Spines attached to one or more sides of a bowl offer a much more interesting element than a standard deck.  They are also great as free-standing obstacles.  A spine is formed when two identical transitional radii rise from opposite sides to meet at a center piece of coping.  The coping can be either a single piece of pipe or two pieces of pipe welded down the center.  The use of two pieces of pipe allows for a flat area at the spine where it is easier for a rider to “stall” before riding back down the same radius or dropping in on the other side.  Use 1.90 outside diameter (O.D.) Schedule 40 tubular steel pipe for spines.  Additional pipe must be attached to the spine in order to suspend the coping for the spine above the ground during substrate preparation. 

For a spine that is part of a bowl or similar element, the interior of the bowl is first finished.  Next, the support system is removed and the excavation continues on the other side.  In that instance, the existing coping will guide the excavation jig and the transitional radius screed from above, and ride on a wooden form board below. 

To build a spine as a free-standing obstacle, weld metal legs to the spine that will allow it to set level and at the correct height above the ground.  After setting the coping for the spine, use the coping to guide an excavation jig, and build up the area with earth and gravel to the proper level.  Prepare the earth and gravel substrate as you would for any transitional radius.  Do not be surprised if the substrate cannot be placed all the way to the level of the spine.  After compacting the substrate, form the lower part of the obstacle using the methods outlined for forming straight and curved areas.  Weld rebar vertically to the spine and tie on additional horizontal sections of rebar placed on 12-inch centers just like a bowl wall.  A transitional radius screed similar to the excavation jig must also be fabricated to provide accurate placement of the concrete.  The screed will slide along the spine from above and will slide along the lower form from below.  Test the screed before attempting to place any concrete.  If everything is satisfactory, place and finish the concrete in the same manner as a transitional wall.

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