94 



but extreme care must be used in investigating local conditions. The principal thing 

 to be noted is exposure to attack: see tables of salinit>' for the locality; note proximity 

 of infested structures. Protection by local contamination, which may in some cases 

 prove an unfavorable environment for pests, is, ho\ve\er, most unsafe to rely upon (see 

 discussion of this factor, p. 57 ff). The purpose for which a structure is to be used 

 will determine whether it will have outlived its usefulness prior to the time of antici- 

 pated collapse. 



EXTER^^^L protections for WOODEN' PILING 



The need for protecting piling against marine borers has aroused the interest and 

 the inventive genius not only of those directly concerned with the building and main- 

 tenance of piling structures, but of a multitude of others. Numerous devices, materials 

 and methods have been suggested. Some of them have been so impracticable that their 

 only value has been to demonstrate the ignorance of their sponsors concerning the 

 results of past experiments and the requirements of a pile protection. Some of them, 

 on the other hand, have considerable merit and extensive experiments with them have 

 been justified. 



All the suggested external protections which have come to the Committee's 

 attention may be placed in one or other of the following classifications: bark, metal 

 nails and sheathings, paint and paint combinations, concrete jackets, and miscellaneous 

 devices or processes. Concrete protections for wooden piles, however, are discussed in 

 the general consideration of concrete, as a matter of coherence in the treatment of 

 that subject. 



B.\RK 



The early discovery that bark, at least in some species, was not attacked by 

 marine borers as readily as the wood led to the belief that the former could be counted 

 on as a protection. Its use became standard practice about the year 1869 and piling 

 specifications of the years following that called for piling timber having a complete 

 bark covering. Furthermore, since bark adheres to the wood more firmly in winter 

 than at other seasons, it was required that the piles be winter-cut. 



Early investigators of the 70's reported that .... "the teredo .... never 

 eats the bark of wood, and only enters the piles after the bark has been removed," 

 and further that — "piles are attacked by the teredo as soon as the bark falls off, which 

 is in about two years after they are driven . . . . " This indicates that they con- 

 sidered that bark increased pile life about two years. 



These statements are quoted as a matter of interest rather than for their accuracy. 

 At best, however, the bark is in danger of being knocked off in patches during the 

 hazardous course of the pile from the stump to its place in the structure. Unless all 

 such piles are scrupulously rejected, this exposes the wood in spots and permits im- 

 mediate attack at those points. Borers working in these patches soon enlarge them by 

 burrowing under the surrounding bark. The softening action of the water, coupled 

 with the abrasive action of driftwood and waves tear off additional bark and soon the 

 pile is in a practically unprotected condition. Furthermore the borers are not entirely 

 repelled by the bark. Specimens of bark were secured from piles during this survey 

 which show large burrows of shipworms. 



A company at Richmond in 1920 reported concerning their past experience, 

 "Where bark is kept intact this class of pile lasts from four to five years in this locality." 



In Forest Service experiments made at Fort Mason in San Francisco Bay, and at 

 San Diego, experimental pile sections of unbarked western hemlock showed attack 

 within six months, both by Limnoria and shipworms. The attack began in patches 



