Assessing the possible future development of the Tessina landslide using numerical modelling

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1 Assessing the possible future development of the Tessina landslide using numerical modelling G. Marcato, M. Mantovani, A. Pasuto, F. Tagliavini & S. Silvano IRPI-CNR - Research Institute for Hydrological and Geological Hazard Prevention, National Research Council of Italy, Padova, Italy L. Zabuski IHEPAS - Institute of Hydro-Engineering, Polish Academy of Sciences, Gdansk, Poland ABSTRACT: The Tessina landslide (North Eastern Alps, Italy) is a complex phenomenon initiated in 1960, which consists of massive rotational slides evolving into a mudflow. The landslide is still active and deformations have led to a constant widening of the source area. In recent years the instability of a large block (called Pian de Cice ) located in the eastern part of the landslide depletion zone has been monitored. It is in fact believed that a collapse of that block would be crucial for the future development of the whole unstable area and could lead to a dangerous enlargement of the depletion zone. The objective of this paper is to forecast possible future development of the Pian de Cice slide using numerical modelling. The numerical simulation using FLAC 2D code based on finite difference method has been carried out. Elasto-visco-plastic model has been used to describe the material behaviour. Visco-elastic part is represented by Burger s model whereas plasticity is described by Coulomb-Mohr law. Thanks to the large-strain Lagrangian formulation it was possible to observe the changes of the slope shape during simulation process. It was assumed that the sudden failure of the slope can occur during the period of extremely unfavourable atmospheric conditions (e.g. heavy rainfall and melting of thick snow cover), causing the rapid raising of the ground water table. Such situation is expressed in the numerical model by substantial worsening of the viscosity and elasticity properties. The numerical simulation provided the results, which can help to evaluate the hazard, caused by this slide. 1 INTRODUCTION The Tessina landslide is located on the southern slope of Mt. Teverone, in the Alpago valley, North Eastern Italy (Fig. 1). Since its beginning in 1960 it endangered some villages in Chies d Alpago municipality. Therefore, the installation of countermeasures to protect the people as well as geological surveys and geotechnical investigations to forecast the landslide behaviour became indispensable (Angeli et al. 1994). In recent years research on the geomorphological risk has been carried out and the geological and physical data have been found and the monitoring system allowed to observe the slide evolution (Silvano & Pasuto, 1991; Pasuto & Silvano, 1995). The cellular automata simulation model and photogrammetry techniques have been employed as well, to assess the geomorphological evolution of the landslide (Avolio et al. 2000; Mantovani et al. 2000; Hervas et al. 2003). The landslide is a complex phenomenon, consisting of massive rotational slide in the upper part transformed into a mudflow in the lower zone. It is in progress and in the consequence of the movement, the source area is widened from the 300,000 m 2 originally to 520,000 m 2 presently. The total displaced volume is equal to about 7 millions m 3. Nowadays, geomorphological evidence indicates the presence of a new collateral landslide, named Pian de Cice, affecting the eastern slope of the depletion zone. It is expected that this slide will be crucial for the future development of the whole unstable area, since its collapse could lead to a rapid enlargement of the source area. The objective of the paper is to assess the stability conditions of the Pian de Cice landslide using numerical modelling. The numerical simulation using FLAC 2D code based on finite difference method has been carried out and the material composing the slip zones in the slope has been modelled as visco-elasto-plastic medium. Figure 1. Location of the Tessina landslide

2 LANDSLIDE HISTORICAL EVOLUTION The first landslide movement with total volume approximately equal to 1 million m 3 has been registered in 30 X. 1960, after exceptionally heavy rainfall.

