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Experiments to Investigate the Hydrological Behaviour of Volcanic Covers
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ScienceDirect Procedia Earth and Planetary Science 9 (2014) 14 – 22
The Third Italian Workshop on Landslides
Experiments to investigate the hydrological behaviour of volcanic covers Luca Paganoa*, Alfredo Redera, Guido Riannab a
DICEA – Department of Civil, Construction and Evironmental, University of Naples Federico II, Via Claudio 21, 80125, Naples, Italy b CMCC-ISC – Impact on Soil and Coasts Research Division- Euromediterranean Center on Climat Change, via Maiorise Capua
Abstract The work investigates experimentally the hydrological response of a volcanic silty-sand involved in the Nocera Inferiore flowslide occurred in 2005. To this aim, an infiltration column and a lisimeter have been developed. In both, the same volcanic silty-sand soil has been placed through the pluvial deposition technique. The former device has been used to carry out infiltration laboratory tests aimed at investigating hydraulic boundary conditions acting at the lowermost surface of the soil column for different contact types (pumices, atmosphere, a geosynthetic material). The latter exposes the layer at atmosphere to investigate water fluxes across the uppermost surface induced by the soil-atmosphere interaction. The work shows how the developed physical models feature the hydrological behavior of a silty pyroclastic layer under various meteorological forcing, allowing to build up frameworks useful to characterize the main factors inducing landslides.
© 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università di Napoli. Universit di Napoli. Keywords: TDR probe calibration; heat dissipation probe calibration; pyroclastic soil; dielectric constant; heat conductivity; suction; soil-water content; soil properties.
1. Introduction In the last two decades significant research activities have been devoted in Italy to investigate the hydrological behavior of silty volcanic soils mantling wide areas of the country. Due to increasingly frequent rainfall-induced
* Corresponding author. Tel.: +39 081 7683478 E-mail address: [email protected]
1878-5220 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Universit di Napoli. doi:10.1016/j.proeps.2014.06.013
Luca Pagano et al. / Procedia Earth and Planetary Science 9 (2014) 14 – 22
landslides involving shallow layers made of these soils, research activities have essentially focused on factors affecting the landslide triggering and post-failure behavior. This latter is often characterized by the sliding mass turning into a rapid flowslide. Experimental and theoretical research activities in this field have mostly been conducted in the context of the interpretation of sadly known flowslide case-histories, such as Sarno (1998), Cervinara (1999) Nocera Inferiore (1997 and 2005) Ischia (2006 and 2009). Investigations have involved: (a) field monitoring of soil variables (essentially soil suction and water content) supposed to control both landslide triggering and post-failure behavior1,2,3; (b) laboratory tests on unsaturated soils4,5; (c) use of isothermal approaches (analytical and or numerical solution of Richards’ equation) to predict rainfall-induced suction changes in the subsoil7,2,8; (d) development of suitable physical models of slopes to reproduce wetting-induced shallow landslides9,10,11,12; (e) interpretation of historical rainfall recorded at landslide sites by the use of statistical13,14,15,16,17 or physically-based approaches8 so as to develop predictive tools for early warning systems. The extensive research carried out has clarified the following points: x the soils involved are non-plastic silty sands with gravel (pumiceous stone); the friction angle ranges between 35 and 39 degrees; the coefficient of hydraulic conductivity ranges between 10-7 and 10-6 m/s at full saturation; it decreases significantly by several orders of magnitude as the saturation degree diminishes; the presence of a significant silt fraction involves small size pores in the soil allow rainwater infiltrating the cover to be retained as meniscus water; x the instability is associated with significant increases in pore water pressures 9,8,18; field monitoring carried out by jet-fill tensiometers indicated that suction exceeds the full scale of the instrument (80 kPa) during dry season; precipitations induce significant drops in soil suction; case-history interpretation and experimentation through physical models indicate that landslides turn into rapid flowslides when suction approaches zero throughout the pyroclastic layer; x high water content is responsible for soil liquefaction upon failure; liquefaction might be involved in the triggering mechanism; it determines a post-failure behaviour entailing a significant acceleration of the sliding mass5; during the wet season the high soil porosity (often 70% or higher) allows one cubic metre of soil to trap hundreds of litres of rainwater; high porosity is also what induces a contractile behaviour upon shearing, involving water loading and increases in pore water pressure; this mechanism may induce soil liquefaction, due also to the lack of inter-particle electrochemical forces, testified by the zero plasticity of the soil; x a rainfall event with a daily rainfall height of at least 80 mm is required to trigger a landslide; triggering also requires that the rainfall event is preceded by significant antecedent precipitations8; most landslides occurring during the wet season (from November to May) were often caused by less intense rainfall than that falling during dry periods, when there were no instabilities; a deeper insight into this topic is provided by the Nocera Inferiore case-history of 4 March 20058; the slide was caused by a rainfall event of 140 mm spread over 15 hours; an interpretation of this case, based on solving the Richards equations, showed how the phenomenon was associated with the vanishing of soil suction throughout a layer two metres thick, at a place where the slope angle was slightly higher than the soil friction angle; the study in question also brought to light the role of antecedent rainfall, which had already reduced soil suction to very low values before the start of the triggering event; it was demonstrated that the same rainfall event would not have caused any instability if it had occurred within a drier period with an initial soil state characterized by higher suctions; precipitation during the previous four months had significantly affected soil suction at the triggering time. The physical explanation of the role played by antecedent rainfall on instability of pyroclastic slopes lies precisely in the above ability of these soils to retain a large amount of precipitation water for a long time. It is well known, however, that water infiltrating the soil, if not lost to deep drainage, is subject to evaporation and transpiration phenomena. In pyroclastic layers evapotranspiration is therefore thought to take place to a significant extent: it reduces the effect of antecedent rainfall, affecting soil state at the beginning of the critical rainfall event. It thus lends a positive contribution to avoid conditions for landslide triggering. In this context, the paper investigates the hydrological behavior of silty volcanic soils through experiments carried out by physical models. Particular emphasis is placed in characterizing the lowermost and uppermost
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boundary conditions acting on a shallow pyroclastic cover, by investigating, respectively, the possible capillary barrier effects and the soil-atmosphere interaction. 2. The physical models An extensive description of the physical model may be found in Rianna et al19. In the following, only the main features of the apparatus are briefly illustrated. It consists (Figure 1) of a wooden tank containing a layer of nonplastic sandy silt volcanic soil, very similar to that involved in the Nocera Inferiore 2005 flowslide. The layer has a horizontal section of 1.15x1.15m and a thickness of 0.75 m. It has been put in place at the high field porosity (around 70%) observed at the triggering zone of the mentioned landslide. To this aim the pluvial deposition technique has been adopted20. The layer has been then exposed at weather elements, in order to investigate the soilatmosphere interaction under one dimensional flow conditions. The monitoring scheme implemented in the physical model (Figure 1) quantifies meteorological forcing (total precipitation and potential evaporation), actual fluxes developing across the uppermost layer surface (actual evaporation and infiltrated precipitation) and, finally, the soil state (matric suction, volumetric water content and temperature at different depths). Actual fluxes have been obtained by measuring the weight changes of the tank through three load cells sustaining the tank itself. Actual evaporation has been obtained also by the monitoring of energetic terms at the soil-atmosphere interface (the net radiation by a radiometer, the soil heat flux by a soil heat flux plate installed at a depth of 5 cm, the sensible heat flux by monitoring air temperature and soil temperature at the layer surface). The bottom of the tank is uniformly holed to allow drainage. A geotextile is interposed between the tank bottom and the soil layer in order to prevent erosion. From a hydraulic point of view it behaves as a capillary barrier, breaking (allowing drainage) at a value slightly lower than the atmospheric pressure; this behavior was observed in the laboratory by carrying out an infiltration experiment through a column containing the same sequence of materials, with the silty layer lower-bounded by a geotextile sheet. The evolutions of seepage flows and pore water pressures were measured at the layer base during the imbibition process from the top surface. Tests conducted by replacing the geotextile with a pumiceous layer (sandy gravel layer) or leaving air exposure at the layer bottom resulted in similar results (Figure 2).
