Design and Construction of Mossyrock Penstocks

Foundation conditions required that the powerhouse be located close to the arch dam, resulting in relatively short, straight runs of penstock. Preliminary computations indicated that maximum downstream, radial deflection of the dam at the level of penstock intersection with the arch intrados, could be in the order of 2 in., accompanied by a 3/4 -in. transverse displacement of the dam at the emerging end of the embedded penstock. It was recognized that some economy might be realized by introducing higher strength steel for construction of the penstocks, resulting in significant weight reductions. The contractor was therefore allowed to bid on supplying, fabricating, and installing the penstocks from either ASTM A-201 steel or T-1, type A Steel. Final choice of T-1 steel was based on alternative prices bid. Construction of the penstocks is described, including problems the contractor overcame in obtaining good weld quality when field-welding T-1, type A steel.