Strength assessment of Tra Vinh clay for offshore wind power project in Vietnam

Abstract

Sample disturbances in offshore wind power projects are inevitable and much higher than onshore projects. Unconsolidated undrained shear tests (UU) results could underestimate the in-situ strength of clay, which would lead to a huge waste in foundation design. Piezocone penetration test (CPTU) can interpret precise soil strength for in-situ soil if appropriate interpretation parameters are chosen. As a case study, laboratory tests and CPTU results of Tra Vinh offshore wind power projects in south Vietnam are analysed to acquire accurate soil strength. SHANSEP (Stress history and normalised soils engineering properties) method are adopted to minimize the sample disturbances. Linear correlations between strength and depth are discovered for Tra Vinh organic clay and Tra Vinh clay. The normalised undrained shear strength (s u/σ v0′)NC for Tra Vinh organic clay at 0m to 20m and Tra Vinh clay at 20m to 80m are approximately 0.17 and 0.34 obtained from DSq test, where (s u/σ v0′)NC of CU test are relatively higher. The normalised undrained shear strength s u/σ v0′ is found to be negatively linear correlated with liquidity index. SHANSEP and OCR-to-CPTU interpretation by using the conservative (s u/σ v0′)NC value from DSq test is in good agreement with laboratory test results of Tra Vinh organic clay at 0m to 20m, and reappears the in-situ undrained shear strength of Tra Vinh clay from 20m to 70m. After calibration by taking the interpretation results of SHANSEP and OCR-to-CPTU method as a reference, N kt values of 25 and 17 are respectively recommended for Tra Vinh organic clay and Tra Vinh clay in the offshore area, which is more convenient for CPTU interpretation in engineering projects.