A transient analysis of slope stability following drawdown after flooding of a highly plastic clay

The stability of slopes at bridge abutments across the Carrot River in east -central Saskatchewan was not influenced significantly by drawdown after fl ooding in the spring of 1995. Traditional methods of analysis for rapid dra wdown predicted the factor of safety of slopes on highly plastic clays of p roglacial Lake Agassiz would drop to 0.65 from an initial value of 1.0. Def ormation along a well-defined slip plane has persisted at a more or less co nstant, slow rate since the bridge was constructed in 1975. The river rose approximately 10 m during a flood in the spring of 1995, yet there was only minimal response in piezometers and no measurable increase in the rate of deformation recorded by inclinometers. Pore-water pressures from a steady s tate seepage model, which was calibrated from piezometer measurements, were integrated into a stability analysis. Changes in pore-water pressures caus ed by flooding and subsequent drawdown were characterized from a transient seepage model using the flood hydrograph as a flux boundary. The stability analysis integrated with the transient seepage model estimated the factor o f safety would drop from 1.0 to 0.91 after drawdown. Field measurements ind icated the reduction in factor of safety was even less.