FROM DEEP SEATED GRAVITATIONAL MOVEMENTS TO ROCK

Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
FROM DEEP SEATED GRAVITATIONAL MOVEMENTS TO ROCK AVALANCHES: THE
ROLE OF THE FAILURE MECHANISM IN SUDDEN ROCK SLOPE COLLAPSE.
Ph.D. candidate: LUCA ZORZI
Tutor: Prof. RINALDO GENEVOIS, Dott. MATTEO MASSIRONI
Cycle: XXV
Abstract
This project is focused on understanding the processes of failure evolution (from triggering to propagation up to
emplacement) and deformation mechanisms of Deep Seated Gravitational Movement(DSGSD) and Rock Avalanches (RA) in
massive brittle rock slopes. In particular, the subject of this work is directed to the failure mechanism and the possible run out
of these phenomena, because of their influence on the slope stability (the former) and on the danger of the settlement (the
latter). Triggering factors, failure mechanisms and run out processes will be studied selecting three mass wasting phenomena
in the Easten Alps (Ridnaun Valley Rock Avalanche, Fadalto Rock Avalanche and Vajont Rockslide). It is expected that the
findings obtained through this investigation will enhance our fundamental knowledge on DSGSD and R.A. processes,
particularly with regard to understanding the key stages of the progressive evolution of rock slope failures from a secondary
creep stage to the sudden collapse.
Introduction
Deep Seated Gravitational Slope Deformation (DSGSD), called also creep or rock flows (RadbruchHall,1978;Agliardi et al.,2001), involve large rock volumes and cause important morphologic changes
due to lateral and vertical movements of the rock mass. Different factors such as structural and geological
settings, seismic events and unloading due to alpine-glaciers retreats may control DSGSDs triggering and
development (Varnes et al.,1989; Corominas, 1990; Bovis and Evans, 1996; Agliardi et al., 2001). These
large-scale mass movements have low probability to evolve into catastrophic events, but in the worst-case
scenario, they cause huge damages (e.g. Vajont, Val Pola, Borca di Cadore, etc. ). Indeed, they constitute
important hazards (Bonnard et al.,2004) since they may represent the incipient stage of possible
catastrophic rock avalanches (RA). A RA is characterized by the sudden release of massive volumes of
rock (> 106 m3; Heim, 1932; Scheidegger, 1973) which is reduced to minute grain size and resembles an
avalanche. Considering a specific area, the danger of these catastrophic events is determined by the
combination of the time factor (“when”will they occur?), and the areal extent of their propagation
(“where” will they arrest?). Due to their parossistic nature, it is extremely difficult to predict when the
catastrophic failure will occur. In addition, determining the propagation of the phenomenon is a complex
matter, which can be tackled only by studying rock avalanche behaviour from past events. Indeed, these
phenomena may affect repeatedly the same areas over time, due to their unfavourable geological and
structural conditions. The risk of recurrences of devastating phenomena over the same places, is often
underestimated because of the large time interval separating two subsequent events. For this reason
towns, villages and infrastructures have been frequently rebuilt over old deposits of past landslide. This
project aims to provide keys for the understanding of the failure processes ruling the DSGSD and RA in
massive brittle rock slopes though structural geology, geomorphology, geophysics, numerical and
experimental modeling.
Methods
Up to date, the mechanics of large gravitational slope deformations are not well understood and only a
few studies on this topic have been carried out using numerical techniques (Agliardi et al., 2001; Kinakin
& Stead, 2005). Up to date, routine analyses treat slope mass as either a continuum (FEM: Finite Element
Methods) or as a discontinuum (DEM: Discrete Element Methods). However, some recent techniques,
developed in hybrid finite-/discrete-element codes (ELFEN), allow to model both intact rock and joint
behaviors (Rockfield, 2001). For this purpose, a reliable geomechanical and hydrogeological model of the
investigated phenomenon is needed. On one hand, geological and geomechanical surveys coupled with
laboratory tests (UCT: Uniaxial Compression Tests; TT: Triaxial Compression Tests) are to be done for a
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
qualitative (rock mass classification in terms of rock quality) and quantitative (friction angle, uniaxial
compressive strength, etc) characterization of rock masses, necessary for the definition of the
geomechanical model. On the other hand, for the definition of the hydrogeological model, the analysis of
the pluviometric regimes, associated with a GIS-based micro-topographical analysis of the slope, are
required.
