| HYDRAULIC ENGINEERING |
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Estimating the maximum installable diameter of offshore cylindrical caisson foundations using standard penetration test blow counts |
| ZHANG Hui1, SHEN Xiaopeng1, SHEN Chen1, MO Weibin2, HUYAN Bin2, WANG Rui2 |
1. CNOOC Research Institute, Ltd., Beijing 100028, China;
2. State Key Laboratory of Hydroscience and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China |
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Abstract [Objective] Vibration-sinking caissons are increasingly popular in coastal engineering because of their low cost, ease of installation, and high stability. However, verifying excitation force and vibration amplitude before installation is essential. Current specifications require extensive geological surveys and geotechnical tests, which are costly and time-consuming. A rapid and reliable assessment of vibration-sinking feasibility with simple on-site tests is essential, especially for large-diameter caissons. [Methods] This study proposes a method for quickly evaluating the vibration-sinking feasibility of steel cylindrical caissons based on standard penetration test (SPT) blow counts. The proposed method correlates soil shear strength with SPT blow counts through empirical relationships to calculate installation resistance during the vibrating penetration process of steel cylindrical caissons. The accuracy of the method is verified by comparing results with current specifications and on-site records. In addition, a quantitative analysis assesses the influence of soil properties and interlayers, represented by SPT blow counts, on the maximum installable diameter of the caisson. [Results] The calculated installation resistance at different depths using the proposed method generally aligns with current specifications. For a steel caisson with a 24-m diameter, the maximum installation resistance calculated during the vibration sinking matches the maximum vibratory force recorded on-site with an 8% error, demonstrating the reliability of the method. When the SPT blow counts are small, installation resistance in clay is greater than that in sand for the same SPT blow counts, whereas higher blow counts yield greater resistance in sand than in clay. For shallow installation depths, vibration amplitude requirements dominate the feasibility analysis, whereas excitation force requirements become primary for deeper sinking depths. The consideration of interlayers is important for feasibility analysis, with a critical number of SPT blow counts identified. The interlayer has little influence on the vibration-installation capacity unless the SPT blow counts exceed the critical number. Beyond this point, the vibration-installation capacity rapidly decreases as SPT blow counts of the interlayer increase. The critical number of SPT blow counts is related to the interlayer thickness and bottom position; deeper and thicker interlayers have a greater impact and lower critical number of SPT blow counts. The impact of the bottom position becomes considerably greater than the thickness when the blow counts exceed the critical number. [Conclusions] The proposed method provides a quick and reliable estimation of the vibration-sinking feasibility of steel caissons based on SPT blow counts. The method effectively predicts the maximum installable diameter of offshore cylindrical caisson foundations based on SPT blow counts and offers valuable insights into the effects of soil conditions and interlayers, thereby enhancing the feasibility and optimization of large-diameter steel caissons.
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| Keywords
steel cylindrical caisson
vibration installation
soil condition
standard penetration test
maximum diameter
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Issue Date: 22 November 2024
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2024,
Vol. 64
