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Assuming that a large void (3' x 3' x 6') in the refuse fill,a curve of required refuse fill/soil cover depth relative to the design final cover grade to prevent ponding on the cover was developed. Based on the analysis, a void of this size would have to be within approximately 10 feet of the surface to develop ponding conditions. (See Figure 4.) This condition can be easily controlled by prohibiting placement of large bulky objects in the upper refuse fill lifts. The Austin Road Landfill does not accept large bulky items such as appliances. 3.2 LOSS OF COVER INTEGRITY The cover system for the vertical and horizontal expansion of the Austin Road Landfill would be a geomembrane type cover system. The concern with 'differential settlement is that the resulting strain in the geomembrane could result in a tear in the material. Various geomembrane materials have different strain characteristics. High-density polyethylene (HDPE) and scrim, reinforce materials have a fairly well-defined yield or breaking point. Other materials such as polyvinyl chloride (PVC) and low-density polyethylene do not have a well-defined yield point and behave in a highly elastic manner. The type of geomembrane material to be used for the cover system will be determined at the time of the closure design. For the purpose of this analysis,a HDPE geomembrane will be assumed. The first step is to establish the limit of acceptable strain on the material. Giroud(4) has proposed a method of estimating the allowable strain on HDPE,based on average tensile strain conditions, effects of scratches, and seams. The maximum allowable design strain would be approximately 1% (see attached Calculation No. 3). assuming the following worst case condition: • biaxial strain on the geomembrane (void settlement condition) • strain at the seam • surface scratches in the geomembrane in the area of a seam. This strain corresponds to a void width to fill thickness ratio of approximately 0.9 (See Figure 4) based on the elastic analysis method(3). For example, a minimum depth from the geomembrane to a 3-foot-wide void should be approximately 3.3 feet. A void with equivalent width at a greater depth should not endanger the geomembrane cover. Again, proper filling procedures in the upper lifts should be able to protect the final cover from the effects of the differential settlement due to voids. The issue of cover integrity should be reviewed and analyzed at the time of final cover design, based on the actual geosynthetic materials and final cover design. WW-1811-CAl-AA/3110 Austin Road Landfill Expansion August 2,1995 NMI033.DOC Landfill Settlement Page 4