2013 – Ashton Dam – meeting the challenge of reconstructing history

Richard Davidson, Jennifer Williams, Roger Raeburn and Jason Boomer

Ashton Dam is a 20-m high embankment dam located on the Henry’s Fork River in Eastern Idaho. It is a high hazard structure licensed with the FERC. The dam was completed in 1916 as a zoned earth and rockfill dam utilizing a low plasticity silt core. Ashton Dam is located approximately 13 km north of Teton Dam and is the sole remaining structure of four similarly designed dams. Over the years, the dam’s condition deteriorated, evidenced by periodic recurrence of sinkholes, sediment plumes and settlement.

PacifiCorp initiated a major 3-year rehabilitation project for the structure. Based on a risk-based design process, a new zoned embankment was reconstructed. Significant structural upgrades were also required for the powerhouse, training walls and gated spillway. To facilitate this construction, a new diversion tunnel and gated outlet structure were built to divert the river and manage flood flows. Cofferdams were required for both the upstream and downstream construction works.

Several challenges were encountered during construction, which were managed with a risk-based process. These included addressing the uncertainties that were known during design and the unknowns that were discovered during construction. Some of the construction challenges covered in the paper include utilization and processing of low plasticity silty material for embankment reconstruction, tunnel construction through fractured basalt with a major shear zone, a lake tap excavation in the wet, dewatering of the embankment excavation, left abutment treatment, real-time redesign of structural features, and fill placement in a constrained excavation.

This paper provides a synopsis of how these design and construction challenges were addressed and overcome on a “blue ribbon” trout stream with high public visibility and interest. Of particular concern was the need for cold weather concrete work, managing flood flows, lake tap and embankment excavation during the very limited construction seasons, and maintaining environmental river controls for the sensitive downstream ecosystem.

Keywords:  Risk-based design, Embankment Reconstruction, Piping, Aged Concrete Repair