October 27 1960: The movable shield shown here bit into the mud while at the same time provided protection against a cave-in. The rams that moved the shield and held back the mud were enormously powerful work units and there were 28 of them. Each could exert a shove of 200 tons and move forward 48 inches. The shield operated like a giant biscuit cutter in removing the hard blue clay through which the tunnel was being dug.