| Chengdao area, northeast of Huanghe subaqueous delta is typically characterized by the geologic hazards , which will threaten the safety of the offshore engineering such as drilling well and drilling oil platform, man-made island and oil transporting pipelines. Many investigations were carried out in this area for the measurement of geologic hazards using the sonic equipment and the records were interpreted. However, the extensive understandings on the classification and distribution of the shallow geologic hazards are still needed, as well as the intrinsic and extrinsic factors responsible for the submarine landslides. Here the data of about 20 well sites in this area are collected and used to study the soil stratum and consequently to analyze the related geologic hazards. The regional seafloor morphologic features are studied qualitatively. The distribution of the shallow stratum geologic hazards is presented according to water depth. Based on the property of the sediment mechanics and the condition of the hydrodynamics, the submarine landslides were quantitatively analyzed facilitated by the Geo-Slope software (GEO-SLOPE International Ltd. Canada).In the Chengdao area many geologic hazards bodies are developed in the soil stratum with the water depth below 16 meters, such as relic dune, collapse pezograph, landslide block, crust disturbing body, palaeochannel, ancient swamp, fossil lake, et al. Bellow the 2-m isobaths, the collapse pezograph is dominant due to many man-made structures that significantly change the flow field in the coastal area. Around the 5-misobaths, owing to the bed scouring induced by stronger tidal currents, a series of mild erosional furrow were developed parallel to the major axes of the tidal currents. Under the circumstance of periodic setup loading the landslides will easily happen. Between 8-m and 12-m isobaths, due to the energy transferring to the seabed from the breaking waves in the surf zone, it develops some geologic hazards such as slip blocks, collapse pezographs, relic dunes and some filled facies stratum such as palaeochannel, ancient swamp and fossil lake. Between 12-m and 16-m isobaths, geologic hazards are mostly crust disturbing body, palaeochannel and fossil lake.The analysis of sediment mechanics properties suggests that the spatial distribution of stratum is discontinuous. The soft stratum with properties of high compressibility and low-intensity is generally detected in the study area that will flow and deform on the declining slope due to the unbalanced pressures. This is the direct triggering factor to the geologic hazards of the shallow stratum, and it is also the base of submarine landslides in this region. The intrinsic factors controlling the submarine landslides are the shearing strength and the thickness of the soft stratum. The extrinsic factors are including many other external conditions such as static pore-water pressure, the actions of wave and tidal current, the excess pore pressure induced by earthquake and the tidal fluctuation. The stability of the subaqueous slope is calculated quantitatively through analyzing the soft stratum using the GEO-SLOPE software. The degree of reliability is analyzed in contrast to soft stratum parametric variation, soft stratum thickness, pore-water pressure and the actions of wave and tidal current. The experimental results show that when the soft stratum shearing strength approaches its minimum, the submarine landslides will happen with failure possibility of 71. 02%; when the soft stratum shearing strength approaches its maximum,the probability for submarine landslides will decrease drastically, even it never happen. The results also indicate that the thickness of the soft stratum is another important factor influencing the submarine landslides. As the thickness increasing, the probability of the slope failure enhances. The critical value of the buckling is founded through analyzing the softstratum shearing strength index, that is c = 10.0, φ =2.5.In this thesis the distribution of the geologic hazards in the Chengdao areas is comprehensively analyzed. The analysis shows that under the circumstance of different hydrodynamics the geologic hazards types are multiple. The submarine landslides are quantitatively analyzed for the first time using the GEO-SLOPE software and the related factors affecting the submarine landslides are discussed.This study extended the understanding on the shallow geologic hazards and the conclusions presented in this thesis are of importance to the offshore engineering and the route design of the oil transporting pipelines.
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