ENGINEERING PROPERTIES ASSESSMENT OF MARINE CLAY IN GUANG-DONG OFFSHORE AREA BASED ON CPTU

As an important part of offshore wind turbine foundation design, the marine soil parameter evaluation would directly affect the safety of wind turbine and project cost. At present, piezocone penetration test(CPTU) is one of the most widely used in-situ testing methods in offshore geotechnical engineering. The disadvantage of CPTU is that soil parameters cannot be directly measured, which could only be interpreted by raw data. How to select appropriate CPTU interpretation parameter of clay strength is a difficult problem to be solved at present. In this paper, based on an offshore wind project in Guangdong offshore area, in-situ CPTU and hand torvane shear, pocket penetrometer, miniature laboratory vane shear, triaxial UU, consolidated quick shear and triaxial CU tests were carried out, the strength profiles were obtained by linear regression of the laboratory test results, where the clay samples under 60m were found to be severely disturbed, consolidated quick shear and triaxial CU test could reduce the effect of disturbance. Based on the hand torvane shear, pocket penetrometer, miniature laboratory vane shear, triaxial UU, consolidated quick shear and triaxial CU tests results, the average values of CPTU interpretation parameters N_kt for clay strength are obtained as 21.68, 19.52, 17.83, 18.63, 15.73 and 13.63, respectively. We suggest using N_kt of 21.68 as the interpretation parameter for the lower bound limit, N_kt of 15.75 to 17.83 as the best estimate, and N_kt of 13.63 as the upper bound limit. By using the calibrated N_kt parameters for clay strength interpretation, it is found that the interpretation results are in good agreement with the linear correlated strength profile. By normalised by the vertical effective stress, the normalised undrained shear strength s_u/σ'_v0 is obtained as 0.298 for normally consolidated clay in Guangdong offshore area. After interpretation of overconsolidation ratio by CPTU data, the clay strength obtained by SHANSEP method is also in good agreement with laboratory test results, which can be applied in offshore wind power projects in the future.