Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Filters are placed next to the soil. If needed, one or more coarser zones areplaced behind the filter to serve as the drain. Collection pipes may be used tocarry the water to a safe outlet.• If the filter has sufficient permeability, it can serve as both the drain and thefilter.• Filters also intercept cracks, openings, or other anomalies where water flow hasthe potential to develop a concentrated leak.• In cracks or openings, filters intercept soil moving in suspension with the water;a filter cake is formed that seals the crack and prevents further erosion.FEMA is sponsoring the development of a “best practices” guidance document forfilters used in embankment dams. This document will contain detailed proceduresand guidelines for design and construction of filters. The design manual will bebased on experience provided from experts in the fields of geotechnical engineeringand construction. The expected publication date is 2007.Filters used in conjunction with conduits through embankment dams generally fallinto three broad categories: chimney filters, filter diaphragms, and filter collars.These filters are discussed in the following sections.6.3 Chimney filtersA chimney filter that extends upward to the highest probable pool level and extendsacross the length of the embankment from abutment to abutment is a commonelement for most high and significant hazard embankment dams. Chimney drainsare also valuable for sites with a high permanent water storage level, because theyintercept and lower the phreatic line and maintain a stronger downstream zone ofunsaturated soil. Figure 87 shows a double gradation zone chimney filter beinginstalled in the construction of a modern embankment dam.Chimney filter zones are a valuable protection against internal erosion in transversecracks that could occur in the embankment dam. Chimney filters are also commonlyused when embankment dams contain zones of widely varying gradation. Thechimney filter provides a transition and filtering capability between these zones.Multiple filter and drain zones may be required in embankment dams that includezones of soils with a wider range in gradation. Examples are embankment damswith zones of impervious, finer grained soils with coarse shell zones of rock or gravelfill.134