Case studies.
Triggering factors, failure mechanisms and run out processes will be studied for the following cases: the
left slope of the Ridnaun Valley (South Tyrol, Italy); the Fadalto Rock Avalanche and the Vajont
rockslide. The extended geological and geomechanical literature dealing with the last two case studies
allow to investigate very specific aspects such as the role of the glacial retreat (Fadalto Rock Avalanche)
and failure mechanisms (Vajont Rockslide) on slopes made up of sedimentary rock successions
(limestones, marls, dolomites). On the other hand, the Ridnaun area (see the next paragraph), chosen for
its complex past and present gravitational evolution, needs a complete geological and geomechanical
characterization and will give me the chance to analyze predisposing causes and failure processes on
highly foliated metamorphic rocks.
The Ridnaun Rock Avalanche
The E-W trending Ridnaun Valley is located in the northern sector of the South Tyrol Province (Italy),
west of Sterzing (Vipiteno), and is carved within the crystalline units of the Austoalpine Nappe of the
alpine orogenic wedge. The left slope of the valley shows evidence of quaternary differential gravitational
evolution: a fully evolved gravitational collapse, having the typical features of a Rock Avalanche,
characterized the central part of the slope; whereas to the east and west of the RA, ongoing gravitational
deformations still involve the slope (figure 1b, c). The RA has an estimated volume of about 0.6 km3,
covering an area of about 2.4 km2, with a crown area having a total length of about 2 km. This mass
wasting event had blocked the valley, resulting in a rock avalanche – dammed lake. This has been
breached and run out at an interval of time not yet defined. The slope is carved within the paragneiss
rocks of the Oetztal - Stubei Unit and the micaschists of the Schneeberg Unit. These two units are
separated by a brittle tectonic limit, well revealed by ultracataclasitic layers following the regional low
angle north –dipping schistosity. These layers, discovered during my field works, affect mainly the
paragneisses of the Oetztal – Stubei Unit, and present a mean spacing of about 0.5 – 1m inside an area
with an upward length of about 300m. In addition, field surveys done this summer, have shown that the
collapsed mass is made up of a grain - supported heterogenic debris (with a grain size going from sand to
gravel) rich in pebbles and boulders which are composed by paragneisses, phyllithic micaschists and,
subordinate amphibolites and marbles. The deposit shows also a coarsening trend in grain – size going
from north to the south, typical of the rock avalanches deposition bodies.
Radiocarbon dating of organic remnants from a sandy deltaic succession shed into the former rockslide
lake yielded an age of 8865 ± 50 year BP; this implies that the rockslide event is older (Ostermann et al.,
2010). Although a big collapse already happened, an ongoing gravitational deformation still involves the
uphill sections of the slope, next to the crown area. In addition, to the west and the east of the Rock
Avalanche, the slope is currently affected by DSGSDs, as demonstrated by the well developed
morphostructural features (double ridge, scarps – counterscarps, trenches; fig. 1), clearly recognized and
mapped during the field surveys and also by the DTM – Lidar analysis. From the first analysis, the
regional schistosity seems not to influence the sliding, except nearby the tectonic contact, where it
changes its dip direction (from N – dipping to SE – dipping) due to the presence of small – scale folds
with sub-horizontal E – trending axes. On the other hand, the recognized fault network seems to play a
major role as a predisposing factor for gravitational failures, because of the increasing degree of damage
approaching each principal fault. In order to evaluate possible variation trends in the physical and
mechanical properties of the rock masses, twelve geomechanical surveys (both “random” and “scanline”
types) were carried out. The collected geomechanical data are currently being studied. These, coupled
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
with the realized geomorphological and structural analysis, will allow the creation of a reasonable
geomechanical model of the slope, necessary for future numerical modelings.
