Shearing behavior of suction caisson wall-sand interface throughout the suction caisson life-cycle

Suction caissons are widely used as foundations for offshore platforms and offshore wind turbines. The life-cycle of a suction caisson includes penetration, service and retrieval. The mechanical characteristics of the interface between sand and suction caisson wall are crucial for assessing the penetration behavior and bearing capacity of suction caissons. Interfacial shear tests under constant normal loading (CNL) and variable normal loading (VNL) conditions were conducted to investigate the shearing behavior of suction caisson wall-sand interface throughout the suction caisson life-cycle. Results show that the interfacial shear stress increases with an increase in normal stress and decreases with an increase in penetration rate throughout the life-cycle of the suction caisson. The value of vertical displacement of the interface decreases with an increase of cycle times during service and the sand particles' movement, rotation and rolling were observed by visualization tests. Under the test conditions, the interface frictional coefficient ranges from 0.46 to 0.80 and the interface friction angle varies between 25.64 degrees and 27.43 degrees during retrieval. Both the interface friction angle and interface friction coefficient during the penetration stage are greater than those during the retrieval stage, indicating that retrieval stage is not an inverse process of penetration stage. In addition, the control groups were carried out to investigate the effect of penetration on service stage and the effect of service on retrieval stage. It was also found that the influence of the penetration stage on shear stress during service stage is mainly observed in the first two cycles. The influence of service stages on shear stress during retrieval stage is more obvious. After prolonged cycling during service stage, the strength of the interface would be reduced.