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Polymeric geogrids have been utilized as a structural reinforcement element within mechanically stabilized earth (MSE) structures for nearly 50 years.
They have been used to reinforce the fill within roadways, embankments, slopes, retaining walls, and any other system where the main function of the geogrid is to provide tensile resistance and confinement of the backfill. For this application the geogrid keeps the backfill from displacing laterally by providing tensile resistance within a composite material called MSE.
The historical argument against the use of polymeric reinforcement for MSE structures has been that these products strain or elongate under a sustained load which is defined as creep. Creep is the load- and time-dependent permanent deformation of a material that occurs when the load exceeds the material yield strength. For polymers it is defined as the creep limited strength.
To effectively utilize geogrids in applications such as retaining walls, which will need to resist the lateral earth pressure, the engineer must understand the behavior of the geogrid under the design loading conditions. This requires the manufacturers of geogrids to evaluate the long-term creep behavior of their products. Without understanding the response of these products to a load there is a risk of excessive uncontrolled deformation resulting in pore performance of the structure if the creep limited strength is exceeded.
For a traditional long-term creep test, a geogrid is tested in a temperature-controlled room and then subjected to a sustained load for 10,000-hours to simulate 1.1 years. The resulting deformation over time is monitored and reported. The result is then extrapolated through two log increments to model the strain of the geogrid for a 100-year design. This is done for several loads to determine the maximum load that can be applied. This test procedure is detailed in ASTM D5262.
AASHTO requires that the long-term design load must be limited to a maximum strain of 10% over 100-year design life. The load that meets this criterion is the creep limited strength which is generally reported as a reduction factor from the ultimate strength of the tested geogrid. This is repeated for the manufacturer’s family of products. This is a time-consuming process which creates a significant constraint for innovation and product development.
In 2003 the “Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method” was published in ASTM D6992. This method takes advantage of polymer behavior when subjected to elevated temperatures. When testing geogrids at elevated temperatures engineers can simulate the deformation that would occur over the design life in a fracture of the time required at normal room temperature. Organizations such as AASHTO allow the use of accelerated testing using ASTM 6992 to evaluate the creep reduction factor for geogrids but the results must be validated against conventional long-term tests using ASTM D5262.
This procedure has made significant impact on the testing of polymeric geogrids. It is also being utilized in other sectors that utilize polymer-based components that are subjected to high loads, and their performance is sensitive to long-term deformation: For example, plastic pipes or storage tanks which are buried and subjected to soil surcharge loads as well as internal hydrostatic pressures.
Creep has been the main argument against the use of polymeric soil reinforcement. Over the past 50 years the geosynthetic industry has made significant advancements in understanding the creep behavior of geogrids and the development of efficient evaluation methods.
Creep is a mechanism that is addressed thorough product testing which results in an allowable load that does not exceed the creep limited strength. In addition, the ultimate geogrid strength is simultaneously reduced by durability and construction damage which results in an allowable strength that is significantly lower than the creep limited strength, therefore creep is not a limitation but a design consideration that is addressed to assure the performance of the structure.
Daniel E. Alzamora, P.E., is a contributing editor for Geosynthetics and has spent more than three decades in the geosynthetics industry in both the public and private sectors.
For additional information:
- ASTM D6992-03, Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method https://store.astm.org/d6992-03.html
- ASTM D5262-21, Standard Test Method for Determining the Unconfined Tension Creep and Creep Rupture Behavior of Planar Geosynthetics Used for Reinforcement Purposes https://store.astm.org/d5262-21.html
- AASHTO Product Evaluation & Audit Solutions, Technical Committee Work Plan for Geosynthetic Reinforcement for Walls and Fills, https://transportation.org/product-evaluation-and-audit-solutions/wp-content/uploads/sites/49/2024/11/AASHTO_REGEO_WorkPlan_24-01_FINAL.pdf