Characteristic of the Marine Site Soil Condition in Bohai Sea, China

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Site soil condition has an important effect on ground motion. The site condition is simplified as an index of the seismic site effect. On the land area the equivalent shear wave velocity at a certain depth range or the site period is often taken as site index, such as the equivalent shear wave velocity at the top 30 m in the US and Europe or at the top of 20 m in China. But it is blank about the site index of marine site. With the development of marine facilities construction the research on marine site condition is necessary for earthquake engineers.

Several hundred offshore platforms have been built in China marine area in recent 2 decades. We achieved lots of engineering geophysical data, such as seismic prospecting data revealing stratum information, downhole data including shear wave velocity and dynamic lab tests revealing the dynamic properties of sub-layers. By the analysis, we found that the marine site condition is far different from the land site. The near surface sub-layer is very soft and is near in liquid or plastic state. The site classification index and method on the continent is not suitable for the marine site. From 112 downhole data we carried out an investigation of the near-surface sub-layer soil including its lithology, shear wave velocity and density and got the character of the near-surface soil. The thickness soft sub-layer is often 5 to 10 m, with a shear velocity of 90 to 110m/s. The shear wave in the sub-layer at 18 to 20m is up to 200m/s. The facility is mainly the jacket-leg platform. The supporting layer is usually at a depth of 60 to 90 m. The near surface layer is almost in liquid state and has tiny effect on the stability of the platform during its operation. However the thick soft layer has an important effect on ground motion parameters. It lead to a high acceleration and long characteristic period of the deign spectrum. We suggest the withdraw of the top soft soil when considering the site effect and site classification of marine site and an index should be given to define the soft soil. The study is still under investigation.

Presenting Author: Yanju Peng

Authors