The most critical part of helix construction is setting the first turn. All of the other turns will be carbon copies of this grade, so getting it right is crucial. This first course can be set just as you would a conventional grade.
Depending on the size of the helix, they can be very heavy constructions. Make sure you have an adequate foundation in the benchwork to support the weight. The helix seen in these photos is eight feet in diameter, contains up to 9 tracks, climbs more than five feet and weighs several hundred pounds. Most of that weight is concentrated toward the outside of the circle. Leaving open access in the center of the helix for access is also a consideration in how you design the base.
The base for this helix was designed with a heavy L-girder construction, using double-L center girders in a "+" design, with additional perimeter supports. Cross braces were laid out on top of these like spokes of a wheel. These allow ample mounting locations for the supports to the subroadbed for the bottom turn. Two spokes were cut out from the center to an intermediate L-girder to allow an access hatch. Finally, vertical supports every 90 degrees (one being a stud in an exterior wall) help distribute the weight of each turn directly to the floor.