NCMA’s Design Manual for Segmental Retaining Walls

Submittal requirements for polyester geogrid reinforcement

By R. Lance Carter and Michael Bernardi

Adopted by the NCMA Segmental Retaining Wall & Hardscape Products Committee

Editor’s note
Lance Carter and Michael Bernardi, although competitors, felt the need to remind geosynthetic users, specifiers, and designers that as manufacturers and marketers of polyester reinforcement products, they are held to high standards when it comes to the quality of these products. They felt it necessary to remind users that not all geogrids are “created equal.” A geogrid product may look and feel like all other geogrid products, but unless it is subjected to the proper testing protocols, the end user may be getting sub-par material. This article was written to emphasize the importance of having documentable certification for geogrid reinforcement products delivered to a construction site for use in any and all civil engineering structures.

Introduction

The NCMA’s Design Manual for Segmental Retaining Walls, 3rd edition, has been instrumental in the growth of geosynthetic reinforced soil walls in commercial markets by providing a fundamental geotechnical-based method for structural stability analysis.

Previous articles from NCMA have noted both AASHTO specifications and FHWA guidelines support the design of geosynthetic reinforced soil structures in public sector markets including structures designed for state DOTs. All these documents provide specification and material testing requirements important to the long-term performance of the geosynthetic reinforcement, and they include specific guidelines relative to the long-term durability of geogrid reinforcement manufactured from PET fiber.

This article discusses the importance of raw material certifications, presents a real-life scenario with structural steel, and outlines to engineers, specifiers, and owners what is required to ensure the long-term durability performance of PET geogrid.

Steel or geosynthetics: Should there really be a difference in material certifications?

The question above should not need to be asked, but what if you read an article about a cable-stayed steel suspension bridge that had been constructed with steel cables that had no quality control requirements or inspections at the time of fabrication? Or what if the engineer failed to specify key performance requirements of the steel? You would question the long-term performance and safety of that structure.

For comparative reference, the purchase of steel fabricated in China for the San Francisco–Oakland Bay Bridge project drew considerable media coverage. Caltrans and other California government officials were subjected to extensive scrutiny for their decision to award the contract to the low-bid, foreign supplier. More than 200 employees, consultants, and contractor representatives were dispatched to China to train workers and to inspect the steel fabrication. Their goal: Take the necessary steps to ensure that the steel met the correct specifications.

So how does this apply to PET geogrids?

Specifiers need to take appropriate steps to ensure the quality of all construction materials in critical applications, including PET used in geogrid reinforcement.

Since the introduction of uniaxial geogrid reinforcement to the U.S. market in the early 1980s, the acceptance of the technology has grown exponentially worldwide and now represents state-of-practice for the majority of MSE structures. Geogrid reinforced structures are typically used in critical long-term applications having design lives exceeding 75 to 100 years.

Using geogrid reinforcement, MSE technology has been used to successfully construct SRWs exceeding heights of 80ft (24.4m) and RSSs to heights of more than 240ft (73.1m). The long-term performance of these structures is directly linked to the quality of the geogrid and, more importantly, the quality of the polymer used to produce the geogrids.

Today there are many different international manufacturers of geogrid reinforcement. Quality control and a quality product on the jobsite are attainable through material certifications.

Access to the now global geosynthetic market is exploding and the availability of global sourcing of polyester fiber and the geogrid itself is becoming more of an engineering question. No different from a structural engineer specifying steel bridge cable or Caltrans purchasing steel from a foreign source is the challenge for MSE designers and specifiers to ensure that the specified geogrid meets industry standards for chemical and molecular composition as well as other physical properties.

Anyone familiar with steel is aware of the many different types of commercial steel: stainless, low carbon, alloy carbon—the list goes on. There seems to be almost endless alloy compositions that are used for different applications.

There are also numerous finishing processes: hot rolled, cold rolled, annealed, hardened, tempered. Each of these types of steel has different chemical and physical properties.

Polyester fibers, as well as most polymers, also come in many different types. Each type has its own unique physical characteristics and, specific to PET fiber, it is recognized that not all PET is appropriate for geogrid application. The molecular composition of PET fiber used to manufacture geogrid is the most important raw material consideration because it serves as the basis for long-term performance, both chemical and mechanical.

