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