Figure 1: a) 3D DTM – LIDAR image of the Ridnaun Valley (view from south – east; b) zoom on the left slope of the
Ridanun Valley (DTM – LIDAR image); in red the morphostructures affecting the slope both outside and inside the crown
area of the Rock Avalanche; c) zoom on the Central Ridnaun Valley (DTM – LIDAR image);in the image are shown the
RA deposition body and the crown area in the uphill sector of the left slope of the valley.
Future activities
Considering the Ridnaun area, the first step after the geomechanical characterization by field analysis, is
the evaluation of the physical – mechanical properties of the rock masses through laboratory tests
(uniaxial compressive and triaxial laboratory tests) on the samples collected on the Ridnaun Valley left
slope. All the collected data will be used to implement a geological and geomechanical reconstruction for
the numerical 2/3D modeling. Both DEM and FEM analysis will be done using the available Itasca
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
softwares, which will be followed by hybrid FEM/DEM modeling for the investigation of the failure
propagation mechanisms. This last modeling phase will be done in collaboration with the engineering
geology research group at the Simon Frasen University (Vancouver, Canada), during a scheduled visiting
period of 4 – 6 month (most probably at the beginning of the third year).
In addition, the collected samples of the Fadalto Rock Avalanche will be tested using mainly the triaxial
apparatus. The main goal is to estimate, if possible, the effect of the glacial cycles of loading and
unloading on microcracks formation and propagation inside the rock mass. The aim is to evaluate if the
glacial loading on the rock masses induced by the Alpine Pleistocene glaciers is or not one of the main
triggering factor of large mass wasting phenomena in the Alps.
References
AGLIARDI, F., CROSTA, G., ZANCHI, A., 2001. Structural constraints on deep-seated slope
deformation kinematics. Engineering Geology 59, 83-102.
BONNARD, C., FORLATI, F., SCAVIA, C., 2004. Identification and Mitigation of Large Landslide
Risks in Europe: Advances in Risk Assessment. Balkema. 317 pp
BOVIS, M.J., EVANS, S.G., 1996. Extensive deformations of rock slopes in southern Coast Mountains,
southwest British Columbia, Canada. Engineering Geology 44(1-4), 163- 182.
HEIM, A., 1932. Bergsturz und menschenleben. Beiblatt zur Vierteljahrsschrift der Naturforschenden
Gesellschaft in Zurich 77, 1 –217.
KINAKIN D., STEAD D., Analysis of the distribution of stress in natural ridge forms: implications for
the deformation mechanism of rock slopes and formation of sackung. Geomorphology 65(2005) 85 – 100.
OSTERMANN M. A., SANDERS D., RODNIGHT H. – Three rockslides shaped the valleys around
Sterzing/Vipiteno, Northern Italy – Egu general assembly 2010 – abstracts;
RADBRUCH-HALL, D., 1978. Gravitational creep of rock masses on slopes. In: Voight, B. (Ed.),
Rockslides and Avalanches Natural Phenomena. Developments in Geotechnical Engineering. Elsevier,
Amsterdam, pp. 608-657.
ROCKFIELD, 2001. ELFEN 2D/3D Numerical modelling Package, Version 3.0. Swansea: Rockfield
Software Ltd.
SCHEIDEGGER, A.E., 1973. On the prediction of the reach and velocity of catastrophic landslides.
Rock Mech. 5, 231-236.
VARNES, D.J., RADBRUCH-HALL, D., SAVAGE, W.Z., 1989. Topographic and structural conditions
in areas of gravitational spreading of ridges in the western United States. US Geological Survey,
Professional, vol. 1496.
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
SUMMARY LAST YEAR’S ACTIVITY
Courses:
Prof. P. FABBRI: “Elementi di Geostatistica” – Dipartimento di Geoscienze, Università di Padova.
Dr. M. FLORIS: “ Introduzione alle tecniche GIS” – Dipartimento di Geoscienze, Universià di Padova.
Dr. S. BOESSO: “ Corso introduzione alla biblioteca” – Dipartimento di Geoscienze, Università di Padova.
