A recent, innovative foundation design is gaining popularity as a cost-effective anchorage for offshore oil drilling platforms: the suction caisson. Suction caissons are structures shaped like an inverted bucket that are allowed to sink into the soft seafloor sediment under their own weight. Water is then pumped from inside the caisson, creating an interior suction and downward force that advances the caisson to full penetration in the seafloor.
Two large deposits of normally consolidated kaolinite were constructed at the University of Texas to investigate the axial uplift capacity of suction caissons. A model caisson composed of anodized aluminum with an aspect ratio of eight was used for model testing. The laboratory facility is described, along with a discussion of the construction and consolidation of one of the testing tanks used for model tests.
A previously consolidated test bed was used for 17 axial pullout tests. The measured axial forces during insertion and pullout of the caisson depend on multiple factors: self-weight of the caisson, weight of soil attached to the caisson, suction pressures inside the top cap of the caisson, side shear along the caisson walls, and end bearing at the caisson tip. Analytical models are developed to quantify the soil resistance during insertion and pullout of the caisson using data recorded from an array of transducers.
Caissons were inserted into the test soil using either full self-weight penetration, or self-weight penetration followed by penetration using suction pressures. Pore pressure sensors attached to the caisson indicated that at least 24 hours are required to dissipate all excess pore pressure around the caisson that are generated during the insertion phase. Axial pullout tests were performed rapidly (undrained soil conditions) or slowly (drained soil conditions), and with a vented top cap (no soil plug) or a closed top cap (soil plug came out with the caisson).
Some data showed a reduction in pullout capacity when suction pressures were used for insertion. However, the available data does not conclusively show a significant difference in capacity following self-weight versus suction installation. Vented pullout tests indicate that little side resistance develops on the inside of the caisson, possibly due to disturbance caused during insertion. External side shear can be modeled using 50 to 80% of the undrained shear strength of the soil (a equals 0.5 to 0.8). Closed top cap pullout resistance depends greatly on the reverse end bearing capacity factor used. Back-calculation of reverse end bearing capacity factors, assuming an exterior a of 0.5 to 0.8, produced factors in the range of 11 to 22. Very little difference in uplift capacity was measured between caissons pulled slowly (drained conditions) and rapidly (undrained conditions). Recommendations are made for further research.
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