Investigation of a pore pressure driven slope failure using a coupled hydro-mechanical model

• The Belvoir Park Forest landslip is situated in Northern Ireland.
• The landslide was monitored with inclinometers, piezometers, etc.
• An influential factor was the elevated pore water pressures along the failure plane.
• Limit equilibrium analysis was not able to predict the effects of rainfall patterns on the landslip.
• A hydro-mechanical model was able to reproduce the movement trends of the landslip.

The Belvoir Park Forest landslip is situated in Northern Ireland. It was monitored for more than five years with inclinometers, standpipe piezometers, and slip indicators providing detailed information on the long term behaviour and rate of movement of a large low angle failure in Glacial Till. A detailed geological model was constructed on the basis of the geomorphology and geology of the area and ground investigation data. A detailed geotechnical model was prepared with estimates of stiffness, permeability, shear strength and unit weight data. The geometry of the failure plane, residual strengths, movement rates, pore pressure fluctuations and rainfall data was used to complete a model of the failure with limit equilibrium methods. This model was able to demonstrate that pore water pressure was the most influential factor when considering the stability of the slope, but could not determine the effect of rainfall fluctuations on the observed rate of movement of the slip. A more sophisticated coupled hydro-mechanical model based on Shetran and Flac-tp flow was used to determine the effect of fluctuations of pore pressures, rainfall and mechanical strains on slip movement. This paper illustrates some of the advantages of using more sophisticated slope modelling approaches alongside more ‘traditional’ modelling of residual slope failures to understand their future evolution.