23 



made tlu- tdllciwiiig significant coninient: "All authorities in writing (in the presor\'a- 

 tion of piles for marine work seem to ignore the existence of the Limnoria terebrans. 

 In my opinion, it is much more (iestructi\e on this coast than is the teredo, and while 

 a pile that has been thoroughly creosoted will resist the teredo, even if somewhat 

 checked, the limnoria will find the slightest opening and destroy the pile." 



In 1900, Pier 10 (remo\'ed in 1915) was built with three-pile concrete cxhnder 

 substructure units, similar t<i thosi.' ol piers 7 and 12 mentioned abo\e, but in which 

 timber casings were used instead of steel. This type was developed and patented b\- 

 Mr. H. C Holmes, Engineer of the Board, and became known as the "Holmes" 

 cylinder. In 1901 the Spreckels Sugar Refiner\' built a wharf with such a substructure, 

 which is still ser\ing after twenty-four years. In 1904 the Harbor Board reported as 

 loUows: ". . . the life of a dock constructed of preser\-ed piles is about ten years . . . 

 but with the achent of the c\lindrical pier constructed of concrete, it looks as though 

 a revolution has been made in the construction of docks and that the foundation is 

 secure for an indefinite period. The oldest wharf (Pier 7) constructed under the new 

 process ... is ten years old and shows absolutely no deterioration. We ha\e eight 

 cylindrical docks. The aim of the Commissioners is to construct no other kind of 

 pier . . ." 



In that same year the continuation of the seawall south of Market Street was 

 made possible by a bond issue and work was started. Fourteen years had elapsed 

 since the portion north of Market Street had been finished, and during this time 

 considerable doubt had been raised that it would e\ er be continued. Wharves had 

 existed in the southern section for fift\- years (fig. 0) liefore the final plan of de^•elop- 

 ment became a certaint\'. 



In 1907 the Southern Pacific Company completed the Dumbarton Bridge across 

 the south end of the Bay. This structure includes six 180-foot and two 40-foot steel 

 spans, one 310-foot swing span and 6,375 feet of timber trestle approaches, consti- 

 tuting a total length of 7,845 feet. The steel spans are supported by concrete cylinder 

 piers fifty-five to seventy feet high; the trestles are of cresoted timber piles, some of 

 which are one hundred and twenty-five feet long. Of these, the piles in shallow water — 

 about one-half of the total — were encased in concrete jackets in 1919 to protect them 

 from marine borers. 



The condition of whar\es on the San Francisco waterfront was reviewed in 190S 

 by the engineer of the Board as follows: "There are practically two kinds of piers in 

 existence on the waterfront of San Francisco. One is the pier resting on creosoted 

 piles and comprises the remains of the old work, completed previous to the use of the 

 Howard Holmes patent. The other comprises the piers resting on piles protected by 

 concrete according to Mr. Holmes' patent . . . 



"Those of the first kind are very hard to maintain, and it has been the policy of 

 the Board for a long time to construct all new piers on the patented piles . . ." 



However, he added: "In a few instances the concrete cylinder piers have failed 

 and fallen from their positions. This latter failure is undoubtedly due to the practice 

 of placing the concrete for a considerable portion of the bottom of the cylinders under 

 water without any special device to preA-ent a separation of the ingredients." The 

 failure was undoubtedly due, not only to this faulty construction, but to the fact that 

 the concrete was not carried down far enough into the mud to prevent exposure of 

 the pile when scouring of the bottom removed the mud and thus opened the way for 

 borer attack. Thus one more type in which great hopes had been placed was showing 

 signs of not being the "permanent solution." The instance is given in detail because 

 it shows, as in manj' similar cases, that the failure was due less to any inherent fault 



