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California Regional Water Quality Control Board Central Valley Region Robert Schneider,Chair Winston H.Hickox C4 Gray.Davis Secretaryfor Fresno Branch Office ': ernor Environmental Internet Address: http://www.swrcb.ca.gov/rwgcb5 Protection 3614 East Ashlan Avenue,Fresno,California 93726 ,� a Phone(559)445-5116•FAX(559)445-5910 15 May 2003 .. Cn W. Michael Carroll,P.E. ce .�, County of San Joaquin Department of Public Works P.O. Box 1810 Stockton, CA 95201 NORTH COUNTYRECYCLING CENTER AND SANITARYLANDFILL AND FOOTHILL SANITARYLANDFILL, SANJOAQUIN COUTY We have reviewed the Use of Crushed Rock for Drainage Layer for the leachate collection and recovery systems at the North County and Foothill Landfill expansions. We have concerns with the particle size that was used in Equation 1 for both facilities. North County Landfill Calculations. The only questionable parameter that significantly affects the outcome is the "effective protrusion height"(H). The proposed specifications for the sieve analysis show 10%between 5/8-inch and 3/4-inch indicating a maximum stone size of 3/4-inch. The value of H you assigned was based on the maximum stone size,which your calculation sheet states is t/z-inch. However, your gradation specifications under 2.10.E. indicate that the maximum particle size is 3/4-inch. If you use the full diameter of the maximum particle size, assuming that a 3/4-inch diameter(19 mm) (H) stone could conceivably stick out beneath a well-packed overburden,you get a factor of safety of 2.6. The design site recommends a minimum factor of safety of 3.0,which in turn would require a minimum 640 g/m2 (19 oz/yd2) cushion geotextile. The next commercially available geotextile that fits this minimum criteria is probably 20 oz/yd2. In this regard,we request that you revise your calculations using an appropriate protrusion height and justify your assumptions. Foothill Landfill Calculations. As above. Assuming the same gradation specifications apply, you need- to eedto re-evaluate using a maximum 3/4-inch diameter particle size. Using 1/4-inch as the protrusion height and the proposed 540 g/m2 geotextile, you get a factor of safety of 1.9. To attain a minimum factor of safety of 3.0, we back calculated with a result of 1085 g/m2 (32 oz/yd2). In this regard, we request that you revise your calculations using an appropriate protrusion height and justify your assumptions. It is recommended that a one foot drainage layer be designed into the liner design for the leachate collection and removal systems. According to Koerner(2002)1 soil drainage layers are constructed of materials that have high hydraulic conductivity and good internal stability. The drainage material must t Xuede Qian,Robert M.Koerner,Donald H.Gray, Geotechnical Aspects of Landfill Design and Construction,Prentice-Hall, Inc.,2002 California Environmental Protection Agency �a Recycled Paper The energy challenge facing California is real. Every Californian needs to take immediate action to reduce energy consumption. For a list of simple ways you can reduce demand and cut your energy costs,see our Web-site at http://www.swrcb.ca.gov/rwgcb5