Dr.A. RASSU, Dr. C. VINANTE, Dr. N. PRATICELLI: “Introduzione a Linux” – Dipartimento di Geoscienze Università di
Padova.
Dr L. SALMASO, Dr L. CORAIN : “Statistica applicata alla sperimentazione scientifica” – Centro Studi per l’Ambiente
Alpino – S. Vito di Cadore – Università di Padova.
Prof. G. ARTIOLI, Dr G. Di TORO, Dr. A. FIORETTI: “Corso di comunicazione scientifica” – Dipartimento di Geoscienze –
Università di Padova.
FIRST EGU SUMMER SCHOOL – Structural analysis of crystalline rocks – Nevessee area – South Tyrol from 22nd to 27th
August 2010.
30th GOCAD MEETING 2010 – TRAINING COURSES – 11th – 12th june 2010 , Nancy, France.
Communications:
ZORZI L., MASSIRONI M., SURIAN N. – Il versante destro dell’alta Val Cismon (Trentino orientale): un esempio di
evoluzione gravitativa differenziale controllata dall’andamento di scistosità multiple - VIII Convegno dei Giovani Ricercatori
di Geologia Applicata (Perugia, 18.02-19.02.2010)
Posters:
ZORZI L., MASSIRONI M., SURIAN N., FLORIS M. – How multiple foliations may control large gravitational phenomena:
the case at the High Cismon Valley (Eastern Trentino, Italy). – Natural Hazard Sessions – EGU General Assembly 2010,
Geophysical Research Abstract, Wien (Austria)
FLORIS M., SQUARZONI C., ZORZI L., D’ALPAOS A. and IAFELICE M. - Using online database for landslide
susceptibility assessment with an example from the Veneto Region (north-eastern Italy). – Natural Hazard Sessions - EGU
General Assembly 2010, Geophysical Research Abstract, Wien (Austria);
DECOSMO P., GENEVOIS R., ZORZI L., BAGLIONI A., Tectonic geometry and seismic role on evolution of Lamosano
Landslide (Belluno, NE Italy) – Workshop: Physical – Chemical Processes in Seismic Faults – to be held on 18th – 20th
November 2010 - Department of Geosciences – University of Padua.
Publications:
FLORIS M., IAFELICE M., ZORZI L., D’ALPAOS A., SQUARZONI C., GENEVOIS R. - Using online database for
landslide hazard assessment with an example from the Veneto Region (north-eastern Italy) (in submission). - Natural Hazards
and Earth System Sciences.
ZORZI L., MASSIRONI M., SURIAN N., FLORIS M., GENEVOIS R., - How multiple foliations may control large
gravitational phenomena: the case at the High Cismon Valley (Eastern Trentino, Italy). – To be submitted on Geomorphology.
SCOTTON P., GENEVOIS R., MORO F., ZORZI L., GIRARDI G., PRATICELLI N. – The new Debris – Flows monitoring
system of Acquabona torrent (Cortina d’Ampezzo, Bl, Italy) – (under review) – Proceedings of the 5th International Conference
on Debris – Flow Hazards Mitigation: Mechanics, Prediction and Assesment – June 14 – 17, 2011 – University of Padua.
Workshop:
-
5th LARAM Workshop – “Advances in landslide risk analysis” – Università degli studi di Salerno - Salerno (Italy) 9
-10 september 2010
-
Physical – Chemical Processes in Seismic Faults – 18th – 20th November 2010 - Department of Geosciences –
University of Padua5
Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010
Meetings:
- VIII Convegno dei Giovani Ricercatori in Geologia Applicata - Perugia, 18th – 19th february 2010;
- EGU General Assembly 2010 - Vienna, Austria, 2 – 7 May 2010;
- 30th gOcad Meeting 2010 – Nancy, France, 8th – 9th June 2010;
Field works:
-
May – October 2010: field works (geological and geomechanical surveys) in the Ridnaun Valley area;
1 – 3 August 2010: field works and sampling in the Lamosano and Fadalto area (Belluno Province)
End October 2010 - couple of weeks in November: arrangement and analysis of the logs relative to the Vajont
Rockslide.
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