Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesFigure B-72.—Seepage entering the downstream conduit.the seepage into the conduit prior to installation of the tap. At this location, it wasdiscovered that a void approximately 12 inches in depth existed behind the conduitwall. The concrete in this section of the conduit, which had previously beendescribed as good, was determined to have 3 to 4 inches of somewhat soundconcrete, backed by approximately 6 inches of loosely bonded aggregate or rubble.In subsequent explorations, it was determined that the void extended a minimum of3 feet upstream and 4 feet downstream from the tap and was approximately 3 feethigh.The repairs amounted to jackhammering the eroded areas and soft spots andpatching holes in the concrete. Repairs were made at locations identified duringman-entry inspections and at some other locations where poor quality concrete wasencountered. A largely unsuccessful attempt was made to grout the void behind theconduit wall located about 190 feet upstream of the outlet portal. This wasattempted using a hand grout pump, which proved to be inadequate (partially due tothe existing seepage gradients). One observation during concrete repairs was thatonly hoop steel was encountered in the concrete conduit.Additional man-entry inspection was performed in May 2001 following thecompletion of the concrete repairs with the intent of evaluating possiblemodification alternatives. The concrete deterioration and erosion in the interior ofthe downstream conduit was considered unusual and possibly an indication ofconcrete with low strength and poor durability. The current condition of theconcrete indicates that deterioration and leakage into and out of the conduit willworsen with time. The condition of the concrete may also be indicating a potentialB-95