Failure mechanisms of suction caisson foundations in clay under vertical and inclined pullout loads

Handayanu, 1963 (2001) Failure mechanisms of suction caisson foundations in clay under vertical and inclined pullout loads. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A suction caisson typically consists of a hollow cylinder or a cluster of cylinders made of steel or concrete with a closed top. This novel foundation consists of massive cells, typically 10 to 30 meters in diameter and up to 40 to 50 meters deep. The caissons will form a tight seal with the soft submarine sediments, and provide the necessary uplift resistance, when pressure reduction is created below the closed top. In this study, finite element method was used to investigate the development of suction force, based on D'Arcy's law, by implementing some user subroutines provided in ABAQUS. Contact between suction caisson and soil surface was simulated using a contact surface model. Soil material was modelled as a porous medium and its plastic behaviour was characterized using a modified camclay model. -- The study to validate the results of the above numerical analysis used four laboratory tests carried out in different locations, for different purposes and with different set-ups. The test data used are the Massachusetts Institute of Technology (MIT) suction caisson tests [Cauble, 1997], University of Texas at Austin suction caisson tests [El-Gharbawy, 1998], Indian Institute of Technology (ITT) Madras (India) friction single pile tests [Prasad, 1992], and suction caisson tests [Rao et al., 1997]. All these tests gave a total of twenty-nine experimental results to validate results from finite element analyses. In addition, scale model analyses for 1g, ng, and prototype models were also carried out. Failure patterns observed in soil were modelled by relevant soil failure models available in literature. Moreover, comparison of 3D and axisymmetric-asymmetric analyses for inclined pullout loads with different inclination angle and anchor point attachments, were also carried out. -- Suction pressure generation in suction caisson model finite element analyses consistently gave close matches with results obtained from laboratory vertical pullout tests carried out at MIT, University of Texas at Austin, and IIT Madras. Suction pressure development, carried out in this study, simulated the suction force or pressure (built up below the caisson top) properly, and the difference between good numerical and experimental results ranging between 0.3% to 7% (based on suction force) for the MIT tests, 10% to 20% (based on suction pressure) for the Texas tests, and 22.7% to 26% for the IIT Madras tests by Rao et al. However, less accurate results ranged from 27% to 70%. -- The finite element models gave a stiffer displacement response and as such did not match very well with some (especially University of Texas) of the experimental measurements. -- Based on the combined contours of shear stress to shear strength ratios (based on direct shear tests and triaxial extension tests) under vertical and inclined pullout loads, failure surfaces similar to the failure surfaces obtained for the Snorre suction caisson model pullout tests (carried out by the Norwegian Geotechnical Institute [Andersen et al., 1993]) were obtained in this study.

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