Fort Peck Dam Spillway Modifications – Rock Anchors in Bearpaw Shale

The nearly one-mile-long (1.6 km) concrete spillway at Fort Peck Dam in northeast Montana was constructed with a cellular concrete terminal (end) structure, but without a formal stilling basin. Periodic use of the spillway, combined with a strong tendency for air slaking of the native Bearpaw Shale bedrock, has eroded the tailrace channel immediately downstream of the terminal structure. Interim repairs were constructed from 2013 through 2016 and included addition of approximately 75-foot-long (22.9 m), slightly inclined anchors to augment connection of the front to back walls of the terminal structure, and vertical anchors to resist unbalanced uplift and hydrodynamic load on a new concrete apron mat constructed immediately downstream of the terminal structure. This paper discusses the unique geotechnical and structural issues that were overcome in the design, construction and testing of the inclined and vertical anchors. 9 pp.