Accelerated Flow Testing of Geosynthetic Drains

The use of the Stepped Isothermal Method (SIM) of accelerated compression creep to measure the time dependent loss of thickness and porosity (and thus flow) of planar geosynthetic drains has increased. As a result, there is opportunity to reach for real-time flow tests at pre-established time-dependent thicknesses. Instead of a time dependent flow test such as the well accepted 100-hour transmissivity measurement, one might achieve the same flow in a very short term test if the thickness of the drain at 100 hours is known and the 100-hour porosity can be established accordingly. The historical challenge associated with this opportunity has been the relative inability to measure the thickness of the geosynthetic drain when encompassed by adjacent materials in the fully designed drainage system as installed in a hydraulic transmissivity test apparatus. Indeed, synthetic geomembranes, filter geotextiles, GCLs and a host of possible cover aggregates and soils interfere with accurate thickness measurements, prohibiting the realization of the necessary thickness vs. flow relationship. Two geocomposite drains were first characterized for their compression creep (thickness vs. time relationship) properties using SIM. The drains were then subsequently tested for 100 hour transmissivity using representative adjacent drainage system components. These 100 hour transmissivity tests were performed using the current industry practice of time dependent flow testing, i.e., 100 hour duration tests. In separate tests, the two drains were positioned in the transmissivity test unit with the same adjacent drainage system components, and then compressed to specified time-dependent thicknesses, including 100 hours, for short term flow testing. Subsequent thickness dependent flow measurements were directly compared to time dependent flow measurements. This paper will present all test results generated, with special emphasis on thickness dependent hydraulic performance relationships to conventional time dependent transmissivity tests. Ramifications of this new testing approach will be explored in the context of existing product specifications, design procedures, and economy of cost.