Undrained interface shear strength of fine-grained soils for near-shore marine pipelines

Abstract

The peak and residual interface shear strengths for a variety of normally consolidated soils were measured using a torsional ring shear apparatus modified to suit soil and interface testing under the undrained condition, which is usually prevalent at soil-marine pipeline interfaces. Five solid surfaces with different roughnesses were utilized in this single-stage shear testing program. Tests were conducted at normal stresses in the intermediate range that are mobilized on soil-pipeline interfaces under near-shore conditions. For comparison, drained peak and residual shear strengths of a representative interface were also measured using a different version of the torsional ring shear apparatus. Based on the analyses and interpretations of the test results, a simple equation is introduced to estimate the undrained peak and residual interface efficiencies as a function of the solid surface roughness. The analyses also revealed that the normalized undrained peak and residual shear strengths of the soils and interfaces are insensitive to the change in soil plasticity. The ratios between these undrained strengths and the corresponding drained strengths decrease with decreasing soil plasticity and increasing interface roughness. Recommendations for the initial assessment of the pipe-soil shearing resistance and the preliminary stability analyses of near-shore marine pipelines are suggested based on the findings of this study.