Pressure Cell Observations—Garrison Dam Project

Pressure cells installed under a powerhouse and stilling basin indicate a reasonable agreement between the design assumptions and the observed values of stress distribution and seem to support the theoretical concept of high edge pressure. The pressure cells were installed to determine the relationship between vertical loads and the resulting foundation stresses in the Fort Union clay-scale. The results cover the period of construction and approximately 5 yr of operation after the completion of construction. The initial stress distribution is fairly equal under the structure, but as time passes, the edge pressures tend to increase, and there is a decrease of pressure in the central part of the foundation. The qualitative results since completion of the powerhouse indicate a reasonable agreement between the observed stress pattern and two assumed design stress patterns, one determined by rigorous analysis, and the other by an approximation.