Fig.1. The physical model: (a) material put in place within the tank; (b) picture of the physical model; (c) vertical section of the physical model (after Rianna et al19)
Luca Pagano et al. / Procedia Earth and Planetary Science 9 (2014) 14 – 22
Previous works had already brought to light the capillary barrier effect exerted by a pumiceous layer lowerbounding a silty layer21,22,23. In the physical model the geotextile thus approximates field conditions of a silty volcanic layers lower-bounded by a formation having well larger pore sizes: intensely fractured calcareous bedrock (roughly equivalent to air exposed surface) and pumiceous layers without silty fraction.
Fig. 2. Pore water pressures and cumulative drainage against time for different types of contacts achieved at the bottom of the infiltration column
3. Soil-atmosphere interaction observed via physical modeling The peculiarity of the experiment is that it merges the benefits of field monitoring and laboratory tests by combining layer thickness and evolution of atmospheric variables typical of the field environment, with full control of boundary conditions and material properties, typical of laboratory conditions. Three hydrological years of monitoring deliver the evolution of precipitation, evaporation, water storage, volumetric water content and suction. The water storage, expressed in Figure 3a as overall volumetric water content of the layer, increases at the beginning of wet periods, due to precipitation which infiltrates the layer17. For prolonged wet periods, as those occurring during the first and third years, water storage tends to stabilize at a wet level (Figure 3a, “drainage” line) placed just below the maximum value associated with the layer saturation (Figure 3a, “saturation” line) and regulated by water pressure for which drainage occurs. Above this wet level drainage has been often observed during and immediately after rainfall events, while below this level drainage has never been observed. During the dry periods the water storage decreases (Figure 3a) under the effects of actual evaporation (figure 3b). For prolonged dry periods it tends to attain a minimum at a value of 0.22. Actual evaporation is intense when potential evaporation is intense (Figure 3b) and top-soil water content is high as well. In Figure 3b top-soil water content is expressed by the evolution of volumetric water content at the depth of 15 cm. It indicates the availability of water within the top soil for the state change. Over each hydrological year the layer experiences a main cycle of water storage increase and decrease. Figure 4 shows that an increase in the top-soil water content reduces the amount of infiltrated precipitation (black bars) as percentage of total precipitation (blue bars). An increase in top-soil water content relates in fact to a reduction of soil suction and, consequently, of hydraulic gradients available for rainfall to infiltrate the layer. This “gradient effect” prevails on the “hydraulicconductivity effect”, which is opposite, as hydraulic conductivity increases with increasing soil water content. The dry state of the topsoil associates with high values of potential infiltration (up to 40 mm per hour), while wet states associate with low values (few millimeters per hour)21.
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Fig. 3. Results obtained through the physical model exposed at elements: (a) water storage and precipitation; (b) potential evaporation, actual evaporation and volumetric water content measured at the depth of 15cm (after Rianna et al19 and Rianna et al24)
Fig .4. Results obtained through the physical model exposed at elements: total rainfall, infiltrated rainfall, volumetric water content measured at the depth of 15cm (after Rianna et al.24)
Luca Pagano et al. / Procedia Earth and Planetary Science 9 (2014) 14 – 22
4. Rainfall history and water storage Some interpretative works of shallow landslides turning into rapid flowslides in silty pyroclastic slopes with inclination angle slightly exceeding the soil friction angle, indicate that triggering does not occur at any times suction vanishes at single depths but, rather, at the specific time suction vanishes contextually at any depth, with the layer experiencing full saturation8. This can be physically explained basing on the fact that full saturation predisposes the slope to extensive liquefaction phenomena, promoting a triggering mechanism of progressive failure under undrained conditions. A different mechanism may however also be invoked. The silty pyroclastic cover often incorporate a pomiceous layer (see for instance the typical schemes reported in Sorbino and Nicotera 25). Under unsaturated conditions the pumiceous layer acts as a capillary barrier, inhibiting downward flow. Rainfall-induced water content increments or suction drops result hence higher in the silty layer placed above the pumiceous layer (shallowest silty layer), due to its inability in draining water, and significantly reduced in the silty layer placed below the pumiceous layer (deepest silty layer), hardly wetted by rainfall-induced seepage flows. Abundant vegetation is also present over the year in these soils, with the root system typically crossing the pumiceous layer. From a mechanical point of view, roots may be supposed to act as reinforcements, binding the shallowest-potentially-instable layer and deepest-resistant layer. Low total stress levels act within the whole system due to the small layer thickness involved, so that the pullout capacity of roots is essentially provided by suction at the root-soil interface. In such a framework, instability could occur when soil suction vanishes throughout the shallow layer, resulting there in the vanishing of the root pullout capacity. The above considerations allow to assume that the layer water storage at full saturation (“saturation line” in Figure 3) may be identified with conditions triggering a rapid flowslide. The hydrological pattern experienced by a silty volcanic layer subject to a rainfall history resulting in flowslide, and, as such, increasing water storage up to the saturation line, may be derived from the experimental results provided by the physical model24. At the beginning of a hydrological year (September) water storage attains the driest level, because of intense and prolonged evaporation phenomena of the summer season. The progression of rainfall events of the wet period increases water storage. This increase is initially enhanced by low evaporation phenomena, high potential infiltration (tens of millimeters) and inhibited drainage (capillary barrier below the layer). The layer behaves as a tank, prone to adsorb rainwater and to store it. At this stage re-equilibrium