Study on the seismic damage mechanism of buildings on the river terrace in Wenchuan earthquake

The seismic ground motion topographic effect, as an important research content in the field of seismic engineering, the study of the mechanism of complex terrain on the ground motion characteristics can provide basis for engineering seismic defense.A large number of post-earthquake site investigations have shown that the complexity of local terrain has a significant impact on the distribution of seismic damage, especially the irregular terrain can change the intensity and spectral characteristics of ground motion. As a type of local irregular topography widely found in nature, the unique geometric shape of river terraces can cause complex scattering and diffraction of seismic waves, resulting in differences of ground motion in its local areas, and then affecting the seismic damage degree of surrounding buildings.

Based on the on-site seismic damage investigation data of the river terraces in Wenchuan M_S8.0 earthquake in 2008, it was found that buildings in the area with thicker alluvial at the front edge of the terrace suffered serious damage, while those in the area with thinner slope deposit at the rear edge of the terrace suffered relatively less damage. In general, the seismic damage at the front edge was significantly greater than that at the rear edge. At the same time, in order to study the mechanism of this seismic damage feature, the river terraces were selected as the research object, and the FLAC^3D finite difference software was used to establish three-dimensional river terrace analysis models with different thicknesses of overburden soil layers, simulating and calculating the ground motion response under impulse loading, further revealing the influence law and internal mechanism of river terrace topography on ground motion characteristics and the distribution of seismic damage to buildings.

For the same level terrace, the peak values of the horizontal and vertical acceleration and the 90% energy duration all show an upward trend with the increase of the overlying soil thickness, reaching the maximum value at the front edge of the terrace and the transition point with the steep slope. Meanwhile, the ground motion level at the front edge of each terrace is significantly higher than that at the rear edge. As the terrace grade decreases, the peak values of the horizontal and vertical acceleration and the 90% energy duration at the corresponding area also gradually decrease. Similarly, the trend of the Fourier spectrum amplitude and ratio at different monitoring points on the same level terrace is basically consistent, but the amplitude and ratio increase gradually as the monitoring point approaches the edge of the terrace facing the steep slope and the front edge of the area with increased soil thickness.As the terrace grade increases （the terrain height rises）, the natural frequency of the structure increases accordingly, thereby significantly enhancing the amplification effect of low-frequency ground motion. The characteristic period value, platform value, and platform amplification coefficient in the standard response spectrum of seismic acceleration for different monitoring points are all affected by the terrace grade and overlying soil thickness. The T_g value decreases gradually as the terrace grade decreases; the platform value increases as the overlying soil thickness at the front edge of the terrace increases; however, the platform amplification coefficient β value decreases as the terrace grade increases, and the amplification coefficient at the front edge of the terrace is significantly larger than that at the rear edge.

River terraces have a significant impact on the propagation of ground motion and the degree of seismic damage to buildings. The change of the thickness of the overlying soil layer leads to different distributions of building damage by affecting the amplification of ground motion, while the number of terrace grades exacerbates or mitigates the seismic damage by affecting the spectral characteristics and overall level of ground motion.Therefore, the number of terrace grades and overlying soil thickness are the key factors affecting ground motion response, and the amplification effect of ground motion is stronger in high terrace and thick covering soil layer, which leads to serious damage of buildings.