Some observations in soil–pipe model tests under vertical uplift loading in compacted clay

Abstract

Physical model tests are conducted to measure the soil resistance exerted on a steel pipe under vertical uplift loading at varying displacement rates in compacted clay. It is observed that the measured soil ultimate resistances are much lower (two to three times in magnitude) than those recommended by existing design guidelines for pipelines buried at shallow depths. This large discrepancy is attributed to the differences in failure modes observed in the tests and assumed in the guidelines. Under vertical uplift loading, the tensile failure mode is more dominant than the shear failure mode in compacted clay at shallow depths, which is not taken into account in the existing guidelines. Furthermore, the results illustrate that the soil–pipe system exhibits a time- or rate-dependent behaviour under vertical uplift loading – that is, ‘isotach’ behaviour. The soil resistance increases with increasing pipe displacement rate, or vice versa. The soil resistance drops by 10–30% when stress relaxation is allowed. Practical implications of this time-dependent behaviour of compacted clay on the performance of buried pipelines subjected to long-term ground movement are addressed.