processes are slow because suction changes link to significant water content changes (re-equilibrium requires a large amount of seeping water) and hydraulic conductivity is held low by the dry state of the soil24. Rainfall-induced suction drops in the top-soil hence delay in propagating downwards, implying that suction evolution at the shallowest and deepest points arise non-synchronous (Figure 5a). The impervious base determines a tendency for pore water pressures to configure at equilibrium as hydrostatic during rainfall interruptions. Fluctuations of suctions indicate however that the hydrostatic state is rarely experienced by the layer, still due to the slowness of re-equilibrium processes. Potential infiltration progressively decreases with increasing the spread between current water storage and driest water storage level. It drops to few millimeters while water storage is near the drainage line. At this stage reequilibrium processes accelerate because suction changes are linked to poor water content changes24 (i.e., reequilibrium requires now poor amounts of seeping water), while hydraulic conductivity is near the maximum value because of the wet state of the soil24. Rainfall-induced suction drops in the top-soil propagate now rapidly downwards, implying that suction evolution at the shallowest and deepest points arise synchronous (Figure 5b). The layer base still behaves as impervious and pore water pressures often experience a hydrostatic distribution , as indicated by suction fluctuations. Above the drainage line the layer experiences drainage. In a process of progressive water storage increase the drainage line represents the last stage at which pore water pressure trend may be hydrostatic. Beyond this line the breakage pressure of the capillary barrier is attained (Figure 5c). A water storage level between the driest level and the drainage line tends to persist within the layer, especially during winter when it is contrasted only by low evaporation phenomena (at this stage drainage does not occur). It may be identified as that level provided by antecedent rainfalls.
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If antecedent precipitations lead to a water storage close to the drainage line, the major event may be identified as that raising water storage to the saturation line, where instability occurs. This path is short but difficult to be experienced by the layer, because of its low potential infiltration and due to the occurrence of drainage phenomena. Since at this stage the layer adsorbs few millimeters per hour at most and drains water, increments in water storage occur slowly. The saturation threshold may therefore be reached only under the effects of a long-lasting event. This event has also to occur without significant interruptions, as they would result in a rapid return of water storage back to W, under hydraulic hydrostatic equilibrium (re-equilibrium processes at this stage are very fast). If occurring with high intensity, the event would result overabundant and generate run-off, because of the low potential infiltration. This feature of the major event is confirmed by rainfall recorded at two main events triggering flowslides: Nocera Inferiore 2005 and Sarno 1998 (Figure 6). For both events antecedent rainfalls occurred consistently with the attainment of a wet threshold. For the Nocera Inferiore flowslide the event was exceptional from an hydrological point of view: 143 mm at the triggering time and 200 mm at the end of the day. The triggering event was however also persistent, matching the basic requirement indicated previously: the phenomenon was triggered after 16 hours of continuous precipitation at an intensity well larger than the potential infiltration. The rainfall history associated with the Sarno flowslides is traditionally considered significant but not exceptional. The main event is in fact not exceptional from a hydrological point of view (87 mm of total precipitation falling in little more than one day), while it could be considered as an extreme event in term of persistency: intensity set around the potential infiltration at saturation maintained for 28 hours, without any interruption.
Fig. 5. Results obtained through the physical model exposed at elements: (a) suction with scale limited to 90kPa; (b) suction with scale limited to 20kPa; schemes of suction distribution at specific water storage thresholds (after Rianna et al 24 and Pagano et al26)
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cumulation at the intensity of hydraulic conductivity at saturation
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triggering (16 hours)
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triggering (28 hours)
0 0
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Time (hours) Fig. 6. Rainfalls triggering flowslides (after Rianna et al24)
5. Conclusions The work has brought to light the key role played by the lowermost and uppermost boundary conditions on the hydrological behavior of a silty volcanic layer. By neglecting the important role that may be exerted by the presence of vegetation, the work has highlighted the importance of some features traditionally not taken into account in the interpretation or in the theoretical prediction of rainfall induced flowslides. They are, on one hand, the capillary barrier effect determined at the layer bottom by the presence of systems with larger voids and, on the other hand, the soil-atmosphere interaction. With regard to this latter, the state of the topsoil is crucial in regulating actual fluxes at ground level, which diverge significantly from meteorological input. Meteorological forcing during the winter would enhance rainfall infiltration with respect to actual evaporation, but the low suction field in the soil mitigates differences by reducing infiltration (low water-driving shallow gradients) and enhancing evaporation (high availability of water for evaporation); on the other hand, during the summer, meteorological forcing would enhance actual evaporation with respect to rainfall infiltration, but the dry state of the topsoil once again mitigates differences by reducing evaporation (scarce availability of water for evaporation) and increasing rainfall infiltration (high waterdriving gradients in the shallow soils). The definition of water storage thresholds aids in interpreting the features of rainfalls resulting in flowslides. These thresholds are the water storage at driest states, the drainage line and the saturation line. The spread between the first and second thresholds represents the rainwater that may be persistently stored by the layer. It has been associated with the effects induced by antecedent rainfalls. The spread between second and third thresholds is related instead to an amount of rainwater that may be easily lost by the layer due to drainage phenomena. This has been associated with the effects of the major event leading to landslide triggering. Antecedent rainfalls and major event are hence provided with a well- defined physical meaning. Potential infiltration evolution has also been discussed in the paper. It progressively reduces with increasing top-soil water content. Taking as a reference the different factors adopted in the experiment, the effects of changes in grain size distribution may be argued. Testing a coarser grained soil (larger voids) should result in a smaller spread between first and second thresholds and a greater spread between second and third ones, involving a minor influence of