2 2 LANDSLIDE HISTORICAL EVOLUTION The first landslide movement with total volume approximately equal to 1 million m 3 has been registered in 30 X. 1960, after exceptionally heavy rainfall. It was possible to distinguish constantly expanded area of depletion, a flat upper accumulation area, a lower accumulation area filled mainly by creeping material and a steep narrow discharge channel, connecting the two latter areas (Fig. 2). In XII reactivation of the mudflow stopped in front of the Funes village. Further movements, summarized in Table 1 caused filling of the Tessina valley with the m thick layer of the material. The historical evolution of the Tessina landslide between 1960 and 1998 and the cumulative rainfall that can be considered as one of the main triggering factors, have been summarized in Table 1. Table 1. Main reactivations due to intense rainfall. Month-year (mm/yy) Cumulative rainfall triggering events Landslide area (m 2 ) 10/ mm (30 days) / mm (30 days) 12/ mm (30 days) 04/ mm (30 days) + snow melt 08/ mm (30 days) / mm (60 days) 06/ mm (60 days) 11/ mm (days) 06/ mm (days) / mm (30 days) + snow melt 04/ mm (60 days) + snow melt 09/ mm (30 days) 01/ mm (30 days) / mm (30 days) / mm (30 days) 04/ mm(15 days) + snow melt / mm (45 days) 06/ mm ( 60 days) 04/ mm (10 days) + snow melt 10/ mm (25 days) In recent years distinct signs of instability in form of fractures and depressions, appeared on left-hand flank of the narrow channel that connects source area with the lower accumulation zone. Therefore, the special program of investigations was carried out in order to keep the movements under constant control. 3 INVESTIGATION AND ACTIVITY MONITORING OF EASTERN BLOCK Figure 2. Geomorphological units of the Tessina Landslide. Important reactivation occurred in 1998 in the upper part of the landslide, in form of large rotational slide with a significant enlargement of the eastern sector of the source area. Following these events, rapid variation in pore pressure due to undrained conditions in the mudflow deposit, caused the mobilization of 1 million m 3 with total displacements of about 200 m in ten days. The landslide did not show any significant reactivation during the first 2000s, although northern regions of Italy were threatened two times by heavy rainstorms and floods. The block, called Pian de Cice, develops between 1100 m and 900 m a.s.l on the eastern slope of the Tessina valley (Fig. 3) and has been partially monitored since The monitoring system of Tessina landslide consists of fully automated topographic station. More than 20 benchmarks were installed in the detachment area and in the upper part of the accumulation zone. The measurements are performed with hourly or daily frequency depending on the movement activity. Since their installation in 1998, three benchmarks (307, 308 and 309) on the eastern block showed a continuous displacement with the rate of movement ranging between 2.8 to 6.3 cm/year. The total displacement recorded by benchmark 309, that is located at the central position on unstable block, has exceeded 50 cm in 10 years. As the stability of the block is crucial for the future behaviour of the whole unstable area, more recently geophysical investigations, geomorphological surveys and inclinometric and piezometric measurements have been carried -328-

out. The lithostratigraphic sequence of the block has been reconstructed on the base of the resistivity model gathered by two tomographic sections and by cores analysis taken from inclinometric

This is composed by rhythmic alternation of marlyargillaceous and calcarenite vertical layers, namely Belluno Flysch Formation (Middle Eocene), covered by moraine deposits of Piave River Wurmian