To properly utilize geogrids in a project, engineers must first understand how to design with them and ensure that the material installed meets the project specifications. Verification of basic material properties used in critical construction projects has been standard practice for literally hundreds of years.

Research establishes PET requirements

In the mid-1990s, the FHWA undertook an extensive research effort to develop testing protocols and recommendations appropriate for durability of polyester geosynthetics. This multi-year FHWA study culminated in the establishment of parameters important to the long-term durability of polyester.

The primary mechanism for PET degradation in naturally occurring soil is hydrolysis. Hydrolysis, usually associated with an alkali or high pH soil in a wet environment, is simply the rupture of molecular bonds resulting in a loss of strength over time. The FHWA identified three key factors affecting PET durability:

1. Soil pH combined with the presence of sufficient water (or moisture)—alkaline soil with a pH of 10 or above representing the environment with greater potential for degradation.
2. Polyester molecular weight—polymer molecular size has a significant influence on chemical durability.
3. Polyester carboxyl end group—PET with fewer “carboxyl end groups” in the molecular structure is less susceptible to degradation.

Molecular weight (MW) and carboxyl end group (CEG) are both specific characteristics of the PET fiber that can be tested and should be specified and verified prior to field installation.

NCMA states minimum PET requirements

The FHWA guidelines on PET durability are adopted by NCMA and state that polyester fiber for geogrid reinforcement used in long-term applications (75 or more years) shall have:

1. CEG count less than 30mmol/Kg as determined in accordance with ASTM D7409 (GRI-GG7).
2. Molecular weight greater than 25,000 g/mol as determined by correlation using inherent viscosity under ASTM D4603 (GRI-GG8).

Certification ensures proper materials

CEG and MW data should be readily available from any PET fiber supplier used in the manufacturing of geogrid for soil reinforcement projects. It should be incumbent upon the project specifier/designer to require the PET fiber manufacturer’s certification from the geogrid manufacturer.

Given the influx of foreign manufacturers of geosynthetics into markets, engineers should require certification that the PET fiber used in the production meets the minimums stated above.

This approach to require PET fiber certifications by the PET fiber manufacturer is consistent with standards of certification applied to other construction materials (e.g., steel used in bridges). It is considered state-of-practice for professional engineers to require “mill certs” for any product component that is used in critical engineering structures. This should include any geogrid used for applications where polyester fiber is relied upon to sustain long-term loads.

Sample specification

All PET geogrid submitted for approval and delivered to the jobsite must be made from polyester fibers that meet durability specifications as follows:

• Molecular weight > 25,000 g/mol
• Carboxyl end groups < 30 mmol/Kg

The contractor on the project must submit certifications on the specific lot of geogrid being proposed for use. The certification, obtained from the geogrid manufacturer, must originate from the actual manufacturer of the fiber, showing conformance to this specification, the date of manufacture, and the fiber production lot number (e.g., fiber merge number). Product labeling should reference the fiber production lot (e.g., fiber merge number) stated in the fiber manufacturer’s certification. Failure to provide proper certification will disqualify the geogrid from use.

It should be noted that a letter of certification from the PET fiber manufacturer, in addition to the PET geogrid manufacturer, is required. Additionally, there should be sufficient documentation supplied from the geogrid manufacturer that demonstrates the product supplied to the project incorporates the certified fiber. In combination, the documentation will assure the engineer and project owner that there is full compliance with the industry standard.

Documentation ensures success

In summary, PET geogrid is commonly used for critical soil reinforcement applications. Existing industry standards are in place to address durability and performance through verification of the PET fiber characteristics.

Accordingly, the material components used in the construction of these structures require the owner, specifier, and engineer to perform the necessary due diligence that ensures the system, including the geogrid, is acceptable for long-term (75 to 100 years) application. PET geogrid supplied to any project should be supported with CEG and MW certifications.

Designers and specifiers can refer to the NTPEP Geosynthetic Reinforcement Evaluation Program, which has evaluated and compiled reports on commercially available geosynthetic reinforcement products to facilitate product selection.

Lance Carter, P.E., technical director at Strata Systems Inc., Cumming, Ga.

Michael Bernardi, P.E., technical manager/engineered structures at TenCate Geosynthetics, Jefferson, Ga.