Abstract
Significant difference between predicted and measured installation resistance of stiffened suction caissons was identified due to the existing uncertainty regarding the mobilized soil flow mechanisms. This paper describes an extensive investigation of square stiffened caisson penetration in nonhomogeneous clays undertaken through large deformation FE (LDFE) analysis to identify the soil flow mechanisms around and between lateral ring stiffeners. A detailed parametric study has been carried out, exploring a range of nondimensional parameters related to stiffened caisson geometry, caisson roughness, and soil strength. The LDFE results were compared with centrifuge test data in terms of soil flow mechanisms, with good agreement obtained. Two interesting features of soil flow inside the caisson were observed including soil backflow into the gaps between the embedded stiffeners and soil heaving at the surface. It shows that the cavity depth can reach ∼5 m. Finally, simple expressions were proposed for estimating the critical depths of soil backflow and cavity formation.
Highlights
E uncertainties have been shown by the significant differences observed between the predicted caisson resistance, based on the presumed soil flow mechanisms, and the measured resistance in the field. e field measurements include the stiffened caisson installation at the Laminaria field in the Timor Sea and at the Girassol field, offshore West Africa [2,3,4], with a detailed discussion reported by Hossain et al [5]
Hr is defined as critical rotational depth, above which soil cannot stand and can rotationally flow into the gaps between stiffeners, and Hc is defined as the limiting cavity depth
When the penetration depth of bottom stiffener reaches Hr, the soil starts to flow back into the bottom cavity, and when the soil heave height inside the caisson is higher than Hc, there exist gaps between soil and structure, and soil does not flow back, that is, when the inner soil heave height is less than Hc, the soil flows into the cavity between the stiffeners with rotational soil failure mechanisms and is trapped, moving downwards together with the stiffeners and the skirt at the same velocity, and there is no relative slide between the pile and the soil and no friction. e internal and external friction calculations need to adapt the equivalent strength after the disturbance instead of the undisturbed soil strength
Summary
To avoid the buckling failure of the thin wall of a long caisson during installation, the skirt is strengthened with internal stiffeners, horizontal rings, and/or vertical flanges (see Figure 1), together with local thickening of the wall in the vicinity of the loading point. e addition of these stiffeners has created significant uncertainties regarding the soil flow mechanisms, side friction, and end bearing and in the prediction of underpressure required for installation [1]. E addition of these stiffeners has created significant uncertainties regarding the soil flow mechanisms, side friction, and end bearing and in the prediction of underpressure required for installation [1]. E uncertainties have been shown by the significant differences observed between the predicted caisson resistance, based on the presumed soil flow mechanisms (see Figure 2), and the measured resistance in the field. Andersen et al [1] discussed the predictions for two different hypothetical installation cases and six case histories of caissons with stiffeners carried out by four predictors using their normal design method. Is paper reports the results from an extensive investigation carried out through large deformation FE (LDFE) analysis in an attempt to provide insight into the failure mechanism of square stiffened caisson penetration in nonhomogeneous clays. Expressions are developed to estimate the critical depths of soil backflow into the gaps between the embedded stiffeners
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