Sewn seams provide transfer of strength and ease of installation
Purpose: Layfield has developed an in-house testing procedure based on ASTM methods partially due to the lack of existing data on the strengths of sewn seams for geotextiles. Layfield tested non-woven, woven and Typar geotextiles of various grades then sent out the more standard grades to a testing laboratory to confirm our results and create a correlation to the existing ASTM wide-width testing method.
Method: Layfield developed a test method based on a combination of the ASTM 4884-96 “Standard Test Method for Strength of Sewn or Thermally Bonded Seams of Geotextiles” and ASTM 4632-91 “Standard Test Method for Grab Breaking Load and Elongation of Geotextiles” test methods. This allowed us to use an 8” long x 4” wide test specimen, which our clamps could accommodate, we then sent some samples to be tested by a certified testing laboratory such as the Texas Research International Company (TRI).
For each test the parent material was also tested and compared to the seam strength along with the stitches per inch and the failure type. Failure modes were placed into 1 of 4 categories: 1. Material failure away from the seam; 2. Material failure at seam; 3. Thread breaking. 4. Thread unraveling.
The types of seams tested include prayer, J seam, J seam with rope, butterfly seam and fell seam with the stitch densities being measured for each test. Each material was tested with every type of seam utilizing the different thread types to find the optimum combination of seam type, thread type and stitch density.
Materials: The geotextiles tested were LP4, LP6, LP8 LP10, LP200, LP315 and Typar 3201, 3401, 3601 and 3801. Different nylon threads were also tested such as the CBB 138, CB 207, CB 346 and the CB 415.
Results: After testing of the non-woven geotextiles it was apparent that the J seam was stronger than the prayer stitch while the strongest seams were achieved using the fell and butterfly stitches, which were very similar to each other. The modes of failure ranged from category 1 to 3 with none failing in category 4. Based on testing done by the TRI on prayer seams it is apparent that, on average, the seam to parent material strength can be anywhere from 60% to 80%.
For the Typar heat bonded, non-woven geotextile, there was no perceptible difference in the strengths between the prayer and J seams, except for the failure modes in which the J seam was much more consistent. The Typar, being a spun bonded material performed much better in the seam tests with a minimum seam to parent material strength of 78% based on 16 tests on 4 different grades.
For the woven geotextiles, as with the non-wovens, the J-seam out performed the prayer seams. In order of performance it was the fell stitch followed by the J-seam, butterfly stitch and the prayer seam coming in last. Based on testing by the TRI on the LP200 and LP315 using the prayer stitch an average stitch to parent material strength of 60% can be expected.
There has also been lack of data on how the thread and stitch densities affect the strength of the seam. To better understand the effect they have several different threads and stitch densities were used in the testing of the geotextiles. The nylon thread of type CB 207 offered better strength values than did the CBB 138, and more consistent failure mode results. The CB 207 also had a higher percentage of failures within the parent material, not in the seam itself. Going to a higher strength nylon thread such as the CB 346 and the CB 415 did not significantly increase the seam strength over the CB 207 and actually lowered it in several samples. The stitch density was also tested with no appreciable difference in the J seam strengths, but a slight increase in the prayer seam strengths at higher densities. All thread and stitch density tests were done on the woven geotextiles.
Conclusion: As a result of testing by Layfield and with collaborative data from the TRI, it can be concluded that Fell and butterfly seams are very effective but cost prohibitive in terms of production time. Further, the CB 207 offered superior and more consistent performance in comparison to the CBB 138 but tests show that stronger and thicker thread does not significantly increase this performance, possibly due to the larger punctures created. The stitch density does not make a significant difference to the performance of the seam unless used in the prayer seams. In general, seams strengths are significantly lower in strength compared to the parent materials, typically in the 60%-80% range and J seams, in general, are superior to the prayer seam, especially when used on woven geotextiles.