf

background image
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
507
DOI: 10.4408/IJEGE.2013-06.B-48
INFLUENCE OF THE GEOLOGICAL STRUCTURE ON
A ROCKSLIDE IN NORTHEASTERN ITALY
D
ario
ZAMPIERI & S
ilvano
ADAMI
University of Padova - Department of Geosciences - Via Gradenigo 6 - 35131 Padova, Italy
tion and communication facilities. In such environment
large landslides are infrequent events. In comparison
with the length of human lifetimes, their occurrence is
low and this can produce a false sense of security that
may pervade even the local administrations. Existing
studies (e.g. E
iSbachEr
& c
laguE
, 1984) have demon-
strated that major landslide disasters can be avoided
if historical experience is evaluated. The Alps of Eu-
rope are a good example of region with a large body of
documented evidence of landslide events.
Geological field investigations are essential in
studying landslides. The lack of appropriate geologi-
cal knowledge can lead to failure of the investigation
process (o
StErbErg
, 1979). In recent years, risk as-
sessment has become an important factor in landslide
hazard reduction. In particular, reliable landslide
hazard maps are of paramount importance since they
should indicate where landslides have occurred in the
past, the locations of landslide-susceptible areas and
the probability of future occurrences.
We document the example of the La Marogna
rockslide in the Astico valley (Venetian Pre-Alps,
Southern Alps), which has not been reported on ex-
isting maps of landslide hazard zonation (a
utorità
Di
b
acino
, 2012). This absence should be considered
critical, because the design of a motorway running
along the narrow valley doesn’t take into account the
vulnerability, elements at risk and specific risk associ-
ated with such potential instability.
ABSTRACT
In the Astico Valley (Venetian Pre Alps, north
east Italy) a rockslide of approximately 10x10
6
m
3
occurred in conjunction with the Verona earthquake
(03.01.1117, I
0
IX MCS, M 7.0). The rockslide seems
to have been favoured by the downhill dip of the car-
bonate beds and the deposit dammed the narrow valley
originating a lake, later emptied by the river incision
of the landslide deposit. Upwards of the crown more
than 10x10
6
m
3
of rocks are still hanging on the valley.
Here we present the results of a preliminary geo-
logical analysis of the slope, showing that the failure
surface corresponds to a thrust surface with a stair
case trajectory only partially coinciding with the beds,
which has been reactivated by the rockslide.
Moreover, the kinematic analysis shows that the
scarp of the landslide could be involved in a quite
huge new landslide.
Along the Astico valley a motorway has been de-
signed, with a viaduct and service areas just at the foot
of this potential landslide. The field investigations
suggest this potential landslide could have a high im-
pact on such infrastructures and that a careful stability
analysis is needed for an appropriate risk assessment.
K
ey
words
: rockslide, thrust, ramp-flat system
INTRODUCTION
Valleys in mountainous regions have recently ex-
perienced a strong demand for expansion of transporta-
background image
D. ZAMPIERI & S. ADAMI
508
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
THE MAROGNA ROCKSLIDE IN THE
ASTICO VALLEY
The Astico valley is a fluvial incision separating
the Sette Comuni (Asiago) Plateau to the east from
the Tonezza Plateau to the west (Fig. 1). The valley
cuts orthogonally the core of the ENE-trending Spitz-
Campolongo anticline (b
arbiEri
et alii, 2007) related
to the Neogene uplift of the plateaus. The northern
limb of the anticline dips towards the valley, since in
its upper part the valley runs WNW changing its direc-
tion from a N-S trend across the fold.
The inclined attitude of the Dolomia Principale
beds represents a structural element favouring sliding
of rock masses along bed discontinuities. Indeed, a
rockslide of c. 10x10
6
m
3
occurred in recent historical
times, since the valley floor was completely filled by
rocks and debris (La Marogna). The triggering of the
rockslide is related to the Verona earthquake, which
has been the strongest seismic event of the northern
Italy (I
0
IX MCS, M 7.0), occurred on 1117.01.03
(g
uiDoboni
et alii, 2005). Two C
14
dating of timbers
collected at the base of the landslide deposit and one
dating of peat buried beneath alluvial gravels up-
stream of the deposit gave ages consistent with that of
the Verona earthquake (b
arbiEri
et alii, 2007).
The La Marogna deposit is composed of a main
body due to a rockslide coupled with falls and a fan-
shaped body due to a later process, probably a rock/
debris avalanche. In the High Middle Age the tract of
the Astico valley affected by the rockslide was prob-
GEOLOGICAL SETTING
The Venetian Pre-Alps are part of the Eastern
Southern Alps, a SSE-vergent retro-belt of the Alps
shortened and uplifted in Neogene times. The South-
ern Alps represent one of the best-preserved passive
continental margins exposed in a mountain belt.
