Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 5—Foundation and Embankment Damsod, topsoil, wood trash, or other foreign material. Other objectionable materialsthat may require removal include very low shear strength, highly compressible andcollapsible soils.If excavation for the conduit is required in earth materials, the trench should be wideenough to allow equipment to perform backfill compaction parallel to the conduit.The side slopes of any excavation may need to be flattened to avoid differentialsettlement. Any excavation for a conduit must consider the potential differentialsettlement that could occur, caused by different properties of the compacted backfillin the excavation and the foundation soils. This problem is most important wherefoundation soils are soft and compressible or collapsible. Flattening the side slopesof excavations may be required to prevent hydraulic fracture of the overlyingembankment. Section 5.2 discusses hydraulic fracture of embankments in moredetail.Conduits may be required to be located on a compacted soil base to provide auniform foundation. Rather than attempting to compact the soil foundation toexactly the required grade, consideration should be given to overbuilding theembankment in the area of the conduit by 1 to 2 feet and excavating down to thestructural grade of the conduit. Depending on the nature of the embankment, it maybe desirable to construct a concrete pad directly over the prepared foundation toprotect the foundation integrity and minimize degradation when exposed to air,moisture, or construction activity. The concrete pad should be placed within thelateral limits of the conduit. If a wider concrete pad is required on both sides tofacilitate construction, it should be constructed with a vertical joint with a bondbreaker. The bond breaker will allow for easy removal of the concrete pad extendingbeyond the conduit edges.When soft foundation soils are encountered, some designers may propose use ofpiles to support a conduit. Use of piles is not recommended, because the conduitmay become undermined, allowing uncontrolled seepage to occur under it. This hasoccurred in at least two pile-supported spillway conduits in Maryland, where voidsup to 5 feet deep were found beneath one structure. In the other structure, completefailure of the spillway conduit occurred less than 2 years after construction wascomplete. For details concerning the latter spillway conduit, see Bohemia Mill Damcase history in appendix B.5.2 Cracking and hydraulic fracture of embankment damsMost embankment dams crack, but only a few develop problems from cracking.Transverse cracks that develop in an upstream and downstream direction are of the115