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Last modified
2/16/2017 11:33:02 AM
Creation date
10/6/2015 9:52:47 AM
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Reference Library
Title
WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 1, ISSUE 3, NOVEMBER 2013
Author/Source
URS
Keywords
EMBANKMENT SLOPE STABILITY, RAIN, DESIGN PRECIPITATION DEPTHS, SPECS, EARTHWORK CONSIDERATIONS
Document Type - Reference Library
Research, Thesis, Technical Publications
Document Date
11/30/2013
Year
2013
Team/Office
Dam Safety
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<br /> <br /> <br /> <br />15 <br />with soils with very low or non-plastic fines, but <br />precautions such as wider cores, more robust filters, <br />and material test pads are appropriate in these cases. <br />There is no absolute limit on any of these criteria, but <br />the goal is to obtain a relatively watertight core which <br />maintains some measure of flexibility under loading. <br />Core sections can be constructed with a broad range of <br />material; ranging from material comprised of nearly all <br />fines to material containing as little as 20-30%. The <br />amount of preferred fines will depend on the plasticity <br />and coarseness of the remaining material gradation. <br />Generally, clay materials of low to moderate plasticity <br />are preferred, as they are quite impermeable and <br />maintain good workability characteristics. Sandy clay <br />soils and clayey sands can also provide a very desirable <br />core section, of both high strength and impermeability. <br />However, as noted above silty sands, silty sands and <br />gravels, and even low plasticity silts can potentially be <br />used with appropriate precautions. <br />Problems of workability can arise if fine grained <br />materials having liquid limits in excess of 50% (CH and <br />MH soils) are allowed. <br />Moisture contents for compacted core material should <br />be specified over the range at which optimal <br />compaction can be best achieved, while still <br />maintaining satisfactory plasticity of the fill. For clay <br />materials, this will generally be between 2% below and <br />2% above optimum moisture content, as defined by <br />ASTM D698. For silty, lower plasticity materials, <br />somewhat lower moisture content is desirable, in the <br />range of 3% below to 1% above optimum moisture, per <br />ASTM D698. <br />Compaction requirements for fine-grained <br />embankment fill materials such as clay core materials <br />are almost universally defined by end-result based <br />specifications rather than method specifications, due <br />to the well-established relationship between moisture <br />content and compacted density under a given <br />compaction effort, and the relatively straight forward <br />means by which the state of compaction is measured. <br />Generally, compaction specifications will be defined by <br />requiring 95% of standard Proctor maximum dry <br />density (relative compaction), as measured by ASTM <br />D698. Under some conditions, such as fill under rigid <br />structures, greater density and resistance to <br />settlement may be desirable, and a higher percentage <br />of relative compaction, such as 98%, may be specified. <br />Alternatively, modified Proctor (ASTM D1557) <br />standards may be used for structural support fill, in <br />which case the required percentage of compaction <br />should be decreased a few points, to 95%. Control of <br />embankment core material using modified Proctor is <br />not commonly used for embankments, due to the <br />shifting of the lower moisture contents required to <br />achieve the higher modified Proctor densities, which <br />has an undesirable effect on core ductility. In addition, <br />experience has shown that the greater compactive <br />effort required to achieve modified Proctor <br />compaction is not generally required for acceptable <br />embankment performance. <br />In addition to the required density and moisture <br />content, acceptable compaction equipment and <br />methodology should be specified. For core materials, <br />this would appropriately involve the kneading action of <br />sheepsfoot or pad foot rollers for mass fill areas. Use <br />of a sheepsfoot or pad foot roller will result in a more <br />homogeneous fill which is compacted from the bottom <br />up, leaving a rough surface for the next layer to adhere <br />to, with less tendency to produce laminations in the <br />fill. <br />In all cases, the specifications should require that <br />placement and compaction of core materials be done <br />in the longitudinal direction parallel to the dam axis <br />rather than across the axis, to avoid the potential for <br />non-uniform fill materials or laminations creating <br />preferential seepage paths through the embankment. <br />Each succeeding lift must be well-bonded with the <br />preceding lift by ensuring the proper fill placement <br />moisture content, and, where necessary, scarifying the <br />preceding lift to prevent slick surfaces which may <br />cause laminations in the fill. Core fill placement <br />specifications also typically require that fill placement <br />shall advance relatively evenly along the length of the <br />core zone, to help prevent the potential for transverse <br />shear surfaces or poorly compacted zones within the <br />fill.
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