The Provo Area Office of the Bureau of Reclamation and its contractor recently completed work on the Hyrum Dam Spillway. That need for the work was discovered in late fall 2003, when a Reclamation safety evaluation of the facility revealed a loss of support in the spillway foundation. This loss was caused by gradual erosion which resulted from small displacements in the concrete spillway and stilling basin.
Further investigations of the spillway with core-sampling and Ground Penetrating radar, as well a water tests in April 2004 determined the extent of voids and sedimentation beneath the concrete slabs which lined the spillway chute. The subsequent movement of these slabs had, over time, begun to interfere with an underlying drainage system for the facility.
“The drain system required immediate repair for it to perform its intended function and prevent detrimental uplift pressures,” said Provo Area Office Manager Bruce Barrett. Because of the loss of some support beneath the slabs, a high spring runoff could’ve caused the spillway to fail. So after the April 2004 test, personnel were dispatched to Hyrum in advance of the impending start of the spring runoff to perform emergency repairs on cracks and joints.
Reclamation monitors the performance of dams in accordance with the Safety of Dams Act, which encompasses two separate programs, the Safety Evaluation of Existing Dams (SEED) and the Safety of Dams (SOD) programs. Together, they ensure Reclamation facilities don’t present unacceptable risks to public safety, property or welfare. Dams are evaluated by how they are likely to perform in earthquakes, large floods and when subjected to normal, long-term seepage. Hyrum’s spillway repair was initiated under required upgrading under Safety of Dams parameters.
Because Reclamation was able to complete all of the necessary paperwork in 60 days, the expedited effort allowed the agency to award a contract for the emergency repair work by the end of June 2004, with a scheduled completion date of November 2004.
Gerber Construction, Inc. of Lehi, Utah, received the contract, and was given the Notice to Proceed in early July 2004. Within a day of mobilizing their equipment to the project, they had removed several sidewall slabs at the lower end of the spillway and created an access ramp directly into the structure itself.
Reclamation’s design called for the complete demolition and replacement of the steep sloping portion of the spillway (the portion in danger of failure), as well as the construction of a new under-drain system consistent with modern design standards. In order to safely demolish the existing floor slabs without pulling down the sidewalls however, the entire structure was first stabilized. By the time Gerber was ready to safely remove the spillway floor, they had inserted 46 steel pipes into the side-slabs and had placed over 100 cubic yards of cement slurry in the foundation voids.
In August, the old spillway floor was demolished and removed. The new drain system required Gerber to excavate another 5 feet below the demolished slab. By September, they had completed all excavation operations and began to construct the new spillway foundation. Eighteen inch thick concrete cutoff walls were formed and placed up the 200-foot slope. Between these walls, Gerber installed a new drainage system consisting of filer sand, drain rock and slotted high density polyethylene pipe.
Within 90 days, the contractor had demolished about 800 square feet of 12-inch thick reinforced concrete, excavated and replaced over 800 cubic yards of earth and installed more than 600 linear feet of pipe. The new, 18-inch thick slabs were heavily reinforced, which made installation difficult. But the work was substantially complete by mid-November and fully complete on Thanksgiving Day. Gerber completed the contract by re-seeding the entire project area just hours ahead of a snowstorm. And, on December 20th, the swollen reservoir spilled, sending water down the new spillway chute just weeks after completion.
Because of the level of communication and cooperation between Reclamation, the South Cache Water Conservancy District and Gerber Construction, the work on the spillway was completed on-time and within 1.7% of the original bid price of $669.018. It has served as an example of how an emergency dam safety issue can be addressed quickly through teamwork.