Long-term water flow capacity of geosynthetic drains and structural stability of their drain cores

The German Federal Institute for Materials Research and Testing (BAM) has established a testing guideline to assess the suitability of geosynthetic or geocomposite drain elements for final landfill cover systems. The BAM guideline uses a uniform procedure for determining long-term water flow capacity. For this purpose, it is necessary to produce data for creep behavior under compressive and shear stresses. The residual long-term thicknesses are extrapolated and the pressures determined that are necessary to enforce the residual thickness in a short-term experiment. The flow capacities measured under these pressures at the various bedding conditions are defined as long-term water flow capacities. The procedure is described for three different geosynthetic drain materials. Two of them use random arrays of PP strands and the third a HDPE geonet as drain cores. The extrapolation of creep curves is permissible only when they follow a simple functional relation over the envisaged time period (at least 100 years). Therefore one has to assume that creep will not trigger a stability collapse of the drain core, and that aging does not invoke any relevant changes in the material strength, which would alter the course of the creep curves significantly. The assumption may be proven by the method of long-term, high-pressure creep and long-term, high-temperature shear strength testing, and by measuring antioxidant depletion in oven-aging tests. The consideration of long-term water flow capacity, given here, is supplemented by a detailed discussion of the structural stability of the drain cores. The long-term aging behavior of the plastic drain cores and the geotextiles will be considered in the second part of the paper.