On the selection of an appropriate consolidation coefficient for offshore geotechnical design

This paper addresses the selection of an appropriate consolidation coefficient for the analysis of drainage beneath foundations and pipelines in offshore geotechnical design. An emerging trend in the design of subsea infrastructure is the consideration of ‘whole life’ effects – namely the changes in soil properties and geotechnical capacity over the operating life. Seabed pipelines that undergo repeated thermal expansion and contraction cause shearing and consolidation in the underlying soil, leading to significant changes in the available seabed friction. Also, foundations that are either fixed or designed to slide on the seabed, are subjected to intermittent loads interspersed with periods of consolidation. These also cause a change in seabed strength and geotechnical capacity. To assess the time over which these effects occur, and therefore their influence on the response and the reliability of the system, it is necessary to perform consolidation calculations, using an appropriate consolidation coefficient. This paper presents observed operative consolidation coefficients drawn from recent model testing measurements and numerical analyses. It is shown that the consolidation rate can vary by more than an order of magnitude for the same soil profile under different loading conditions, due to the differences in stiffness and permeability. Meanwhile, design parameters are commonly drawn from one-dimensional oedometer compression tests. This compendium of data highlights the potential variation in consolidation coefficient for different loading types and through the ‘whole life’ of infrastructure. A key conclusion is that consolidation effects generally occur faster than is commonly assumed, meaning that changes in strength and stiffness – that are commonly beneficial in design – may be more readily relied on than is done so in current practice.