Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter Zonessoil. This can be checked by mathematically dividing the grain-size curve intotwo gradations. If the coarse gradation does not meet the filter requirementsfor the fine gradation, the filter is internally unstable (ASDSO, 2003).• Avoid placement of filter materials in freezing temperatures. Frozen filtermaterial cannot be properly compacted.Recently, an additional method has been proposed that expands on the existingresearch on protective filters. The method (Foster and Fell, 2001, pp. 398-407)establishes a “continuing erosion boundary” that is based on the analysis of the resultsof laboratory tests and the characteristics of dams that have experienced internalerosion incidents. The method can be used to determine whether filters that arecoarser than required by modern filter criteria will eventually seal, or experiencecontinuing erosion leading to possible failure of the embankment dam in the eventthat internal erosion begins. The method is intended to help evaluate filters in existingembankment dams only and should not be used to design new filters for dams.6.7 Construction of the filterConstruction of the filter should be performed carefully to ensure that high quality isobtained. The construction needs to ensure that the filter is placed completelyaround the conduit. Sloppy placement techniques can result in voids in the filter orinadequate bond with the conduit encasement.Placement of the sand filter and adjacent materials in the embankment must beperformed to avoid contamination of the filter. During construction, the sand filterzone should be maintained above adjacent materials to preclude contamination.Construction traffic crossings over filter zones should be minimized. The surface ofthe filter at the crossing can be covered with plastic and the plastic covered withearthfill to reduce contamination at the crossing.Placement techniques should ensure that segregation of the filter does not occur.Segregation will cause portions of the filter to be overly coarse, which can allowembankment material to flow through, negating the purpose of the filter.Segregation can be avoided by careful selection of the handling equipment. Thefollowing has been adapted from Reclamation’s Embankment Dams—Protective Filters(1999, p. 16).A common cause of segregation is the manner in which material is handled. Materialplaced in a pile off a conveyor, or loaded from a chute, or from a hopper segregatesbecause the larger particles roll to the side of stockpiles or piles within the haulingunit. Material dumped from a truck, from loader, or other placing equipment willalmost always segregate, with the severity of the segregation corresponding to theheight of the drop. When material is dumped on the fill, segregation occurs.149