Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsSegregation can be minimized in several ways. First, the designer should avoid usingwidely graded filters that are more prone to segregation. Rather than using a single,widely graded filter, a designer could specify a dual band of filters. A fine filter layerto protect the finer embankment materials would be outletted into a coarser layerused to outlet the collected flow. Secondly, construction techniques to controlsegregation should be specified and enforced. Use of rock ladder, spreader boxes,and tremies or “elephant trunks” for loading hauling units, and hand working theplaced materials will help prevent segregation. If material is dumped, limiting theheight of drop will help. Often the height of drop is limited to 4 feet maximum.Placing filter/drain material with belly dumps is a better method than others becausethe height of drop of materials is limited by the equipment. Limiting the width of thebelly dump opening by chaining or other means can increase their ability to limitsegregation. Using baffles in spreader boxes and other placing equipment can helpreduce segregation. The personnel inspecting the filter/drain production, placement,and compaction should be trained in the techniques effective in preventingsegregation. They should be aware of contract provisions in specifications that areintended to prevent segregation and be prepared to enforce those specifications.Filter zones must be compacted properly to avoid problems. Vibratory compactors(usually smooth drum or plate), are more efficient in densifying filters than“kneading” compactors (such as sheepsfoot or padfoot), without causing muchbreakdown. Breakdown of the filter material’s particles can cause the gradation offilters to change. The most harmful result of breakdown is the increase in thepercentage of fines (usually defined as the material passing the No. 200 sieve size).Excessive fines in the material will drastically reduce the material’s permeability andadversely affect other of the filter’s attributes. Overcompaction should be avoided.Often the gradation of the filter is specified as “in-place, after compaction,” thusensuring that the intended gradation is obtained. Also, specifying filter material thatis comprised of hard and durable particles is important in helping minimizebreakdown.Previous sections have discussed factors that can affect the integrity and quality ofthe filter diaphragm or collar around a conduit. A paper by Hammer (2003) containsvaluable additional guidance on constructing drain zones within embankment dams.Recommendations in that paper include other factors in addition to those discussedin previous sections of this document. Recommendations are included for methodsto avoid contamination of filter zones, advantages and disadvantages of variousschemes for constructing vertical drainage zones, and others. The recommendationsin that paper should also be considered when constructing filter collars anddiaphragms associated with conduits, as well as when constructing other types ofembankment drainage zones.150