Journal Article

Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea

Liu, Changling, Hu, Gaowei, Chen, Qiang, Wu, Nengyou, Li, Chen’an, Li, Yanlong

2019

Overview

The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations. In situ piezocone penetration tests (CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China’s first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments (HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays, the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays, while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.

• Publisher: The Chinese Society of Oceanography,Springer Nature B.V,Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China,College of Information and Control Engineering, China University of Petroleum, Qingdao 266580, China,Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China%Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao 266071, China,Laboratory for Marine Mineral Resource, Pilot National Laboratory for Marine Science and Technology(Qingdao), Qingdao 266071, China,Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao 266071, China
• Subject: Adaptability / Area / Bearing / Clay / Climatology / Coefficients / Deep water / Design parameters / Distribution / Earth and Environmental Science / Earth Sciences / Ecology / Empirical models / Engineering Fluid Dynamics / Environmental Chemistry / Exploitation / Format / Hydrates / Intervals / Laboratories / Laboratory experimentation / Laboratory experiments / Laboratory tests / Marine & Freshwater Sciences / Mathematical models / Oceanography / Penetration tests / Penetrometers / Pore pressure / Prediction models / Pressure / Safety / Safety engineering / Sediment / Sediments / Shear strength / Statistical methods / Strata / Tests / Triaxial tests