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The GSE Drainage Design Manual Chapter 4 — Design Methods And Concepts FSD = overall factor of safety for drainage (dimensionless), RFcR = reduction factor for long-term creep (dimensionless), RFcc = reduction factor for chemical clogging (dimensionless), RFBc = reduction factor for biological clogging (dimensionless). The allowable transmissivity in Equation 4.24 should be compared with the 100 -hour transmissivity value obtained from a test. The specified 100 -hour transmissivity value should be equal to or higher than the allowable value of transmissivity. Typical 100 -hour transmissivity data for some products is presented in Appendix A. The designer should contact manufacturers for the most up-to-date information on their products. In addition to the reduction factors in Equation 4.24, a review of the chemical compatibility of the resin used in the geonet to the fluid that the geonet will be exposed to, should be performed. Most geonets are made of HDPE, which is very resistant to most environments where they are used; therefore, this is seldom an issue. Ultimately, the engineer's review should result in a "go no-go" determination, in which the designer decides that the material is acceptable for use or not. If it is deemed acceptable, no reduction in the required transmissivity is applied due to the chemical degradation associated with chemical compatibility issues. The allowable transmissivity discussed here should not be confused with the index transmissivity used for purposes of quality control or conformance testing. The latter is typically performed between steel plates for 15 minutes at a pressure of 10,000 to 15,000 psf and is completely unsuitable for the purpose of design. Project specifications can include both performance and index transmissivity requirements provided that a clear distinction is made between them. 4.3.1 Drainage Factor of Safety, FSD The overall drainage factor of safety should be applied to take into account possible uncertainties in the selection and determination of the design parameters. When selecting an appropriate factor of safety for drainage, the designer should evaluate the criticality of the project, considering such factors as: (i) the cost of repair; (ii) the potential for loss of life; (iii) the certainty of the design parameters; and (iv) the probability of failure. Recommended values of FSD are typically between 2.0 to 3.0 [Giroud, et al., 2000a], with a larger value resulting in a larger required geocomposite transmissivity, and therefore, higher cost. The authors of this design manual believe that lower -end values (i.e., 2.0) are acceptable for most projects, since the performance of geosynthetic materials is fairly well established. Narejo & Richardson [2002] propose a value of 2.0 as an overall factor of safety for drainage. 4.3.2 Chemical Clogging Reduction Factor, RFcc The chemical clogging reduction factor is intended to account for the clogging of the geocomposite due to precipitates deposited from high alkalinity soils (typically calcium and magnesium). The designer should evaluate the soils and waste he/she anticipates being placed on the drainage system. GRI-GC8 recommends using values in the range of 1.0 to 1.2 for chemical clogging in the final cover system (see Table 4.4). If the designer finds that high alkalinity soils or other precipitants are not present, use of the lower value (i.e., 1.0) should be acceptable. Page 4-14