MODELLING OF HYDROLOGICAL TRIGGERING MECHANISMS OF LANDSLIDES IN VOLCANIC ASHES (COLOMBIA)

M.T.J. Terlien (International Institute for Aerospace Survey and Earth Sciences (ITC), Kanaalweg 3, 2628 EB Delft, The Netherlands)

Hydrological investigations of landslide-prone ash-covered slopes were executed north-east of Manizales (Colombia) to establish hydrological triggering mechanisms of different landslide types. Tensiometer measurements revealed the presence of saturated zones in the upper meter of the soil profile during and shortly after rainstorms and perched water tables and a groundwater table deeper in the ashes present during the entire rainy season. The saturated zones and perched water tables were related to changes in soil moisture and hydraulic conductivity while the groundwater was caused by the presence of semi-impermeable bedrock. A detailed survey of five landslides demonstrated that the zones where saturated conditions developed form potential slip surface zones. Two hydrological models were used to predict the spatial and temporal variation of pore pressures on potential slip surfaces and to determine return periods of critical pore pressures. The pore pressure fluctuations during infiltration and percolation of rainstorms were modelled with a 1-dimensional finite element model. To model the daily groundwater fluctuations in the ashes a 2.5- dimensional finite difference model was developed. Both models were calibrated and verified with field data. The results of the hydrological modelling were used in limit-equilibrium slope stability calculations to calculate failure probabilities.