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design and prioritization of spillway gate repairs

design and prioritization of spillway gate repairs
client
J.F. Brennan Company, Inc.
location
Montana and South Dakota

developing a tiered repair approach results in a more efficient use of funds

The dam of the Fort Peck hydroelectric project in Montana is the highest of six major dam facilities on the Missouri River. Flooding in 2011 caused record releases through the spillway that significantly damaged the plunge pool, requiring emergency repairs and spillway rehabilitation. The Big Bend hydroelectric project in South Dakota, also on the Missouri River, experienced similar conditions during the flooding. As a subcontractor to J.F. Brennan, Barr provided inspections, structural analysis, and design for gate repairs at both sites in order to restore the spillway gates and adjacent concrete to pre-flood conditions.

For each gate, we performed visual inspections of the structures, associated concrete surfaces, and counterweights, as well as nondestructive weld inspections. We also conducted finite element analysis to determine plate and member stresses under different loading conditions and to understand how gates would behave after repairs were performed. The rehabilitation plan we designed for each gate included defect removal, weld repairs, cover plate replacement, connection detail modifications, ladder replacement, and spillway concrete repairs.

After the field inspections, it became evident that applying the specification developed by the U.S. Army Corps of Engineers would result in budget overruns, so we developed a tiered gate-repair approach that resulted in a more efficient use of funds. The final repair plans and specifications we developed included structural weld repairs; replacement of structural steel sections, cathodic protection anodes, and gate seals; concrete surface repairs; ultrafine grout injection; and gate painting.

For construction, special considerations had to be taken into account because work had to occur in a remote area, and a confined-space and air-monitoring program had to be maintained during blasting and painting. In addition, use of a single set of dewatering bulkheads made project scheduling and work sequencing key to project success.