3 out. The lithostratigraphic sequence of the block has been reconstructed on the base of the resistivity model gathered by two tomographic sections and by cores analysis taken from inclinometric boreholes. This is composed by rhythmic alternation of marlyargillaceous and calcarenite vertical layers, namely Belluno Flysch Formation (Middle Eocene), covered by moraine deposits of Piave River Wurmian glacier with variable thickness. nearly 300 m. Its maximum width equals about 150 m. In order to verify and quantify the relation between rainfall depth, groundwater level and displacement, the data retrieved from meteorological station and continuously acquired by piezometer P1, both located in the nearest vicinity of the analysed block have been considered. The analysis of a series of 10 years observations shows very low fluctuations (0-0.5 m) of ground water level (GWL) which remains almost constant at the contact between the moraine deposits and the flysch bedrock. The GWL can be considered as homogenous in entire slope, where the moraine material covers the bedrock, as it has been also verified by several manual measurements performed in inclinometric boreholes. These results suggest a high permeability of the moraine deposit and high capacity of pore water pressure dissipation. It can be thus concluded that only extremely unfavorable meteorological conditions could be the reason of instability phenomena triggering in this zone. Figure 4. Schematic geological profile of the eastern block. 4 NUMERICAL SIMULATION OF LANDSLIDE CREEPING Figure 3. Benchmarks network and ERT sections. The measurements carried out in bohehole I1 shown a slip surface at the depth of 26 m and annual cumulative displacement of 4.5 cm. The same slip surface appears in the borehole I2 at the depth 10 m, with similar movement rate. The rate recorded in the boreholes is almost identical as measured by the topographic station (benchmark 309) and has a direction of 230 with respect to the North. No failure surface is detected in the borehole I3. According to the inclinometric results the slip surface of the Pian de Cice landslide seems to be approximately linear and located along the contact between disaggregated flysch layer and moraine deposits. The unstable slope has rock and soil volume approximately equal to m 3 and it extends Numerical simulation using finite difference method (FLAC code; ITASCA C.G. 2000) has been carried out to predict possible future behaviour of the Pian de Cice landslide. Elasto-visco-plastic (EVP) model of the medium composing the sliding zones is assumed, allowing to determine the relation between time and displacement (Borgatti et al. 2007, Zabuski 2004). In other words, the real process could be reconstructed in numerical way if the creep (viscous) phenomenon is taken into account. Thanks to this, prediction of the future movement can be done. In addition, large-strain mode of the solution allows to simulate the changes of the slope shape. The Burger s model has been applied to simulate the creep (Fig. 5) whereas commonly known modified Coulomb-Mohr law described plasticity conditions (Marcato et al. 2007). The ideally plastic behaviour has been assumed, where maximum and residual parameters are identical. The creep Burger s model is composed of Kelvin and Maxwell members (ITASCA C.G. 2000). Both of them are arranged as -329-

4 sets of springs and dashpots (Fig. 5), which simulate elastic and viscous behaviour respectively. The model is described by four parameters, namely Kelvin and Maxwell viscosity coefficients and elasticity shear and bulk modules. 4.1 Formulation of the numerical model and simulation procedure The model of the slope has been splitted into finite difference zones (Fig. 6) and divided into geotechnical layers (Fig. 7), determined on the base of geological investigations described above. The model dimensions are properly designed to avoid an unfavourable boundary effects. The boundaries do not disturb the model behaviour, as the failure is mainly concentrated in the narrow zones (No. 1 and No. 2) and between the zones 4 and 5 (see Fig. 7). Initial stress distribution is hydrostatic; there are not any data at the disposal, to assume any particular disturbances in this field. Figure 5. Burger s model simulating creep behaviour. The measured quantities, such as displacement and groundwater level variations allowed to calibrate the model. Although displacement in the past developed linearly with time, it could be expected that catastrophic events might occur in the future, as occurred in the previous collapse events of the eastern source zone in 1992 and 1998.The objective of the simulation is therefore connected with the prediction of the possible behaviour of the slope in extremely unfavourable conditions, i.e. when the medium composing the slide zone is substantially weakened and behaves as fluid-like material (liquefaction) and the viscosity and elasticity parameters decrease very significantly and rapidly. It is possible that such conditions might occur in the period of very heavy rainfall, especially when it follows a snow melting. The historical data recorded by the monitoring system have been used to analyze longterm trends in the behaviour of the landslide block and to provide information to test the simulation results (Avolio et al. 2000). At first instance the geometry of the model has been defined and the corrections introduced in the 2D model compared to a true 3D phenomenon. Comparing the intensity and directions of the superficial and in-deep deformations recorded during the years with the results of the geophysical survey it has been possible to detect the main direction, along which the slide primarily evolves. The analysis in this paper is based on two main assumptions: - rapidly increasing groundwater table at 1.0 m above the average constant level; - subsequent rapid worsening of viscosity and elasticity properties due to the material liquefaction. Figure 6. Splitting the model into finite difference zones (the numbers 2-9 sign the points in which horizontal displacement is recorded during the simulation). Figure 7. Division of the model into layers, having different geomechanical properties. The simulation procedure has been as follows: - ten-years creep with constant viscosity and elasticity parameters; - 24 hours creep after increasing water table and decreasing the parameters; - next 24 hours creep after subsequent decreasing of the parameters (totally 48 hours creep); - next 24 hours creep after further decreasing of the parameters (totally 72 hours creep). The parameters of the model in all stages are set in Table 2. These parameters are not realistic and have speculative character. It has to be pointed out that the simulation produces the dangerous situation, but such situation could only occur if the parameters would substantially decrease to the values equal to or lower than these, which are set in the table. It is probable that after long period of creep with the constant rate the material begins to behave as fluidlike medium in consequence of extremely unfavourable external conditions (Zabuski 2004). Moreover, fast increasing of the water table causes lowering of the frictional resistance of the material, although according to the assumption the strength parameters do not change

