Filter Design

“Filters are placed in embankment zones, foundations, or other areas of hydraulic structures for two purposes:

"To intercept water flowing through cracks or openings in a base soil and block the movement of eroding soil particles into the filter. Soil particles are caught at the filter face, reducing the flow of water through the cracks or openings and preventing further erosion and enlargement of the cracks or openings.
"To intercept water flowing through the pores of the base soil, allowing passage of the water while preventing movement of base soil particles.^[1]

"Without filters, piping of susceptible base soils can occur when seepage gradients or pressures are high enough to produce erosive discharge velocities in the base soil. The filter zone is generally placed upstream of the discharge point where sufficient confinement prevents uplift or blowout of the filter. Drains consist of sand, gravel, or a sand and gravel mixture placed in embankments, foundations, and backfill of hydraulic structures, or in other locations to reduce seepage pressure. A drain’s most important design feature is its capacity to collect and carry water to a safe outlet at a low gradient or without pressure buildup. Drains are often used downstream of or in addition to a filter to provide outlet capacity. Combined filters and drains are commonly used. The filter is designed to function as a filter and as a drain."^[1]

“The filter design for the drainage layers and internal zoning of a dam is a critical part of the embankment design. It is essential that the individual particles in the foundation and embankment are held in place and do not move as a result of seepage forces. This is accomplished by ensuring that the zones of material meet ‘filter criteria’ with respect to adjacent materials. The criteria for a filter design is presented in Appendix B. In a zoned embankment the coarseness between the fine and coarse zones may be such that an intermediate or transitional section is required. Drainage layers should also meet these criteria to ensure free passage of water. All drainage or pervious zones should be well compacted. Where a large carrying capacity is required, a multilayer drain should be provided. Geotextiles (filter fabrics) should not be used in or on embankment dams”.^[2]