Selecting High Quality Bi-Axial Geogrids

The Importance of Selecting High Quality, Punched and Drawn, Bi-Axial Geogrids in Road, Rail and Trafficked Other Applications

Background

Integrally formed (or punched and drawn) biaxial geogrids have been the preferred style of geogrid for pavement reinforcement and soft soil mitigation since introduction of the technology in the 1980’s. This type of geogrid has become the stand-by of engineers and contractors because it has demonstrated consistent, successful performance over nearly thirty years use. Although researchers have not been able to pinpoint a single or group of measurable properties that correlate directly to performance, many have observed that the unique combination of base polymer, manufacturing process, physical properties and robustness of these geogrids all interact to produce the observed results.

Until late in the 2000’s, manufacturing and/or sale for use of this type of geogrid in roadway applications was patented around the world. Therefore, punched and drawn biaxial geogrids were only available from a single, high-quality manufacturer. Other types of biaxial geogrids using varying manufacturing processes (woven, welded, multi-layer extruded) and base polymers (polypropylene, polyester, poly-vinyl alcohol, etc.) were available during this time. However, none were able to match the performance, value proposition or market penetration of the integrally formed geogrids. In fact, with patent protection expired today, all manufactures and sellers of the alternate types of geogrids identified above also sell integrally formed geogrids.

Without patent limitations, many new manufacturers have started making and selling punched and drawn geogrids that closely resemble the historically trusted geogrid style. Although some manufacturers do understand the importance of resin quality, product design, manufacturing quality control, robustness, etc., most are only concerned with quick market penetration based on low price. This leads to corner cutting, low quality and eventually poor performance. Therefore, the design, specifying and using community must now evaluate integrally formed geogrids for quality before accepting them for use.be concerned with selection and approval of only high-quality geogrids that will perform. The remainder of this Technical Note provides guidance on evaluation and selection of only high quality geogrids for use in roadway applications.

 

Product Properties

In an effort to reduce selling cost, many companies are selling light-weight “punched and drawn” polypropylene geogrids that look like the historical products sold in this class, but are not nearly as robust. Although exact correlations between geogrid performance and a particular physical property do not currently exist, it is well understood that certain physical properties act in combination to produce the superior performance of punched and drawn biaxial geogrids. As summarized by Webster (1992), these properties include: rib thickness, shape and stiffness, aperture size, modulus and junction strength.

Today, many punched and drawn geogrids with ultimate strengths less than 18 kN/m in BOTH principle directions are being offered for sale in North America. These products are generally produced using less than traditional products in the class. This reduction in mass correlates directly to increased risk of damage during installation (Wrigley, et al, 2012). If a geogrid is severely damaged during installation, it cannot possibly perform its intended function. Furthermore, design properties such as aperture stability modulus (Giroud & Han, 2004a and 2004b) are also less than historically based norms in the lighter products.

Based on historical evidence and the publications referenced above, the property value ranges shown in the table below are recommended as guidance in writing geogrid specifications. If a geogrid distributor or agent cannot provide certification (based on test results that can be tied to the specific lot or batch of product being delivered) it is likely that their product will not perform well in the field.

 

 

U.S. Standard

Metric

PROPERTY

PROCEDURE

MD

XMD

MD

XMD

Geometric1

Aperture Shape

Observed

Square or Rectangular

Open Area

Measured

78%

Aperture Size (opening)

Measured

1.0 to 1.6 in

1.4 to 1.6 in

25 mm to 40 mm

37 to 40 mm

Rib Depth (height or thickness)

Measured

0.05 in

0.03 in

1.2 mm

0.75 mm

Rib Width

Measured

0.08 in

0.09 inch

2.0 mm

2.4 mm

Rib Shape (cross section)

Observed

Rectangular

Mechanical 2,3

Tensile Strength - Ultimate

ASTM D6637-09

Procedure B

850 lbs/ft

1300 lbs/ft

12.4 kN/m

19.0 kN/m

Tensile Load @ 2% Strain

280 lbs/ft

450 lbs/ft

4.1 kN/m

6.6 kN/m

Tensile Load @ 5% Strain

580 lbs/ft

920 lbs/ft

8.5 kN/m

13.4 kN/m

Junction Efficiency4

GRI-GG2-05

93%

Flexural Rigidity

ASTM D1388 mod

300,000 mg-cm

Aperture Stability5

US COE

3.3 cm-kg/deg = 0.32 m-N/deg

Durability

UV Degradation Resistance1,6,8

ASTM D4355/D6637

100%

Carbon Black Content3

ASTM D1603

0.5%

Chemical Damage Resistance1,7,8

EPA 9090A

100%

Installation Damage Resistance1,8,9

ASTM D5818/D6637

SM ≥ 95%, GP1 ≥ 95%, GP2 ≥ 90%

Footnotes:
1Nominal value(s)
2 All Mechanical properties are based on the manufacturer’s laboratory test results at 21 ±1° C
3 Unless indicated otherwise, values shown are minimum average roll values determined in accordance with ASTM D4759-02
4 Expressed as a comparison of GRI-GG2 strength to GRI-GG1 strength of the same sample
5 Resistance to in-plane rotational moment of 20 kg-cm
6 500 hour exposure
7 120 day emersion testing
8 Expressed as a percentage of Ultimate Tensile Strength
9 SM – Silty sand, GP1 – Poorly graded gravel with sand, GP2 – Coarse, poorly graded gravel

Please note that these ranges have been developed to ensure adequate resistance to installation damage and some level of performance benefit. Specific values within the ranges shown are often warranted to achieve the specific result. Consult www.layfieldgroup.com and/or your Layfield Sales Representative for additional references and guidance.

