91 



COMI'ARISOX OK I'llVSICAl. l'R( Jl'EKTlES OP 

 BLACK COTTONWOOD AND DOUGLAS FIR 



(From r. S. Dept. of Agriculture Bulletin 556) 



Black Douglas 



Cottonwood Fir 



Weight per cu. ft., green (lbs.) 46 38 



Weight per cu. ft., oven-dry (lbs.) 20 28 



Moisture content (percent of oven-dry wt.) 132 ,^6 



Tension fibre stress at Elastic Limit (lbs. per sq. in.) ... 6,200 10,600 



Compression parallel to grain at Elastic Limit (lbs. per 



sq. in.) 4,410 9,260 



Compression perpendicular to grain at Elastic Limit (lbs. 



persq. in.) 460 1.220 



In August, 1923, several 4-foot lengths of cottonwood were submerged for test 

 purposes at the Committee biological station at Oakland Pier. While this was about 

 a month too late for the heavy breeding season of Teredo, the specimens were foimd 

 to be attacked quickly by both Teredo and Limnoria, that of the latter being general. 

 The final inspection of October 10, 1924, showed extremely heavy attack by Limnoria, 

 and additional attack b\' Teredo and Bankia. some of the latter ha\ing attained a 

 length of 18 inches. 



In October, 1923, 30 cottonwootl piles were placed by the Southern Pacific 

 Company at 7 wharf locations in Oakland and carefully observed at about 3-month 

 intervals. These were placed too late for the 1923 breeding season of Teredo. However, 

 they were immediately attacked by Limnoria. The final inspection of October 10, 

 1924, showed extremeh- heavy attacks by Limnoria. the erosion being to a depth of 

 about 1^^ inches. Attack by Teredo and Bankia was found in a minor degree, probably 

 due to the fact that the Limnoria had taken complete possession of the pile surface. 



The evidence of these tests shows that such piling will be completely destroyed 

 in from 2 to 3 years and that, even if obtainable, it can in no way be justified for 

 use in marine structures where Douglas fir is available. 



Eucalyptus 



There are more than two himdred species of Eucalyptus. Among them are some 

 which, like the Australian Jarrah (Eucalyptus marginata), ironbark (E. sideroxylon) 

 and grey box (£. heniipliloia), have a high reputation for their resistance to marine 

 borers and their general suitability for piling purposes. The most common one in 

 California, and the only one in fact at all widely available, is the blue gum (Eucalyptus 

 globulus). 



Blue gum Eucahptus is Aery hea\'y, hard and strong, comparing when sound 

 with American hickory, except in elasticity. In exposed use, however, such as piling, 

 it shows an abnormal tendency to split and check, both under initial dri\ing and under 

 subsequent weathering and impact of boats. In 1909 about 1200 piles of this species 

 were driven in a San Francisco ferry slip. These failed within 4 years, either from 

 borer attack or from checking and Ijreakage. Of the large number used in the original 

 trestle of the San Francisco-Oakland Terminal Railways (now the Key System Transit 

 Co.) a few still remain in place in the car storage trestle stub after at least 12 years' 

 service. These are of course the exceptional ones which resisted destructi\'e checking. 

 In respect to the borers, the known fact that the conditions at the Key System piers 

 on the Oakland side of the bav are much less faxorable to Teredo and Bankia than are 



