Abstract:
Landslides induced by heavy rainfall pose a common geotechnical hazard in Sri Lanka. Sloping
grounds in Sri Lanka are made of rocks at different levels of weathering, residual and colluvial
soils. The parent rock's mineralogical composition influences the heterogeneous nature of these
weathering profiles. Slopes in the region have deep groundwater tables and unsaturated soil
profiles with high matric suction during dry periods. However, prolonged rainfall leads to slope
instability due to the loss of matric suction and the potential formation of a perched water table
or the rise of the groundwater table.
National Building Research Organisation (NBRO) has developed the landslide hazard zonation
maps of Sri Lanka at 1:50000 and 1:10000 for districts where landslide risk is high based on
the overall hazard rating (score) determined by terrain factors. The terrain factors are bedrock
geology and geological structures, type and natural soil cover and thickness, slope range and
category, hydrology and drainage, land use and management and landform. In landslide hazard
zonation maps, sloping grounds are divided into regions of different hazard levels: Safe slope,
landslide not likely to occur, a modest level of landslide hazard and landslides are to be
expected.
To supplement that zonation, it is essential to identify the threshold rainfall intensities causing
a disastrous situation in zones of different landslide susceptibility. Currently, the threshold
rainfall values are determined by previous experience.
Researchers have developed different empirical statistical relationships to identify threshold
intensities that vary with local landslide factors and site-specific. Given the significant material
variations in Sri Lankan slopes, relying solely on statistical approaches is inappropriate.
Landslide susceptibility level-specific thresholds based on numerical studies would be
necessary to make reliable decisions regarding early warning and remedial measures. In this
research, landslides where the triggering rainfall data is available are back analysed to establish
a relationship between landslide susceptibility zonation and threshold rainfall intensities.
The SEEP/W 2018 software was used to model rainwater infiltration and the consequent
changes in the pore water pressure. The results of the above seepage analysis were then
transferred to SLOPE/W software to analyse the slope stability. Initially, recent landslides
(Pinnawala landslide and Kithulgala landslide) where rainfall records are available was back
analysed and threshold rainfall values for these two slopes was established. The level of hazard
of these two landslide areas was identified using the landslide hazard zonation maps of Sri
Lanka that developed by National Building Research Organisation (NBRO). A relationship
between landslide susceptibility zonation and threshold rainfall intensities can be established
by doing this analysis on several landslides that rainfall data was available.
