Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Other joints.—Some HDPE pipe products have integral threads or snap jointsthat allow sections to be easily joined without special equipment. However,these should only be used for nonpressurized applications in low hazardembankment dams due to the potential for pullout. Some types of plastic pipeuse gasketed or glued bell and spigot joints. HDPE pipe cannot be joined bythreading or solvent bonding.Flotation.—When grouting an HDPE slipliner within an existing conduit, it is likelythat the slipliner will “float” or be displaced upward by the fluid pressure of thegrout in the annulus between the existing conduit and slipliner. Due to the relativelylight weight of HDPE pipe, floatation can be more pronounced with this material.Floating of the slipliner may not allow for grout to completely encase the HDPEpipe and therefore reduce the overall strength of the structure. Floatation can alsoresult in vertical misalignments, which may alter the hydraulics of a conduit,especially one that would flow under open channel conditions.Steps should be taken to address this floating potential, such as using spacers orblocking between the existing conduit and the slipliner. Figure 169 shows anexample of spacers being attached to the HDPE slipliner by extrusion welding.Some manufacturers have recommended that the HDPE pipe be filled with water toreduce flotation of the pipe during grouting. However, this does not always preventflotation, because water is not as dense as the surrounding grout, and blocking is stillnecessary. Other manufacturers strongly advise against filling the HDPE pipe withwater and instead recommend properly installed blocking and staged grouting.HDPE pipe is flexible and can conform to alignment changes; therefore, a largerHDPE section is more applicable than a rigid slipliner section, such as steel. Thistends to greatly minimize the potential distance an HDPE slipliner can float (i.e., byreducing the size of the annulus) and reduces the potential adverse effects of anydisplacement. One caution is that since the HDPE slipliner is more flexible, it mayrequire more spacers than a rigid liner for the same span lengths, to controlfloatation, and provide sufficient room to fully encase the liner. However, if thealignment in the existing conduit varies, then the flexible liner will adapt more easilyto the alignment, but will still require sufficient spacers to ensure adequateencasement. Spacers extending the full length of the HDPE slipliner are notrecommended. Spacers should be designed to allow grout to fill the annulusbetween the existing conduit and the HDPE slipliner. The type and spacing ofspacers will vary depending on the standard dimension ratio (SDR) of the HDPEslipliner and should be based on the recommendations of the HDPE pipemanufacturer.Entrance and terminal structures.—The sliplining of an existing conduit mayrequire partial or full removal and replacement of certain structures to improverelease capabilities or to facilitate construction. Figure 170 shows an intake structure302