The relationship between slope stability and pore-water pressure in highly plastic clay in the Carrot River Basin

Abstract

Slope instability in glaciolacustrine clays has been a problem since 1950 at two sites in the Carrot River valley. Movement at the Carrot River and Smoky Burn bridge sites persists, compromising the integrity of the bridge structures. In addition, movement persists along an embankment constructed between 1990 and 1992.

Residual shear strength parameters, Φᵣ' = 7.6° with c' = 0 were determined for three landslides by back analysis. Laboratory tests showed Φᵣ' ranged from 6.0 to 9.1 °. Pore-water pressure distributions were characterized with models of steady state groundwater flow, drawdown following flooding, and of induced pore-water pressure caused by construction. The flood hydrograph was represented as a total head versus time boundary condition for the transient groundwater seepage model. Factors of safety were calculated throughout the course of the flood event with pore-water pressures characterized by the transient seepage model. Factors of safety predicted with the transient seepage model following drawdown were greater than for traditional approaches of rapid drawdown analyses used for dams and reservoirs. This is supported by the observation that, historically, flooding has not resulted in catastrophic failure of slopes. Heavy precipitation during flooding was found to decrease stability according to the modelling. Pore-water pressure induced during construction of an embankment was predicted with an effective stress analysis utilizing the pore pressure coefficients A and B. Predicted pore-water pressure induced along the shear zone of the landslide compared favourably with pore-pressure measured during construction.

Stabilization of active landslides at Carrot River and Smoky Burn bridge abutments may be achieved with the construction of a berm in the river channel to resist landslide movement. The berm would consist of granular fill, with a culvert to discharge low flows. The berm would function as a weir during flooding.