Overview of Geological and Geotechnical Characteristics of Offshore South China Sea Sediments-Paper

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The paper presents geological and geotechnical findings from an offshore site investigation and laboratory testing program at Liu Hua Field in the Pearl River Mouth Basin, approximately 210 km southeast of Hong Kong. The study characterizes the sediments’ monotonic, cyclic and dynamic behavior to support offshore facility design and installation.<br /><br />The field lies on the Dongsha uplift, within a tectonically complex basin shaped by rifting, subsidence and changing sea levels. Currents, tides and waves influence sediment distribution. Regional delta and deep-water fan deposits include loose, potentially unstable sediments, and the area is also exposed to typhoons.<br /><br />Investigation to 150 m below the seabed identified six geotechnical soil units, ranging from very soft to very hard calcareous lean clay; one unit also contains interbedded silty fine sand. The clays are generally low plasticity, with carbonate content of 10–40%.<br /><br />Laboratory and piezocone data were used to assess consolidation, stress history and shear strength. The coefficient of consolidation ranged from 7 to 90 m²/year, and compression indices from 0.283 to 0.424. Strength testing showed moderate anisotropy: triaxial compression strengths were 1.09–1.17 times direct simple shear strengths, while extension strengths were 0.80–0.92 times those values. Thixotropic strength gain reached 23–35% after 30 days, and soil sensitivity was approximately 3.5–5.<br /><br />Cyclic direct simple shear tests established cyclic strength and failure relationships. Resonant-column, bender-element and cyclic shear tests characterized dynamic response, including shear-modulus degradation and damping. The site-specific dynamic properties differed from some published clay correlations, and interpreted shear-wave velocities exceeded 200 m/s below 28 m beneath the seabed.<br /><br />Overall, the study provides site-specific geological, strength, consolidation and dynamic parameters, alongside comparisons with other offshore sediments, for use in deepwater geotechnical design.

Keywords

offshore site investigation

geotechnical testing

Liu Hua Field

Pearl River Mouth Basin

calcareous clay

cyclic shear strength

consolidation properties

dynamic soil behavior

shear-wave velocity

deepwater geotechnical design