Use of Geosynthetics for Separation and Stabilization in Low-Volume Roadways

About the report:

The implementation of geosynthetics in road construction in the United States dates back to the 1970s. Although the benefits of these materials are widely recognized, actual design and construction practices are not universal.  Guidelines for use are often developed locally based on specific field experience.

Currently, the Virginia Department of Transportation (VDOT) provides specifications (but no design guidance) for only one function of subgrade geotextile: stabilization.  Pavement layer separation is not explicitly addressed by VDOT.  This study reviewed corresponding practices at other transportation agencies, focusing on subgrade and pavement applications.  In addition, observations gathered from field testing of two known sites with geosynthetics in Virginia were analyzed.  Finally, life cycle cost analyses (LCCAs) were performed on two common secondary road designs, each with the options of separator geotextile and preventive maintenance, to estimate comparative costs under three different rates of subbase contamination by fines migrating from the subgrade. 

The LCCA employed actual VDOT cost data and conceptual pavement layer deterioration curves that are based on AASHTO curves augmented by differentiated subbase deterioration resulting from three hypothetical contamination rates. Unlike the current LCCA used by VDOT’s Materials Division, the LCCA model developed in this study explicitly recognizes the fact of potential subbase layer deterioration because of contamination by subgrade fines, comporting with the widely recognized function of geotextile separator.  The results indicate that separator geotextile imparts long-run cost-effectiveness relative to pavements without geotextile separators at contamination rates of 0.1 in per year and greater.  At contamination rates of 0.05 in per year and lower, the addition of separator geotextile is not cost-effective, although the results vary slightly depending on the treatment of estimated pavement design life remaining at the end of the analysis period.

The study recommends that VDOT revise its current specifications to include subgrade separation and subgrade stabilization geosynthetics as separate and distinct pay items, and it provides suggested specification language to effect this change.  Guidelines to implement these changes are also enumerated.

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