Deformation modelling of the Valoria earth slide - earth flowF.Ronchetti (2), W.Schädler (1), J.Meier (1), L.Borgatti (2), A.Corsini (2), and T.Schanz (1)

EGU General Assembly 2008 Wien 14th-18th April 2008

ReferencesReferencesRonchetti, F., Borgatti, L., Cervi, F., Lucente, C.C., Veneziano, M.M., Corsini, A., 2007.

The Valoria landslide reactivation in 2005-2006 (Northern Apennines, Italy). Landslides 4 (2007), pp. 189-195, Springer.

Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes. In: Turner, A.K, Schuster, R.L. (Eds.), Landslides: Investigation and Mitigation, Transp. Res. Board, Spec. Rep., vol. 247. National Academy Press, Washington, D.C., pp. 36– 75.

AcknowledgementsAcknowledgementsThe German Academic Exchange Service (DAAD) and the Association of the Rectors of the Italian Universities (CRUI) are acknowledged for funding traveling expenses through a VIGONI exchange project. The second author acknowledges the support by the Konrad-Adenauer-Foundation via a postgraduate scholarship.

UNIVERSITA’ DEGLI STUDIDI MODENA E REGGIO EMILIA

IntroductionIntroduction

Slope EvolutionSlope Evolution

MonitoringMonitoring

(1) Laboratory of Soil Mechanics, Bauhaus-Universität Weimar, Germany.(2) Earth Sciences Department-University of Modena and Reggio Emilia, ItalyEmail: (1) winfried.schaedler@uni-weimar.de (2) ronchetti.francesco@unimore.it

dstMODENA

Numerical modelling was performed on the basis of two geometries in the framework of continuum mechanics, comprising a constitutive approach that is based on a rheological model. The different entities of the landslide were discretized as soft homogeneous blocks, showing only little internal deformations and moving along thin, soft and highly plastic shear zones, which exhibit a pronounced time-dependency in their material behaviour. Thus, the sliding bodies themselves are considered only in the form of the load they impose onto the creeping shear zones. Finite-Element calculations performed by means of a well-established commercial code (PLAXIS), using the Soft Soil Creep Model as constitutive model for theshear zone material, were able to reproduce qualitatively the distribution of displacements and stresses at different stages of the slope evolution. Thereby, the consequent topographic variations could be explained: pre-reactivation phase 1973–2001 (obtained by existing 1:5000 topographic maps), post-reactivation phase 2001 and pre-reactivation phase 2005 (obtained using a 1: 2000 topographic map of 2003) and post-reactivation phase 2005 (obtained from Lidar data of 2007). As the actual spatial distribution of the material properties along the shear zones is unknown, each of them was divided into two homogeneous sub-zones characterised by a specific set of material parameters (smeared approach), considering the engineering geological setting. The numerical model was verified with a large reference dataset obtained from continuous monitoring with inclinometers and wire extensometers.

A Finite Element deformation model has been developed for the head area of the Valoria earth slide – earth flow. The evolution of the landslide from 2001 on was mapped, investigated and monitored with various systems. On this basis, two different 2D geometry models were set up along a representative section through the head area in order to account for different plausible interpretations of the field and monitoring data. The first model is simplified and more robust. The second one is more complex and detailed, but implies a higher number of assumptions.

Case StudyCase Study Complex earth-slides – earth-flows triggered from the Lateglacial to date represent about 80% of the landslides known in the Emilia Romagna Region. In many cases, their gentle deposition areas have been chosen as favourable location for villages or are crossed by important roads. This results in considerable potential damage in terms of both human lives and economic losses. The Valoria landslide reactivated partially or totally for several times in the last 60 years. Reactivations are controlled by rainfall and snow melting and normally occur in autumn and spring. After decades of relatively calm phases, the Valoria landslide was reactivated in 2001 during a period of abundant rainfall. After this event, new reactivations were recorded in the years 2005 and 2007. Every reactivation was characterized by the retrogression of the crown zone and successively by further destabilization of the whole head area. The reactivations of the head area triggered the advancing of the earthflows in the middle and lower part of the slope. The downslope propagation of the activity is related to the transfer of mass from the head area and to undrained loading mechanisms developing during the sudden overriding of landslide deposits.

Figure 1: Location map and panoramic view of the Valorialandslide in February 2006.

Figure 2: Panoramic view during the 2005 reactivation.

Height max. [m]

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Valoria 1.350 520 830 24 3,5 0,7 40 1,1 30

Table 1. Morphometric characteristics of test sites

Landslide descriptionLandslide descriptionThe Valoria landslide affects Cretaceous to Miocene rock masses such as sandstone dominated flysch, and silty to clayey shales (Fig. 3). Inside the slope, these rocks are deformed by overthrusts and faults (Fig. 4). The landslide deposits can be described asblocks in a silty-clayey matrix. The basic geotechnical characteristics of these materials are summarised in Table 2.

