Interface Friction Angle from Ring Shear Tests on Sand: A Round Robin

The interface friction angle is a crucial design parameter for various foundations, including piles, suction buckets, anchors, gravity-based structures, pipelines, cables, and more. In the past decade, the exponential growth of offshore wind projects has led to a higher demand for multistage ring shear tests to mimic installation effects. Moreover, there is a need to obtain reliable interface friction angles, due to the potential savings in manufacturing, transportation, and installation of foundations, such as monopiles. To investigate the reliability of the interface friction angle obtained by laboratories worldwide, a round-robin testing program on interface shear tests of sand against rough steel is presented. This study focuses primarily on the most common ring shear devices, including the Bishop type and the Bromhead. Results from 42 ring shear tests conducted on 30 different devices across 19 laboratories (using the same sand and similar procedures) are presented and compared. The data suggest that the residual interface friction angle (δres) obtained from Bromhead can be 10% to 35% higher than that obtained from the Bishop ring shear apparatus. Significant scatter was observed in the results, as the Bishop ring shear yielded a δres=26.2°±0.8° (mean and standard deviation), while for the Bromhead devices, a δres=30.5°±3.5° (min=23.7° and max=39.4°) was derived, indicating that the Bromhead device could overestimate δres, in sand, by an average of approximately Δδres=4° or more. Additional tests revealed that careful specimen reconstitution is necessary to avoid creating extra friction between the sand and the confining rings in the Bromhead ring shear device. Furthermore, as suggested in previous studies, decreasing the settlements during consolidation (to <0.75 mm) does aid but does not eliminate side friction of the Bromhead device when testing a typical North Sea offshore sand in multistage shear testing. Finally, best practice recommendations are provided in this paper for obtaining more reliable interface friction angles on sand using Bromhead-type ring shear devices.