Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter Zonesbedrock surfaces, are usually addressed with chimney filter zones in addition toa filter diaphragm.• The ability of the filter diaphragm to prevent propagation of a crack through the zone.—Thepurpose of a filter diaphragm is to intercept cracks in the earthfill and collectand filter any flow eroding the walls of the crack. The filter zone must be thickenough to prevent a crack from propagating through the filter. Many designersconsider a thickness of 3 feet as adequate to satisfy this requirement. Otherfactors important in crack propagation include the gradation of the filter usedfor the diaphragm, its degree of compaction, and the potential for cementationof the filter. For less favorable conditions, wider filter diaphragms may beadvisable.Intergranular seepage passing through the filter diaphragm may be collected andconveyed downstream to the toe of the embankment dam with various designapproaches. The outlet drain to convey the collected flow may be a combination ofgranular filters and it may or may not include a perforated collector pipe. Figure 93shows one type of outlet drain for a filter diaphragm. This figure shows an outletdrain consisting of a zone of gravel surrounded by a fine sand filter, without acollector pipe. Collector pipes may also be included in the designs for outlet drainsfor filter diaphragms, particularly to provide a safety factor for conveying larger thanexpected flow quantities. Many designers contend that outlet drains should bedesigned to have a capacity to convey all of the collected flow in the granular zonesalone, without considering the additional capacity provided by a collector pipe—thereason being that the collector pipe could eventually be damaged or otherwisebecome inoperative, and the granular zone would still be functional.The estimated flow quantity that filter diaphragms are required to convey dependsprimarily on the predicted quantity of intergranular seepage, not flow through cracksthat are intercepted by the diaphragm. If properly designed, the filter diaphragm willform a seal on the face of any intercepted cracks, and subsequent flow through theface of the crack at the filter will be similar to intergranular seepage.In addition to the dimensions of filter diaphragms, designers must also decidewhether to use a sloping zone or a vertical configuration for the diaphragm. Eachconfiguration has advantages and disadvantages:• Sloping configuration.—Filter diaphragms and chimney filters may also beconstructed with a sloping configuration, as illustrated in figure 88 (top). Thisconfiguration is more common on larger embankment dams and those withdistinct zones in the dam. The filter zone is often placed at the juncturebetween the core and shell zones in the dam as shown in figure 88 (top). Thisconfiguration reduces the effect of differential settlements between the filterzone and the adjacent embankment zones. Because sloping zones are typically143