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The GSE Drainage Design Manual Chapter 4-Design Methods And Concepts GRI-GC8 recommends using values in the range of 1.1 to 1.3 for biological clogging in the leachate collection system (see Table 4.4). The designer should evaluate the conditions anticipated in the landfill to select an appropriate value for the biological clogging reduction factor. In the absence of available information, the designer is urged to choose a conservative value(i.e., 1.3). GRI-GC8 recommends using values in the range of 1.1 to 1.3 for biological clogging in the leakage detection system(see Table 4.4).The designer should evaluate the conditions anticipated in the landfill to select an appropriate value for the biological clogging reduction factor. In the absence of available information,the designer is urged to choose a conservative value(i.e., 1.3). 4.3.4 Creep Reduction Factors, RFcR Performance transmissivity tests are typically conducted for up to 100 hours, as required by GRI test procedure GC8. The decrease in transmissivity with time levels off within 100 hours, and usually much sooner than that, indicating that much of the initial compression and intrusion has already taken place (see Figure 2.5). Reduction factor for creep,RFS% accounts for the decrease in transmissivity beyond the first 100 hours covered by the performance transmissivity test. The quality of the geonet core, including its structure, thickness, mass and density can have a significant influence on creep reduction factors. Table 4.5 presents creep reduction factors for geonets manufactured by GSE Lining Technology, Inc. Products from other manufacturers can have creep factors different from those given here. Regardless of the type of product, creep increases with increasing overburden stress. For this reason, creep reduction factors for cover applications (i.e., cover drainage and gas removal layers) are significantly smaller than those for liner system applications (i.e., LCRS and LDS). As shown in Table 4.5, a creep reduction factor of 1.1 is adequate for most cover projects. Creep reduction factors should be selected on the basis of normal stress for LCRS and LDS. A conservative value of creep reduction factors may be 2 for landfill liner systems with overburden stress of up to 15000 psf. Table 4.5 Creep reduction factors(RFm)for geonets manufactured by GSE Lining Technology, Inc., are'o and Allen,20041. Pressur=e,kPa: sk *tuatitractorR= . 48 1000 1.1 240 5000 1.2 478 10000 1.3 7I8 15000 1.6 4.4 Seepage Forces and Cover Soil Stability The selection of a geosynthetic drainage layer in the previous section (Section 4.3) was based solely on impingement rate (q;) and the resulting required transmissivity, O q. The use of Equation 4.24 for calculating allowable transmissivity would ensure that liquid head stays within the drainage layer, i.e., all percolation is adequately handled by the drainage layer. Relative to slope stability, this criterion is important to insure that no excess seepage forces need be considered when performing stability calculations. Page 4-16