Soil Response for Pipeline Upheaval Buckling Analyses: Full-Scale Laboratory Tests and Modelling

Abstract

ABSTRACT A pipeline is subjected to a compressive load when its thermal expansion is axially restrained. This load can cause the pipeline to buckle when the lateral and axial friction resistances are not sufficient to prevent pipeline movement. The resulting movement will usually be upwards, since this direction has the smallest lateral restraint. The likelihood of occurrence of this upheaval buckling phenomenon is largely determined by the capability of the soil to resist pipeline movements. Detailed understanding of uplift and axial friction resistance thus contributes to the improvement of reliability and economy of pipeline design against this failure mode. This paper presents the results of a full-scale laboratory test programme on the uplift and axial resistance of a 4 inch pipe embedded in saturated soil for a wide range of soil conditions. The soil types include dense and loose sand, remoulded clay and rock. These soils were used for cover conditions ranging from homogeneous to trenched, over depth-to-diameter ratios between 4 and 12. Although most tests were quasi-static, a number of uplift tests specifically studied time-dependencies. All tests were performed under described and well defined conditions and test procedures that approach offshore conditions as closely as possible. INTRODUCTION When a pipeline is operated at higher than ambient temperatures it will try to expand. If the pipeline is axially restrained, for instance by the friction of the surrounding soil, the pipeline will be subjected to an axial compressive load. The pipeline will then tend to buckle with displacements in the direction of the smallest lateral restraint, the extent of which depends on pipeline material behaviour, shape and imperfections, bedding and cover conditions. This is usually in the vertical plane upwards. This phenomenon is called upheaval buckling (offshore) or referred to as overbend (in)stability (onshore). Pipeline response might in that case be unacceptable in terms of vertical displacements (pipe protruding through the cover or moving out of the trench), excessive yielding of the material due to second order loading effects, or both. Upheaval buckling is hence a failure mode that has to be taken into account in the design of trenched and buried pipelines. Its likelihood of occurence is largely determined by the capability of the soil to resist pipeline movements. Consequently, the availability of accurate soil resistance models is critical to the safe and economical design of these pipelines. As part of an extensive upheaval buckling research programme (ref. 1, 2, 3) a two-phase study has been carried out at Delft Geotechnics to define the uplift and axial soil resistance as a function of soil type, pipeline characteristics, trench geometry and loading type. Phase 1 of the study consisted of a review of relevant prediction models and the available high-quality data. One result of which was that the ratio of pipe diameter and cover height H/D is sufficient to describe the pipeline characteristics within the range of interest of the study. Phase 2 included the performance of a complementary test programme to complete the data base for the pipe and soil conditions of interest.