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�AGRA <br />Earth & Environmental <br />21 December 1994 <br />1 1-0839 0-0 2 <br />Sterling Asphalt, Inc. <br />P.O. Box 412 <br />Kenmore, Washington 98028 <br />Attention: Mr. Sam Johnson <br />Subject: Proposed Corrugated Metal Pipe <br />Stormwater Detention System <br />Sterling Asphalt Batch Plant <br />Everett, N.'ashington <br />pear Mr. Johnson: <br />AGRA Earih & <br />Environmental, Inc. <br />77335 NE 122nd Way <br />Suite 700 <br />Kirkland, Washington <br />U.S.A. 98034•6918 <br />Tel (206) 820•4669 <br />Fax(206)821-3914 <br />This letter presents our comments and opinions regarding fines (soil fraction passing the U.S. <br />J/200 sieve) migration around the proposed corrugated metal pipe (CMP) stormwater detention <br />vauit due to leakage from the vault. We understand the CMP vauit will be S feet in diameter <br />and be constructed out of aluminum. Furthermore, we understand that the pipe will leak on <br />the order of 300 gallons per inch, per mile, per day (gal�n/mi/day). In our opinion, this <br />leakage will not cause substantial migration of the fi�e-grained portion of the soils which will <br />provide support of the pipe. The on-site soils are itlatively clean with fine contents varying <br />between approximately 5 to 15 percent. <br />In order to minimize the risks of any settlement, we recommend that the exposed subgrade <br />beneath the pipe be compacted to a minimum of 90 percent of the its modified Proctor <br />maximum dry density. Furthermore, we recommend that the backfill around the entire pipe <br />be compacted in accordance with the manufacturer's recommendations or 95 percent of the <br />modified Proctor maximum dry density, whichever is greatest. We recommend that the <br />foundation subgrade soils beneath the pipe be inspected during compactian in order to <br />determine if there are any soft or pumping soils which require overexcavation and <br />replacement. If the exposed subgrade is too wet to be ��ompacted, we recommend that the <br />subgrade ba covered with a woven geotextile fabric such as Mirafi 500X or equivalent to <br />improve the load carrying characteristics of the subgrade soils. <br />We understand that the excavation soils will be used as backfill around the pipe. If these soils <br />are over optimum in moisture and cannot be compected, we recommend that a crushed gravel <br />be used. We recommand that the material consist of '/.-inch minus crushed gravel or pit-run <br />� Engineenng 6 Environmenlal Services <br />