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antecedent rainfalls. A more significant major event is needed, which intensity should be higher, consistently with a supposed higher potential infiltration. Testing a finer grained soil should instead result in a greater spread between first and second thresholds and a smaller spread between second and third ones, involving opposite considerations with respect to the previous hypothesis. References 1. Evangelista A, Nicotera MV, Papa R, Urciuoli G. Field investigation on triggering mechanisms of fast landslides in unsaturated pyroclastic soils. In: Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils - advances in geoengineering, Durham, UK, 2-4 July 2008, London: CRC Press - Taylor & Francis Group. p. 909-915. 2. Cascini L, Sorbino G. The contribution of soil suction measurements to the analysis of flowslide triggering. In: Picarelli L, editor, Proc Int Workshop on Occurrence and Mechanisms of Flow-Like Landslides in Natural Slopes and Earthfills. Bologna: Patron; 2004. p. 77-86. 3. Damiano E, Olivares L, Picarelli L. Steep-slope monitoring in unsaturated pyroclastic soils. Eng Geol 2012;137-138:1-12. 4. Papa R, Pirone M, Nicotera MV, Urciuoli G. Seasonal groundwater regime in an unsaturated pyroclastic slope. Geotechnique 2013;63(5):420426. 5. Olivares L, Picarelli L. Susceptibility of loose pyroclastic soils to static liquefaction: some preliminary data. In: Kühne M, Einstein HH, Krauter E, Klapperich H, Pöttler R, editors, Proc. Int. Conf. on Landslides – Causes, Impacts and Countermeasures. Davos: 2001. p. 75-8. 6. Nicotera MV, Papa R, Urciuoli G. An experimental technique for determining the hydraulic properties of unsaturated pyroclastic soils. ASTM Geotech Test J 2010; 33(4):263-285. 7. Cascini L, Guida D, Nocera N, Romanzi G, Sorbino G. A preliminary model for the landslides of May 1998 in Campania Region. In: Evangelista A, Picarelli L (eds). The Geotechnics of Hard Soils - Soft Rocks. Rotterdam: Balkema, 2000. Vol.3, p. 1623-1649. 8. Pagano L, Picarelli L, Rianna G, Urciuoli G. A simple numerical procedure for timely prediction of precipitation-induced landslides in unsaturated pyroclastic soils. Landslide 2010;7(3):273-289. 9. Olivares L, Damiano E. Post-failure mechanics of landslides: a laboratory investigation of flowslides in pyroclastic soils. J Geotech Geoenviron Eng ASCE 2007;133(1):51-62. 10. Pagano L, Zingariello MC, Vinale F. A large physical model to simulate flowslides in pyroclastic soils. Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils- advances in geo-engineering, Durham, UK, 2-4 July 2008, London: CRC Press - Taylor & Francis Group. p. 111-115. 11. Olivares L, Damiano E, Greco R, Zeni L, Picarelli L, Minardo A, Guida A, Bernini R. An instrumented flume for investigation of the mechanics of rainfall-induced landslides in unsaturated granular soils. ASTM Geotech Test J 2009;32(2):108-118. 12. Greco R, Guida A, Damiano E, Olivares L. Soil water content and suction monitoring in model slopes for shallow flowslides early warning applications. Phys Chem Earth 2010;35:127-136. 13. Sirangelo B, Versace P. A real time forecasting for landslide triggered by rainfall. Meccanica 1996;31:1-13. 14. Calcaterra D, Parise M, Palma B, Pelella L. The influence of meteoric events in triggering shallow landslides in pyroclastic deposits of Campania, Italy. In: Bromhead E, Dixon N, Ibsen ML (eds) Proc 8th Int Symp on Landslides. Rotteram: Balkema, 200. p. 209-214. 15. Evangelista A, Scotto di Santolo A, Lombardi G. Previsione dell’innesco di fenomeni franosi nelle coltri piroclastiche della città di Napoli. In: Proc XIII Convegno Nazionale di Geotecnica, Bologna: Patron, 2007. p. 227-234. 16. Picarelli L, Versace P, Olivares L, Damiano E. Prediction of rainfall-induced landslides in unsaturated granular soils for setting up of early warning systems. In: Proceedings of the 2007 International Forum on Landslide Disaster Management. Hong Kong, 2009. p. 643-665. 17. Brunetti MT, Peruccacci S, Rossi M, Guzzetti F, Reichenbach P, Ardizzone F, Cardinali M, Mondini A, Salvati P, Tonelli G, Valigi D, Lucani S. A prototype system to forecast rainfall-induced landslides. In: Picarelli L, Tommasi P, Urciuoli G, Versace P, editors, Proc 1st Italian Workshop on Landslides. Napoli, 2009. p. 157-161. 18. Toll DG, Lorenco SDN, Mendes J, Gallipoli D, Evans FD, Augarde CE, Cui YJ, Tang AM, Rojas Vidovic JC, Pagano L, Mancuso C, Zigariello MC, Tarantino A. Soil suction monitoring for landslides and slopes. Quart J Eng Geol Hydrogeol 2011;44(1):23-33. 19. Rianna G, Pagano L, Urciuoli G. Investigation of soil-atmosphere interaction in pyroclastic soils. J Hydrol 2014;510:480-492. 20. Lo Presti DCF, Pedroni S, Crippa V. Maximum Dry Density of Cohesionless Soil by Pluviation and by ASTM D 4253-83: A Comparative Study. ASTM Geotech Tes J 1992;15(2):180-189. 21. Damiano E, Olivares L. The role of infiltration processes in steep slopes stability of pyroclastic granular soils: laboratory and numerical investigation. Nat Haz 2010;52(2):329-350. 22. Mancarella D, Simeone V. On Capillary barrier effects in unsaturated layered soils, with special reference to pyroclastic covers of Pizzo d’Alvano in Campania. Italy Bulletin of Engineering Geology and Environment 2012;71(4):791-801. 23. Greco R, Comegna L, Damiano E, Guida A, Olivares L, Picarelli L. Hydrological modelling of a slope covered with shallow pyroclastic deposits from field monitoring data. Hydrol Earth System Sci 2013;17:4001-4013. 24. Rianna G, Pagano L, Urciuoli G. Rainfall patterns triggering shallow flowslides in pyroclastic soils. Eng Geol 2014; in press. 25. Sorbino G, Nicotera MV. Unsaturated soil mechanics in rainfall-induced flow landslides. Eng Geol 2013;165:105-132. 26. Pagano L, Reder A, Rianna G. Processi di infiltrazione ed evaporazione nei terreni piroclastici illustrati attraverso la selezione di alcuni eventi rappresentativi. Rivista Italiana di Geotecnica (in press).
