Deformation of the Deer Creek bridge by an active landslide in clay shale

Abstract

Two parallel, concurrently active slip surfaces of a landslide in clay shale of the Cretaceous Lea Park Formation are causing deformation of a bridge structure across the North Saskatchewan River near Deer Creek, Saskatchewan. The upper slip occurs at the contact between the shale and glacial deposits, which is common in this region. However, the second slip occurs deep in the shale, 24 m below the upper slip zone. This multilevel landslide mechanism, not reported previously in this region, is resulting in a complex deformation pattern where components of the structure are moving at different rates. The multilevel slip mechanism is related to a unique combination of the hydrogeology and geologic structure at this site. Under an upward groundwater gradient, slip surfaces occur at discontinuities in available shearing resistance at different elevations in the shale. The discontinuities are gouge zones in the clay shale, which are the result of a combination of glacial shear and regional tectonism where parameters have been reduced to a residual state ([Formula: see text] and c′ = 0). The pore-water pressures for the slope stability analysis were generated from a site specific finite element seepage model using boundary conditions determined from a regional finite element seepage model. The groundwater models were calibrated from piezometer data and from hydrochemistry of water from farm wells, piezpmeters, and natural surface ponds. The hydrochemistry was used to delineate groundwater, discharge areas from recharge areas. The validity of the landslide mechanism is supported by a stability analysis integrated with the finite element seepage analysis, which demonstrates that two separate parallel slip surfaces at different depths can be at a state of limiting equilibrium concurrently. Key words : bridge deformation, Cretaceous shale, integrated models, residual strength, multilevel slips.