EXPERIMENTAL INVESTIGATION OF RING FOOTING BASED ON REINFORCED SOIL WITH GRANULATED RUBBER AND GEOGRID UNDER REPEATED LOADING

In present study, the behavior of ring footing subjected to repeated loading has been investigated by several experimental tests. These tests have been done by ring footing with outer diameter of 300 mm and optimum inner to outer diameter ratio of 0.4 based on unreinforced bed, rubber-reinforced bed, geogrid reinforced bed and geogrid-rubber reinforced bed. Three levels of repeated loading (240, 560 and 840 kPa) have been applied. The results showed that the residual plastic settlement is reduced about 48% and 17.9% for the reinforced bases with rubber particles and with geogrid layer, respectively, compared to the unreinforced bed, but this reduction is about 63.5% for reinforced base with combination of rubber particles and geogrid layer. Also simultaneous use of rubber particles and geogrid reinforcement can be more effective in increasing the portion of resilient settlements compared to using each of them alone. The resilient settlement ratio increased from 5.95% in unreinforced soil up to 24.2% in geogrid-rubber reinforced beds. In fact using reinforcements in soil bed can decrease the plastic deformations of footing bed during few numbers of loading cycles. This behavior is a result of shakedown phenomenon which could lead to steady-state condition and the system behave more resilient in reinforced beds compared to unreinforced bed. Also the results of tests indicated that in both rubber-reinforced bed (without any geogrid layer) and geogrid-rubber reinforced bed, the optimum value of the thickness of rubber-soil mixture layer to improve the behavior of ring footing, is equal to 0.5 times the outer diameter of ring footing. The enhancement of the thickness of rubber-reinforced layer beyond its optimum value can lead to negative effects (increase in plastic settlement or decrease in the portion of resilient settlement) due to increase in compressibility of the bed. Although the adding of geogrid layer could activate the positive reinforcing effects of deeper layers of rubber-soil mixture; but these positive effects are not big enough to withstand the negative effects of using a thick layer of rubber-soil mixture.