COMPLIANCE INFO_FINAL WMU FU-23 DESIGN RPT & CONCURRENCE LTRS

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WMU FINAL WMU FU-23 DESIGN REPORT 2-18-23 5-3 The leachate generation analysis was performed using the Hydrologic Evaluation of Landfill Performance (HELP) computer model (Schroeder et al, 1988) (see Design Calculations, Appendix B). The cross section analyzed by the HELP model corresponds to a condition of maximum anticipated leachate generation. The maximum leachate generation condition was assumed to occur during placement of the first 10-foot-thick lift of refuse in WMU FU-23. The rainfall rate for the 1,000-year, 24-hour storm is 4.79 inches/day. The maximum leachate impingement rate estimated by the HELP model for a maximum drainage length of 150-ft and a LCRS gravel permeability of 0.1 cm/sec is 0.32 inches per day. The HELP analysis shows that the designed LCRS will maintain less than one foot of head over the liner. 5.2.1 Slope LCRS The LCRS on the 2:1 liner slope will consist of a GDN placed directly upon the HDPE geomembrane. The GDN is comprised of a sheet of geonet drainage material with a nonwoven geotextile filter fabric bonded to one side. To maintain its position on the slope, the GDN is anchored at the slope crest. The geonet component of individual panels will be joined with plastic ties, and the geotextile component will be sewn. Because the placement of the protective operations layer will not occur immediately after placement of the GDN, a protective plastic covering will be placed over the GDN to protect the geotextile component from ultraviolet (UV) deterioration. The protective plastic covering will be progressively removed and replaced with the protective operations layer in conjunction with refuse placement. The GDN transmissivity was calculated based on the anticipated leachate generation obtained from the HELP analysis. The required transmissivity is specified under the imposed loading conditions. The GDN will tie directly to the base granular drainage layer. 5.2.2 Base LCRS The base LCRS will be constructed of a 12-inch-thick layer of clean granular material with a minimum hydraulic conductivity of 0.1 cm/sec draining towards leachate collection pipe trenches. The particle size of the granular drainage material has been limited to a maximum of 3/8-inch to protect against puncture of the underlying geomembrane and the overlying geotextile filter fabric, and to a maximum of 3 percent of particles finer than the No. 200 sieve to maintain its permeable nature. The granular drainage material is also specified to consist of rounded sub-angular gravel (i.e., no crushed rock will be allowed) to protect the underlying geomembrane from puncture or tears. In addition, a protective geotextile cushion will be placed between the geomembrane and granular drainage material for added geomembrane puncture resistance. If the granular drainage material appears more angular than desired by the Engineer and QA/QC Consultant, the Contractor will be required to demonstrate the suitability of the material by utilizing an on-site test area. The test will consist of placing a 1-foot thickness of granular drainage material on a 15-foot by 30-foot piece of 60-mil HDPE geomembrane and cushion geotextile using the same equipment and procedures as the Contractor is planning to use to place the granular drainage material in WMU FU-23. The QA/QC Consultant will then inspect and, if necessary, test the 60-mil HDPE geomembrane for damage and either accept or reject the angularity of the proposed granular drainage material. The leachate collection trenches will contain 6-inch diameter SDR 11 perforated HDPE leachate collection pipes (see pipe sizing and loading calculations in Appendix B). The pipes will be placed within each leachate collection trench to increase the drainage capacity of the LCRS. Each pipe will be surrounded by coarse gravel wrapped with a nonwoven geotextile to prevent migration of fine