عنوان مقاله [English]
The loading rate is an effective factor influencing the interaction between soil and foundation. Increasing the loading rate on different footings, particularly those resting on saturated or partially saturated soils may result in a rapid build-up of pore pressure and foundation failure due to a sudden decrease in effective stresses. The problem would be of greater importance in many special foundations, such as those designed to resist explosions, or missile launching platforms, which are under shock and impact loadings. Numerical modeling of this case for dry soils is not possible without developing an appropriate behavioral model, since, in most numerical modeling, the soil is assumed to be a continuum medium. The existing routine models usually evaluate the influence of the loading rate on the liquid phase, and, then, assess the counter effect of the liquid phase on the whole medium. Because development of a suitable model needs a physical modeling, similar to real conditions, to be undertaken, the present study may play an effective role, in this respect, for backing by a new physical model. In the current research, first, some tests are carried out on soil and the geogrid to determine the physical and mechanical characteristics of the materials, and, then, the initial tests for modeling are implemented using a new physical model developed in the soil laboratory of Amirkabir University of Technology (Mir Hosseini \& Abrishami). The capability of the model was checked and evaluated during these tests, as well as the design and planning of the main test programs, to investigate the different effects of soil-foundation interaction, such as loading rate, on reinforced soils. The main tests in this study consist of two groups, namely: reinforced and unreinforced. The major objective of the unreinforced tests was to get some experimental data as a base reference for evaluation and comparison of the behavior of the footing on reinforced soil. In the group of reinforced tests, a different series of experiments were carried out for different purposes, one of which was planned to study the effect of loading rate on ultimate bearing capacity, and, also, the behavior of footings on sands reinforced by geogrids. Sand density and the method of testing were quite the same as those used in unreinforced tests. The only difference in this series was the presence of a geogrid layer located at optimum depth, which was revealed to be half of the footing width, according to the results of the initial experiments. Based on the results of the performed tests in this series, the ultimate bearing capacity of reinforced sand, in the opposite direction to unreinforced sand, decreases as the loading rate increases. The amount of bearing capacity reduction is about 12 percent compared to that of unreinforced soils. However, the final settlement of the footing on reinforced sand increases about 10 percent, and the total stiffness of the system is reduced. The main reason for these findings may be attributed to the lack of sufficient time which is needed for the interaction between soils and geogrids to happen entirely. Increasing the loading rate, together with soil inertia, may cause the stress distribution in the mass to attain the state close to punching failure. As a result, a higher stress magnitude is applied to a small area of geogrid, leading to an increase in strain and settlement, and a reduction in bearing capacity, accordingly. Some other important findings associated with the loading rate on footings resting on reinforced sand have been resulted, a detailed description and technical discussion of which are presented in the full paper.