ScienceDirect Procedia Earth and Planetary Science 9 (2014) 14 – 22
The Third Italian Workshop on Landslides
Experiments to investigate the hydrological behaviour of volcanic covers Luca Paganoa*, Alfredo Redera, Guido Riannab a
DICEA – Department of Civil, Construction and Evironmental, University of Naples Federico II, Via Claudio 21, 80125, Naples, Italy b CMCC-ISC – Impact on Soil and Coasts Research Division- Euromediterranean Center on Climat Change, via Maiorise Capua
Abstract The work investigates experimentally the hydrological response of a volcanic silty-sand involved in the Nocera Inferiore flowslide occurred in 2005. To this aim, an infiltration column and a lisimeter have been developed. In both, the same volcanic silty-sand soil has been placed through the pluvial deposition technique. The former device has been used to carry out infiltration laboratory tests aimed at investigating hydraulic boundary conditions acting at the lowermost surface of the soil column for different contact types (pumices, atmosphere, a geosynthetic material). The latter exposes the layer at atmosphere to investigate water fluxes across the uppermost surface induced by the soil-atmosphere interaction. The work shows how the developed physical models feature the hydrological behavior of a silty pyroclastic layer under various meteorological forcing, allowing to build up frameworks useful to characterize the main factors inducing landslides.
© 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università di Napoli. Universit di Napoli. Keywords: TDR probe calibration; heat dissipation probe calibration; pyroclastic soil; dielectric constant; heat conductivity; suction; soil-water content; soil properties.
1. Introduction In the last two decades significant research activities have been devoted in Italy to investigate the hydrological behavior of silty volcanic soils mantling wide areas of the country. Due to increasingly frequent rainfall-induced
* Corresponding author. Tel.: +39 081 7683478 E-mail address: [email protected]
1878-5220 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Universit di Napoli. doi:10.1016/j.proeps.2014.06.013
Luca Pagano et al. / Procedia Earth and Planetary Science 9 (2014) 14 – 22
landslides involving shallow layers made of these soils, research activities have essentially focused on factors affecting the landslide triggering and post-failure behavior. This latter is often characterized by the sliding mass turning into a rapid flowslide. Experimental and theoretical research activities in this field have mostly been conducted in the context of the interpretation of sadly known flowslide case-histories, such as Sarno (1998), Cervinara (1999) Nocera Inferiore (1997 and 2005) Ischia (2006 and 2009). Investigations have involved: (a) field monitoring of soil variables (essentially soil suction and water content) supposed to control both landslide triggering and post-failure behavior1,2,3; (b) laboratory tests on unsaturated soils4,5; (c) use of isothermal approaches (analytical and or numerical solution of Richards’ equation) to predict rainfall-induced suction changes in the subsoil7,2,8; (d) development of suitable physical models of slopes to reproduce wetting-induced shallow landslides9,10,11,12; (e) interpretation of historical rainfall recorded at landslide sites by the use of statistical13,14,15,16,17 or physically-based approaches8 so as to develop predictive tools for early warning systems. The extensive research carried out has clarified the following points: x the soils involved are non-plastic silty sands with gravel (pumiceous stone); the friction angle ranges between 35 and 39 degrees; the coefficient of hydraulic conductivity ranges between 10-7 and 10-6 m/s at full saturation; it decreases significantly by several orders of magnitude as the saturation degree diminishes; the presence of a significant silt fraction involves small size pores in the soil allow rainwater infiltrating the cover to be retained as meniscus water; x the instability is associated with significant increases in pore water pressures 9,8,18; field monitoring carried out by jet-fill tensiometers indicated that suction exceeds the full scale of the instrument (80 kPa) during dry season; precipitations induce significant drops in soil suction; case-history interpretation and experimentation through physical models indicate that landslides turn into rapid flowslides when suction approaches zero throughout the pyroclastic layer; x high water content is responsible for soil liquefaction upon failure; liquefaction might be involved in the triggering mechanism; it determines a post-failure behaviour entailing a significant acceleration of the sliding mass5; during the wet season the high soil porosity (often 70% or higher) allows one cubic metre of soil to trap hundreds of litres of rainwater; high porosity is also what induces a contractile behaviour upon shearing, involving water loading and increases in pore water pressure; this mechanism may induce soil liquefaction, due also to the lack of inter-particle electrochemical forces, testified by the zero plasticity of the soil; x a rainfall event with a daily rainfall height of at least 80 mm is required to trigger a landslide; triggering also requires that the rainfall event is preceded by significant antecedent precipitations8; most landslides occurring during the wet season (from November to May) were often caused by less intense rainfall than that falling during dry periods, when there were no instabilities; a deeper insight into this topic is provided by the Nocera Inferiore case-history of 4 March 20058; the slide was caused by a rainfall event of 140 mm spread over 15 hours; an interpretation of this case, based on solving the Richards equations, showed how the phenomenon was associated with the vanishing of soil suction throughout a layer two metres thick, at a place where the slope angle was slightly higher than the soil friction angle; the study in question also brought to light the role of antecedent rainfall, which had already reduced soil suction to very low values before the start of the triggering event; it was demonstrated that the same rainfall event would not have caused any instability if it had occurred within a drier period with an initial soil state characterized by higher suctions; precipitation during the previous four months had significantly affected soil suction at the triggering time. The physical explanation of the role played by antecedent rainfall on instability of pyroclastic slopes lies precisely in the above ability of these soils to retain a large amount of precipitation water for a long time. It is well known, however, that water infiltrating the soil, if not lost to deep drainage, is subject to evaporation and transpiration phenomena. In pyroclastic layers evapotranspiration is therefore thought to take place to a significant extent: it reduces the effect of antecedent rainfall, affecting soil state at the beginning of the critical rainfall event. It thus lends a positive contribution to avoid conditions for landslide triggering. In this context, the paper investigates the hydrological behavior of silty volcanic soils through experiments carried out by physical models. Particular emphasis is placed in characterizing the lowermost and uppermost
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boundary conditions acting on a shallow pyroclastic cover, by investigating, respectively, the possible capillary barrier effects and the soil-atmosphere interaction. 2. The physical models An extensive description of the physical model may be found in Rianna et al19. In the following, only the main features of the apparatus are briefly illustrated. It consists (Figure 1) of a wooden tank containing a layer of nonplastic sandy silt volcanic soil, very similar to that involved in the Nocera Inferiore 2005 flowslide. The layer has a horizontal section of 1.15x1.15m and a thickness of 0.75 m. It has been put in place at the high field porosity (around 70%) observed at the triggering zone of the mentioned landslide. To this aim the pluvial deposition technique has been adopted20. The layer has been then exposed at weather elements, in order to investigate the soilatmosphere interaction under one dimensional flow conditions. The monitoring scheme implemented in the physical model (Figure 1) quantifies meteorological forcing (total precipitation and potential evaporation), actual fluxes developing across the uppermost layer surface (actual evaporation and infiltrated precipitation) and, finally, the soil state (matric suction, volumetric water content and temperature at different depths). Actual fluxes have been obtained by measuring the weight changes of the tank through three load cells sustaining the tank itself. Actual evaporation has been obtained also by the monitoring of energetic terms at the soil-atmosphere interface (the net radiation by a radiometer, the soil heat flux by a soil heat flux plate installed at a depth of 5 cm, the sensible heat flux by monitoring air temperature and soil temperature at the layer surface). The bottom of the tank is uniformly holed to allow drainage. A geotextile is interposed between the tank bottom and the soil layer in order to prevent erosion. From a hydraulic point of view it behaves as a capillary barrier, breaking (allowing drainage) at a value slightly lower than the atmospheric pressure; this behavior was observed in the laboratory by carrying out an infiltration experiment through a column containing the same sequence of materials, with the silty layer lower-bounded by a geotextile sheet. The evolutions of seepage flows and pore water pressures were measured at the layer base during the imbibition process from the top surface. Tests conducted by replacing the geotextile with a pumiceous layer (sandy gravel layer) or leaving air exposure at the layer bottom resulted in similar results (Figure 2).
