Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Sardis DamLocation: MississippiSummary: A sinkhole developed over an outlet works conduit due to materialbeing eroded through a jointSardis Dam is a hydraulic fill embankment dam constructed by the Corps ofEngineers and was placed in service in 1940. Sardis Dam is 15,300 feet in lengthwith an average height of 97 feet. The outlet works is located in the left abutment.The outlet works consists of an intake tower with four gated passages, and thesepassages transition in a 64-foot long monolith to a single “egg” shaped reinforcedconcrete conduit. The 18.25- by 16-foot conduit is founded on fine Tertiary sandand was cast in place. The walls of the conduit are 3.25 feet thick. The conduitconsists of 17 monoliths, each 30 feet in length. Copper waterstops were placed ateach monolith joint. The conduit discharges into a concrete stilling basin, which hasbaffle blocks for energy dissipation.In December 1974, a sinkhole occurred above the monolith joint at the junction ofthe intake tower and the upstream end of the transition monolith. Figure B-74shows the location of the sinkhole. Sinkhole investigation revealed that the intaketower is founded on piles, but the transition is not founded on piles. This allowedthe transition monolith to settle about 1 inch more than the intake tower. Thisdifferential settlement was enough to rupture the copper waterstop. With water inthe conduit being free flowing (nonpressurized conduit), and water pressure outsidethe conduit being near lake stage, a large pressure differential existed across thisjoint. This large pressure differential caused flow through the joint after thewaterstop ruptured. The water flowing through the joint carried enough material toeventually cause the sinkhole to occur.The solution to this problem was to fill the sinkhole with impervious material and todrill grout holes in this monolith joint all the way through the concrete into thesurrounding soil along the entire perimeter of the joint. The holes were drilled frominside the transition. Neat cement grout was then pumped through these holes to fillany voids outside the transition and to seal the waterstop as well as possible. Prior togrouting, the gates were closed and sealed with saw dust, air compressors and agrouting machine were set up in the backfill area of the stilling basin, and supplylines were run up the conduit to the transition monolith to be grouted. The lakestage was at its normal level for that time of the year, and the elevation of the lakewas about 20 feet above the invert of the conduit. Twelve grout holes were drilled inthe monolith joint, four in the invert, four in the crown, and two in each side wall.Each hole was installed as follows: (1) A hole about 2 inches in diameter was drilledB-98