: A spillway stilling basin was originally excavated downstream of several buttresses at Pueblo Dam. Geologic information indicated the presence of weak shale seams in the foundation that daylight in the stilling basin excavation. The reservoir had not filled completely since original construction, and therefore the foundation was not fully tested. Analyses indicated the foundation could have a very slim margin of safety if the reservoir filled to normal levels. Due to the large population at risk downstream from the dam, there was strong justification to reduce the risk, and modifications were undertaken to improve foundation stability. The modifications consisted of buttressing the foundation with roller-compacted concrete (RCC) and rock bolts. Additional drainage was also installed. Although the design was completed using limit equilibrium techniques, unique aspects of the geometry were investigated using UDEC and DDA analyses. Both confirmed the design provided adequate resistance. This paper focuses on the design and analysis of the foundation treatment system.
Pueblo Dam is a composite earthfill and concrete massive head buttress dam (see figure 1) on the Arkansas River just upstream of Pueblo, Colorado. The dam, completed in 1975, is about 53.3 m high. A spillway stilling basin, 168 m long and 13.7 m deep was originally excavated at the downstream toe of Buttresses 8 through 14, which form the uncontrolled overflow spillway for the dam.