Laboratory Evaluation of Geogrid-Reinforced Flexible Pavements

An experimental laboratory program to assess the effectiveness of biaxial and triaxial geogrid-reinforced flexible pavements to reduce roadway section was carried out. Six laboratory tests were conducted using a steel cylindrical mold with dimensions of 1.8 m (6 ft) in diameter and 2.1 m (7 ft) in height. The studied reinforced and unreinforced (without geogrid) sections consisted of a locally-obtained subgrade with a minimum thickness of 1.5 m (5 ft) and an asphaltic surface course of 7.6 cm (3 in). The base thickness of three tests was 30.5 cm (12 in) while for the other three tests used base thickness of 40.6 cm (16 in). A layer of biaxial or triaxial geogrid was placed at the mid-depth of aggregate base course for the tests with 40.6 cm (16 in) of aggregate base layer. For the tests which the base layer was 30.5 cm (12 in), the geogrid was placed at the subgrade-base interface. The instrumentations included pressure cells placed at different locations of the test sections, foil strain gauges installed on the ribs of geogrids, and LVDT placed on top of the loading system. These devices were connected to a data acquisition system. A hydraulic actuator provided 40 kN (9 kips) cyclic load through a 305 mm (12 in) circular steel plate at a frequency of 0.77 Hz. The repeated loading was continued for at least 3 million cycles for each test. Performances of biaxial-reinforced and triaxial-reinforced sections were compared with that of companion unreinforced sections. Test results revealed that inclusion of both biaxial and triaxial geogrids in flexible pavement reduced the surface rutting and vertical stresses in the subgrade-base interface. For the studied geogrid-reinforced pavement sections, no tensile strain was experienced by the strain gauges installed on the ribs of the geogrids. The vertical pressure at the center of subgrade-base interface reduced by an average of 18 and 24% for biaxial and triaxial geogrid-reinforced pavement sections, respectively. Using the results of rutting depth, it was found that use of geogrid increased the number of load applications by a factor of 1.5 to 7 depending on the test section and geogrid type, as well as rutting depth experienced at different loading applications. Using Base Course Reduction (BCR) method and the obtained rutting depth, inclusion of geogrid resulted in the base thickness reductions of 11 to 44 percent depending on the above-mentioned variables