Determination of Load Equivalency for Unpaved Roads

Abstract

The load equivalency method is widely used to consider the effect of traffic loading on pavement design, and the equivalent axle load factor (EALF) for paved roads has been studied often. For unpaved roads, however, EALF is not well understood because it is not necessarily the same as it is for paved roads. In this study, cyclic plate load tests were conducted on unpaved road sections (six base-over-subgrade sections and four subgrade-only sections) constructed in a geotechnical box (2 m × 2.2 m × 2 m) to investigate the load equivalency for unpaved roads. The base-over-subgrade sections were constructed as unstabilized, T1 geogrid–stabilized, and T2 geogrid–stabilized base courses of 15% California bearing ratio (CBR) with thicknesses of 0.23 m and 0.30 m over weak subgrade of 2% CBR. The subgrade-only sections were constructed with CBR values of 6.2%, 7.4%, 9.5%, and 11.0%. The intensities of the cyclic loads were increased from 5 kN to 65 kN, at increments of 5 kN. For each load intensity, 100 cycles were applied on one test section. The EALFs were analyzed in terms of permanent deformation. The results showed that the regression powers of the ratios of axle loads for unpaved roads with aggregate bases over weak subgrade ranged from 1.9 to 2.9, which were lower than a power of 4, the typical value used for paved roads. The powers for subgrade-only sections had an even wider range, from 1.1 to 3.4. The increase of base thickness, the presence of geogrid, and the use of a higher-grade geogrid increased the power.