121 



Pile-Cluster Concrete Cylinders with Permanent Steel Forms 



This type of substructure support is formed by driving a cluster of timber piles; 

 a steel cylinder form is then lowered over the cluster and dri\en into the mud; the 

 form is then ]uimix'd dry and filled with concrete, completely encasing the piles. 



Tills type, using .vpile clusters, was introduced at San Francisco in 1895-96, 

 when 1,^0 were used in constructing Pier No. 5 (Old Pier No. 7— Pacific .St. Wharf), 

 and 120 in Pier No. 20 (Old Pier No. 12— FolsomSt. No. 1). The steel forms were made 

 of ,^15 in. boiler plate, were 4 ft. in diameter and averaged 45 ft. in length. Old wire 

 cable was used as reinforcement for the concrete. The cost of Pier No. 5 (exclusive of 

 the shed) was about $48,000 or 80.80 per sq. ft. It was reported at the time that the 

 steel cylinder forms cost S4.50 per lin. ft. Most of these units are still in service after 

 29-30 years, having had the longest life of aiu' substructure supports on the San 

 Francisco waterfront, although a number ha\e been replaced due to the lailure of 

 both the steel shell and the concrete, and while in some which remain the concrete 

 has disintegrated but is protected by the steel form. 



Holmes Concrete Cylinder Piers 



The so-called Holmes cylinders developed by Mr. H. C. Holmes, former Chief 

 Engineer of the Board of Harbor Commissioners, were modifications of the 3-pile 

 cylinders used in Piers 5 to 20 described above, timber forms being used, for reasons 

 of economy, instead of steel. The forms were Iniilt of 2-in. and ,^-in. wood staves 

 bound with steel hoops. Those for single piles were about 2 ft. in diameter, and tor 

 3-pile clusters from 3 ft. 6 in. to 4 ft. 10 in. The sequence of construction was to drive 

 the piles, lower the form into position, pump out the mud and water, place the re- 

 inforcing bars, and pour the concrete. The forms were left in position and were 

 eventually destroyed by borers. 



These cylinders built with 3 piles were first introtluced in the Howard St. Whart 

 No. 3 (Old Pier No. 10) in 1900. During the next six years several whar\es were liuill 

 with the 3-pile type aggregating a total of about 3,000 cylinders. 



In the next few years some of the cylinders failed and fell from position. By 1915 

 their condition had become such that the majority were removed or replaced. Pier 

 10, mentioned above, was completely removed in 1915. The life of the majority 

 ranged from 7 to 15 years — a few^ are still in place after 25 years' serYice. 



In rcY^iewing causes of these unsuccessful results it appears that the original 

 intention to pump the forms dry before placing concrete could not be carried out in 

 actual construction. It was found impossible to prcY-ent leakage of the forms and to 

 seal the bottoms, bags of cement placed at the bottom in an effort to accomplish the 

 latter being onl\' partialK- effecti\-e. As a result, much of the concrete was placed, 

 or went through its setting process, in water. 



Subsequent investigation indicated the following causes of failure: (1) Poor 

 concrete because of setting in salt water; (2) mud and sand pockets due to failure to 

 seal the bottom; (3) pockets formed by arching of concrete due to faulty workmanship; 

 (4) laitance pockets and joints due to excess water and to failure to pour each unit 

 completely without stopping; (5) failure to carry cylinder concrete down far enough 

 into the mud to allow for scouring; (6) failure to place adequate reinforcement to 

 prcY'ent separation of parts when ruptures of concrete occurred. 



These construction difficulties led to the abandonment of the 3-pile type. Mr. 

 Holmes later developed a single-pile unit used in the construction of Pier 34 in 1910. 

 The forms for this reduced size were easier to handle and could be kept more nearly 



