This paper describes the results of an experimental study conducted in the laboratory to evaluate the suction force, which develops beneath cylindrical anchors during pullout. Model cylindrical anchors were embedded to different depths in model test tanks filled with soft clay and were pulled out at different pullout rates. The behaviour of anchors with a solid base was compared with that of anchors with a vented base to evaluate the suction force. Suction breakout factors are observed to lie in the range of 3.5 to 7.5 and they increase with both loading rate and embedment depth. They appear to reach a limiting value at an embedment depth of 8 times the anchor diameter.
Uplift forces, transmitted by the moorings of floating offshore structures, are resisted by various types of anchors embedded in the seabed such as pile anchors, superpile anchors, plate anchors and gravity anchors. Large-diameter, cylindrical shaped anchors in the form of superpile anchors (Albert et al. (1989)), concrete foundation templates (Stove et. al. (1992)) and skirted mudmats (Cottrill (1992)) have been proposed or recently installed as foundations of tension leg platforms and their usage is likely to increase in the years to come. In cylindrical anchors embedded in clay, a suction force is generated beneath the anchors during uplift. This suction force, though temporary in nature, can be significant in magnitude because of the large base area of the anchors. Such suction force is usually neglected during design even though it can be used to withstand short-term loads such as cyclic loads during ocean storms. The suction force beneath plate anchors embedded in soft clay has been studied by Bemben & Kupferman (1975), Nhiem (1975), Davie & Sutherland (1977), Baba et.al. (1989), Shin et. at. (1994), Datta & Suryanaryana (1994) and Das et. al (1994).