Alternating flows in the ground have a detrimental effect on the internal stability of the ground at the bottom of bodies of water, at offshore structures, coastal protection structures, and revetments. A test apparatus for alternating flow was constructed for the purpose of investigating various problems relating to alternating flow in the ground. It was used to conduct investigations into the stability of granular filters for offshore wind turbines subjected to high levels of alternating hydraulic loads. The design criteria for granular filters subjected to oscillating loads must be considerably more stringent than those for granular filters subjected to unidirectional flow. It was also possible to demonstrate that the hydraulic loads due to waves have a significant effect on the filter stability in the area relevant for offshore structures.