SHAKING TABLE TESTS AND THEIR SIMULATIONS BASED ON SURVEY ON COLLAPSED RESIDENTIAL RETAINING WALLS DURING THE 2016 KUMAMOTO EARTHQUAKE

Abstract

 Many retaining walls for residential houses were damaged by the 2016 Kumamoto earthquake. There is always a risk that collapsed wall blocks would create damage to structures as well as people and block the road for evacuation or restoration work. Therefore, we need to clarify the collapse mechanism and then construct a method how to design more ductile retaining walls.

 

 In this study, we first carried out the survey on the collapsed retaining walls in Mashiki Town, Kumamoto prefecture, Japan. From the results of the damage survey, we found that concrete-block walls were most severely damaged, and it seems that the degree of the damage were affected by the height and the direction of them.

 

 Second, in order to understand the dynamic behavior and find out the collapse mechanism of retaining walls, we conducted the shaking table tests with small-scale retaining walls and tried to reproduce the dynamic behavior through their simulations. As a result of the first experiment using the shaking table in DPRI, Kyoto University, the model wall was not collapsed because of the stronger model wall than the design based on the regulations. One reason is that the strength of the concrete behind the model wall did not follow the scaling low. The other reason is that the model wall was supported by the steel container due to the frictional force. Therefore, we conducted the second experiment with a re-designed model wall. In the second experiment, even though the friction problem was solved, the model wall was not collapsed either, because of the smaller maximum input acceleration due to the capacity of the shaking table in KIT. However, since the frictional force was eliminated, we could simulate the results of the second experiment with the dynamic DDA (Discontinuous Deformation Analysis) nicely.

 

 Finally, we analyzed the nonlinear behavior of the retaining walls to find key factors leading to the collapses by the dynamic DDA of the real-scale retaining walls. In our parametric study, we made two types of models which have different heights, and different strengths of the concrete behind the blocks. As a result, we found that the degree of the damage is affected by the factors such as height, direction of walls, and the strength of concrete behind the blocks. Furthermore, it is inferred that the overburden pressure from structures too close to the wall, which are not allowed in the current standards, affects the dynamic behavior of the walls.

 

 Since the actual blocks were found to be completely separated whenever the walls were collapsed or severely damaged, it is suggested that whether blocks of walls were connected with concrete or not is much more important than the strength of concrete in order to prevent severe damage of block walls. Therefore, we strongly recommend that the construction practice for and the immediate inspection of “the quality of the concrete behind the blocks” must be implemented as soon as possible, which will ensure the inside integrity of the retaining walls and prevent their severe damage.