DWR_2801672 - Laserfiche WebLink

Home Search

14 avoid the potential for liquefaction if saturated. All of these requirements can be met by compacting to around 70% relative density (ASTM D4254), which is not particularly difficult to accomplish. Overcompaction beyond this point should be avoided, as this can lead to excessive particle breakage and increased fines content, which can negatively affect permeability and the desired self-healing nature of filters. In general, it is easier to use a method specification for filter and drain materials, in which a minimum size and weight of compaction equipment, and a minimum compaction effort (e.g. number of passes with the required equipment), are specified. In addition to the compaction equipment and effort, it is also recommended that the placement specification for the filter include wetting the material both during handling, which may help prevent segregation, and prior to compaction. Compaction is most effectively achieved when water is added to the filter material as it is placed to produce a moisture content near saturation. This can be effectively accomplished with a water bar mounted on the compactor or by applying water with a water truck or hose just in advance of the compactor. The filter material should not be oversaturated in locations where the water cannot readily flow away during compaction. Vibratory compaction equipment, such as smooth drum vibratory rollers, should be specified for compacting granular filter and drain materials in order to achieve uniform, complete compaction. Construction of filters and drains within dams generally requires that the designed width and alignment of these features conform to the types and methods of construction to be used. As a practical issue, alignments of filters and drains should be kept as reasonably straight as possible across the width of the dam section to ensure that continuity of those features is maintained, and thicknesses/widths of filters and drains should be specified to match up with the types of construction equipment to be used. Maintaining alignment, width and vertical continuity of filter and drain zones is of vital importance, and should be covered in detail within the specifications. Placement specifications should be written to provide for accurate surveys of filter and drain locations during construction, so that these locations are reasonably certain during fill placement operations. The correct geometry must be maintained at all times to ensure vertical continuity of filter and drain zones. Accurate and precise placement of filter material, in lifts of limited thickness, can help prevent the development of “Christmas tree” shaped filter zones within the embankment, thus minimizing the expense of placing excess filter material while ensuring that the design width of the filter is maintained. Some degree of variation in the filter boundaries will occur despite the best efforts of the contractor and specified widths should be sufficient to maintain continuity with an expected variation in these boundaries. To prevent the potential for contamination of filter and drain zones, placement and compaction of materials in those zones should be advanced one lift thickness ahead of materials in surrounding core and shell zones, to ensure that surface drainage is away from the filter/drain. Also, traffic of construction equipment across the filter and drain zones should be eliminated or very carefully controlled to prevent contamination of the surface. Those areas where traffic over the filter/drain zone is allowed require special treatment to remove contaminated granular materials before the next lift is placed. Core Placement Specifications for low permeability core materials generally need to consider the following: 1. Minimum fines content and plasticity 2. Moisture requirements 3. Compaction requirements 4. Special compaction 5. Protection from drying or overwetting Embankment core sections are generally constructed of the most fine-grained, highest fines content soils available on or near site, although there are some exceptions which may arise due to unworkable materials. Specifications should require that core materials possess a certain minimum content of fines (minus #200 sieve size fraction). It is also desirable that the fines maintain a required minimum plasticity, as measured by Atterberg limits; however, in some locations soils with plastic fines may not be available. Embankment cores can be successfully constructed