Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Loveton Farms DamLocation: MarylandSummary: Failure of an embankment dam by internal erosion along the spillwayconduitLoveton Farms Dam is a 23-foot high earth embankment dam in Baltimore County,Maryland. The dam was constructed in 1985 as a stormwater management structureto attenuate increased runoff due to commercial and residential development of thewatershed. The embankment dam is a “dry structure” which does not normallyimpound any water. The spillway consists of large diameter (78-inch diameter) CMPconstructed through the embankment dam. A vertical section of CMP about 16 feethigh (riser) was constructed at the upstream end of the spillway pipe. Low flowspass through a small (1-foot) opening at the base of the riser. Flows in excess of the100-year storm bypass the embankment dam via an emergency spillway channelexcavated in the left abutment.The embankment dam is essentially a homogeneous embankment constructed oflocal residual soils. These soils are micaceous silty fine sands and sandy siltsweathered from the parent rock (Piedmont Geologic Province). They are classifiedas SM and ML under the Unified Soil Classification System. Typical liquid limits areabout 30 percent, with a plasticity index of about 7.The embankment dam failed less than a year after it was completed, when a relativelysmall storm filled the pool to the top of the riser. Failure was attributed to internalerosion of embankment fill along the outside of the pipe (figures B-50 and 51). Theoriginal spillway pipe was likely placed in a vertically sided trench excavated throughthe nearly completed embankment dam. This construction technique is notrecommended, as it makes compaction of the soil under the sides of pipe verydifficult.Poorly compacted fill in this area results in poor support of the pipe, which causesexcessive deformation of the pipe and may cause the joints to separate. In addition,the sides of the trench may tend to support the fill, allowing it to “bridge” across theexcavation, preventing the fill from consolidating under its own weight. This cancreate areas of low soil density under the pipe where seepage can occur. In addition,differential settlement and hydraulic fracture can result.The embankment dam was redesigned to include seepage controls. The structurewas rebuilt in 1990 using essentially the same embankment dam and spillwayB-60