Figure 3: Longitudinal section of the landslide.

a b

Table 2: Geotechnical characteristics of bedrock and landslide deposits.

Figure 4: Bedrock of the Valoria landslide. a) Flysch formation; b) Clayshale formation

The Valoria landslide can be subdivided into different zones with different types of movements and materials involved (Fig. 5). The upper rock- and earth- sliding area extends between 1375 m and 1200 m. Roto- translational movements involve claystones and flysch-type rock masses outcropping in the crown. The earthflowsource area is located between 1200 m and 925 m, where the displaced rock masses are completely dismembered and then incorporated into earthflows. The earthflow track extends from 925 m to 650 m. The landslide toe is located between 650 m and 520 m. Drill-holes and refraction seismics have shown that the thickness of the rock masses involved into the slides at the landslide crown is between 5 and 40 m, and that the thickness of the earthflowdeposits along the slope varies from a few meters in high slope-gradient regions to more than 30 m in the low slope-gradient regions of the track and toe areas.

A B C

An age of about 7800-7580 cal yr BP was obtained for a wood fragment collected close to the bedrock interface in the landslide toe zone.

Figure 5: Pictures of different landslide zones during the 2005 reactivation. A) Crown zone; B) Track zone; C) Toe zone.

The evolution of the slope has been evaluated through the analysis and the comparison of different Digital Elevation Models (DEMs), obtained in different years starting from the 70’s. In general, from the subtraction of these DEMs, it is possible to assess the erosion in the landslide source area and the accumulation in the toe area. The height difference observed due to the erosion in the upper part of the slope is around 30 m.

The Valoria landslide is a complex earth-slide – earthflowlocated in the Northern Apennines of Italy, in the upper Secchia River basin, in the Emilia Romagna Region. It extends from 1413 m to 520 m in elevation and affects an area of 1.6 km2 over a length of about 3.5 km (Fig. 1).

Subtraction of DEM Product

Name: CGR (year 2003)- CTR (year 1973)

Altimetry change 2003-73 (event 2001)

Legend

Altimetry change [m]

Subtraction of DEM Product

Name: Lidar (year 2006)- CGR (year 2003)

Altimetry change 2006-2003 (event 2005)

Legend

Altimetry change [m]

ModellingModelling the evolution of shear strength and geometrythe evolution of shear strength and geometry

After the 2005 event, only few of the monitoring instruments had “survived”: one inclinometer (B8A), depth 54 m, and one piezometer (B8B), screening between 5.5 and 24.5 m and equipped with electric transducer, located in the non-active head zone; one inclinometer (B2A), depth 81.5 m, and one piezometer (B2B), screening between 6 and 41 m and equipped with electric transducer, located in the active track zone.During summer 2007, new instruments were installed in the crown zone and source area of the landslide. Five wire extensometers (Ex1, Ex2, Ex3, Ex4, Ex5) were placed into the active crown zone across open fractures, two inclinometers (B9A, depth 50 m; B9B, depth 41.5 m) and two piezometers (B9C, 30 m; B9D 15 m), equipped with electric transducers wereset up in the active head zone. The piezometers are monitoring the groundwater table in the landslide body and in the bedrock. These monitoring instruments recorded the deformations and the groundwater fluctuation before and after the 2007 event, and some of them continue monitoring at the moment.

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CONTINOUSCONTINOUS WIRE WIRE EXTENSOMETRSEXTENSOMETRS

CONTINOUSCONTINOUS AND NON AND NON CONTINOUSCONTINOUSINCLINOMETERSINCLINOMETERS

CONTINOUSCONTINOUS PIEZOMETERSPIEZOMETERS

Model setupModel setup

Conclusions and future researchConclusions and future researchThe model proved to be able to simulate the past and the ongoing deformations of the head zone of the Valoria landslide, and suggested that the evolution of this area is determined not only by groundwater condition, that is the main triggering factor, but also by the progressive reduction of friction and cohesion along the shear zones, linked to the recent and ongoing sliding processes. Future research will concentrate on calibrating the model by means of inverse parameter identification strategies, based on the monitoring data gathered in the field. Once the model is successfully calibrated, it will be used for simulating the masses that can be mobilised by possible reactivation scenarios, which are the indispensable input for any kind of run-out simulation.

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Creep deformation in rock

Deep rock slide – around 35 m depthShallow earth slide – 0 to 25 m depth

Deep rock slide – around 35 m depthShallow earth slide – 0 to 25 m depth

Model Cross section

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Figure 8: Monitoring carried out at the Valoria landslide

A) Location of instruments, B) Piezometer readings,C) Extensometer plots, D) Inclinometer data

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Figure 6: Location of the section considered in the FE-Model

Figure 7: Simple and complex model of the landslide head area.