The Difference Of Seabed Instability And Its Impact On The Engineering Facilities In Chengdao Sea Area

Chengdao sea area, in which a Lot of sediments were deposited here since huanghe changed its channel, is located in the north-east of the Huanghe Delta, south-west of Bohai Sea. The abandoned sub-delta came through a serious erosion, forming a complex underwater landscape, meanwhile, strong hydrodynamic is developed here, which leads to the instability of a large number of sediments. Thus geological disasters are very serious in this area. Previous researchers have conducted a lot of study in this area:underwater slump gullies, residual Harbour Hill Gang, ancient river, ancient lakes, swamps and many other geological disasters exist here, which is a great threaten to the security of the ocean engineering facilities in this area. The research object of previous studies was often limited to a region or a platform, leading to the lack of a comprehensive understanding of the instability of the whole sea area. Therefore, it is of great significance for the construction and maintenance works of drilling platforms, subsea pipelines, cables and other facilities to study the instability of the entire sea area.The seabed topography in chengdao area is divided into three major geomorphological units. The first unit, with water depth ranging from 0 to 5 meters, is relatively flat, topography is dominated rough seabed which is caused by erosion. The second unit, with water depth ranging from 5 to 14.5 meters, has a gradient of about 2 %o. This section is in the upper of delta front, lots of slopes and submarine slumps, landslides, debris flows and many other instabilities are well developed. A variety of micro-landforms also exist in this unit, such as smooth seabed topography, submarine sliding instability landforms, erosion depression landforms, erosion processes over landscapes and so on. The third unit, with water depth ranging from 14.5 to 19 meters, has a gradient of about 0.4%o. This section belongs to the bottom of the slope of the front delta. The grain size of the sediment become coarse from the bottom to the surface and the wedge-shaped delta debris which become thinning and pinch out to the sea. The structure of one mouth and two pieces of mud was formed in Huanghe delta due to the shallow water in this area, which caused the existance of plenty of soft soil in the subaqueous delta plain. Meanwhile, the huanghe channel changed frequently, which caused the superposition of different geomorphological units, leads to the occurrence of instability, such as Landslide, subsidence and so on.In this paper, high resolution acoustic instruments including multi-beam, side scan sonar, bottom profiler were used to survey in this area. Combined with the well drilling data, field and laboratory experiment, the analysis and interpretation of the sea-bed instability were conducted aiming to study the differences of instability in chengdao Sea area. Base on the route surveys and drilling data, the geological hazards caused in different areas were studied in this paper. The near-shore currents change a lot due to the existance of lots of facilities in Erosion area. Meanwhile plenty of collapse pits, gully exist in this area. The slope unstable zone has a large gradient. the upper sediment moves laterally while the lower part moves vertically. A lot of unstable collapse and sliding exist in this area. Diverse types of geological hazards occur in this zone which has a unstable topography widely distributed. The erosion pits, gullies exist in this area. Stratigraphic zone was filled with ancient buried river, ancient lakes and so on.Based on the analysis of unstable geological disaster, a further discussion was made to study the instability and its impact on artificial structures caused by different geological disasters.