DEFORMATION OF THE DEER CREEK BRIDGE BY AN ACTIVE LANDSLIDE IN CLAY SHALE

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 Cree k, Saskatchewan. The upper slip occurs at the contact between the shal e and glacial deposits, which is common in this region. However, the s econd slip occurs deep in the shale, 24 m below the upper slip zone. T his multilevel landslide mechanism, not reported previously in this re gion, is resulting in a complex deformation pattern where components o f the structure are moving at different rates. The multilevel slip mec hanism is related to a unique combination of the hydrogeology and geol ogic structure at this site. Under an upward groundwater gradient, sli p surfaces occur at discontinuities in available shearing resistance a t different elevations in the shale. The discontinuities are gouge zon es 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 (phi' = 6.5 degrees and c' = 0). The pore-water pressur es for the slope stability analysis were generated from a site specifi c finite element seepage model using boundary conditions determined fr om a regional finite element seepage model. The groundwater models wer e calibrated from piezometer data and from hydrochemistry of water fro m farm wells, piezometers, and natural surface ponds. The hydrochemist ry was used to delineate groundwater discharge areas from recharge are as. The validity of the landslide mechanism is supported by a stabilit y analysis integrated with the finite element seepage analysis, which demonstrates that two separate parallel slip surfaces at different dep ths can be at a state of limiting equilibrium concurrently.