Q:What is the typical overlap for a geotextile used as a separator in a highway project?
A: This is a very simple question requiring an involved answer. Overlap seam specifications for geotextile separators are all over the place depending on material type and application. It is best to check the plans and specification, consult the project engineer for details or check the manufacturer’s recommendations. Note that if you install an overlap “too small,” the mistake is not reversible. With the above written, there are general rules of thumb. The typical overlap for a geotextile used as a separator on “firm” subgrade (i.e., CBR >2) is 12 to 18 inches. The overlap for a geotextile separator on “soft” soil subgrade (i.e., CBR < 2) is 24 to 36 inches. However, in addition to the overlap width, one needs to shingle the overlap correctly plus push the backfill in the right direction. This discussion takes a nasty turn when considering curves. In such cases, the geotextile is either pleated or overlapped. I would strongly recommend consulting Propex Operating Co. LLC’s “GEOTEX Installation Guidelines for Separation and Stabilization.” It is available online and is excellent.