Determination of transmissivity of synthetic drainage materials at low gradients

The relationship between hydraulic gradient and transmissivity for most synthetic drainage materials is known to be non-linear. Performance transmissivity tests, often required for the purpose of design and material acceptance, must be performed at site-specific gradients. Since the accuracy of measured transmissivity values at low gradients (less than 0.1) can be poor, this article suggests an alternative approach where transmissivity at the lower gradients is calculated based on the measured values at a series of higher gradients. Rather than perform a test at a single low gradient, this approach proposes performing exactly the same test but taking measurements at three or more arbitrary higher gradients. The resulting relationship is typically non-linear and can be used to calculate transmissivity at any site-specific gradient. This method significantly improves the accuracy of transmissivity values at low gradients and, as such, helps with resolving disputes during product conformance testing.