Evaluation of AASHTO default values for pullout friction factor, F^∗, for steel grid mat reinforcement

This paper presents an evaluation of default AASHTO values for pullout friction factor, F∗>, for steel grid reinforcement versus measured pullout behavior determined from a laboratory program of 397 pullout tests on welded steel grid mats of various geometries. Within the context of internal stability analyses associated with mechanically stabilized earth wall design, the laboratory test program for this study established pullout friction factors for grid mats relative to several independent variables including backfill type, overburden pressure, reinforcement length, bar size, and grid geometry that included both transverse and longitudinal bar spacing. Appropriate statistical analyses were used to interpret the pullout test data relative to published AASHTO design guidance for inextensible reinforcements. As expected, the results show that pullout capacities for steel grid mats in properly compacted sandy and gravelly backfill are conservative compared with the default pullout friction factors provided by AASHTO. The data suggest that current AASHTO equations for pullout friction factor for steel grid reinforcement may not adequately capture the influence of transverse bar spacing, longitudinal bar spacing, and backfill particle size.