Slope stability evaluation by limit equilibrium and finite element methods

A case study is presented for a natural slope situated in Nepal, where a hydropower project is being constructed. A 40 in high concrete dam is situated close to a 65 in high slope, consisting of river and glacier deposits. The long-term safety of the slope is evaluated by the software codes PLAXIS and GeoSlope, mainly changing the groundwater conditions in the slope. Variations of these conditions have also been studied by Janbu's Direct method. The computed factors of safety from PLAXIS are based on Mohr-Coulomb model, whereas the results from GeoSlope utilize the limit equilibrium methods. Safety factors obtained from these methods for a number of water level variations are discussed and compared. The groundwater variation is studied both for dry and wet seasonal conditions. Apart from this, conditions with pseudo-static earthquake loading in GeoSlope and dynamic loading in PLAXIS have been presented. The dynamic analyses have been performed based on prescribed displacements at the base of the model and selected acceleration-time series. The output results from the dynamic analyses are discussed in terms of displacements, acceleration and generated pore pressure in the slope. The authors have concluded that potentially unstable slope conditions exist due to combined groundwater variations and earthquake impact.