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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />5 <br />Figures 2a and 2b. Conventional Filter Construction: Placing <br />Sand Filter (Left) and Ballasting With Gravel (Right) at <br />Bureau of Reclamation Dam [2]. <br />Inverted Filter Blanket. An “inverted filter” <br />(sometimes referred to as a “reverse filter”) should be <br />considered where the concentrated seepage discharge <br />rate, and by extension the head within the dam or <br />foundation driving the discharge, are already higher <br />than can be controlled by a conventional sand filter <br />blanket or another direct discharge intervention (e.g., <br />a “sandbag ring” as discussed below). <br />With this intervention, rather than first placing <br />erodible filter sand, a material with a coarser gradation <br />is placed as a diffuser to lower the velocity of the <br />concentrated seepage at the exit point so that the <br />filter sand can be placed without washing away. The <br />coarser material must be heavy enough not to be <br />washed away by the seepage flow, sufficiently <br />permeable to pass the flow at lower velocity but still <br />relatively freely, and not so open-graded that the filter <br />sand will be at risk of falling into the rock and being <br />washed away. A judgment will have to be made as to <br />what material to use as the diffuser zone of the <br />inverted filter based on local, timely availability, as well <br />as design considerations. It may be that more than one <br />coarse layer will have to be considered in the diffuser <br />zone. <br />Once the lower coarse layer (diffuser) is in place, the <br />overlying filter sand (as described above under <br />Conventional Filter Blanket) can be placed, fully <br />enveloping the diffuser layer. The filter zone prevents <br />(or at least minimizes) escape of the internally eroding <br />soil, while still allowing relatively free passage of the <br />seepage water. If the filter is too fine and its <br />permeability too low, there is a risk that the seepage <br />flow that is currently freely discharging will back up <br />behind the filter. <br /> <br />Figure 3. Inverted Filter Berm at Salt Fork Dam [4]. <br />The ballast/drain material is typically gravel <br />(sometimes up to small cobble sizes) that protects the <br />filter from external damage (erosion from wind and <br />rain and equipment access/passage where required). <br />This zone also serves as a drain, allowing the seepage <br />flow to escape freely downstream. If the lower coarse <br />(diffuser) zone and filter material function as intended <br />the filter compatibility of the filter and ballast/drain <br />may not be critical, but this condition must be <br />evaluated. Figure 3 shows a completed inverted filter <br />blanket controlling an area of concentrated seepage. <br /> <br />Figures 4a and 4b show conceptual designs of an <br />inverted filter for concentrated flow from a sinkhole <br />and from surface seepage discharge at or beyond the <br />toe of the dam. If possible, grade the base of the filter <br />system to direct as much seepage as practical to a <br />common discharge at the downstream side of the <br />system, and install a weir to monitor discharge from <br />the concentrated seep (see, for example, the case <br />study of Needwood Dam at the end of this article). <br />Unintentionally choking seepage discharge could <br />result in locally high gradients that may well induce <br />internal erosion in another area(s), or possibly lead <br />to a slope failure on the downstream face of the <br />dam due to saturation and increased, uncontrolled <br />pore pressures in the downstream embankment fill. <br />