 

Product Design

Design of a manufacturing plant that can efficiently mass produce consistent, high quality integrally formed geogrids is not a simple process. Those responsible must have unique understanding of- and experience with manipulation of plastic films to gain the desired result. Given the product distortion during the manufacturing process required to produce such geogrids, it is not simple to reverse engineer the process, polymers and tools required. Although a plant may be able to produce geogrids that are visually similar to the traditional integrally formed style, products produced in a plant designed without the proper expertise will not produce a product that performs like the traditional punched and drawn geogrids.

Layfield E’GRID and RX geogrids are manufactured by BOSTD Geosynthetics (BOSTD) at their state-of-the-art plant in Qing Dao, China. It is important to note that BOSTD commissioned NewGrids Ltd (NewGrids) to gain the expertise of Mr. Nigel Wrigley, a founding Principal, to design and build the BOSTD plant. This commissioning entailed input from conceptual design through operations start-up and including product development. Mr. Wrigley is recognized as the world’s leading authority on manufacturing integrally formed geogrids. His career includes an extensive time at ICI Plc spent in the development, manufacture and engineering of oriented plastic film. From there, he spent 14 years as the Technical & Manufacturing Director of Netlon Ltd. (inventor of-and original patent holder for integrally formed geogrids) where he provided guidance for design and construction of the world’s first three plants to produce punched and drawn geogrids under the patented process. Mr. Wrigley’s knowledge and this combined experience enabled him to establish the BOSTD facilities as the first continuous-process integrally formed geogrid production facility in the world. This significant improvement in manufacturing processing results in high efficiency and superior quality control capability.

Before specifying or using a geogrid product, it is important to understand the background of the company manufacturing that geogrid. If the distributor or agent cannot provide the credentials and experience of those involved in the manufacture of their product, their product is not likely of sufficient quality.

 

Product Manufacturing

Many companies selling biaxial geogrids in North America today source their products from brokers or multiple manufacturers then label the product under their own brand. This practice can lead to high variation and lack of consistent quality of product delivered to project sites.

E’GRID and RX geogrids sold by Layfield are only produced at the ISO 9001 Quality Certified facility operated by BOSTD in Qing Dao, China. This gives Layfield the ability to trace and document all geogrids delivered to the jobsite from base resin through finished product and shipment. In specifying or using a geogrid product, it is important to understand the manufacturing source of the product and be able to obtain documentation related to production of specific lots of material. If the distributor or agent cannot provide the details regarding the manufacturing facility and lot/batch specific manufacturing and QC data, it is likely that they are sourcing material from the lowest bidder.

 

Manufacturing Quality Control

Often, biaxial geogrids are manufactured at contract facilities producing all types of plastic products. These facilities rarely include documented quality control programs or in-house laboratories properly staffed and equipped to perform geosynthetic QC testing.

The BOSTD manufacturing plant responsible for producing E’GRID and RX geogrids sold by Layfield includes an in-house quality control (QC) laboratory that is properly equipped to perform geosynthetic specific, ISO and ASTM testing. This facility is constantly staffed during plant production. The BOSTD quality control department assures that only products meeting BOSTD’s and/or the customer's requirements are released for shipment by rigidly following their documented Quality Control Program. The quality control personnel are directly responsible for monitoring, testing, and providing feedback to the manufacturing department ensuring the production of the specified product quality. Each member of the quality team must participate in detailed training that includes factory exposure. The total BOSTD quality team consists of the quality control laboratory personnel, engineering personnel and manufacturing personnel. The combination of expertise and experience from these groups provide BOSTD with the proper tools to maintain the highest level of product quality and customer service in the industry.

It is critical to understand the manufacturing QC procedures employed to ensure that only high-quality, consistent product meeting the required specifications are delivered to a project site. If a geogrid distributor or agent cannot provide evidence of documented quality control program and lot/batch specific manufacturing and QC data, it is likely that their product source (or sources) do not have an adequate (or any) quality control program in place.

 

References

Giroud, J.P. and Han, J. (2004a). “Design method for geogrid-reinforced unpaved roads–Part I: theoretical development”, ASCE Journal of Geotechnical and Geoenvironmental Engineering, 130(8), 776-786.

Giroud, J.P. and Han, J. (2004b). “Design method for geogrid-reinforced unpaved roads–Part II: calibration and verification”, ASCE Journal of Geotechnical and Geoenvironmental Engineering, 130(8), 787-797.

Webster, S.L. (1992). “Geogrid reinforced base courses for flexible pavements for light aircraft: Test section construction, behavior under traffic, laboratory tests, and design criteria.” Final Report, DOT/FAA/RD-92/25, U.S. Department of Transportation and Federal Aviation Administration, 91 pp.

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