Abstract:
Landslides are a major global hazard that affects human and economic losses every year. The
spatial and temporal distribution of landslides is one of the fundamental tools for landslide
susceptibility, hazard and risk assessment to establish land use policies and mitigation
measures. Sri Lanka has been identified as a hotspot for rainfall-induced landslides,
considering the heavy and prolonged rainfalls received during tropical monsoon seasons.
Further, favourable preparatory factors for landslides such as ground conditions,
geomorphology and anthropogenic activities prevails in the country. The significant
weathering in the tropical region due to temperature and humidity can form thick soil layers
and geological structures of high variability. The ground conditions in the central hill country
in Sri Lanka is highly complex due to the weathering of the crystalline metamorphic rock of
Precambrian age which exists in this region. Colluvium soils are a heterogeneous mixture of
soil and rock particles accumulated at the toe area of the slope due to previous downslope
movements under gravity. Movements within these layers can get easily reactivated due to
porewater pressure variations or human interventions. Currently, landslide susceptibility in
the mountainous regions of Sri Lanka has been identified and mapped based on a model
considering six terrain factors and factor classes on the scale of 1:10,000 and 1:50,000.
Landslide early warnings are issued for the high susceptible areas based on the empirical
thresholds identified for the whole nation and these can lead to false alarms. Hence it is vital
to implement early warning based on rainfall thresholds identified using process-based
methods, which is more accurate and reliable. In this study, the suitability of limit equilibrium
(LE) and finite element (FE) methods to assess slope stability with rainfall infiltration was
evaluated using two case studies. The first case study was conducted on the Kithulgala
landslide which occurred on 15th May 2016 in Kithulgala area along the Avissawella-Haton
main road. Sobasiripura landslide which occurred on the same day in Hanwella North was
selected as the second case study. For the LE study SLOPE/W software was utilised along with
the SEEP/W software for FE seepage analysis. Alternatively, PLAXIS 2D software was used
to conduct a FE based coupled hydro-mechanical stability analysis. Rainfall for 10 days prior
to the failure was considered for the analysis to allow the antecedent rainfall conditions and the
equilibrium of the groundwater table. From the results of the study, it was evident that both
methods can be used to effectively verify the failure and triggering rainfall that closely
resemble the failure surface identified at the site. The failure occurred along the colluviumweathered
rock interface in both cases and the matric suction loss in the colluvium layer was
the main reason for the failure rather than the rise of the groundwater table.
