ALBERT

Description: High-rise buildings in downtown Edmonton have generally been founded on spread footings in glacial till. Early geotechnical work proposed a geological model comprising uniform, very dense basal (lodgement) till. However, recent work shows that the till is more complex, being variable in both material type and consistency. A case history is described that exemplifies this variability. It is concluded that early experience with the dense tills has led to an overconfident approach to soil investigation in this area and that a thorough site investigation is required.

Description: Westburne Dam, located near Coronation, Alberta, is a 30 m high earth-fill dam with a crest length of approximately 200 m. It was constructed in 1969, for water storage purposes, inatributary valley of the Battle River. Local geology comprises sand, silt, and glacial clay till overlying sandstone and clay shale of the Bearpaw Formation. A large landslide occurred at the north abutment in 1971. In order to assess the performance of the dam, a monitoring program was initiated in 1972. A total of 28 slope indicators, 30 piezometers, and a number of standpipes were installed in the dam, abutments, and slide area. A ground survey network was established in 1974 and has since been surveyed annually.Cumulative ground movements up to 4 m have been observed in the slide mass adjacent to the dam, at rates as high as 900 mm/year. Movements of up to 20 mm/year have been measured in the dam. Piezometric conditions in the slide have been found to be related to the reservoir level. Boreholes, drilled as part of a stability review in 1979, confirmed a shear surface, previously identified by slope indicators, and showed it to extend beneath the dam. It is thought to be associated with the mechanism of a larger, older, slide underlying the dam site.Remedial action taken in 1980 included lowering the dam crest and the maximum operating reservoir level. Piezometric levels and ground measurements have since declined somewhat; the slide mass has shown rates of movement of 100–200 mm/year. Through implementation of the detailed monitoring program and remedial measures, the dam has been maintained in an operational capacity. The paper presents and discusses the results of the monitoring program, providing a comprehensive 10-year record of dam performance. Keywords: earth-fill dam, landsliding, instrumentation, ground movements, pore pressures, slope stability analysis.