29 



structural usefulness for periods up to three years; in water containing 

 sewage or other contaminations where borers are known to be present 

 untreated piling appears in some cases to last longer than that- Lim- 

 noria, however, does not seem to be affected by sewage contamination 

 as much as Teredo and Xylotrya. Untreated piles in uncontaminated 

 salt water have been destroyed in as short a time as six months by 

 Teredo and Xylotrya. Untreated piles in salt water have had longer 

 than the average life in locations removed from other infestation or 

 where local conditions were unfavorable for marine borers. 



Metal Protective Coatings 



This type of protection is no longer in as common use as it is 

 known to have been in former years. No new information has come 

 to light during the period covered in this report to alter the conclu- 

 sions stated in the 1921 report. These conclusions are summarized 

 at the end of this section of the present report. 



Paint and Batten Protective Coating's 



The usefulness of this method of protecting untreated piles has 

 been shown in last year's studies and report. The report of the Chief 

 Engineer of the Northwestern Pacific Railroad Company covering an 

 installation at Petaluma Creek indicates some of the primary considera- 

 tions that determine their selection of this type of pile. These, brief- 

 ly stated, were: (1) lower initial cost as compared with creosoted 

 piles; (2) possibility of salinity being so reduced by a return of wet 

 years, and increased fresh water flow, as to kill off marine pests; (3) 

 possibility of structure becoming obsolete before the expected life of 

 pile would be reached; (4) sheltered location. 



The difficulty in determining the life of piles protected by surface 

 coatings in this harbor, as noted in last year's report, is that most of 

 the installations have been removed without adequate record having 

 been kept. The several extensive installations of such piles made dur- 

 ing the years 1920 and 1921, as shown in the accompanying tabulation, 

 it is hoped can be kept under continuous observation and record. 



With protected piles the importance of position of mud line cannot 

 be overlooked, as called to attention in last year's report. In the use 

 of this type of pile the mud line cannot be lowered or the protection 

 loses its effectiveness. A study into the causes that could bring about 

 a change of bottom conditions must be made before deciding in favor 

 of this type of bearing pile. The desirability should also be borne in 

 mind of using such construction in waters sheltered from excessive 

 storm action, on account of its susceptibility to abrasion. 



The service records covering paint and batten protective coatings, 

 were pretty thoroughly canvassed and evaluated in the 1921 report. 



