72 



and careful workmanship and driving methods used. A much surer partial length 

 protection is obtained by types such as the precast jacket, in which the timber pile is 

 first dri\-en and a precast reinforced concrete shell then slipped over it and dri\'en into 

 position. The application of this method to the protection of wooden piles in an exist- 

 ing structure requires the removal of the superstructure, or at least of the flooring, 

 and springing the piles out from under the superposed stringers. When, as in most 

 cases, this is impracticable, practically the only alternati\-e for piles already in place 

 is the use of one of the methods, like the Camp process, for applying the concrete shell 

 in short sections which are individually lowered in turn to position until the whole 

 protection has been built up. All types of partial length protection of timber piling 

 involve the hazard of the possible subsequent lowering of the mud line to below the 

 protection, with the resultant exposure of the wooden piles and their destruction by 

 borers. 



Length of Pilk 

 The distance from wharf to mud line, plus penetration distance, may be so great 



Fig. 23. Handling lOU-ft. concrete piles during pier construction. 



that a single pile may be impracticable. On the Pacific Coast Douglas fir can be secured 

 in lengths of 100 feet or more, which is adequate for the great majority of cases. Some 

 of the piles used in the trestle approaches to the Dumbarton Bridge of the Southern 

 Pacific Company were 125 feet long, the bottom penetration being 60 feet. Splices 

 are sometimes used where single lengths are too short. The usual practice in cases of 

 great depth is to drive a cluster of timber piles to below mud line and superimpose upon 

 them a concrete column. Reinforced concrete piles ha\-ing lengths of 100 feet or more 