The rifting begun in the Late Triassic and ended in
the Middle Jurassic, when the drifting phase begun
(b
Ertotti
et alii, 1993). Extension was controlled by
a set of c. N-S trending normal faults, which are now
exposed and well recognizable in the field. During
the development of the fold- and thrust-belt these
normal faults were reactivated as strike-slip faults
and acted as lateral or oblique ramps of the thrust
sheets (Z
ampiEri
& m
aSSironi
, 2007).
The Neogene uplift of the Pre-Alps has widely ex-
posed the 800 metres-thick unit of the “Dolomia Prin-
cipale” (Hauptdolomit). This Late Triassic shallow
water deposit is composed of a lower unit of peritidal
well-bedded carbonates and an upper unit of thicker
subtidal beds (b
oSEllini
& h
arDiE
, 1988). Given
its thickness, the Dolomia Principale constitutes the
frame of the Pre-Alps plateaus, which are separated by
deep valley incisions with steep slopes.
Fig. 1 - Location of the study area. The yellow line out-
lines of the La Marogna accumulation; the red
line is the contour of the rock mass upstream of
the crown (La Gioia) potentially involved by a
future landslide. The white line is the de
signed
motorway trace (continuous: surficial; dashed:
underground)
Fig. 2 - Oblique view of the La Gioia upper slope and the
upper part of the La Marogna accumulation. The
rockslide occurred on the northern limb of an an-
ticline fold, of which the southern limb is visible
in the background (upper right corner of the pic-
ture). DP: Dolomia Principale. The white circle is
a temporary water spring
background image
INFLUENCE OF THE GEOLOGICAL STRUCTURE ON A ROCKSLIDE IN NORTHEASTERN ITALY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
509
ded carbonate rocks have been described by Z
ampiEri
&
m
aSSironi
(2007) only few kilometres to the SW.
LA GIOIA NORTH FACE
La Marogna crown, called La Gioia, is a very steep
rock wall ENE trending and reaching 100 metres in el-
evation. The wall is affected by some set of inclined
and sub-vertical faults and fractures parallel and at
high angle to the wall face. In particular, the scarp and
the upstream slope are dissected by a main subverti-
cal fracture trending N340° recognizable also on the
east-facing slope (Fig. 2). Therefore, La Gioia scarp is
divided into two parts: an eastern part, which represents
a recess, and a western part corresponding to a salient
(Fig. 4). The eastern part of the wall shows an inclined
fault zone dipping 70° towards ENE. The geometry
of the fault, which is characterized upwards by splay
faults, typically points to a normal fault according also
to the kinematic indicators collected on the fault plane.
In the Venetian Pre-Alps such faults affecting
Mesozoic rocks are generally of Jurassic age, but nor-
mal faults of Palaeogene age has been also reported
(Z
ampiEri
, 1995; Z
ampiEri
& m
aSSironi
, 2007).
The main sets of sub-vertical fractures affecting
the La Gioia wall trend N50° (K2), N70° (K3), N310°
(K4), N340°-360° (K5), the most persistent being the
K5 set, which controls the development of straight
and deep incisions affecting the slope above the land-
slide crown (Fig. 4). The slope is segmented into two
main blocks separated by a prominent fracture along
which a 15 metres wide trench has developed. Minor
sub parallel trenches have also found on the eastern
ridge (Fig. 4).
ably free from permanent settlements. The accumula-
tion dammed the valley and a lake originated upstream.
In 1278 the natural dam collapsed and an alluvial event
damaged the road, a church and a spinning-mill just
downstream (p
Erin
, 1899); eventual casualties are not
reported. The emptying of the lake allowed the re-es-
tablishment of a road along the valley floor, which re-
mained the only artifact until the end of the 20
th
century.
GEOLOGICAL STRUCTURE
LA GIOIA EAST FACE
The structure of the slope where the Marogna rock-
slide occurred is the northern limb of an anticline fold
where beds, dipping towards NNW, increase their dip
angle towards the valley floor. Upstream of the main
scarp (elevation from 750 m to 1169 m), the beds dip
less than 20°. At the base of the La Gioia main scarp (el-
evation from 750 m to 850 m) the beds dip 20° to 25°,
while downstream the dip angle progressively increases
up to 50° (elevation 550 m) (Fig. 2).
