Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 13.—An example of a decant intake structure.26employed to combat this problem, but there also were problems with thewatertightness of the joints. This problem became particularly apparent whendam safety regulators began to require pressure testing of the pipes.As a result of problems with corrosion and joint watertightness, and to dealwith increasing fill heights over the pipes, designers turned to two other typesof decant pipes: thick-walled welded steel pipe, and high density polyethylenepipe (HDPE). Pipes were often designed to withstand the fill height loadingfrom several stages of construction, and the pipe would be replaced byinstalling another pipe at a higher elevation, with the original pipe being filledwith grout.Designers considered corrosion-protected welded steel and HDPE pipes to bebeneficial for the type of foundation conditions and construction practicesfound at these dams. The locations of these dams are limited to areas near theprocessing plants, meaning that designers need to deal with varied, and oftenless than ideal, foundation conditions. Furthermore, as these pipes may beextended up- or downstream, their length can become relatively long,sometimes exceeding 1,000 feet. Over such lengths, a flexible pipe couldtolerate some differential movement due to varying foundation conditions.Additionally, many of these dams have underground mining in their vicinity andthe possibility of mining-induced ground movement needs to be considered.The pipes used for the decanting systems have typically been installed withoutbeing encased in concrete. In most cases, the pipes have been installed withhand compaction of the backfill in the haunch area. Hand held compactionequipment has often been used. Flowable fill has been used in a few cases. A