Model Tests on the Adhesion Factor of Full-Scale Polyethylene-Coated Pipes in Soft and Very Soft Clays

Abstract

In subsea pipeline engineering practice, the total stress or alpha approach, which assumes that the axial resistance to pipeline walking in fine-grained soils such as soft and very soft clays for a specified type of coated pipe varies primarily with the undrained shear strength of the material and contact area, has been increasingly applied in axial stability analyses. The adhesion factor between the subsea pipeline and soil is an important design parameter in the total stress approach. In this paper, a series of axial pipe–soil interaction model tests were conducted to study the effects of the buried depth of a polyethylene (PE)-coated pipe, the undrained shear strength of clays, the pipe diameter, the loading rate, and the setup period on the adhesion factor using full-scale steel pipes coated with PE material. The results reveal that the adhesion factor increases with increasing loading rate, buried depth, and pipe diameter. However, the effect of the pipe diameter on the adhesion factor is not obvious for very soft clay. The range of the adhesion factor of PE-coated pipes is analyzed based on the model test results. Because the displacement-resistance curve of very soft clay shows hardening behavior based on model test results, the adhesion factor associated with the ultimate resistance of very soft clay varies from 0.2 to 0.38. Because the displacement-resistance curve of soft clay shows soft behavior based on model test results, the peak adhesion factor ranges from 0.15 to 0.26, and the residual adhesion factor is approximately 0.85 times the peak adhesion factor for soft clay. Finally, an empirical formula for determining the adhesion factor of PE-coated pipes in soft and very soft clays is recommended based on the model test results. A discussion is also provided to explain to why existing studies based on the Coulomb friction law have generally arrived at seemingly different results.