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<br /> <br /> <br /> <br />6 <br />to below the sinkhole to stop the flow of water <br />through the sinkhole while repairs were developed and <br />implemented. <br /> <br />Photo 7: Sinkhole when first observed at Lake Ann, CO <br />A reverse filter was designed and constructed to repair <br />the sinkhole. The sinkhole area was excavated to <br />approximately 6 feet deep, 20 feet in diameter at the <br />surface and 9.5 feet in diameter at the bottom. The <br />subgrade was compacted before application of the <br />filter material. The reverse filter was constructed of <br />five layers, each 12 inches thick followed by a layer of <br />native material at the surface. The properties of the <br />five layers were as follows (Layer 1 was placed at the <br />bottom of the excavation): <br />1. 1.5-inch screened rock <br />2. No. 67 Aggregate AASHTO M43 <br />3. 50% C33 concrete sand and 50% 3/8 inch <br />screened rock <br />4. Clayey material <br />5. Clayey material <br />(Layers 4 and 5 formed a two-foot thick compacted <br />clay liner/cap). <br /> <br /> <br />Figure 2: Elevation of sinkhole repair patch