40 BULLETIN 155, U. S. DEPARTMENT OF AGRICULTURE. 



was only partially filled. This portion was badly decayed in five 

 years, and 1,100 feet of it was replaced by redwood in 1895. With 

 the exception of a 100-foot section near the middle of the new part, 

 the redwood was painted with hot coal tar. The 100-foot section was 

 painted with asphaltum paint. According to Mr. C. K. McHarg, 

 secretary of the company, the pipe coated with tar was found to be 

 perfectly sound when examined in 1910, while the 100-foot section 

 was badly decayed and had to be replaced. 



Contrary to the theories commonly held 30 years ago, it has been 

 found that the durability of wood pipe is usually dependent on the 

 life of the wood rather than on the life of the bands. Only in rare 

 instances, some of which have been cited, have the bands failed first. 

 Corrosion of the bands being a chemical action, requires the presence 

 of moisture and oxygen. It usually occurs most rapidly where pipes 

 are buried and the backfill is wet, under conditions which, as a rule, 

 are most favorable for the life of the wood. Corrosion is greatly 

 accelerated by the presence of alkali in the soil. The early failure of 

 bands in the few instances cited was due chiefly to this cause. Under 

 such conditions the bands almost invariably fail at the bottom of the 

 pipe. 



Wearing out of the wood as a factor in the durability of pipe is a 

 matter of small consequence, though it must at times be recognized. 

 A 48-inch spruce pipe on the Catlin Canal in Colorado, in 23 years' 

 use, was worn nearly through the staves for a distance of nearly 100 

 feet at the outlet end. The inlet end of a redwood siphon near North 

 Yakima, Wash., had to be lined with sheet iron to preserve it from 

 wearing action of grit, and one of the large pipes on the King's 

 Hill project in Idaho was nearly cut in two at one place by the cir- 

 cular movement of chips and debris floating on the surface near the 

 intake where the pipe was not full. 



With so many influences affecting the life of wood pipe no attempt 

 should be made to strike an average of durability except in cases 

 where attending conditions are known to be the same. "Where pipes 

 are fully exposed and supported free from all contact with the soil 

 the conditions are much less variable than otherwise, and a life of at 

 least 20 years may be quite reasonably expected for either fir or red- 

 wood if properly maintained. If placed in the ground or in contact 

 with the soil, the life of wood pipe may, under very favorable condi- 

 tions, be much greater than 20 years, otherwise it may be a great 

 deal less. In contact with soil the durability is nearly always a mat- 

 ter of some uncertainty. 



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