Undrained vertical bearing capacity of perforated shallow foundations

Headnote

ABSTRACT: Perforated shallow foundations are commonly used as mudmats to support subsea infrastructure such as pipeline end manifolds and terminations. The perforations may be included in the foundation design to allow water to escape during installation, or to reduce uplift resistance on decommissioning. Perforated geometries, often involving a single perforation, can also be efficient for larger gravity-based foundations. However, perforations decrease the available foundation bearing area, which reduces the capacity of the foundation during operation. This paper examines the impact of the degree of perforation on undrained vertical bearing capacity. Centrifuge model testing has been undertaken to investigate both surface and skirted foundations on undrained clay, with a perforated area ranging from 0 to 64% of the total foundation area. A corresponding numerical study has been undertaken using adaptive finite element analysis. The results allow the reduction of vertical bearing capacity for a given degree of foundation perforation to be estimated.

(ProQuest: ... denotes formulae omitted.)

1INTRODUCTION

Perforated shallow foundations are commonly used for subsea infrastructure such as pipeline end terminations and manifolds (Fig. 1a). Perforations reduce hydrodynamic forces during deployment as water can pass through the perforated area. Furthermore, the pullout resistance can be reduced during decommissioning (Fisher & Cathie 2002). Perforated foundations can also provide a functional design solution for larger gravity-based foundations, with advantages such as increased moment capacity for a given bearing area. In this case a single central perforation might be used to form an annular foundation with a square or rectangular shape (Fig. 1b).

While perforations can have a number of benefits, they reduce the foundation contact area over which bearing resistance can be mobilised. There may also be additional reductions, over and above that simply due to the contact area reduction, and these should be accounted for in the foundation design. To date, relatively little research has been undertaken to examine the bearing capacity of perforated foundations, so there is limited design guidance available.

This paper presents the results of numerical analyses and experiments that have been undertaken to better understand the undrained bearing capacity of perforated foundations under vertical loading. Both surface and skirted foundations have been investigated. Finite element analysis (FEA) has been carried out using adaptive mesh refinement to improve the accuracy of the...