= Kelvin viscosity coefficient. Layer Model Time (stage) ρ [t/m 3 ] K G M G K c φ [deg.] η M [kpa. s] η K [kpa. s] 1 EVP 1.9 4.17E5 1.92E5 2.5E5 0 15 5.0E11 5.0E11 2 EVP 10 1.9 4.17E5 1.92E5 2.5E5 0 20 5.

5 Table 2. Geomechanical parameters of the layers: ρ= Density; K= Bulk modulus; G M = Maxwell shear modulus; G K = Kelvin shear modulus; c= Cohesion; φ= Friction angle; η M = Maxwell viscosity coefficient; η K = Kelvin viscosity coefficient. Layer Model Time (stage) ρ [t/m 3 ] K G M G K c φ [deg.] η M [kpa. s] η K [kpa. s] 1 EVP E5 1.92E5 2.5E E11 5.0E11 2 EVP E5 1.92E5 2.5E E11 5.0E11 3 EVP years E5 1.92E5 2.5E E13 2.1E13 4 EVP E5 3.84E5 2.5E E13 2.1E13 5 EP E6 1.15E EVP E5 1.15E5 1.0E E8 5.0E8 2 EVP E5 1.15E5 1.0E E8 5.0E8 3 EVP 24 h E5 1.15E5 1.0E E10 2.1E10 4 EVP E5 2.30E5 1.0E E10 2.1E10 5 EP E6 1.15E EVP E5 7.68E4 5.0E E6 5.0E6 2 EVP E5 7.68E4 5.0E E6 5.0E6 3 EVP 48 h E5 7.68E4 5.0E E8 2.1E8 4 EVP E5 1.54E5 5.0E E8 2.1E8 5 EP E6 1.15E EVP E4 3.84E4 2.5E E4 5.0E4 2 EVP E4 3.84E4 2.5E E4 5.0E4 3 EVP 72 h E4 3.84E4 2.5E E6 2.1E6 4 EVP E5 7.68E5 2.5E E6 2.1E6 5 EP E6 1.15E Results of simulation Figure 8 presents the horizontal displacement field in the last simulation stage (72 hours fast-rate creep). As it can be seen, the most intensive movement occurs along the shallower zone. A few meters displacement in a deeper zone of fractured flysch is also visible. The presence of two slip surfaces is also evident in Figure 9, where the zones of maximum shear strain increments are drawn. deeper zones and is significantly greater in the surface proximity. Figure 8. Field of horizontal displacement after 72 hours creep. Figure 9. Location of the zones of maximum shear strain increments after 72 hours creep. Figure 10 shows the curves of horizontal displacement of section located at X= 170 m. These curves illustrate that the deformation initiates in the Figure 10. Curves of horizontal displacement in three 24-hours stages of creep. Coordinate X = 170 m. Possible progression of the displacement is illustrated by the curves in Figure 11. The maximum horizontal and total displacement in each creep stage are set together and curves, which approximate the results, are drawn. They have a parabolic shape with high correlation coefficients. It has been possible to determine the equations which relate displacement with time and hence the prediction of the future displacement can be made under the condition that the same relations would be valid in the next days. Table 3 presents the results of such prediction for 144 hours, i.e. for 6 days. Both the curves in the Figure 11 and the values in the Table 3 have to be consid

6 ered as the lower limits of the possible displacements. The real movement can be more fast and intensive, since the lowering of the strength parameters would produce additional negative effects, accelerating the failure. The investigated block moved with approximately constant rate during the last few years. However, the results of numerical simulation carried out using elasto-visco-plastic model proved that highly unfavorable atmospheric conditions (high precipitation, melting of thick snow cover) could result in a substantial decreasing of rock mass mechanical properties. It could cause sudden acceleration of the movement and eventually the failure of entire slope in a very short time (i.e. in few days or even hours). The moving material would reach the main channel with significant increasing if the pore pressure in the lower accumulation area (Hutchinson & Bhandari, 1971). However, it should be taken into account that strong assumptions were done regarding the lowering degree of the model parameters. In fact, it is only probable that the parameters fall down very significantly. Therefore one can consider the results of the simulation as information on the potential mechanism, which can appear on the analogy of the large events, which appeared in the past. Despite these limitations the results can provide useful input to decision makers in order to better understand the mechanism of the most probable landslide development, to manage the emergency situations and to safeguard the public safety. ACKNOWLEDGMENTS This paper is a part of the research Project MoVeMit: studio e modellazione di movimenti di versante finalizzati alla formulazione di interventi di mitigazione, financed by the funding of the Fondazione Cassa di Risparmio di Verona,Vicenza, Belluno e Mantova. REFERENCES Figure 11. Horizontal and total displacement in three stages of creep. Table 3. Simulated and predicted horizontal and resultantmaximum displacement U x and U tot. Time [day] Time [hour] U x [m] U tot [m] Remark Calculated Calculated Calculated Predicted Predicted Predicted 5 FINAL REMARKS AND CONCLUSIONS Avolio, M.V., Di Gregorio, S., Mantovani, F., Pasuto, A., Rongo, R., Silvano, S. & Stataro, W Simulation of the 1992 Tessina landslide by a cellular atomata model and future hazard scenario. In JAG 2(1): Angeli, M.G., Gasparetto, P., Menotti, R.M., Pasuto, A. & Silvano,.S A system of monitoring and warning in a complex landslide in Northeastern Italy. In Kyoji Sassa (ed.) Landslide of the world. Kyoto University Press: Landslide news 8: Borgatti, L., Corsini, A., Marcato, G., Pasuto, A., Silvano, S. & Zabuski, L Numerical Analysis of Countermeasure Works Influence on Earth Slide Stabilisation: A Case Study in South Tyrol (Italy). 1 st North American Landslide Conference Landslides and Society: Integrated Science, Engineering, Management, and Mitigation. Proc. intern. conf. Vail, 3-8 June USA: AEG 23: Hervás, J., Barredo, J.I., Rosin, P.L., Pasuto, A., Mantovani, F., Silvano, S., Monitoring landslides from optical remotely sensed imagery: the case history of Tessina Landslide, Italy. Geomorphology, 54(1-2), Hutchinson, J.N. & Bhandari, R.H Undrained loading, a fundamental mechanism of mudflows and other mass movements. Geotechnique 21: ITASCA FLAC: Fast Lagrangian Analysis of Continua (Version 4.0). User s Manual. ITASCA Consulting Group Inc. Minneapolis. Mantovani, F., Pasuto, A., Silvano, S. & Zannoni, A Collecting data to define future hazard scenarios of Tessina landslide. In JAG 2(1): Marcato, G., Mantovani, M., Pasuto, A., Silvano, S., Tagliavini, F., Zabuski, L. & Zannoni, A Site investigation and modelling at La Maina landslide (Carnian Alps, Italy). Natural Hazards and Earth System Sciences 6: Pasuto. A. & Silvano, S The Tessina landslide. In Horlick Jones, T., Amendola, A., Casale, R. (eds), Natural Risk and Civil Protection. Chapman & Hall, London. Silvano, S. & Pasuto. A Geotechnical investigations applied to the study of mass-movements. In M. Panizza, M. Soldati & M.M. Coltellacci (eds), European Experimental Course on Applied Geomorphology. Istituto di Geologia, Università degli Studi di Modena, Italy. Zabuski, L Prediction of the slope movements on the base of inclinometric measurements and numerical calculations. Risks Caused by the Geodynamic Phenomena in Europe. Proc. Int. Conference, Cracow, May Warsaw: PGI 15:

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