Slope Failure in Cretaceous Clay Shale in Western Manitoba: A Case Study

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Fiebelkorn, Jeremy
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Slope instabilities have been affecting the grade slope of Provincial Trunk Highway 5 near the junction with Provincial Trunk Highway 10 in northwestern Manitoba for over 50 years. In recent years, the instabilities have resulted in significant damage to the highway pavement surface. In 2011, Manitoba Infrastructure and Transportation initiated a geotechnical investigation to gain a better understanding of the failure, identify possible failure mechanisms, and explore various remedial design alternatives in order to stabilize the slope. The site was instrumented with slope inclinometers and vibrating wire piezometers, and monitored over a period of two years. An extensive laboratory testing program was completed to compare the results of direct shear tests and torsional ring shear tests for determining the shear strength of the underlying Cretaceous clay shale. Measured values were compared with values back analyzed using limit equilibrium analysis. A coupled finite element model was used to model the expected excess porewater pressure response, and therefore the stability of the slope, during construction of a stabilization berm. It was subsequently calibrated to agree with the measured porewater pressure responses from the instrumentation. Finally, spring flood conditions were simulated to determine the effect of multiple flash flood events on the stability of the slope.
Slope Stability, Ring Shear, Flooding, Elastic Stiffness