305 



The other sample tested by Mr. Russell, a somewhat i)etter specimen containing 

 larger aggregate, showed a crushing strength of 3070 pounds per stiuare inch. The 

 dimensions of this sample were 3^ o" ^ -5^2". 5^2" high. 



The average crushing strength of a number of six-cimens of these jackets tested 

 by Mr. W. H. Sadler was 2870 pounds per square inch. 



Thus, while these jackets were not of high grade composition judged by present 

 standards, a number of them were of sufficient hardness to make it a matter of some 

 surprise that the>' should be attacked by molluscan borers. 



We have been informed by Mr. O. Thompson of Seattle that a wharf built by 

 him near Anchorage, Alaska, in 1901, of piling jacketed with concrete after driving, 

 showed similar att.ick by rock boring moUusks. In 1914 a piece of one of the jackets 

 was broken off accidentally about low tide level, revealing the presence of a number 

 of rock borers about the size of a man's thumb. According to the statement of Mr. 

 Thompson, the wooden pile thus exposed was foimd to contain schik' large shi])worms 

 and a few Limnoria, which had apparently gained entrance tiimugh tlu' ci])enings 

 made by the rock borers in the concrete jacket. 



It has commonly occurred in jacketing piles in place that the concrete has been 

 "drowned" either by the presence of too much water in the mi.\ or by depositing the 

 material in water, causing segregation and laitance and rendering such structures 

 specially susceptible to borer action. Whether or not concrete of greater hardness, 

 containing approximately 90 per cent of assorted aggregate in excess of the 50 mesh 

 screen, as contrasted with mortar having only 7 per cent abo\-e 50 mesh, is a matter for 

 further investigation. An inspection of concrete jacketed piles at Pier 34 and at 

 Fisherman's Wharf in San F"rancisco Bay failed to disclose any borers. The tvpe of 

 piling inspected here was similar in construction to the Los Angeles t>pe in that 

 they were jacketed in the water, but the aggregate consisted of rock anil sand and 

 produced a fairly sound concrete. It is possible that these piles ha\e not been exposed 

 to attack because the mud shores of San Francisco Bay do not harbor pholad liorers. 

 However, the piles at Fisherman's wharf are located within one-half mile of Fort 

 Mason, at which location rock borers have been reported in shale rocks. 



Fig. 122. Pholadidea penita Conrad, in place in a loose block of sandstone from Fort Mason. 



