Axial force–displacement behaviour of a buried pipeline in saturated and unsaturated sand

Several pipeline projects have been proposed in the world in recent years owing to the rapid growth of the petroleum industry. Onshore pipeline systems are usually buried above the natural groundwater table, where the soil is typically in an unsaturated condition. Rational interpretation of these pipelines is possible by extending the mechanics of unsaturated soils. In this paper, the axial force exerted by way of the relative movement on a prototype steel pipeline in sand under both saturated and unsaturated conditions is determined from laboratory investigations using specially designed equipment. The test results under the unsaturated condition showed significantly higher axial force on the pipeline compared to the saturated condition, attributable to the contribution from matrix suction. Two analytical models proposed in this study, which take into account the influence of matrix suction and dilation, successfully predicted the peak and residual skin friction along the pipe surface. The framework developed, based on experimental studies and analysis of the results and the proposed models that use saturated shear strength parameters and the soil-water characteristic curve, is promising for generating simple tools for the design of pipeline infrastructure buried in sands to ensure their stability and safety, and at the same time provide economical solutions.