The NCMA 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. The manual contains documents providing specification and material testing requirements which are important to the long-term performance of the geosynthetic reinforcement. Specific guidelines relative to the long-term durability of geogrid reinforcement manufactured from polyester (PET) fiber are included.
This article discusses the importance of raw material certifications, presents a real-life scenario with structural steel, and outlines to the engineer, specifier and owner 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?
For comparative reference, the purchase of steel fabricated in China for the San Francisco-Oakland Bay Bridge project drew considerable media coverage. The California Department of Transportation (CALTRANS) and California government officials were subjected to extensive scrutiny for their decision to award the contract to the low-bid, foreign supplier. Over 200 employees, consultants, and contractor representatives were dispatched to China to train workers and inspect steel fabrication. This was all to ensure the steel met the required specifications.
So how does this apply to PET geogrid? 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 mechanically stabilized earth structures. Geogrid–reinforced structures are typically used in critical long-term, applications, having design lives exceeding 75 to 100 years where the geogrid is under constant tension. Utilizing geogrid reinforcement, mechanically stabilized earth (MSE) technology has been used to successfully construct segmental block retaining walls (SRW) exceeding heights of 80 ft (24.4 m) and reinforced soil slopes (RSS) to heights over 240 ft (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 now more than ever a major concern. 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 and more an engineering question.
There also is an almost endless number of alloy compositions that are used for different applications. With numerous finishing processes: hot–rolled, cold–rolled, annealed, hardened, and tempered, each has different chemical and physical properties. Polyester fiber, as well as most polymers, comes in with its own unique physical characteristics. 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 as it serves the basis for long-term performance, both chemical and mechanical.
Research Establishes PET Requirements
In the mid–1990s, the U.S. Federal Highway Administration (FHWA) undertook an extensive research effort to develop testing protocols and recommendations for polyester geosynthetics durability. This multi-year study identified three key factors affecting PET durability,:
- 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.
- Polyester molecular weight – polymer molecular size has a significant influence on chemical durability.
- Polyester carboxyl end group – PET with fewer “carboxyl end groups” in the molecular structure is less susceptible to degradation.
Molecular weight 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
NCMA adopted the FHWA guidelines and created recommendations for polyester fiber in geogrid reinforcement used in long-term applications:
- CEG count less than 30 mmol/Kg as determined in accordance with ASTM D7409 (GRI-GG7).
- 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 molecular weight data should be readily available from any PET fiber supplier used in the manufacturing of geogrid for soil reinforcement projects. Given the influx of foreign manufacturers of geosynthetics into markets using the NCMA Guidelines, the engineer should require geosynthetic manufacturers to prove the PET fiber supplied meets the minimums stated above. This approach to require certifications by the PET fiber manufacturer is consistent with standards of certification applied to other construction materials. 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.
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. PET geogrid supplied to any project should be supported with CEG and molecular weight certifications. Designers and specifiers can refer to the National Transportation Product Evaluation Program (NTPEP) for Geosynthetic Reinforcement, which has evaluated and compiled reports on commercially available geosynthetic reinforcement products to facilitate product selection.
This is another installment in the Segmental Retaining Wall (SRW) Market History Articles Series. You can find the original article here.