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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />8 <br />Fill Sinkholes/Entry Points. Depending on seepage <br />velocity, a downstream filter blanket alone may be <br />insufficient to stop the internal erosion of soil caused <br />by concentrated seepage. If the condition assessment <br />identifies specific locations where reservoir water is <br />entering the dam, abutments, or foundation, <br />consideration should be given to slowing or stopping <br />flow into the entry. It is important to understand; <br />however, that this is often a challenging and <br />sometimes unsuccessful emergency intervention. This <br />approach might be successful where a clearly <br />identified entry point (i.e., a sinkhole or whirlpool) is <br />close to the dam or reservoir rim in an area that is or <br />could be made accessible without extraordinary effort <br />(see Figure 5). The objective of at least partially <br />plugging the open entrance is to slow (and if possible <br />effectively stop) the entry of reservoir water and <br />thereby reduce or stop the downstream concentrated <br />seepage and associated internal erosion. However, <br />there are many examples of situations where blocking <br />upstream sinkholes with a blanket do nothing more <br />than cause the sinkhole to move. Materials that might <br />be used depending on availability, access, equipment <br />and labor to deploy them include: heavy gage sheeting <br />(e.g., geomembrane, geotextile, reinforced plastic tarp, <br />etc.) weighted down by sufficiently large rock (coarse <br />gravel, cobbles and/or small boulders); the large rock <br />without the sheeting (preferably a mix of sizes to <br />minimize large voids); large sand or gravel filled <br />geosynthetic bags; hay bales weighted with concrete <br />blocks; concrete blocks or demolition debris; or <br />random, low permeability fill that effectively increases <br />the length of the seepage path. The material would <br />likely be deployed by a tracked hydraulic excavator <br />(ideally with extended reach capability), a crane and <br />bucket, or in the direst of circumstances by a <br />helicopter and bucket. If there is available time, other <br />options may be considered, such as grouting or <br />placement of a layered backfill, as shown in the <br />Western Dam Tech Notes article “Sinkholes: The Hole <br />Story…Issues are Deeper than you Think”. <br /> <br />Figure 5. An Accessible Sinkhole in the Early Stages of <br />Development – a Good Target for Plugging <br />If reservoir water is entering in a large and/or diffuse <br />area (e.g., through fractures in an extensive bedrock <br />unit), measures to address such a condition while the <br />reservoir is full (or is at least submerging the problem <br />area) are limited. However, consideration should be <br />given to placement of an upstream “choke filter” to <br />slow the entrance of reservoir water and thereby slow <br />the seepage discharge downstream. The choke filter <br />comprises sand and gravel and is placed as a berm <br />over the area of seepage entry. The filter material can <br />be placed through shallow water by dozing from the <br />shoreline, or more ideally as the reservoir pool is being <br />drawn down. The Bureau of Reclamation successfully <br />employed this technique at AV Watkins and Deer Flat <br />Dams. <br /> <br />Figure 6. A Placement of Upstream Berm at AV Watkins <br />Dam [4].