Geosynthetic Seaming

Does your project require geosynthetic seaming or welding? The decision to use a seam is based on the following:

• The weakness of the subgrade
• The cost of the extra geotextile overlap versus the costs of seaming and installing large, fabricated panels
• The feasibility of on-site sewing or welding
• The reality of installing the geotextile or geomembrane panels at the jobsite

AASHTO Guidelines

Ultimately, soil CBR will determine if overlapping or sewing is the correct option. AASHTO offers these general guidelines for sewing versus overlapping:

Soil CBR > 3 Minimum overlaps of 0.3 - 0.45 meters

Soil CBR 1-3 Minimum overlaps of 0.6 - 1.00 meters

Soil CBR < 0.5 Must be sewn

Types of Seams

Geotextiles can be seamed on-site with hand held machines. Most field-sewn seams are prayer seams, formed by placing two sections of geotextile together and joining them with one or more rows of stitching. The prayer seam is the easiest to make and is commonly used for seam strengths of 42 kN/m (240 lb/in) or less. The J and butterfly seams are more difficult to make and are used to develop higher seam strengths. The J-Seam is made by placing two sections of geotextile together and then folding them over (see illustration bottom at of page).

Seam type can also be defined by the number of rows of stitching. Typically, one or two rows of stitching are used. Hand held sewing machines sew only a single row of stitches. It is difficult to hand-sew two parallel rows of stitches. In this case, a second row of stitching is really nothing more than a safety mechanism. More sophisticated equipment used in shops can result in significantly higher seam strengths than those made by hand held machines in the field. When on-site fabrication is not an option, US Fabrics offers in-house geotextile seaming.

Seam Location

Seam location is also important. Seams in nonwoven geotextiles should be sewn 1 to 2 inches from the edge. Woven geotextiles often have finished edges, or selvege. As such, woven geotextile seams should be sewn 1-1.5 inches from the edge of the fabric. If a woven has no selvege, the geotextile should be folded and the seam sewn within this area.

Stitch Type

The most commonly used stitch types are the Federal 101 or 401 Chainstitch. The 101 is a single-thread stitch and the 401 is a two-thread stitch. As such, the 401 offers superior seam strength and will not unravel if the seam is cut.

Stitch Count

Stitch count also plays a role. A standard geotextile stitch count is 3 to 7 spi. A higher stitch count creates a stronger seam to a point. Eventually, the increased number of stitches will break too many fibers and result in reduced seam efficiency.

Thread Type

A seam is no stronger than the thread used to form it. Remember, the thread will also be exposed to the same environmental conditions as the geotextile and therefor must have the same or similar durability as the geotextile. Please consult a thread manufacturer for the proper thread.

ASTM D-4884

Seam strength is typically evaluated using ASTM D-4884 "Test Method for Seam Strength of Sewn or Thermally Bonded Seams of Geotextiles." This method requires a 8" specimen and results of this test correlate accurately to anticipated field seam strength.

Seam Efficiency

With most geotextiles, a 60-90% seam efficiency is typical.

Welding

Welded seams can be an option for geotextiles. These technologies use a heated wedge and pressure rollers to provide a thermal fusion seam. BPR technology seals against a base of suspended tensioned geotextile, compacted sub-soil or an external seaming board. OPR technology seals against two opposing rollers. The BPR is a preferred method since it permits welding on rough, uneven surfaces. Also, OPR requires both layers to be raised as they pass through the rollers. This can create equipment jams, material stretching and other mechanical issues. Wedge welding creates seams strengths comparable to, or better than, sewn seams.

Geomembranes

On-site welding or taping of geomembranes is also an option. US Fabrics offers project specific custom panel sizes for our various liner products when on-site welding is not an option.

Call US Fabrics today to discuss your projects' geotextile and geomembrane seaming or welding needs.