Technical Manual: Conduits through Embankment Dams (FEMA 484)

IntroductionFigure 12.—An example of a tailings dam.before the impoundment is constructed. Reinforced cast-in-place concrete,precast concrete, steel, and plastic conduits have all been used. Some designersprefer to avoid having a conduit pass through the dam and use either floatingpump installations or siphons to decant the water. Use of these options isespecially favored in areas where the impoundment can be located high in thewatershed to minimize runoff inflow, and in areas, such as portions of thewestern United States, where rainfall is low. Designers also cite the advantagein this approach of eliminating potential problems with decant conduit risers,such as structural stability and debris clogging.Some tailings dams are required to be provided with impervious liners due tothe acidity of the leachate. In these cases, if a decant conduit is used, awatertight connection must be achieved between the liner and the conduit. Thepresence of the liner affects these installations by limiting the potential forseepage along the conduit.Some tailings dam failures and problems have been attributed to problems withcompaction of the backfill around the decant conduit. A notable occurrencewas a failure at a phosphate tailings dam in Florida in 1994. While this caseinvolved CMP, it highlighted the difficulty in obtaining adequate compaction inthe haunch area under the pipe. Postfailure investigation of two other decantsthat had been installed at the same facility indicated gaps or loose areas in thehaunch area backfill. Interestingly, although plastic and steel pipes have beenused extensively in slurry impoundments, no failures are known to haveoccurred, and only a few problems have been attributed to inadequate hauncharea compaction in these applications.In the past, decant pipes were constructed of CMP. However, in many casesthe acidity of the refuse caused corrosion problems. Protective coatings were25