2017-01-18_INSPECTION - M1992069

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Aggregate Industries-WCR, Inc. TETRA TECH January 11, 2017 Page 2 Given that Al's reclamation was in compliance with the permit raises the question,why did the breach occur where it did? Our review of the aerial photography available from Google Earth indicated that the breach occurred just east of a small pond. Review of the 2011 Ditch Improvements design drawings indicates that a 15-inch diametercorrugated metal culvert(CMP)was present in the area of the breach. The culvert likely acted as an overflow/spillway for the pond. The design called for the removal and reinstallation of the culvert. The 2011 Ditch Improvements also called for the placement of fill in the area of the 2013 breach, see sheet 4. The specification for the backfill was a non-plastic material that would be highly susceptible to erosion. In our review of the 2011 Ditch Improvements design we noticed that the design called for 48-inch diameter SDR 35 PVC pipe. This is a pipe that is used for gravity flow sewers. The design of the 2011 Ditch Improvements is an inverted siphon pipeline, a pressure pipe design, not gravity. Our field observation and photographs of the failed pipe revealed that the as built pipe is actually a corrugated plastic pipe, trade name Duramax. This pipe is also gravity sewer pipe not intended for use in pressure applications.All pipes of this size, regardless of gravity or pressure conditions, should have joint restraint, thrust blocks and be properly backfilled. Those details are not provided in the 2011 Improvements Design document available to us. The actual pipe installed if not properly backfilled and joint restrained would be susceptible to leakage at the joints that can lead to erosion around the pipe and subsequent settlement above the pipe. The leakage condition was observed by DRMS during the DRMS inspection. See page 2 of 7 the last paragraph second sentence. "The pipeline was not operational at the time of the inspection due to leaking joints in the pipeline." In summary, we believe Al is in compliance with their permit. The origin of the 2013 erosion that led to the breach and damage to the pipeline, could have been caused by three modes of failure: 1. Simple sheet flow down the reclamation slope, 2. Leaky joints In the 48-inch inverted siphon that resulted in settlement above the pipeline, or 3. The removal of the 15-inch CMP that resulted in uncontrolled overtopping of the reclamation slope. We recommend that the design of the 48-inch inverted siphon be reviewed to insure the integrity of the pipeline and protection of reclamation slope. The reclamation slope in the area of the breech should be armored to protect from future floods. Sincerely, TETRA TECH By L By- Tom Hesemann P.G., C.E.G William J Rapp III, P. . SVP, Water, Energy& Infrastructure Eng. Project Mgr. II Civil P.M. P\23514\133-23514-17001\Docs\Reports\AI F Street DR MS WWE Response 2017 01 06 RevA