The study focuses on the evaluation of retaining walls stiffness using Surface-Wave Method, in terms of both field test and numerical simulation. It is generally accepted that the stiffness of retaining wall is proportional to S–wave velocity. S–wave velocity is used to estimate the variation in stiffness of retaining walls with depth. The retaining walls consist of very heterogeneous mixture of stone, concrete and soil. The 2005 Fukuoka-Ken Seiho-oki Earthquake with magnitude 7 caused several damages to residences on slope facing south of the Genkai Island. The houses, roads and retaining walls were damaged due to landslides and ground deformations induced by the earthquake, and the fact that the hypocenter was located close to the Genkai Island. Most of damaged slope structures were masonry retaining walls. In order to evaluate the safety of retaining walls in future earthquakes, it is important to investigate damage of retaining walls in the Genkai Island.
With the purpose of evaluating the stiffness of retaining walls, the Surface-Wave Method was applied. The Surface-Wave Method was carried out for sixteen sites in Genkai Island in this study. A survey line length is 11m and 12 receivers were deployed. A 10-kg sledgehammer was used as a source. Geophones with natural frequency of 28 Hz were used as receivers. An OYO McSEIS–SXW equipment was used for data acquisition. Phase–velocity calculation is based on a multi–channel analysis of surface waves and the dispersion curves were inverted to one-dimensional S–wave velocity models. In spite of the heterogeneity of the retaining walls, clear waveforms and dispersion curves were obtained. It seemed that the resultant S-wave velocity models reflected the difference of retaining wall stiffness.
In order to evaluate the reliability of the method, we have carried out a numerical modeling of surface waves propagating in the retaining walls, using a three–dimensional finite–difference method. The numerical modeling has simulated the surface waves propagating in the heterogeneous retaining walls consisting of stone, concrete and soil. The size of model was 15m x 6m x 6m. The S–wave velocities of stones, concrete, soil are 1500m/s, 1000m/s and 200m/s respectively. Theoretical waveforms were calculated by the velocity–stress staggered grid three–dimensional finite–difference method. Dispersion curves were obtained through the multi–channel analysis of surface–waves. A one–dimensional inversion has been applied to the dispersion curves in order to obtain one–dimensional S–wave velocity models. The result of numerical modeling revealed that the surface–wave method can be used for evaluating average S-wave velocity of heterogeneous retaining walls.