The N10° trending face delimiting to the east the La
Gioia slope is a c. 150 metres-high wall, on which the
Dolomia Principale formation is well exposed. A careful
investigation of the wall face shows that a thrust with
typical “stair case” trajectory (r
ich
, 1934) affects the
fold limb (Fig. 3). The geometry of this type of thrust
sheets is the product of rigid-body translation with mi-
nor internal bending strains as the sheet moves over the
ramp-flat structure of the fault surface (e.g. S
uppE
, 1983).
In a contractional context ramps are steeper reverse fault
tracts connecting flats (fault tracts subparallel to the bed-
ding) at different stratigraphic levels. Ramps may also
give rise to thrusts with opposite sense of displacement
(backthrusts). Typically, ramps tend to form in stiff lay-
ers, while flats or detachments in weak ones.
This is what one can observe on the east-facing
wall of the La Gioia slope (Figs. 2 and 3). More in
detail, the lower exposed flat is located at the tran-
sition between the well bedded lower peritidal unit
of the Dolomia Principale and the upper subtidal
unit, which is characterized by much thicker bed-
ding (Fig. 2). The NNW dipping ramp above this flat
cuts through the thick and relatively stiff beds of the
dolomite rock, giving rise also to minor SSE dipping
ramps (backthrusts) (Fig. 3).
The origin of the thrust can be referred to flexural
slip, i.e. slip along bedding interfaces during folding. In
the same area, ramp-flat geometries at a fold core in bed-
Fig. 3 - N-trending face of the eastern slope of the La Gio-
ia showing the staircase geometry of a main SSE-
vergent thrust with flats and ramps. HW: hanging
wall, FW: footwall
background image
D. ZAMPIERI & S. ADAMI
510
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
wall lying above the slickenside shows a 1-meter thick
damage zone with very poor strength. The striae are also
recognizable looking inside the cavities below and later-
ally to the wall, not only in the depletion zone. This latest
observation, along with the other features of the sliding
surface, suggests that on the upper part of the surface of
rupture sliding has occurred on a pre-existing fault sur-
face cutting the beds, i.e. on a thrust ramp surface.
Water circulation along the upstream prolongation
Several trenches at low angle to the trend of the
main scarp affect the upper slope, confirming the per-
vasiveness of the K2 set.
At the base of the main scarp, where the beds dip
20°- 25° towards NNW, the sliding surface is well ex-
posed and cuts the beds since it has a dip angle of 37° to-
wards N310° (K1 in Fig. 4). This plane has a striated sur-
face locally coated by a few cm-thick breccia and shows
lineations plunging 35° towards N337°. The rock of the
Fig. 4 - a) Structural sketch of the La Gioia upper slope of the La Marogna rockslide. b) Plots (lower hemisphere) of the main
fracture/fault sets recognized on the eastern sector of the La Gioia upper slope and of the slope face with the failure plane
background image
INFLUENCE OF THE GEOLOGICAL STRUCTURE ON A ROCKSLIDE IN NORTHEASTERN ITALY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
511
by the quarrying of the landslide accumulation the pre-
liminary project has designed a viaduct, a toll station,
a maintenance centre, a service centre and a car park.
The lack of knowledge about the geological set-
ting of the slope generating the rockslide has probably
led to underestimate the risk. Given the effects of the
1117 rockslide, a new event with comparable volume
of rocks could produce similar effects, i.e. sudden
complete infill of the valley bottom with destruction of
any development structure.
The results of the geological investigations of the
La Marogna rockslide provide the basis for the stabil-
ity analysis of the La Gioia slope. The structural set-
ting of the slope presented here suggests that an ac-
curate site characterization including identification of
the geometry of relatively homogeneous zones and the
constitutive properties of the material within the zones
(with laboratory testing) is needed.
In addition, accurate investigations and modelling
are needed in order to perform the risk assessment be-
fore to plan the development of the La Marogna area.
ACKNOWLEDGEMENTS
This research was executed within the research
project GEORISK (n. STPD08RWBY_001) of the
University of Padova. We thank R. Genevois, M. Mas-
sironi and an anonymous reviewer for their construc-
tive criticism.
of the sliding surface is shown by the occurrence of a
temporary water outflow, which locally cleans up the
debris accumulated on the slip surface at the base of the
main scarp (Fig. 2).
DISCUSSION AND CONCLUSIONS
The analysis of the natural cross section exposed
on the N10° trending face of the La Gioia (Figs. 2 and
3) shows that in its lowermost part the La Marogna
failure surface is localized along a folded dolomite
bed, while in the middle and upper parts it cuts
through the beds (Fig. 5). Therefore, the failure sur-
face was not controlled by the downhill dip of the bed-
ding surfaces, as an inaccurate analysis could suggest.
