Investigations of Blocks in Foundations and Abutments of Concrete Dams

This paper discusses investigations of potential block hazards in ten existing American concrete arch and buttress dams. Block theory was used to identify and describe all removable blocks, based on the study of construction photos and logs, and new mapping, often done on steep valley sides. Water forces on the block faces were calculated for stability analysis using a map of the phreatic surface or an estimated flow regime along the subsurface boundary of the block. In some cases, block hazards were shown to be nonexistent because there were no real intersections of significant discontinuities of sufficient extent to cut out a block, or there was no kinematically possible mode of failure with the prevailing force system. Several dams had blocks that were removable but judged sufficiently safe with the applicable friction angles. Three structures demanded attention: A spillway structure was armored to prevent further erosion that diminished the resistance of a key block in its foundation; a dome and buttress structure received a reinforced concrete buttress to add passive support against a block critically located in the abutment; and a drainage adit and deep drains drilled from the surface and from the adit were constructed to raise the safety factor of a large block beneath the abutment of an arch dam.