A plasticity model is presented to describe the drained vertical load-displacement response of untrenehed offshore pipelines on calcareous sand. The predicted plastic response during monotonic loading and elasto-plastic behaviour during reloading is shown to compare well with experimental data. The model can be used to evaluate total settlement of a pipeline under both monotonic and cyclic loading conditions. Using the framework presented in this paper, it is possible to extend the model to simulate the vertical load-displacement response of other types of shallow foundations and to cover undrained conditions.
The response of pipelines to monotonic and cyclic vertical loading is an important aspect of offshore pipeline engineering, as this response affects the on-bottom stability of untrenched pipelines. Previous investigations into pipeline behaviour under vertical cyclic loading have shown a minor increase in settlement and bearing capacity (Dunlap et al., 1990), compared to that deduced from cyclic horizontal loading (Morris and Dunlap, 1988). Hence the load-displacement response of pipelines under cyclic vertical loading has received relatively little attention, compared to that of pipelines under cyclic horizontal loading. However, with the development of analytical methods having a strong theoretical background (Zhang et al., 1999), accurate modelling of the vertical load-displacement response of pipelines becomes necessary. While pipeline response under cyclic vertical loading has not been studied a great deal, some progress has been made in the modelling of vertical load-displacement response of shallow foundations. Results from model tests on a strip footing on dense (Dr = 86 %) sand reported by Gottardi and Butterfield (1995) have shown very little hysteresis, and an essentially linear response during a single unloading and reloading cycle. The general form of the footing load-displacement response mirrors the virgin compression and unloading-reloading response seen in soil compression tests.
