Seismic Response Improvement of Non-yielding Retaining Walls Using Polymeric Seismic Buffers

Isolating the earth structures such as retaining walls, bridge abutments and buried pipes using the compressible materials is a novel solution to reduce the lateral earth pressure. In this technique, a layer of the compressible material with relatively small stiffness and limited thickness is implemented between the retaining wall and the backfill. This material acts as a seismic buffer due to its high compressibility, which absorbs the excess dynamic earth pressure significantly and attenuates the transmitted force to the retaining structure. Choosing the appropriate materials for construction of seismic buffers is based on their physical and mechanical properties as well as cost-effective considerations. Most of the previous studies were focused on some specific materials such as expanded polystyrene (EPS) foam blocks and tire chips.
This paper investigated the performance of polymeric seismic buffers made from Polyurethane (PU) foam on seismic response of non-yielding retaining walls. PU foam has appropriate properties and eliminates some of limitations on materials used in previous studies. The purpose of current study was to evaluate the applicability of PU foam as a new option for construction of seismic buffers with regard to its benefits. Hence, the behavior of non-yielding retaining walls was investigated in two conditions of with and without presence of the seismic buffers by conducting of a series of 1g shaking table tests. Seismic buffers included PU foam blocks, which were prepared by injecting foam into the cubic molds and spraying a certain amount of water on the specimens. A total of 13 tests were carried out on two models (retaining wall with and without seismic buffer) with changing the input base acceleration from 0.07g to 0.46g. The input motion was a horizontal sinusoidal excitation with a constant frequency of 3.6 Hz, which was applied for 10 seconds to the longitude direction of the model. The model responses including wall force and backfill soil displacement were measured during the excitation in each test.
The results showed that the implementing seismic buffers made from PU foam reduce the total and dynamic horizontal wall forces on average of 30% and 45%, respectively. The force attenuation and backfill soil displacement have an inverse relationship to each other. For an equal Normalized compressible inclusion stiffness, this type of foam has a better performance in comparison with similar materials such as expanded polystyrene foam (EPS). Moreover, it is identifying that the force attenuation is not uniform along the height and the maximum attenuation occurs at the top of the retaining wall. The force distribution is triangular for static conditions. As the peak base acceleration is increased and the contribution of dynamic loads on upper elevations is increased, the force distribution becomes nonlinear. Therefore, at earthquakes with moderate to high intensity, the point of application of total horizontal force is transferred to the upper elevations of the retaining wall. Moreover, it is revealed that the efficiency of this technique increases for moderate to high-intensity earthquakes (acceleration amplitude more than 0.24g).