Deformation dependent states in cyclic disturbed sand

Offshore wind turbine are slender structures, performance of which depends on foundation stiffness. Wind turbine foundations embedded in sand episodically lose and recover stiffness due to soil being disturbed by cyclic loads. To understand and model this phenomenon, new methods of testing and modeling of disturbed sand were attempted.

To observe the phenomenon governing disturbed sand states, dynamic tests were conducted using a frictionless triaxial apparatus and a small scale mono-bucket foundation. Experimental observations were then summarized into an original model. While developing the original concept, new paradoxes and anomalies were predicted: soil states of lower stiffness at higher density. The anomalies are confirmed to exist experimentally.

The new knowledge allows to control cyclic stiffness of sand in real-life real-time testing. Methods adequate to control a phenomenon, should be adequate to model it. Because deformation allows to control cyclic stiffness in experiment, deformation envelopes were proposed for modeling. Remarkably, the final result is one set of rules which are applicable in both drained and undrained soil states, combining loss and recovery of stiffness, during irregular cyclic loads.