Study On The Differences And Geneses Of Geotechnical Properties Of Cohesive Soil In Typical Offshore Areas Of China

Marine cohesive soil widely distributes in the offshore areas of China, mainly inarea with high developed marketing economy and intensive offshore marineengineering. However, because cohesive soil has the characteristics of high watercontent, high compressibility, and low shear strength, it is poor foundation soil andeasily causes the instability of seabed. Therefore, the study of geotechnical propertiesof cohesive soils in these areas has important theoretical and practical significance.This paper takes the cohesive soil in Laizhou Bay, the central South Yellow Sea,Zhejiang-Fujian coastal area as research object, studies about their geotechnicalproperties and differences, microstructure and the geneses of geotechnical properties.Through analysis of the geotechnical properties and their differences of cohesivesoil in the three areas, it is found that, the cohesive soil in Laizhou Bay is mainlymuddy clay, with a small amount of mud and silty clay. On the whole, cohesive soil inLaizhou Bay has the characteristics of low water content, high density, low void ratio,low plasticity, low compressibility, high shear strength, high vane shear strength, andhigh miniature penetration resistance. Cohesive soil in the central South Yellow Sea ismainly mud, contains some muddy clay and a small amount of clay, silty clay, and hasthe characteristics of high water content, low density, high void ratio, high plasticity,high compressibility, low shear strength, low vane shear strength, low miniaturepenetration resistance. Cohesive soil in Zhejiang-Fujian coastal area is mainly mud,with a small amount of muddy clay, clay and silty clay, its geotechnical properties isbetween the above two areas.The principal component analysis finds that, the physical and mechanical indexesthat influce geotechnical properties of cohesive soil in the three areas all can extractfour principal components, water content, density, void ratio, liquid limit andminiature penetration resistance all enter the first principal component with high load,the main loads of other principal components vary greatly. Through comparison of the evaluation function of the principal component, it is found that, the geotechnicalproperties of cohesive soil in Laizhou Bay mainly affect by physical property indexes.The geotechnical properties of cohesive soil in the central South Yellow Sea affect notonly by physical property indexes, but also compressibility indexes. The geotechnicalproperties of cohesive soil in Zhejiang-Fujian coastal area mainly affect by physicalproperty indexes, compressibility and internal friction angle.The electron microscopic analysis of the microstructure of cohesive soil in thethree areas shows that, the cohesive soil of Laizhou Bay has three kinds ofmicrostructure: granular cementation structure, granular link-bond structure, floccu-lent link-bongd structure, the dominant types is flocculent link-bongd structure, soilwith this structure, medium to high strength, low to medium porosity, low to mediumcompressibility. The cohesive soil of the central South Yellow Sea also has three kindsof microstructure, they are granular link-bond structure, flocculent link-bongdstructure and clay matrix structure, the dominant types is clay matrix structure, soilwith this structure, low strength, high porosity, high compressibility. The cohesive soilof Zhejiang-Fujian coastal area has the structure of granular link-bond structure,flocculent link-bongd structure and clay matrix structure, flocculent link-bongdstructure and clay matrix structure are the main structures, cohesive soil in this area,medium strength, medium porosity, medium compressibility.By using IPP image processing software and Matlab image processing function,the microstructure images of cohesive soil in Layzhou Bay and Zhejiang-Fujiancoastal area are processed. The differences of parameters characteristics of differentmicrostructural types and regression analysis results between microstructuralparameters and geotechnical index of cohesive soil in the two area are analyed,analysis results are as follows: the average pore size of granular link-bond structure ofcohesive soil in Laizhou Bay are larger than cohesive soil in Zhejiang-Fujian coastalarea. The pore of flocculent link-bongd structure of cohesive soil in Laizhou Bay issmaller than cohesive soil in Zhejiang-Fujian coastal area. The particle of granularlink-bond and flocculent link-bongd structure of cohesive soil in Laizhou Bay are alllarger than cohesive soil in Zhejiang-Fujian coastal area. Multiple linear regression analysis on the microstructural parameters and geotechnical indexes of cohesive soilin the two area indicates that the physical indexes of cohesive soil in the two area aremainly related to the parameters that characterize pore size and number, themechanical indexes not only affect by the size, number of pore, but also the geometryof pore.Laizhou Bay is a semi-closed bay, it plays a limit role in the spread of sedimentto the open sea. Lager amount of Yellow River source material deposits in this areawith high rate, and is easy subjected to the late strong hydrodynamic conditions,sediment in this area usually has high silt content, mineral component is maily ofprimary mineral, this makes the cohesive soil in this area is mainly muddy clay withlow water content, and has relatively good geotechnical properties. The central SouthYellow Sea is far from the mainland, low-lying and has high water depth, materialfrom the Yellow River, old Yellow River and Yangtze River that can spread to thisarea is mainly clay material with smaller particle size, its deposition rate is very low,hydrodynamic conditions is weaker and for reductive environment, sediment in thisarea usually has high clay and organic matter content, and is not easy subjected to thelate hydrodynamic conditions. This makes the cohesive soil in this area has high watercontent, and is mainly of mud, geotechnical properties is poor. Zhejiang-Fujiancoastal area is relatively close to the mainland, coastal rivers and islands are numerous,material from the Yangtse River and coastal rivers that can spread to this area ismainly clay particle, under weaker conditions occurs flocculation and sedimentation,sedimentary speed is high, and is not easy subjected to the late strong hydrodynamicconditions, the cohesive soil is mainly of mud. But because of it is closer to themainland, the clay particle, clay minerals and organic matter content are lower thancohesive soil in the central South Yellow Sea. Its hydrodynamic conditions is alsostronger than the central South Yellow Sea, so geotechnical properties of cohesive soilin this area is better than soil in the central South Yellow Sea.