Axial behaviour of underground steel pipeline buried in unsaturated soils

Soils surrounding underground pipelines are often unsaturated in field conditions. However, the effects of the unsaturated condition on the axial interaction between pipelines and soils are unclear and not considered in current design guidelines. This limitation may lead to unsafe designs, as the load from potentially moving soil to pipelines could be underestimated. To address this, six large-scale physical modeling tests were conducted to examine pipe pullout behavior using a rough steel pipe buried in saturated and unsaturated completely decomposed granite (CDG). Matric suctions around the pipe (0 ∼ 70.2 kPa) were adjusted by varying the initial water content and measured using tensiometers. The results show that axial pullout resistance, under constant nominal overburden pressure, increases with suction. At a suction of 70.2 kPa, the resistance was 1.69 times greater than in the saturated condition, highlighting significant risks in current design guidelines. 68 % of this axial resistance increment is attributed to the additional interface contact pressure induced by capillary forces of soil-pipe interface liquid menisci. The remaining 32 % is related to net interface contact pressure increases mainly due to suction effects on constrained dilatancy. A new and simple model was developed for calculating axial resistance in the unsaturated condition, based on elastic expanding cylinder theory, considering suction effects on Bishop's stress, stiffness, and dilatancy.