Australian Research Council Centre of Excellence Geotechnical Science and Engineering

The changing strength of clay around subsea infrastructure

Summary

This research is to explore and quantify the beneficial effect of consolidation in response to in-service cyclic loading, and establish a framework to incorporate this mechanism in design. The framework will be set against a background of critical state soil mechanics, which provides the building blocks to quantify the balance between undrained cyclic loadings – leading to pore pressure build up – and drainage and consolidation – leading to strength regain. Centrifuge modelling and numerical analysis will be performed, using soft clay soils. A novel aspect of the centrifuge tests will be the philosophy of ‘whole life’ modelling. Realistic forms of cyclic loading based on practical situations will be applied over realistic time periods, allowing the true level of consolidation-induced strength gain to be explored for the first time.

Why my research is important

Oil and gas developments in deep water, where the seabed sediment is usually soft clay, require subsea foundations such as piles, pipeline, anchors and mudmats. These foundations are subjected to cyclic loads from a range of sources. These cyclic loads occur throughout the life of the foundation and lead to changes in the strength of the surrounding soil. The weakening effect of cyclic loading is well recognised throughout design practice, and methodologies for determining the cyclic ‘fatigue’ of clay during undrained loading are well established. S-N curves that account for the nature and magnitude of the loading cycles can be constructed to determine an operative soil strength. However, such undrained assessments neglect the beneficial effect of drainage and consolidation that will inevitably occur over the life of the foundation, and this effect will benefit industrial design.