| dc.description.abstract | Site effect is one of the influential factors of seismic intensity. It also plays a crucial role in seismic hazard analysis. The average shear wave velocity of the upper 30 meters of a soil profile (Vs30) has been widely used for assessing site characteristics, such as site classification and ground motion prediction equation used Vs30 to describe the site conditions. Taipei Basin is an area with high population density and active economic activities in Taiwan. Site effect happen here which increase the earthquake disaster because of the basin topography and the overlying soft sediments. Although the distribution of Vs30 has been proposed by previous studies, as the borehole data gradually accumulates, it is necessary to develop a new Vs30 map in Taipei Basin.
There are a lot of borehole data in Taipei Basin. Those data can be converted into shear wave velocity(Vs) by empirical equations then get more Vs30 data. The existing empirical equations usually use Standard Penetration Test N value or depth as independent variable. However, Standard Penetration Test is easily affected by man-made operations or differences of equipment, which makes the N value inconsistent. Previous studies have pointed out that there is a strong correlation between void ratio, effective stress and Vs in sandy soil. Therefore, this study tried to establish new empirical equations which use void ratio and vertical effective stress as independent variable, and we expect the new equations can replace the old one with lower prediction error.
In this study, we constructed the Vs empirical equations for gravel, sand, clay and silt in Taipei Basin. The soil physical properties test data and Vs measurement data we used come from EGDT(Engineering Geological Database for TSMIP). In order to reduce the uncertainty of Vs estimation, the quality of Vs measurement data was checked and the soil data was separated by (1) whether the sample belongs to Holocene sediment; (2) classification of soil sample.
By using this empirical equation, the Vs of each depth of the numerous engineering borehole in Taipei Basin were computed to obtain more Vs30 data. Finally, the “Kriging with varying local means” method was applied to spatial interpolation and the distribution of Vs30 in Taipei Basin was mapped.
Compared with previous studies, the Vs empirical equation for sand, clay and silt suggested in this study have lower prediction uncertainty and less bias. Regarding the distribution of Vs30 in the Taipei Basin, the Vs30 at most of the area is around 210m/s to 300m/s. In Beitou, Shilin, Zhongshan, Songshan, Xinyi and Daan district, Vs30 is lower than 210m/s. In the southwest and southeast part of the basin, Vs30 is relatively higher, ranging from 210m/s to 440m/s. In addition, previous studies believe that there is a low-Vs30 area which Vs30 is lower than 180m/s in Zhongshan, Zhongzheng, Daan and Songshan. However, as this study adds new data to the analysis, the results show that this low-Vs30 area should not exist.
Finally, we calculated the Vs30 uncertainty caused by Vs estimation at engineering borehole site in Taipei Basin. The results show that this kind of uncertainties at most of the engineering borehole sites are 50m/s to 70 m/s. Nevertheless, there are higher uncertainties when borehole is located in southern basin or near the edge of basin, because most of the materials in these area are gravel and rock. | en_US |