Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter Zonesseepage or leakage along the surface of the structure or through adjacent impervious zones is the use of a properly designed filter and drainage zones aroundthe conduit downstream of the impervious core along with quality constructedfill adjacent to the structure.Current policy is that cutoff collars should not be used as a seepage controlmeasure and any other protruding features on a conduit should be avoided.• Natural Resources Conservation Service.—From NRCS’s Earth Dams and Reservoirs(1990, p. 6-7):Use a filter and drainage diaphragm around any structure that extends throughthe embankment to the downstream slope. . . . It is good practice to tie thesediaphragms into the other drainage systems in the embankment or foundation.Foundation trench drains and/or embankment chimney drains that meet theminimum size and location limits are sufficient and no separate diaphragm isneeded.• U.S. Army Corps of Engineers.—From USACE’s General Design and ConstructionConsiderations for Earth and Rock-Fill Dams— (2004a, p. 6-6):When conduits are laid in excavated trenches in soil foundations, concreteseepage collars should not be provided solely for the purpose of increasingseepage resistance since their presence often results in poorly compactedbackfill around the conduit. Collars should only be included as necessary forcoupling of pipe sections or to accommodate differential movement on yieldingfoundations.. . . Drainage layers should be provided around the conduit in the downstreamzone of embankments without pervious shells. . . . In embankments having arandom or an impervious downstream shell, horizontal drainage layers should beplaced along the sides and over the top of conduits downstream of theimpervious core.Filter zones are provided in embankment dam designs to meet various requirementsand conditions. Filters serve the following purposes (ASDSO, 2003):• For water seeping through the natural voids of the soil (embankment dam orfoundation), a drainage system is designed to intercept this seepage and carry itto a safe outlet.• A filter consists of a graded sand and/or gravel material designed to prevent themigration of soil particles from the base soil being drained.• The filter supports the soil discharge face, and no movement of soil occurs withwater flow.133