Installation of Bucket Foundations and Suction Caissons in Sand - Geotechnical Performance
Authors: Erbrich C.T. , Tjelta T.I. 
Published: October 2008

This paper describes the important geotechnical principles relating to the installation of bucket foundations in dense sand, where 'suction' pressures are applied to both increase the static driving force, and more importantly, to degrade the penetration resistance. The paper will present a series of finite element analyses that demonstrate various important mechanisms. These will be compared with results from an extensive series of model tests and with data collected during the installation of the bucket foundations for the Draupner E platform (formerly Europipe 16/11E).

Introduction

In order to penetrate the skirts of a bucket foundation into the seabed, a 'suction' operation is performed. During this process, pumps are used to evacuate water from within the sealed skirt compartments, which creates a differential water pressure. This has two beneficial effects in sand soils:Water seeps down and around the skirt tip, and then upwards within the skirt compartments towards the base plate. Given sufficient time, approximately steady state seepage gradients will form (complete steady state conditions never develop since the skirt continuously penetrates). The downwards seepage gradient outside the skirt, acts to increase the effective stresses in the soil and consequently to increase the external skirt friction. Conversely, the upward seepage gradient inside the skirt acts to reduce the effective stresses in the soil. This reduces the internal skirt friction, but most significantly, degrades the skirt tip resistance. The net effect of these processes is a substantial reduction of the total penetration resistance.The differential pressure created across the base plate by the 'suction' operation provides an additional force that helps drive the skirts to the desired penetration.

In addition to these two beneficial effects, one important and potentially detrimental effect must also be considered. The degradation process described occurs because the effective stresses within the skirt compartment are reduced by the upward seepage gradient. However, this gradient is limited since the effective stresses can never be less than zero. The onset of this state occurs at a 'critical gradient', and is quite commonly referred to as a 'quick' condition. It is defined by:(Mathematical equation) (Available in full paper)

In related applications, soil liquefaction or 'boiling' has been observed at this state, and this is quite often accompanied by the formation of 'piping' channels.

For bucket foundations, these phenomena appear highly undesirable. Extensive soil liquefaction would cause a large amount of soil heave which could impair skirt penetration but, more importantly, would seriously affect the in-place foundation performance. The formation of 'piping' channels would cause a breakdown in the hydraulic seal across the base plate that would halt, at least temporarily, further penetration.

Given these consequences, it could be considered that a large safety factor against the formation of 'quick' conditions should be specified. However, from the model test results described in subsequent sections, it will be shown that very large internal gradients were nearly always required to cause skirt penetration and in some cases, the critical gradient was reached. However, except where deliberately provoked, extensive soil heave did not occur, nor were piping channels formed. This suggests that some in-built safety mechanism acts to prevent these unstable phenomena.