Interfacial shearing characteristics between siliceous sand and the various scales of bioinspired scaled suction caisson during penetration

Abstract

Siliceous sand is widely encountered in the Yellow Sea, China, in which offshore wind and photovoltaic farms have been gradually constructed. The suction caisson has been proven to be suitable to support the upper structures due to its easy installation and retrieval. However, the high strength and compactness of the siliceous sand may increase the difficulty in the suction caisson installation. Also, the soil parameters are the key factors in designing the suction caisson. Therefore, this study proposes an innovative scaled suction caisson (SSC), which features easy penetration, high bearing capacity, and elimination of grouting in the gap between the soil plug and the cap. Compared with the traditional suction caisson, a bio-inspired design concept on its outer sidewall mimics the characteristics of snake belly scales to reduce resistance during installation and to increase resistance when being subjected to uplift. Smooth penetration to the design depth is crucial under reasonable arrangement of the scales. Therefore, the arrangement of scales on the outer sidewall in the design of the SSC is examined through a series of direct shear tests. The interface shear behaviors of sand-sand and sand-scaled steel are investigated with various scale types, scale heights, scale aspect ratios, sand grain gradations, and normal stresses. Results show that the sand-sand interface exhibits strain softening. The amount of sand roundness exceeding 0.7 for sizes 0.5–1.0 mm and 1.0–2.0 mm is 67.92 % and 41.55 %, respectively. Under identical sand particle size and normal stress conditions, the interface shear stress at the sand-scaled steel increases first and then decreases with increasing scale height for similar types of scales. Under normal stresses of 200 kPa and 400 kPa, the number of sand particles in 0.5–1.0 mm increases by 7.96 % and 8.29 %, respectively, after shearing for sand size 1.0–2.0 mm with scaled steel plates. In addition, as the scale height increases during shearing, two shear zones are observed: the sand-scaled steel shear zone and the sand-sand shear zone. Meanwhile, the theoretical formulas for calculating the critical shear stress are obtained. Finally, the peak shear stress, stress ratios and sand vertical movement at the sand-scaled steel interface are analyzed, determining the optimal geometric characteristics and arrangement of scales. This study can provide guidance for the construction of foundation structure in marine geological engineering of siliceous sand.