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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />6 <br /> <br />Figure 4a. Conceptual Design of an Inverted Filter (Concentrated Seepage). <br /> <br />Figure 4b. Conceptual Design of an Inverted Filter (Disperse Seepage). <br />An alternative approach to consider at a concentrated <br />seep is the use of geotextile as the filter material in <br />place of sand (see Lake Needwood Dam case history <br />below). With this intervention, the geotextile material <br />would be deployed either directly over the area of <br />concentrated seepage or on an initial layer of coarse <br />(preferably not too angular) gravel. Deployment would <br />ideally begin well upslope of the area of active seepage <br />as protection against enlargement of the area of <br />seepage over time. Also, deployment would be easier <br />and safer using gravity where possible to roll out the <br />material. As the geotextile is deployed it should be <br />initially ballasted with sand bags at close enough <br />spacing to counteract any tendency for billowing and <br />lifting due either to the concentrated flow or wind. The <br />placement of the sand bags would follow immediately <br />along with the deployment. If ground conditions and <br />accessibility allow, the sand bags may be deployed <br />manually; otherwise a hydraulic excavator or other <br />equipment with suitable reach may be used. <br />Immediately following or together with the <br />deployment of the geotextile and sand bags, a layer of <br />gravel should be placed over the fabric to more <br />uniformly ballast the geotextile, while not impeding <br />free drainage from the geotextile. <br />One potential concern with the use of geotextile as the <br />filter is that it tends to clog more readily than a <br />properly designed earthen (i.e., sand) filter. If possible, <br />a non-woven geotextile with the greatest apparent <br />opening size (AOS) should be used to enhance <br />permeability of the filter. This clogging factor will have