Sliding stability of dams and other mass concrete structures founded on rock is dependent on the shear strength of the foundation rock mass, the concrete-rock interface, and the horizontal construction joints in the structure. In many studies reviewed by the authors, apparent cohesion (representing the interface bond and/or the strength of intact material) was neglected, in which case shear strength estimates rely on shear friction with the failure plane’s effective friction angle as the sole parameter to represent the shear strength. This approach simplifies a complex interaction that can result in overly conservative estimates of a hydraulic structure’s sliding stability.
This paper will compare current guidelines on shear strength criteria from various national and international organizations (USACE, Reclamation, FERC, FEMA, CDA) and provide an overview of methods for estimating shear strength with and without apparent cohesion to illustrate potential methods to develop shear strength estimates that more accurately represent natural conditions. Several case studies that include examples of desktop reviews of historic construction records and photographs, geologic field studies to better characterize the rock mass and expected foundation parameters, coring of concrete or masonry and rock, and laboratory testing are presented to estimate surface roughness and support more realistic interpretations of concealed sliding failure planes that are typically idealized by assuming horizontal planar surfaces. The case studies will include narratives describing the reduction or elimination of stabilization measures that would be required using more conservative shear strength parameters and estimates of the resulting cost savings to the owners.
