Technical Manual: Conduits through Embankment Dams (FEMA 484)

Introductionmore recent practiced has been to place the pipe high in the dam cross section,so that it is above the normal phreatic level; then the pipe is grouted andreplaced by another pipe when the next stage is constructed. A concern for apipe placed high in the dam is that a large storm could result in a raised phreaticlevel which may subject the pipe to a situation analogous to “first filling” of thedam. That is, a problem with seepage along the pipe may only become evidentat a critical time with respect to the amount of water stored in theimpoundment.In an attempt to address potential problems with seepage along a conduit, olderinstallations made use of antiseep collars. In more recent years, filterdiaphragms have been installed. In spite of installing the pipes with handcompaction of the haunch area, a practice that has led to problems in otherapplications, no significant problems have been attributed over the last 25 yearsto piping or excessive seepage through the backfill of a decant conduit forslurry impoundments.The International Commission on Large Dams (ICOLD) has prepared anumber of technical publications (Bulletins Nos. 44, 74, 97, 98, 101, 103, 104,106, and 121 [1989a, 1989b, 1994b, 1995a, 1995b, 1996a, 1996b, 1996c, and2001]) related to the design, construction, and operation of these types of dams(many of these have been developed in partnership with the United NationsEnvironment Programme [UNEP]).Tailings and slurry dams have inherent differences compared to embankmentdams used for the storage and control of water. The reasons that tailings andslurry dams do not fit within the normal context of “embankment” damsinclude (see ICOLD publications for further information):1. They are designed to be abandoned and not operated.2. Construction is usually simultaneous with its operation.3. Generally constructed with mill tailings, mine waste, and earth- or rockfill.4. The primary use is the disposal of waste and slurry from the processingoperations. They usually impound water only for sedimentation,reclamation, and mill operation. Water retention is considered to beincidental to their intended operation of waste disposal.5. The waste is typically discharged along the upstream slope of the dam,forming a delta of settled fines, with the water pushed back away from thedam.27