Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesLessons learned• The designers must be actively involved onsite during the construction phase asthe actual subsurface conditions are revealed. Designers should not rely onconstruction personnel to make critical judgments about the need for fieldchanges—especially where filter and drain features are involved.• The highest risk to the embankment dam occurred when the excavation was atthe maximum extent, and before the new drains were installed. The contractorwas required to prepare a contingency plan for mobilization of a horizontaldrilling specialist to minimize the time the excavation had to be open to themaximum depth. When it was determined that a horizontal drain would berequired, the drilling contractor was mobilized. The drilling contractor wasonsite by the time the excavation was completed, resulting in minimum impacton the duration of the critical phase of the excavation.• This type of construction, due to subsurface conditions, is difficult toinvestigate. Since as-built records from that construction era were lacking, ahigh degree of uncertainty was involved. Anticipating field changes is required,as well as carrying a higher contingency than might be necessary forabove-ground construction.• Any evidence of slime bacteria deposits should be addressed as a potentiallyserious problem for filter and drain features.ReferenceU.S. Army Corps of Engineers, Summary Report CENAB-EN-GF, WaterburyDam—Seepage Control Modifications, January 30, 2003.B-111