Axial behaviour of steel pipelines buried in sand: effects of surface roughness and hardness

Surface roughness and coating hardness of underground pipelines are expected to play decisive roles in their axial pullout behaviour, which is an important aspect of pipeline design. Existing guidelines and previous studies underestimated or ignored these effects, resulting in potentially unsafe design. To address this problem, the current study conducted nine large-scale physical modelling tests on pipes in dry and dense sand. Five steel pipes with varying normalised roughness (0.04-1.01) and coating hardness (32.6-59.0 HRA) were used and instrumented with a novel type of film-like piezoresistive sensors for measuring soil-pipe contact pressure. The measured pullout resistance of rough pipes is 2.70-2.85 times of smooth pipes, significantly greater than the value specified in current design guidelines (i.e., 1.17 times). This substantial increase stems from an increase in interface friction coefficient (accounting for 72-79%) and a contact pressure increase induced by constrained dilation and soil arching (contributing the remaining 21-28%). Regarding coating hardness, a critical hardness was observed (around 35 HRA). Due to equivalent roughness from particle embedding, pipes with hardness below this value exhibited similar behaviour to rough pipes. Finally, a new and simple method was proposed for calculating the pullout resistance with consideration of the effects of roughness and dilatancy.