Preparation of Foundations for Concrete Caisson Sea Structures
Authors: Gerwick Ben C. 
Published: October 2008

The increasing actual and proposed use of bottom-founded concrete caissons for offshore storage vessels, drilling and production platforms, Arctic Ocean structures, offshore terminals, and ocean breakwaters, necessitates a concentrated investigation of methods and procedures for preparing the foundations to provide adequate stability and bearing.

Preparation of an accurately graded and compacted sea bed at a water depth of several hundred ft at an exposed offshore site, is obviously extremely difficult to perform and to control. Although some authorities have recently stated that such bottom preparation is virtually "impossible", methods have been developed to carry out such work in both relatively shallow and deep water, at least to depths of 200 tg300 m.

The caisson itself can be a stable platform for performance of these operations, both prior to and after touchdown on the sea floor, but efficient and positively effective methods are essential.

Granular soils and preplaced rock-base courses, being permeable, are subject to considerable variation in pore pressure under wave action, and this must be considered in combination (in proper phase relationship) with vertical and lateral wave forces imposed on the structure itself.

Soft silts and clays are common to many deep-sea floor sites and require either removal or confinement. In the latter case, loading rates must be controlled.

Irregular and rocky sites present special difficulties, requiring removal of high spots and/or covering with a stable rock fill. Performance of the foundation under maximum earthquake or overload must also be considered. Liquefaction and mud slide under seismic or storm action must be prevented.

Grout injection, tremie concrete, and preplaced aggregate concrete may be used effectively in some situations provided means are taken to prevent escape to the sea, minimize bleed problems, and ensure essentially complete filling. Performance of a new thixotropic admixture is described.

A self-compensating column or "spar" screeding and trimming device, suitable for water depths over 100 m, is described. For shallow water, a mechanized screeding frame may be employed.

Specific solutions, with suggested equipment and procedures, are given for a variety of subsurface soil conditions and depths, and a preliminary logic diagram is presented by which the operations, tasks, and methods required may be determined from the parameters that described a particular concrete sea structure.

INTRODUCTION

Large concrete sea structures are now well accepted for use in the deep waters of the North Sea for drilling, production, and storage. A major offshore terminal is under construction offshore Queensland, Australia, founded on large concrete caissons. The protective breakwater for the Atlantic Generating Plant, the floating nuclear power plant off the coast of New Jersey, is to be built upon a core of large concrete caissons, and the floating power plants will be moored to other caissons.

The indicated success in the North Sea is generating interest in other offshore areas to see if concrete caisson structures can be applied in weaker soils as well. Particularly for the Arctic Ocean, such caissons offer promise if the foundation can be suitably prepared to support the caisson.

Finally, bridges are now proposed to cross deep arms of the sea, e.g., Honshu-Shikoku, where caissons must be properly founded despite the adverse sea conditions and great depths.