Canvas Over the Canyon

It is the second largest tent in the world, and represents the successful completion by the Seattle Tent & Awning Co. of one of the most difficult tent-making jobs of all time.

Plans for construction of the Mud Mountain Dam, deep in the White River Canyon, east of Enumclaw, Washington, call for a rock-filled, watertight core. The material to be placed in the core must have a certain maximum water content to insure its success in holding back the flood waters of the White River. Obviously, rain and snow would play havoc with such material, so the tent for covering the entire canyon where the dam is to be built was ordered. The Seattle Tent & Awning Company undertook the construction of the tent, and came through with flying colors in record time.

Above,  Union Special Samson Canvas Seamer in operation at the Seattle Tent & Awning Company.

Costing approximately $35,000 and measuring 328 by 196 feet, the tent was made with provisions for a tight fit against the canyon walls at three elevations: 1,000 feet, 1,050 feet and 1,100 feet. Three separate eaves lines are necessary, one for each elevation, and each one has a radically different contour because of the variation in width and contour of the canyon walls.

Because of the fact that once this tent is in the air, it can never be lowered and must be constantly raised as the dam is built upward, the usual method of building a square-end tent could not be employed. The eaves lines intersect the end-eaves at different points, and so there could only be one endeave stretching across the canyon at each end. This meant that the square ends of the tent had to be cut and roped like a round-end tent, with all of the ropes converging at the bale rings.

Style 51500 D Canvas Seamer, at work on one of the huge tent sections.  Note two men helping operator shift canvas.

The dam site before construction began. Above, Finishing operations in progress on the big tent.

By this design, one rope is the corner rope at the first elevation; another rope is at the corner at the second elevation; and a third corner rope at the top elevation. When any of these ropes are not serving as corner ropes, they become regular up-ropes on the sides.

Other problems encountered in the tent's construction were, in the words of Mr. E. L. Mc Hugh, of the Seattle Tent & Awning Company: "too numerous to mention".  However, Mr. Mc Hugh did not encounter difficulties with the Union Special Canvas Seamers used on this job.

A Style 51500D machine was used to join the narrow sections of the tent together, and in many places sewed through sixteen layers of 10 oz. Army 0. D. material "without a murmur". Quoting Mr. Mc Hugh again: "This was a very severe test, but the Style 51500D came through admirably and did a wonderful job. The machine was not babied in any way. It took only seven minutes to sew more than 100 feet of these heavy seams, and this included all the time required by two men to shift the big section along as the machine sewed.  I have found through many years of actual experience that these large tents, seamed and joined with Union Specials, stand up 100% and last much longer. I would consider myself in a real jam if I ever had to build a large tent without the aid of Union Special Machine." NE April 1941

Going up! This photo was taken as the tent was being lifted into position on the 1,000-foot level.