Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsequipment, and the side slopes should be flat to reduce differential strain. TheUSACE (2004a, p. 8-2) recommends:Special attention must be given to the junction of embankments with concretestructures, such as outlet works . . . to avoid piping along the zone . . . . A 10 verticalon 1 horizontal batter on the concrete contact surfaces will ensure that the fill will becompressed against the wall as consolidation takes place. . . . It may be desirable toplace material at higher water contents to ensure a more plastic material which canadjust without cracking, but then the effects of increased porewater pressures mustbe considered.Factors that can contribute to hydraulic fracture in embankment dams include:• Differential settlement that exceeds about 1 foot per 100 feet (measuredlongitudinally along the embankment dam). Settlement that exceeds this limitof acceptable strain can lead to concern for hydraulic fracture.• Trenches that are transverse to the embankment dam create differentialsettlement, especially if the trench backfill has different compressibility thanfoundation horizons. Conduits often require excavations to provide a uniformfoundation for the structure. Shaping the side slopes of an excavation that istransverse to the embankment centerline is essential. USACE (2004a, p. 6-6)recommends:Excavations for outlet conduits in soil foundations should be wide enough toallow for backfill compaction parallel to the conduit using heavy rollingcompaction equipment. Equipment used to compact along the conduit should befree of framing that prevents its load transferring wheels or drum from workingagainst the structure. Excavated slopes in soil for conduits should be no steeperthan 1 vertical to 2 horizontal to facilitate adequate compaction and bonding ofbackfill with the sides of the excavation.The above recommendation suggests that side slopes of excavations should be2H:1V. This recommendation is appropriate for favorable soil properties.Flatter side slopes should be used for less favorable conditions. Excavationslopes of 3H:1V to 4H:1V are commonly recommended for unfavorablesituations. Flatter than normal side slopes for excavations are advisable whenthe following situations exist:1. When an excavation that is transverse to the centerline of an embankmentdam is backfilled, the compacted soils in the excavation may haveconsiderably different stress/strain properties than the foundationhorizons that have been excavated. These differences can result in118