2019-03-22_REVISION - C1982056 (5)

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Subsurface and Surface Drainage Recommendations March 20, 2019 Wash Plant —Thickener Tank Job No,: 18-11283 Page 2 of 2 excavation. We strongly recommend that a drain pipe be installed within the crushed limestone aggregates and at the base of the tank excavation, so that any water that collects in the granular fill materials can be adequately removed. The portion of the pipe placed within the limestone aggregates should be perforated 6 - inch diameter PVC pipe that meets or exceeds ASTM D-3034 requirements. The perforations should range from'/. to 318 -inch in diameter with the holes orientated at 8 and 4 o'clock. The piping should be graded to daylight at a minimum of 1 percent. The piping that is located outside of the limestone aggregates should consist of solid 6 -inch diameter PVC pipe that meets or exceeds ASTM D-3034 requirement. Caution should be taken when backfilling the piping so as not to damage or disturb the installed underdrain. Surface Drainage Recommendations: Proper surface drainage at this site is of paramount importance for minimizing the infiltration of surface drainage into the tank excavation fill materials. The following surface drainage precautions should be observed during construction and at all times after the structure has been completed: 1) The ground surface surrounding the tank should be sloped to drain away from the structure in all directions to a minimum of 10 feet. Ponding must be avoided. If necessary, raising the top of foundation wails to achieve a better surface grade is advisable. 2) Non-structural backfill placed around the structures should be compacted to at least 95% of the maximum standard Proctor density at or near the optimum moisture content in order to minimize future settlement of the fill. The backfill should be placed immediately after the braced foundation walls are able to structurally support the fill. Puddling or sluicing must be avoided. 3) The top 2 to 3 feet of soil placed within 10 feet of the foundation should be impervious in nature to minimize infiltration of surface water into the wall backfill. If you have any questions regarding this report, our recommendations or if we may be of further service, please do not hesitate to contact Sincerely, NWCC, INC. A,7 25750 Brian D. Len, P.E. Principal Engineer NWCC, Inc.