Influence of soil-atmospheric boundary conditions on slope stability analyses

Shubhra Pk.

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: GEOHAZARDS - VI Climate Change Floods & Landslides

ABSTRACT: In slope stability analyses,the complexflux boundary conditionsthat occur atthe soil-atmosphere interfaceare oftensimplified.Asimplified soil-atmospheric boundarytypicallyconsidersonlyinfiltration andignoresevaporation andtranspiration processes.However in certain contexts, such as landslide early warning systems and future climate changeinstability predictions, the actual fluxes at the soil-atmospheric boundary become critical.Inthis study,the effectsofdifferent soil-atmospheric boundary conditions on a slope•s stability are examined.Anumericalfinite elementslopestability analysis coupledwithan actualsoil-atmospheric boundary are compared to one coupled with asimplifiedboundary. The results of this studyshowthat over a period of one month, theslope•s stability ishighly dependentonthesoil-atmosphere conditions of the modelthat become critical during peak precipitation events.It was found thatover agreater length of time, the soil-atmospheric conditions had a lesser effect on the stability of the model slope, with peakdifferences occurring during dry to wet season transition periods.The results of this studydemonstrate the importance ofcarefullyselecting theappropriatesoil-atmosphericmodel for the given purpose and application.R†SUM†Dans les analyses la stabilit‡ des pentes, les conditions aux limites de flux complexes qui se produisent … l'interface sol-atmosph—re sont souvent simplifi‡es. Une limite du sol atmosph‡rique simplifi‡e consid—re g‡n‡ralement que l'infiltrationet ignoreles processus d'‡vaporation et de transpiration. Cependant, dans certains contextes, tels que les glissementsde terrain syst—mes d'alerte pr‡coce et les pr‡dictions du changement climatique d'instabilit‡, les flux r‡els … la limite dusol atmosph‡rique deviennent critiques. Dans cette ‡tude, les effets des diff‡rentes conditions aux limites du solatmosph‡rique sur la stabilit‡ de la pente sontexamin‡s. Un ‡l‡ment fini pente analyse de stabilit‡ num‡rique coupl‡e …une limite du sol atmosph‡rique r‡elle sont compar‡s … un coupl‡e … une fronti—re simplifi‡e. Les r‡sultats de cette‡tude montrent que sur une p‡riode d'un mois, la stabilit‡ des pentes est fortement d‡pendante des conditions du sol etl'atmosph—re du mod—le. Il a ‡t‡ constat‡ que, sur une plusgrande longueur de temps, les conditions du solatmosph‡rique ont un moindre effet sur la stabilit‡ de la pente du mod—le, avec des ‡carts de pointe se produisantpendant des p‡riodes s—ches de transition de la saison des pluies.Les r‡sultats de cette ‡tude d‡montrent l'importancede choisir les conditions de d‡limitation des sols atmosph‡rique correctes pourl'utilisation des applications.1INTRODUCTIONA soil-atmospheric boundary refers to a flux boundary atsoil-atmosphere interfaceacross which flowor fluxoccurs.The common fluxes observed in soil-atmosphereinterface are infiltration, evaporation and transpiration.These fluxes areaffected by several factors, such asclimatic conditions, hydrological influence, geologicalactivities, weathering action, vegetation, topography andhuman interventions(Rahardjo et al. 2013).Estimationofthese fluxesand application of soil-atmosphericboundariesare challenging, and thereforemanygeotechnical analyses commonly ignorethiscomplexityand simplify the soil-atmospheric boundary conditions.However, the actual soil-atmospheric conditionscan be ofparticular importance for certain design applications, suchas soil cover system in a waste disposal site(e.g.Wilsonet al. 1993), seepage analyses(e.g.Gitirana et al. 2006),and slope stabilityanalyses and monitoring(e.g.Rahardjoet al. 2013).Current literature shows that slope stability analysesusuallyconsiderrainwater infiltration andsimplify and/orignoreother fluxes(e.g.Rahardjo et al.2001, Liew et al.2004, Li et al. 2005, Gitirana et al. 2006, Evangelista et al.2008, Ng et al. 2008). Only a few studies have consideredthe actual soil-atmospheric boundary conditionsandanalyzed the effects of flux boundarieson slope stabilityanalyses(e.g.Rahardjo et al. 2013, 2014).These studiesfocused their analysis on either a small number ofrainfalleventsora very short period of time.However, manyslope stability analyses need to run for aprolongedperiodof time, such as continuous monitoring of a vulnerableslopes,climate change impact studies,and landslideEarly Warning System (EWS).Landslideearly warning systems workasariskmitigation tool, working to try and remove the riskelements (people) away from the landslide in time.Because landslidesareoftentriggered byintense andprolonged precipitation,significant works has been doneto developEWSs.The firstsetof EWSswerebased onansimple statisticalrelationship betweenthe temporalincidencesof pastprecipitation andlandslide events (e.g.Caine 1980;Keefer et al. 1987;Wieczorek 1987;Larsenand Simon 1993;Aleotti 2004;Baum and Godt 2009).However, the performance of these empirical thresholdswasoftenunsatisfactory due totheirover simplification.To compensate for this simplification,physical thresholdsbecame more widespread, which works to combinethewhole hydrogeological,geotechnical and climatologicalprocess of the site (e.g.Montgomery and Dietrich 1994;

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