Fig.1. The physical model: (a) material put in place within the tank; (b) picture of the physical model; (c) vertical section of the physical model (after Rianna et al19)
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Previous works had already brought to light the capillary barrier effect exerted by a pumiceous layer lowerbounding a silty layer21,22,23. In the physical model the geotextile thus approximates field conditions of a silty volcanic layers lower-bounded by a formation having well larger pore sizes: intensely fractured calcareous bedrock (roughly equivalent to air exposed surface) and pumiceous layers without silty fraction.
Fig. 2. Pore water pressures and cumulative drainage against time for different types of contacts achieved at the bottom of the infiltration column
3. Soil-atmosphere interaction observed via physical modeling The peculiarity of the experiment is that it merges the benefits of field monitoring and laboratory tests by combining layer thickness and evolution of atmospheric variables typical of the field environment, with full control of boundary conditions and material properties, typical of laboratory conditions. Three hydrological years of monitoring deliver the evolution of precipitation, evaporation, water storage, volumetric water content and suction. The water storage, expressed in Figure 3a as overall volumetric water content of the layer, increases at the beginning of wet periods, due to precipitation which infiltrates the layer17. For prolonged wet periods, as those occurring during the first and third years, water storage tends to stabilize at a wet level (Figure 3a, “drainage” line) placed just below the maximum value associated with the layer saturation (Figure 3a, “saturation” line) and regulated by water pressure for which drainage occurs. Above this wet level drainage has been often observed during and immediately after rainfall events, while below this level drainage has never been observed. During the dry periods the water storage decreases (Figure 3a) under the effects of actual evaporation (figure 3b). For prolonged dry periods it tends to attain a minimum at a value of 0.22. Actual evaporation is intense when potential evaporation is intense (Figure 3b) and top-soil water content is high as well. In Figure 3b top-soil water content is expressed by the evolution of volumetric water content at the depth of 15 cm. It indicates the availability of water within the top soil for the state change. Over each hydrological year the layer experiences a main cycle of water storage increase and decrease. Figure 4 shows that an increase in the top-soil water content reduces the amount of infiltrated precipitation (black bars) as percentage of total precipitation (blue bars). An increase in top-soil water content relates in fact to a reduction of soil suction and, consequently, of hydraulic gradients available for rainfall to infiltrate the layer. This “gradient effect” prevails on the “hydraulicconductivity effect”, which is opposite, as hydraulic conductivity increases with increasing soil water content. The dry state of the topsoil associates with high values of potential infiltration (up to 40 mm per hour), while wet states associate with low values (few millimeters per hour)21.
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Fig. 3. Results obtained through the physical model exposed at elements: (a) water storage and precipitation; (b) potential evaporation, actual evaporation and volumetric water content measured at the depth of 15cm (after Rianna et al19 and Rianna et al24)
Fig .4. Results obtained through the physical model exposed at elements: total rainfall, infiltrated rainfall, volumetric water content measured at the depth of 15cm (after Rianna et al.24)
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4. Rainfall history and water storage Some interpretative works of shallow landslides turning into rapid flowslides in silty pyroclastic slopes with inclination angle slightly exceeding the soil friction angle, indicate that triggering does not occur at any times suction vanishes at single depths but, rather, at the specific time suction vanishes contextually at any depth, with the layer experiencing full saturation8. This can be physically explained basing on the fact that full saturation predisposes the slope to extensive liquefaction phenomena, promoting a triggering mechanism of progressive failure under undrained conditions. A different mechanism may however also be invoked. The silty pyroclastic cover often incorporate a pomiceous layer (see for instance the typical schemes reported in Sorbino and Nicotera 25). Under unsaturated conditions the pumiceous layer acts as a capillary barrier, inhibiting downward flow. Rainfall-induced water content increments or suction drops result hence higher in the silty layer placed above the pumiceous layer (shallowest silty layer), due to its inability in draining water, and significantly reduced in the silty layer placed below the pumiceous layer (deepest silty layer), hardly wetted by rainfall-induced seepage flows. Abundant vegetation is also present over the year in these soils, with the root system typically crossing the pumiceous layer. From a mechanical point of view, roots may be supposed to act as reinforcements, binding the shallowest-potentially-instable layer and deepest-resistant layer. Low total stress levels act within the whole system due to the small layer thickness involved, so that the pullout capacity of roots is essentially provided by suction at the root-soil interface. In such a framework, instability could occur when soil suction vanishes throughout the shallow layer, resulting there in the vanishing of the root pullout capacity. The above considerations allow to assume that the layer water storage at full saturation (“saturation line” in Figure 3) may be identified with conditions triggering a rapid flowslide. The hydrological pattern experienced by a silty volcanic layer subject to a rainfall history resulting in flowslide, and, as such, increasing water storage up to the saturation line, may be derived from the experimental results provided by the physical model24. At the beginning of a hydrological year (September) water storage attains the driest level, because of intense and prolonged evaporation phenomena of the summer season. The progression of rainfall events of the wet period increases water storage. This increase is initially enhanced by low evaporation phenomena, high potential infiltration (tens of millimeters) and inhibited drainage (capillary barrier below the layer). The layer behaves as a tank, prone to adsorb rainwater and to store it. At this stage re-equilibrium processes are slow because