This report documents and presents the results of a multifaceted field exhuming and laboratory study that focused on the long-term performance of geosynthetic drainage systems. During the field phase of the project, 91 sites involving six types of drainage systems were exhumed and evaluated. Eighty-four of the sites involved some type of highway edge drain system. Problems were mainly encountered with construction/maintenance items and the geotextile filters surrounding the various drainage materials, mainly soil retention problems. Recommendations are made to address the construction/maintenance problems as well as the soil retention problems with the geotextile filters. Notably, recommendations to change the method of installation of prefabricated geocomposite edge drains are made whereby the product is to be moved to the shoulder side of the trench and is to be backfilled against and beneath the pavement slab using sand puddled in place with water. Three laboratory test methods were utilised and/or developed. The long-term flow tests were instructive in illustrating various problematic soils. The fine-grained soils, either in the upstream soil or in the permeating liquid, resulted in very low flow rates. This test method should be utilised for selecting geotextiles for fine cohesionless soil applications of a severe and/or critical nature. The fine fraction filtration and dynamic fine fraction filtration tests were not particularly relevant insofar as the thrust of this study is concerned. They are not recommended for further evaluation. Using the field exhumed sites as "ground-truth", insofar as acceptable or nonacceptable performance of the various geotextile filters was concerned, a critique of the various geotextile design criteria was undertaken. This involved analysing numerous criteria for permeability, soil retention and excessive clogging. It was determined that the currently used Federal Highway Administration criteria are excellent predictors (i.e., design methods) in all cases. These criteria should be disseminated to the widest possible audience and utilised accordingly. The findings of this study strongly support that the state of the art in long-term performance of geosynthetic drainage systems is very positive. Geosynthetics are indeed providing the lifetime performance behaviour that they suggest and promise. However, continued vigilance so as to obtain intimate contact of geotextiles to their adjacent soil and an ever-alert awareness of maintenance problems must be assured. (A) Appendices B, C, and D as contained in the research agency's final report are not published herein, but are available from the Transportation Research Board Business Office.
Samenvatting