An Experimental Investigation on the Effect of Loading Frequency on the Settlement and Bearing Capacity of the Footing over the Soil Reinforced by One Geogrid Layer

The soil under footings is generally under cyclic loading, which causes that the footing operation becomes different from that in static loading condition. The change in the behavior corresponds to the bearing capacity and the settlement of the footing under cyclic loading. To compensate such limitations, it is common practice to improve the soil behavior by considering reinforcementlayers in depths under the footing region. By this approach, the reinforcement layers, which are able to resist tensile loads, decrease the soil settlements and consequently, cause to increase the tolerable applied pressure with respect to the condition where the soil is not reinforced with such layers. In the literature, there are several researches that studied and compared the behavior of non-reinforced and reinforced soils under both static and cyclic loads. It is noted that most of these studies were performed by considering small dimensions of footings by using dimensional analysis. Furthermore, all previous studies have only focused on the post-cyclic settlement of the footings and no investigation was done on the effectiveness of the bearing capacity or applied load under the footing. In the present study, the mechanical behavior of a footing under static and cyclic loading condition is investigated by paying attention to both the settlement and bearing capacity. It is also aimed to find the effect of loading frequency on the footing behavior. In order to have more effective results to be used as practice, experimental tests were performed by using an equipped plate load test (PLT) system. The diameter of the circular plate is 300 mm that is equal to traditional PLT equipment in common practice. The plate was thick to behave rigidly and it is equipped with monitoring system including LVDTs, load cells, pressure gauges and strain gauges installed in different parts of the plate. The test were done in a reservoir with diameter of 1400 mm and height of 900 mm. The soil inside the reservoir was uniform-graded sand whose relative density was 72%. In order to assure having a uniform-compacted soil in the whole reservoir, a portable curtain rain system was utilized. The loading equipment includes a loading steel frame with high capacity and a loading jack with 50-ton capacity. The loading system works hydraulically with closed-loop algorithm. In the experimental tests, only one layer of geogrid with the commercial name of CE121B was used, which was installed at 50 mm below the footing surface (equal to the ratio of the embedment depth to the plate diameter of 0.17). The ultimate bearing capacity of the footing (plate) was obtained experimentally for both non-reinforced and reinforced soil conditions. In order to investigate the footing behavior under cyclic loading, the footing was first loaded under static force equal to 33% of the corresponding ultimate bearing capacity and then, the cyclic loading was applied.The dynamic loading was harmonic with three different frequencies of 1, 2, and 4 Hz along with 1000 cycles with the amplitude of 20% ofthestatic ultimate bearing capacity of the footing. The results show that the variation of the cyclic settlement is almost linearwith the logarithm of number of cycles. In addition, comparison of the results shows that although the frequency has an increasing effect on the cyclic settlement, the effect of frequency is not so much. The cyclic settlement increases 18% if the frequency augments from 1 to 4 Hz. The applied load under the footing at the post-cyclic condition was also investigated for different footing settlements for all loading frequency levels. It was observed that generally, there is the same trend regardless of the level of footing settlement. There is a small increase in the loading (less than 10%) at the frequency of 2 Hz, such that it can be concluded that the frequency has very small effect on the pressure under the footing. The induced tensile strain in the geogrid layer was also investigated and it was found that for the applied loading conditions in this study, the loading frequency has no effect on it.