Stress-induced response from both natural and construction-related processes in the deepening of the Peace River valley, B.C.

The Peace River rises in the Rocky Mountain Trench and flows easterly through the Rocky Mountains before flowing onto the Plains in northeastern British Columbia near the Alberta border. Stress measurements conducted at two of three damsites near the boundary of the Foothills and the Plains reveal high remnant lateral stresses and a K0 of about 2. In proceeding 90 km downriver from the W.A.C. Bennett Dam, past the Peace Canyon Dam to the proposed site C dam near Fort St. John, the stratigraphic section changes from competent, massive sandstones of Lower Cretaceous age to soft, weak shales at the top of the Lower Cretaceous section. Valley downcutting by the Peace River through these highly stressed rocks has led to valley rebound features that have both similarities and differences where observed at each of the three damsites. Differences in rock strength at each site contribute to the variations in features noted. These valley rebound features include vertical relief joints, horizontal bedding plane joints, upward arching of the riverbed, and shear zones that both parallel and cross the bedding. The challenge for the geotechnical engineer is first to find these features and account for them in the design, as they tend to govern the foundation and slope strength even though they may only be a few millimetres in thickness. A second challenge is to appreciate that the remnant stresses that led to the formation of the features are still present and these may cause further response when excavation proceeds at a site. A geologic model for the Peace Valley is used to explain the natural and construction response to deepening of the river valley.