Ring penetrometer for interface shear testing on sand under low-stress conditions

Accurate characterisation of near-surface soils is essential for the design of subsea cables, pipelines and shallow foundations. Conventional laboratory testing equipment is not well adapted to these applications where the operative stress levels are typically <10 kPa. This paper presents and interprets a comprehensive test campaign using a novel flat ring penetrometer. This device measures interface friction at stress levels as low as 1 kPa. Drained friction parameters for a range of sands obtained from the ring penetrometer are compared to other friction measurements from centrifuge sled (CFS) and interface shear box (ISB) tests. The results demonstrate the repeatability of the ring penetrometer, and the benefit of the simple but rigorous interpretation approach. The residual friction measured in the ring and CFS tests shows no dependency on stress level, which contrasts with the ISB results. It is shown that the apparent stress dependency could be an artefact of system friction in the shear box. A correlation between friction coefficient and device settlement rate is identified, with the maximum friction mobilised as the settlement rate approaches zero, consistent with the combined bearing–sliding failure mechanism. These findings highlight the value of the ring device for interface friction measurement.