Controlling cyclic stiffness of a foundation, by manipulating the deformation history

Offshore wind turbines are slender structures, dynamic response of which depends on foundation stiffness. Unfortunately, foundations embedded in sand can become disturbed, their stiffness can increase and decrease episodically. To investigate the phenomenon governing loss and recovery of stiffness, an original testing program was implemented. A prototype of an offshore foundation was repeatedly disturbed and re-stabilized: the stiffness curve generated while testing peak strength was disturbed and reset back to initial state, multiple times, in one loading sequence.

The ability to reset the "initial stiffness path" was achieved after observing a new physical phenomenon: converging Stiffness hysteresis loops. During cycles of constant deformation amplitude, stiffness hysteresis loop were observed to converge in proportion to applied deformation amplitude. Thus, by controlling position and amplitude of deformation cycles, the stiffness hysteresis loops can be "stretched" and "repositioned" along the deformation axis. Therefore, allowing to de-facto control cyclic stiffness of a foundation - to disturb and reset the initial soil state. The observations provide new factual evidence, which suggests some features of cyclic stiffness in sand could be governed exclusively by deformation history. Furthermore, inputs adequate to control a phenomenon in practice, could be adequate to model the phenomenon in theory. Thus, the new observations could be a precursor to a new generation of numerical models.

The ability to reset the "initial stiffness path" was achieved after observing a new physical phenomenon: converging Stiffness hysteresis loops. During cycles of constant deformation amplitude, stiffness hysteresis loop were observed to converge in proportion to applied deformation amplitude. Thus, by controlling position and amplitude of deformation cycles, the stiffness hysteresis loops can be "stretched" and "repositioned" along the deformation axis. Therefore, allowing to de-facto control cyclic stiffness of a foundation - to disturb and reset the initial soil state. The observations provide new factual evidence, which suggests some features of cyclic stiffness in sand could be governed exclusively by deformation history. Furthermore, inputs adequate to control a phenomenon in practice, could be adequate to model the phenomenon in theory. Thus, the new observations could be a precursor to a new generation of numerical models.