fig3 and its leading edge lies flush against the outer surface of the inner lap panel 29i as best seen in fig4 thus the knife edge sheers residual concrete from the outer surface of panel 29i when the cylinders 30a 30f are retracted each of the cylinders 30a 30f is provided with a check valve to hold it in the expanded position so that the retainer form will not collapse should hydraulic pressure accidently be lossed when the cylinders are contracted they reduce the diameter of the retainer form by approximately two inches or six inches in circumference the throw of the hydraulic cylinders limits the expansion and contraction of the retainer form each of the nine panels 29a 29i has six concrete fill ports the fill ports for panel 29a are designated respectively 40a 40f there are two fill ports five feet above the bottom of each panel two at ten feet above the bottom of each panel and two at fifteen feet above the bottom of each panel thus there are eighteen fill ports at each of the three levels each fill port is separated from the adjacent one by a four feet circumferential distance on center each of the fill ports is similar in structure such.