suction changes link to significant water content changes (re-equilibrium requires a large amount of seeping water) and hydraulic conductivity is held low by the dry state of the soil24. Rainfall-induced suction drops in the top-soil hence delay in propagating downwards, implying that suction evolution at the shallowest and deepest points arise non-synchronous (Figure 5a). The impervious base determines a tendency for pore water pressures to configure at equilibrium as hydrostatic during rainfall interruptions. Fluctuations of suctions indicate however that the hydrostatic state is rarely experienced by the layer, still due to the slowness of re-equilibrium processes. Potential infiltration progressively decreases with increasing the spread between current water storage and driest water storage level. It drops to few millimeters while water storage is near the drainage line. At this stage reequilibrium processes accelerate because suction changes are linked to poor water content changes24 (i.e., reequilibrium requires now poor amounts of seeping water), while hydraulic conductivity is near the maximum value because of the wet state of the soil24. Rainfall-induced suction drops in the top-soil propagate now rapidly downwards, implying that suction evolution at the shallowest and deepest points arise synchronous (Figure 5b). The layer base still behaves as impervious and pore water pressures often experience a hydrostatic distribution , as indicated by suction fluctuations. Above the drainage line the layer experiences drainage. In a process of progressive water storage increase the drainage line represents the last stage at which pore water pressure trend may be hydrostatic. Beyond this line the breakage pressure of the capillary barrier is attained (Figure 5c). A water storage level between the driest level and the drainage line tends to persist within the layer, especially during winter when it is contrasted only by low evaporation phenomena (at this stage drainage does not occur). It may be identified as that level provided by antecedent rainfalls.
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If antecedent precipitations lead to a water storage close to the drainage line, the major event may be identified as that raising water storage to the saturation line, where instability occurs. This path is short but difficult to be experienced by the layer, because of its low potential infiltration and due to the occurrence of drainage phenomena. Since at this stage the layer adsorbs few millimeters per hour at most and drains water, increments in water storage occur slowly. The saturation threshold may therefore be reached only under the effects of a long-lasting event. This event has also to occur without significant interruptions, as they would result in a rapid return of water storage back to W, under hydraulic hydrostatic equilibrium (re-equilibrium processes at this stage are very fast). If occurring with high intensity, the event would result overabundant and generate run-off, because of the low potential infiltration. This feature of the major event is confirmed by rainfall recorded at two main events triggering flowslides: Nocera Inferiore 2005 and Sarno 1998 (Figure 6). For both events antecedent rainfalls occurred consistently with the attainment of a wet threshold. For the Nocera Inferiore flowslide the event was exceptional from an hydrological point of view: 143 mm at the triggering time and 200 mm at the end of the day. The triggering event was however also persistent, matching the basic requirement indicated previously: the phenomenon was triggered after 16 hours of continuous precipitation at an intensity well larger than the potential infiltration. The rainfall history associated with the Sarno flowslides is traditionally considered significant but not exceptional. The main event is in fact not exceptional from a hydrological point of view (87 mm of total precipitation falling in little more than one day), while it could be considered as an extreme event in term of persistency: intensity set around the potential infiltration at saturation maintained for 28 hours, without any interruption.
Fig. 5. Results obtained through the physical model exposed at elements: (a) suction with scale limited to 90kPa; (b) suction with scale limited to 20kPa; schemes of suction distribution at specific water storage thresholds (after Rianna et al 24 and Pagano et al26)
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cumulation at the intensity of hydraulic conductivity at saturation
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triggering (16 hours)
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triggering (28 hours)
0 0
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Time (hours) Fig. 6. Rainfalls triggering flowslides (after Rianna et al24)
5. Conclusions The work has brought to light the key role played by the lowermost and uppermost boundary conditions on the hydrological behavior of a silty volcanic layer. By neglecting the important role that may be exerted by the presence of vegetation, the work has highlighted the importance of some features traditionally not taken into account in the interpretation or in the theoretical prediction of rainfall induced flowslides. They are, on one hand, the capillary barrier effect determined at the layer bottom by the presence of systems with larger voids and, on the other hand, the soil-atmosphere interaction. With regard to this latter, the state of the topsoil is crucial in regulating actual fluxes at ground level, which diverge significantly from meteorological input. Meteorological forcing during the winter would enhance rainfall infiltration with respect to actual evaporation, but the low suction field in the soil mitigates differences by reducing infiltration (low water-driving shallow gradients) and enhancing evaporation (high availability of water for evaporation); on the other hand, during the summer, meteorological forcing would enhance actual evaporation with respect to rainfall infiltration, but the dry state of the topsoil once again mitigates differences by reducing evaporation (scarce availability of water for evaporation) and increasing rainfall infiltration (high waterdriving gradients in the shallow soils). The definition of water storage thresholds aids in interpreting the features of rainfalls resulting in flowslides. These thresholds are the water storage at driest states, the drainage line and the saturation line. The spread between the first and second thresholds represents the rainwater that may be persistently stored by the layer. It has been associated with the effects induced by antecedent rainfalls. The spread between second and third thresholds is related instead to an amount of rainwater that may be easily lost by the layer due to drainage phenomena. This has been associated with the effects of the major event leading to landslide triggering. Antecedent rainfalls and major event are hence provided with a well- defined physical meaning. Potential infiltration evolution has also been discussed in the paper. It progressively reduces with increasing top-soil water content. Taking as a reference the different factors adopted in the experiment, the effects of changes in grain size distribution may be argued. Testing a coarser grained soil (larger voids) should result in a smaller spread between first and second thresholds and a greater spread between second and third ones, involving a minor influence of
