From this source

Screw stacks are usually created from structural steel. The tremie method of positioning concrete involves decreasing a pipeline to the base of the excavation, the pipeline end on top of flynn piling north west the excavation is attached to a concrete receptacle. The hoper as well as the pipe are then charged with the wet concrete mix. The pipeline is then slowly raised out of the dug deep into opening, placing concrete as it goes. During the lifting procedure, concrete is constantly restored to the hopper in order to maintain the system completely billed.