Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiesdownstream end of the conduits was deteriorated, resulting in exposed aggregate andrebar.Dam safety modifications were begun in 1993, consisting of injection ofpolyurethane grout into cracks and conduit joints. A two-man crew from McCabeBrothers Drilling of Idaho Falls, Idaho, mobilized to the job site. They installedventilation ductwork into the two outlet works conduits and began drilling injectionholes in the south conduit. Existing cracks (mostly at construction joints) upstreamof station 1+27 were injected with polyurethane resin grout to stop leakage throughthe cracks. This was done prior to repair of spalled concrete in the conduits. Thesubcontractor used a ratio of polyurethane to water of 1.3:1, which effectivelystopped 90 percent of the seepage. However, after completing injection of cracks inthe south conduit, seepage began to migrate downstream and appear in cracks thatwere previously dry.During drilling of the injection holes, two voids were discovered, one in the crownof each conduit at station 0+13. The voids were approximately 12 inches deep and24 inches wide and seemed to be connected to each other. Old constructiondrawings showed this as the location where concrete counterfort walls, whichsupport the intake tower, meet the conduits. No voids were found behind any of theother cracks. The voids at station 0+13 were injected with polyurethane. Asinjection of the south conduit was completed, some migration of polyurethane wasnoted through the crown and divider wall of the middle conduit.In mid-November, McCabe Brothers Drilling completed injecting polyurethane resininto cracks in the outlet works conduits. They injected a total of 305 gallons into thetwo conduits (the specified quantity was 50 gal). As the injection operationprogressed from upstream to downstream, cracks that had been previously dry nearthe canal outlet began to seep water. Therefore, these cracks were injected also.Because the seepage appeared to be following the exterior of the conduits andexiting farther downstream, the seepage continued to be unfiltered and may increasethe internal pressures in the embankment. A decision was made to install weepdrains in the conduit and to construct a filter collar about the exterior of the walls. Amodification to the contract was issued to provide for this additional work.After the polyurethane injection was completed, the conduits were unwatered andinspected. Repair areas were marked, and the contractor began chipping out andpreparing the surfaces of the repair areas for epoxy-bonded concrete.Approximately 30 small repairs and one large repair at the conduit outlet (splitterwall) were done to complete the conduit repairs option of the work. Smaller andshallow areas were repaired using an approved two-part epoxy material. Larger areaswere repaired with epoxy-bonded concrete.B-73