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The GSE Drainage Design Manual Chapter 4 — Design Methods And Concepts ® = ®— nd .Ld .td 4.22 req d ( ) id Td The minimum required transmissivity computed above should be compared with that calculated based on leakage rate and any minimum regulatory requirements. The required transmissivity of the leak detection layer should then be based on the largest of the three values. 4.3 Allowable Transmissivity The allowable transmissivity included in the specifications is the minimum transmissivity that must be met during testing. The allowable transmissivity is unique for a given stress and gradient; therefore, the laboratory testing must be performed at the same stress and gradient. Alternatively, if higher gradient is used for testing then allowable transmissivity value at a lower gradient can be computed based on the empirical relationship presented in Section 2.3. The allowable transmissivity will include an increased value as compared to the required transmissivity in order to account for: (i) a design factor of safety; and (ii) anticipated changes due to environmental effects on the in-place geocomposite. The design factor of safety for drainage is applied to account for uncertainties in the design related to material properties, weather and other factors that may increase the impingement rate or decrease the flow capacity of the geocomposite. The application of the factor of safety for drainage results in the allowable transmissivity as shown below. ®alloy„ = 8,,q • FSD (4.23) where: Barrow = minimum allowable transmissivity of geocomposite (m2/sec), B,,q = required transmissivity for a site as explained in Section 4.2 (m2/sec), FSD = overall factor of safety for drainage (dimensionless). Environmental factors such as biological clogging, chemical clogging and long-term creep of the geocomposite drainage layer will decrease the in-place capacity of the geocomposite. To account for these reductions, the allowable transmissivity will be increased further using reduction factors for each component. The magnitude of each reduction factor (which should be equal to or greater than 1) should reflect a correction that provides a best estimate of the anticipated reduction. It should not be inflated to a larger value to account for uncertainty, since this has already been accounted for in the factor of safety. The allowable transmissivity is shown in Equation 4.24 below (see also GRI procedure GC8): Oml.w = ®r.,. FSD - RFCR - RFCC - RFBC (4.24) where: Ballow = minimum allowable transmissivity of geocomposites or geonet (M2 /sec), Page 4-13