Soil water content and suction monitoring in model slopes for shallow flowslides early warning applications R. Greco a,b, * , A. Guida a , E. Damiano a , L. Olivares a,b a Dipartimento di Ingegneria Civile, Seconda Università di Napoli, via Roma 29, 81031 Aversa (CE), Italy b CIRIAM – Centro Interdipartimentale di Ricerca in Ingegneria Ambientale, Seconda Università di Napoli, via Roma 29, 81031 Aversa (CE), Italy article info Article history: Received 14 November 2008 Received in revised form 8 October 2009 Accepted 21 December 2009 Available online 4 January 2010 Keywords: Flowslides Pyroclastic granular soils Physical modelling TDR Early warning abstract The results of laboratory infiltration experiments on instrumented model slopes are presented. Loose granular volcanic ashes from the mountainous area north-eastern of Naples, responsible for several large flowslides during the last decades, have been tested. The experiments were aimed at a better understand- ing of the hydraulic processes leading to slope failure, in order to identify the most useful variables to be monitored for building up effective early warning systems. With respect to this, useful information was provided by coupled measurements of soil suction and volumetric water content, respectively carried out by minitensiometers and TDR, and by slope surface settlement measurements, made with optical laser sensors. In particular, monitoring of soil volumetric water content seemed more useful than soil suction monitoring for early warning purposes, since water content grew smoothly during the entire infiltration processes, while soil suction showed abrupt steep fronts. Furthermore, the obtained results showed how rainfall infiltration was significantly affected by soil volumetric collapse, in turn related to its initial porosity. In addition, during infiltration flume tests with soils showing volumetric collapse, suction resulted nearly in all cases much smaller than expected from laboratory water retention curves. Since steep slopes equilibrium is often guaranteed by cohesion increment due to suction under unsaturated conditions, the obtained results indicate that soil volumetric collapse may be responsible for flowslide triggering when soil is still unsaturated. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Risk associated with occurrence of fast landslides has generally grown during the last decades, due to the increased density of set- tlements, industrial plants and infrastructures. Such problem is particularly worrying in Campania (Southern Italy), where fast population growth led to diffuse building activity without plan- ning: indeed, recent flowslides caused hundreds of victims and heavy damage to buildings, roads and other infrastructures. Flowslides occurrence in Campania is quite frequent, due to the presence of mountainous areas with steep slopes covered with loose pyroclastic granular soils (Calcaterra et al., 2002). For very steep slopes, with inclination comparable or greater than internal friction angle, stability under unsaturated conditions is guaranteed by the apparent cohesion due to soil matric suction. Whereas rain- fall infiltration causes soil to approach saturation, suction de- creases and eventually slope failure occurs (Olivares and Picarelli, 2003). Flowslides involve primary pyroclastic deposits consisting in alternating layers of unsaturated volcanic ash (non-plastic silty sand) and pumice (gravely sand), with maximum overall thickness of a few meters. Flowslide movement can attain a peak velocity of tens of meters per second (Faella and Nigro, 2003), behaving like fluid mud rather than soil: the flowing mass eventually stops only on flat areas or after impact against structures. According to several authors (i.e., Sladen et al., 1985) flowslide generation is the result of soil liquefaction, the shear strength de- crease due to fast accumulation of positive excess pore pressure. Flowslide triggering may be favoured by concurrent causes, such as either stratigraphical (Crosta and Dal Negro, 2003) or geometri- cal discontinuities, such as scarps and road cuts (Guadagno et al., 2005), as well as flow accumulation due to slope morphology or to springs at soil–bedrock interface (Cascini et al., 2008). Undrained shear tests, carried out either on natural or reconsti- tuted specimens, have shown that liquefaction is typical of loose gravely sands and silty sands (Hunter and Fell, 2003) having a non-plastic fine-grained component (Yamamuro and Lade, 1998). Furthermore, for liquefaction to take place, soil should be nearly saturated and soil deformation rate should be so fast to avoid dis- sipation of excess pore pressure, induced by first soil movement at the beginning of slope failure (Fleming et al., 1989). 1474-7065/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2009.12.003 * Corresponding author. Address: Dipartimento di Ingegneria Civile, Seconda Università di Napoli, via Roma 29, 81031 Aversa (CE), Italy. E-mail address: roberto.greco@unina2.it (R. Greco). Physics and Chemistry of the Earth 35 (2010) 127–136 Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce