A continuous curve arch shape cross section corrugated injection molded thermoplastic chamber is provided and includes a body having a multiplicity of peak and valley corrugations running along the curve of the arch shape, wherein the multiplicity includes opposing sidewalls, each having a foot, opposing chamber ends connected to the body, the ends shaped to form joints with the ends of like chambers, a multiplicity of sprues, within spaced apart valleys, at elevations intermediate the foot and apex of the chamber, wherein each sprue has a first runner running upwardly alone the valley in direction of the apex, and a second runner running downwardly along the valley toward the foot, a base runner connected to the bottom of the second runner, running lengthwise along the foot of the chamber; and at least one third runner connected to the base runner and running upwardly along a peak adjacent the valley.