Thrace-LINQ TECH NOTE #2

GEOTEXTILE SEWING TECHNIQUES

Geotextile connections can be a critical part of any project or specification requirement. This technical note is meant to clarify potential confusion regarding geotextile seams and to provide general guidelines for specifying and sewing geotextiles.

In some applications, sewing is used as a means of eliminating the expense and time of overlaps. Normally, a sewn seam will require a few inches of geotextile from each roll, compared to an overlap which may use two to three feet. Both woven and nonwoven geotextiles can be sewn.

If an application requires stress transfer from one geotextile roll to another, sewing becomes necessary and critical. As the strength required across the seam increases, details of the sewn seam, such as seam type, sewing equipment, sewing thread, and field vs. factory sewing, become more important.

The three seam types commonly used for geotextiles are shown in attachment A. Each of these seam types can be sewn with varying threads, stitch types, varying numbers of stitch lines and number of stitches per inch of seam. The easiest and most common seam type is the flat or prayer seam. For optimum strength and seam efficiency, a "J" type seam is generally used.

Two stitch lines are sometimes required when a high degree of seam integrity confidence is necessary. Lightweight geotextiles with low strength requirements only warrant one stitch line.

Normal stitch counts, or stitches per inch, range from 3 to 7, depending on sewing machine, seam type, and geotextile type, and geotextile strength.

Sewing machinery is divided into two categories. The first is portable or hand-held equipment, used for lightweight, low-strength geotextile seams. These hand-held machines sew one stitch line and, depending on the manufacturer, utilize either a lockstitch (two-thread stitch) or chainstitch (single-thread stitch). Lockstitches are more resistant to unraveling. Field-sewn seams requiring two stitch lines will need two passages with a lightweight sewing machine.

When high seam strengths (300 lbs/in or greater) are required, or when heavier geotextiles are specified, a higher-powered sewing machine must be used. This is required for the needle to penetrate the geotextile, and to accommodate a larger needle to carry the sewing thread. These machines normally are table- or equipment-mounted and come with both single-head and double-head connections. The standard stitch type is a lockstitch.

Lightweight sewing threads are composed of nylon, polypropylene or polyester. Polyester thread is used for sewing medium- to heavy-weight geotextiles. For extremely high seam strengths (500 lbs/in wide-width strength or greater), Kevlar thread or equivalent is used.

Specification requirements for tensile strength must be clearly understood because the geotextile selected is dependent upon whether the seam strength and efficiency are measured using grab-tensile or wide-width test procedures. The two most common geotextile tests are Grab Strength (ASTM D-4632) and Wide-Width Strip Tensile (ASTM D-4595).

The ratio of tensile strength across the seam to the strength of the intact geotextile is defined as seam efficiency. When measured using grab-tensile testing procedures, 90% to 100% seam efficiency is possible. When wide-width testing procedures are specified, the seam efficiency tends to be lower. For polypropylene geotextiles, efficiency ranges from 50% to 70% seam strength. When woven polyester fabrics are used, 40% to 50% seam efficiency is common.

A 50% seam efficiency results in the geotextile being twice as strong as the required seam strength.

Factory sewing generally provides the highest quality seam. Table-mounted factory sewing machines traditionally sew two parallel rows of stitch lines. All Thrace-LINQ factory-sewn seams have two parallel rows of stitch lines unless otherwise requested.

High seam strengths and efficiencies can be achieved in the field; however, this is a much more difficult operation due to the uncertainty of any field construction procedure.

Many projects will require both factory and field sewing, and this must be considered in both the design and the bidding phase.

Several Thrace-LINQ distributors can provide field sewing and installation expertise and assistance.

The table below gives suggested sewing details for Thrace-LINQ GTF Woven Geotextiles.

Attachment A

Seam Types