Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsfor loss of structural strength over time. Water tests were performed to determine ifthe water in the reservoir and the conduit were acidic or contained anything thatwould be detrimental to the concrete. The water tests results were negative, and itwas concluded that the condition of the concrete could be a result of poorconsolidation and possibly other poor construction practices.The upstream conduit had never been inspected, since the reservoir would need tobe drawn down to the level of the intake structure. In lieu of reservoir drawdown,an underwater inspection was performed in September 2001 to assess the conditionof the concrete. The divers found the concrete in the intake structure to be in verygood condition with no signs of deterioration. The concrete in the upstream conduitdid not appear to be in as good of condition as the intake structure. The diversinspected and videotaped approximately 80 linear feet of the conduit. A knife wasused to probe cracks and deteriorated areas in the concrete. Figure B-73 shows thediver using a knife to probe a crack in the conduit. In general, the crown of theupstream conduit was in the best condition, with most of the concrete being smoothand free of voids. The floor of the conduit is mostly smooth to 1 /8-inch relief. Thesides of the conduit were in the poorest condition with concrete relief being 1 /8 to1/4 inch thick with localized areas to 3 /4 inch thick. Some areas of unconsolidatedconcrete were observed on the sides of the upstream conduit, but did not appear tobe as severe as what was observed on the downstream conduit. The divers alsoinspected the upstream sides of the two 3-foot by 4-foot 6-inch cast-iron slide gates.During the inspection, the divers took care not to stir up particles on the invert, toavoid reducing the water visibility to near zero.Due to the poor condition of the concrete within the downstream conduit and voidson the outside of the conduit, a decision was made to perform outlet worksmodifications to mitigate the existing dam safety deficiencies. In 2003, a 48-inchinside diameter steel liner was installed, and the annulus between the steel liner andexisting concrete conduit was backfill grouted. As part of the outlet worksmodifications, a filter collar was installed around the downstream end of the conduit.The upstream conduit was determined to be of adequate condition to continue inservice without repair, but underwater inspection should be made at regular 6-yearintervals.Lessons learned• Man-entry inspection should be used to evaluate the condition of the conduitfor both temporary repairs and permanent renovations.• Where feasible, divers should be used to perform underwater inspections ofconduits that cannot be dewatered.B-96