Pavement model tests to investigate the effects of geogrid as subgrade reinforcement

The stabilization of weak subgrade is commonly required before placing the pavement structural layers. It is well known that geogrids can be used to reinforce the weak subgrades, however, verification using local conditions is needed to convince road authorities. In this study, two identical model pavements: one without composite geogrid and the other with composite-geogrid reinforcement at the interface of the subgrade-base layer were prepared in a steel box with length, width and height of 1m, 1m and 1.2m, respectively. The 500mm thick subgrade was prepared to achieve a CBR value less than 3% and then a 200mm granular layer was compacted on top of the subgrade to achieve 91% of its maximum dry density. The model pavements were instrumented with Linear Variable Displacement Transducers (LVDTs), soil pressure transducers and moisture sensors. Both pavement models were subjected to repeated loading (more than 110, 000 cycles) at the centre using a 200mm diameter plate to simulate the maximum tyre pressure of 550kPa. The test results showed that the inclusion of composite geogrid at the base-subgrade interface can significantly reduce the rutting depth of granular pavement on weak subgrade (CBR < 3 %). At 50mm rutting, an approximate Traffic Benefit Ratio (TBR) of 5 could be achieved by using composite-geogrid-reinforced subgrade. Furthermore, the pressure transmission to the subgrade was significantly reduced by the composite geogrid at the interface.