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antecedent rainfalls. A more significant major event is needed, which intensity should be higher, consistently with a supposed higher potential infiltration. Testing a finer grained soil should instead result in a greater spread between first and second thresholds and a smaller spread between second and third ones, involving opposite considerations with respect to the previous hypothesis. References 1. Evangelista A, Nicotera MV, Papa R, Urciuoli G. Field investigation on triggering mechanisms of fast landslides in unsaturated pyroclastic soils. In: Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils - advances in geoengineering, Durham, UK, 2-4 July 2008, London: CRC Press - Taylor & Francis Group. p. 909-915. 2. Cascini L, Sorbino G. The contribution of soil suction measurements to the analysis of flowslide triggering. In: Picarelli L, editor, Proc Int Workshop on Occurrence and Mechanisms of Flow-Like Landslides in Natural Slopes and Earthfills. Bologna: Patron; 2004. p. 77-86. 3. Damiano E, Olivares L, Picarelli L. Steep-slope monitoring in unsaturated pyroclastic soils. Eng Geol 2012;137-138:1-12. 4. Papa R, Pirone M, Nicotera MV, Urciuoli G. Seasonal groundwater regime in an unsaturated pyroclastic slope. Geotechnique 2013;63(5):420426. 5. Olivares L, Picarelli L. Susceptibility of loose pyroclastic soils to static liquefaction: some preliminary data. In: Kühne M, Einstein HH, Krauter E, Klapperich H, Pöttler R, editors, Proc. Int. Conf. on Landslides – Causes, Impacts and Countermeasures. Davos: 2001. p. 75-8. 6. Nicotera MV, Papa R, Urciuoli G. An experimental technique for determining the hydraulic properties of unsaturated pyroclastic soils. ASTM Geotech Test J 2010; 33(4):263-285. 7. Cascini L, Guida D, Nocera N, Romanzi G, Sorbino G. A preliminary model for the landslides of May 1998 in Campania Region. In: Evangelista A, Picarelli L (eds). The Geotechnics of Hard Soils - Soft Rocks. Rotterdam: Balkema, 2000. Vol.3, p. 1623-1649. 8. Pagano L, Picarelli L, Rianna G, Urciuoli G. A simple numerical procedure for timely prediction of precipitation-induced landslides in unsaturated pyroclastic soils. Landslide 2010;7(3):273-289. 9. Olivares L, Damiano E. Post-failure mechanics of landslides: a laboratory investigation of flowslides in pyroclastic soils. J Geotech Geoenviron Eng ASCE 2007;133(1):51-62. 10. Pagano L, Zingariello MC, Vinale F. A large physical model to simulate flowslides in pyroclastic soils. Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils- advances in geo-engineering, Durham, UK, 2-4 July 2008, London: CRC Press - Taylor & Francis Group. p. 111-115. 11. Olivares L, Damiano E, Greco R, Zeni L, Picarelli L, Minardo A, Guida A, Bernini R. An instrumented flume for investigation of the mechanics of rainfall-induced landslides in unsaturated granular soils. ASTM Geotech Test J 2009;32(2):108-118. 12. Greco R, Guida A, Damiano E, Olivares L. Soil water content and suction monitoring in model slopes for shallow flowslides early warning applications. Phys Chem Earth 2010;35:127-136. 13. Sirangelo B, Versace P. A real time forecasting for landslide triggered by rainfall. Meccanica 1996;31:1-13. 14. Calcaterra D, Parise M, Palma B, Pelella L. The influence of meteoric events in triggering shallow landslides in pyroclastic deposits of Campania, Italy. In: Bromhead E, Dixon N, Ibsen ML (eds) Proc 8th Int Symp on Landslides. Rotteram: Balkema, 200. p. 209-214. 15. Evangelista A, Scotto di Santolo A, Lombardi G. Previsione dell’innesco di fenomeni franosi nelle coltri piroclastiche della città di Napoli. In: Proc XIII Convegno Nazionale di Geotecnica, Bologna: Patron, 2007. p. 227-234. 16. Picarelli L, Versace P, Olivares L, Damiano E. Prediction of rainfall-induced landslides in unsaturated granular soils for setting up of early warning systems. In: Proceedings of the 2007 International Forum on Landslide Disaster Management. Hong Kong, 2009. p. 643-665. 17. Brunetti MT, Peruccacci S, Rossi M, Guzzetti F, Reichenbach P, Ardizzone F, Cardinali M, Mondini A, Salvati P, Tonelli G, Valigi D, Lucani S. A prototype system to forecast rainfall-induced landslides. In: Picarelli L, Tommasi P, Urciuoli G, Versace P, editors, Proc 1st Italian Workshop on Landslides. Napoli, 2009. p. 157-161. 18. Toll DG, Lorenco SDN, Mendes J, Gallipoli D, Evans FD, Augarde CE, Cui YJ, Tang AM, Rojas Vidovic JC, Pagano L, Mancuso C, Zigariello MC, Tarantino A. Soil suction monitoring for landslides and slopes. Quart J Eng Geol Hydrogeol 2011;44(1):23-33. 19. Rianna G, Pagano L, Urciuoli G. Investigation of soil-atmosphere interaction in pyroclastic soils. J Hydrol 2014;510:480-492. 20. Lo Presti DCF, Pedroni S, Crippa V. Maximum Dry Density of Cohesionless Soil by Pluviation and by ASTM D 4253-83: A Comparative Study. ASTM Geotech Tes J 1992;15(2):180-189. 21. Damiano E, Olivares L. The role of infiltration processes in steep slopes stability of pyroclastic granular soils: laboratory and numerical investigation. Nat Haz 2010;52(2):329-350. 22. Mancarella D, Simeone V. On Capillary barrier effects in unsaturated layered soils, with special reference to pyroclastic covers of Pizzo d’Alvano in Campania. Italy Bulletin of Engineering Geology and Environment 2012;71(4):791-801. 23. Greco R, Comegna L, Damiano E, Guida A, Olivares L, Picarelli L. Hydrological modelling of a slope covered with shallow pyroclastic deposits from field monitoring data. Hydrol Earth System Sci 2013;17:4001-4013. 24. Rianna G, Pagano L, Urciuoli G. Rainfall patterns triggering shallow flowslides in pyroclastic soils. Eng Geol 2014; in press. 25. Sorbino G, Nicotera MV. Unsaturated soil mechanics in rainfall-induced flow landslides. Eng Geol 2013;165:105-132. 26. Pagano L, Reder A, Rianna G. Processi di infiltrazione ed evaporazione nei terreni piroclastici illustrati attraverso la selezione di alcuni eventi rappresentativi. Rivista Italiana di Geotecnica (in press).