Importance of Product-Specific Testing in Determining Durability Reduction Factor for Polyester Geogrids in High pH Conditions

As alternative backfills become more commonplace in the construction of mechanically stabilized earth (MSE) walls, the selection of reduction factors in calculating the long-term design strength (LTDS) should be evaluated. Alternative backfill materials have the potential to approach the FHWA default reduction factors for durability. The FHWA default reduction factors for durability are based on the minimum molecular weight (Mw) and maximum carboxyl end group (CEG) concentration of the high tenacity polyester fibers used in geogrid production in conjunction with limiting the pH range of the soil backfill for polyester (PET) geogrids. However, these default reduction factors are conservative, and the default pH range is based on limited testing of a particular coated PET geogrid in environments with a pH over 9. This paper outlines the importance of product-specific testing in elevated pH backfill environments. A detailed multi-year investigation of the reduction in tensile strength of two different manufacturers’ coated PET geogrid exposed to pH values ranging between 10 and 11.4 was performed. The comparison between the two different coated PET geogrids shows the reduction in tensile strength due to submersion in elevated pH aqueous solutions can vary, even with similar Mw and CEG products. This paper illustrates the importance of product-specific testing and the conservatism built into the FHWA default durability reduction factors.