Technical Manual: Conduits through Embankment Dams (FEMA 484)

IntroductionThe “best practices” provided in this document should be applied to the decantconduits installed in tailings and slurry dams. However, these best practicescreates a dilemma in the case of tailings and slurry dams. As previouslydiscussed, there are benefits to having a conduit that can tolerate somedeformation in these dams. Furthermore, these impoundments do not typicallyhave an “impervious core,” and the added cost of reinforced cast-in-placeconduits concrete is not as suitable for the shorter life of impoundmentconduits, as compared to conduits through traditional embankment type dams.While the absence of significant problems does not rule out future problems,the existing record does provide some indication that alternatives to concreteencasement may be reasonable in tailings and slurry impoundment applications.The following recommendations are provided for installing conduits in thesetypes of dams:1. Although extensive problems have not been encountered with decantpipes through these dams, good conduit design and installation practicesneed to be followed. A primary recommendation is that designersrecognize the large body of evidence that indicates that adequatecompaction cannot be achieved in the haunch area by conventional handheld compaction methods. Using these methods, full contact between thepipe and the backfill cannot be ensured. For guidance on compaction, seesection 5.3.2. Decant conduits should be provided with an adequately designed filter.The filter should extend far enough out from the conduit to interceptareas where cracks may occur due to hydraulic fracturing or differentialmovement of backfill/embankment materials. For guidance on filters, seechapter 6.3. The filter should not be considered as an adequate defense, by itself,against problems with seepage along the conduit. The permeability of thebackfill material and its level of compaction need to be sufficient to restrictseepage and reduce the hydraulic gradient along the conduit. The filter isintended to collect the limited seepage that occurs through well compactedand suitable backfill. The filter could be overwhelmed and renderedineffective by excessive seepage.4. If the pipe is not to be encased in concrete, with sloping sides that allowcompaction by heavier equipment, then an alternate construction method,that provides for adequate compaction and full contact in the haunch area,needs to be specified.5. Use of an alternate construction method should only be considered whereit can be shown that the combination of hydraulic gradient and backfill29