An algorithm was developed to evaluate the pull-out capacity or resistance of open-bottom or closed cylinders under water and embedded in the sea floor, with full account of suction effects. The basic mechanics are very simple, but the combinations of geometry, soil conditions and failure modes are legion.
The method provides for easy parameter studies and thus for an optimal use of the suction effects in the design of small offshore platforms, submersible CPT-rigs, suction anchors for mooring buoys, etc. It works as a small 'expert system' and needs very little computing power.
Attempts to analyse the installation procedure and the pull-out capacities for suction piles showed so many variations that it became advantageous to use a computer.
It was first required to analyse the pulling resistance of the three hollow foundation piles, open at bottom level, of a monotower offshore platform. For these piles, shorttime vertical tension (pull-out) forces are of particular interest. These forces are generated by a ship's impact or waves and vortex shedding on the superstructure.
The methods used in the program are presently being compared with conventional small-scale model tests as well as small-scale centrifuge tests. Lately also field tests on a larger pile model are planned.
There are various failure modes both for sand and for clay. These mechanisms will be mentioned as well as the main parameters which control the magnitude of the suction effect. The capability of the program will be visualized through the analysis of two structures for which the suction effects are essential.
In addition to geometry, weight and shear strengths, certain assumptions concerning the suction effects are required. Various types of analysis may be used; of these the processing of test data will not be discussed.
Geometry parameters : The geometry parameters_ are defined on Fig 1. The vertical dimensions are the water depth, the total pile height, the penetration measured on the outer side of the pile, and the height of the soil plug in- side the pile.
For the horizontal pile section the data are the total (convex) circumscribed area and pe- rimeter, and the interior area and perimeter. Basically these numbers are assumed to re- present an exterior and an interior circler i.e. 'equivalent' circles if the pile section is of another shape.
Weight parameters : The total as well as the buoyant weight of the pile proper are required. The difference between these two values would show for instance whether the construction had any empty watertight com- partments.
Also the unit weight of water, and the atmospheric pressure expressed in the chosen unit system are required if any consistent unit system Is to be allowed, or if the analysis of centrifuge model tests is of interest.