The recognition of slickenlines inside cavities
located along the surface of rupture uphill and later-
ally to the depletion zone suggests that the slip surface
originated along a pre-existing thrust fault, i.e. along a
plane of weakness.
The sliding surface corresponds to different
tracts of a thrust plane, parallel to the beds (flat) in
the lower slope, while truncating them (ramp) in the
upper part. The flat and the ramp corresponding to
the failure surface are part of a thrust structure af-
fecting all La Gioia-La Marogna slope, i.e. the entire
northern limb of the Spitz-Campolongo anticline.
Other secondary discontinuities such as inclined
faults and subvertical fractures contribute to the
structural fabric of the slope.
In recent years the La Marogna slide deposit was
quarried on the left bank of the Astico creek, while the
mining of the right slope of the valley is ongoing. This
activity has produced a flat area on valley floor, which
is now suitable for development, since the Basin Au-
thority (public agency charged to define the zone of
respect for geologic and hydrogeologic hazards) didn’t
include La Marogna rockslide in the regulated areas,
because its age > 300 y led to classifying it as a natu-
rally stabilized “paleo-slide”.
In the meantime, a plan for construction of an inter-
regional motorway (A 31 Nord) along the valley has
been proposed. More in detail, in the flat area produced
REFERENCES
a
utorità
Di
b
acino
(2012) - Piano Stralcio per l'assetto idrogeologico dei bacini dei fiumi Isonzo, Tagliamento, Piave, Brenta-
Bacchiglione e corrispondenti misure di salvaguardia. http://www.adbve.it/Documenti/brenta_bacchiglione2.htm
b
arbiEri
g., c
ucato
m., D
El
p
iEro
W., D
E
Z
anchE
v., g
ianolla
p., g
ranDESSo
p., m
iEtto
p., r
oghi
g., S
chiavon
E., S
tEfani
c., v
iSonà
D., Z
ampiEri
D. & Z
anfErrari
a. (2007) - Note illustrative della Carta Geologica d'Italia alla scala 1:50.000 -
Fig. 5 - Natural cross section of the La Marogna rockslide
showing the relationship between bedding and
sliding surface
background image
D. ZAMPIERI & S. ADAMI
512
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
Foglio 082 Asiago. APAT-Servizio Geologico d'Italia - Regione del Veneto, 1-135, Firenze: S.EL.CA.
b
Ertotti
g., p
icotti
v., b
Ernoulli
D. & c
aStEllarin
a. (1993) - From rifting to drifting: tectonic evolution of the South-Alpine
upper crust from the Triassic to the Early Cretaceous. Sedimentary Geology, 86: 55-76.
b
oSEllini
a. & h
arDiE
l.a. (1988) - Facies e cicli della Dolomia Principale delle Alpi Venete. Mem. Soc. Geol. It., 30 (1985):
245-266.
E
iSbachEr
g.h. & c
laguE
J.J. (1984) - Destructive mass movements in mountains: hazard and management. Paper 84-16.
Geological Survey of Canada, Ottawa, Ontario, 230 pp.
g
uiDoboni
E., c
omaStri
a. & b
oSchi
E. (2005) - The ‘exceptional’ earthquake of 3 January 1117 in the Verona area (northern
Italy): A critical time review and detection of two lost earthquakes (lower Germany and Tuscany). J. Geophys. Res., 110:
B12309: 1-20.
o
StErbErg
J.o. (1979) - Failures in exploration programs. In: D
oWDing
C.H. (
ED
.). Site characterization and exploration.
American Society of Civil Engineers, New York, 3-9.
p
Erin
g. (1899) - Il Miracoloso Crocefisso d’Araceli in Vicenza. Prem. Stab. Tip. S. Giuseppe, Vicenza
r
ich
J. l. (1934) - Mechanics of low-angle overthrust faulting as illustrated by Cumberland thrust block, Virginia, Kentucky and
Tennessee.
Bull. Am. Ass. Petrol. Geol., 33: 1643-1654.
S
uppE
J. (1983) - Geometry and kinematics of fault bend folding. Am. J. Sci., 283: 684-721.
Z
ampiEri
D. (1995) - Tertiary extension in the southern Trento Platform, Southern Alps, Italy. Tectonics, 14: 645-657.
Z
ampiEri
D. & m
aSSironi
m. (2007) - Evolution of a poly-deformed relay zone between fault segments in the eastern Southern
Alps, Italy. In: c
unningham
D. & m
ann
p. (
EDS
). Tectonics of strike-slip restraining and releasing bends. Geol. Soc.,
London, Special Publication, 290: 351-366.
Statistics