The Bearpaw formation (Upper Cretaceous) which occurs over large areas of Western Canada presents problems in connection with engineering works. These clay shales were consolidated under high pressures, but the load has been removed. A rebound occurred which is a maximum at the shale surface and decreases with depth. The rebound is made up of elastic rebound which occurred immediately upon removal of load and a “time rebound” which is still occurring slowly and involves a softening of the material.
The characteristics of the shale and the depth of the rebound zone are being studied by soil-mechanics methods. Routine tests were made for water content, density, and Atterberg limits; water content is the best indicator of consistency. The results of laboratory strength tests do not correlate well with observed field behavior, and this has led to the conclusion that observations of existing slopes will supply more reliable strength data for long-time stability studies. Field investigations involve piezometric measurements and observations of slides, particularly creep. The softened surface zone is unstable, and slides have occurred on flat slopes. Slickensides are common in this zone, but their intensity appears to decrease with depth. The underlying shale is firm and stable.
Rebound is also caused by local excavations for structures. Observations are being taken on light concrete structures to correlate predictions from laboratory swelling tests with observed heave.