Soil Reconsolidation Following the Installation of Suction Caissons

Abstract

Abstract Experiments are being performed with a prototype suction caisson in tanks of soft clay. This paper contains recent test results and numerical simulations showing the reconsolidation behavior of the soil around the installed caisson. The 100-mm diameter caisson prototype was proportionally dimensioned to represent a typical offshore suction caisson. Tests were conducted in normally consolidated deposits of kaolinite clay, which were formed to simulate common seafloor conditions. Large tanks of the soil were prepared by consolidating a slurry to a final thickness of about 1.1 m, a process that took about eight months. Caissons were then installed using dead weight and/or suction. Instrumentation was used to record displacements, axial forces, and pore water pressures during installation and extraction of the caisson. Tests with the prototype caisson, as well as tests with a flat plate inserted vertically into the test bed deposit, show an increase in pullout capacity with time (set-up) as the soil reconsolidates following installation. Data are presented showing the dissipation of excess pore water pressures on the interior and exterior of the prototype caisson. The measured pore pressures are also compared to results from a finite element simulation of the reconsolidation process. These results demonstrate how the pullout capacity of a suction caisson may be limited if large loads are applied prior to full soil reconsolidation. Introduction A suction caisson is a closed-top steel tube (typical diameter larger than two meters) that is lowered to the seafloor, allowed to penetrate the bottom sediments under its own weight, and then penetrated further with differential pressure produced by pumping water out of the interior. Suction caissons are an attractive option for providing anchorage for floating structures in deep water. Suction caissons are easier to install than driven piles and have higher load capacities than drag embedment anchors. Suction caissons can be inserted reliably at pre-selected locations and penetration depths, and can be recovered for re-use. The large axial and lateral capacity of a suction caisson permits the design of taut mooring systems with smaller seafloor footprints, which are less likely than catenary systems to encroach upon adjacent tracts or facilities. Sparrevik [1] estimates that there are over 300 suction caissons in operation around the world. Most worldwide experience with this technology has involved relatively short, large-diameter caissons with vertical loads applied at the center of the top plate. However, many deep-water locations have soft clay deposits where foundations are more efficient if smaller diameters and larger penetrations are used. Further, to provide maximum resistance to lateral loads, the connection for the mooring line is typically moved from the top center to a padeye in the lower half of the installed caisson. The offshore industry is currently designing and installing deep-water structures moored to arrays of suction caissons of this type. However, a number of design issues remain unresolved [2, 3]. There is a paucity of published performance data on suction caisson behavior, especially for long, slender caissons, with little information addressing set-up times and behavior after consolidation of the surrounding soils.