Centrifuge modelling of upheaval buckling in sand

Abstract

When operating at high temperature and internal pressure, initial imperfections in a cold laid pipeline can lead, through expansion and an increase in axial stress, to imperfection· growth and upheaval buckling. Efficient design against upheaval buckling requires accurate prediction of soil uplift resistance. However, current design methods are based on empirical uplift factors chosen from soil classification data, and do not capture the strong influence of backfill density on uplift resistance. A series of tests are reported, in which the pipe uplift is modelled in a 0.8 m diameter mini-drum centrifuge. A key advantage of the mini-drum centrifuge is that uplift tests can be carried out on small soil samples taken from boreholes along the pipeline length. Pipe uplift can be considered as a routine laboratory test. The results demonstrate the strong influence of backfill density on peak uplift resistance. Observation of the failure mechanism suggests that existing design methods based on vertical slip planes are unrepresentative of the failure. Instead, a new solution is presented, which more closely represents the observed mechanism, and provides excellent correlation with the measured data of peak uplift resistance across a range of backfill densities. Design curves are presented, which require element test results as input parameters. However, upheaval buckling is an unusual example in which centrifuge models, standing alone, provide the most efficient design tool.