EXAMPLES OF WALL SECTIONS. 157 



therefore 31 feet, the width of the wall being 11 feet, equal to 

 the length of the concrete blocks. 



A precaution against undue ultimate settlement, universally 

 adopted by French engineers, in structures similar to this wall, 

 is to load the concrete wall with blocks before adding the 

 superstructure. 



Pola Basin Walls. 1 The following course was pursued by 

 Mr. H. E. Towle in constructing the enclosing walls of the Pola 

 Dock basin, to provide for anticipated settlements. 



The conditions obtaining required that, in order to ensure 

 a water-tight joint with the bottom, the wall should be founded 

 on a bed of tenacious mud, varying from 2 feet to 12 feet in 

 depth, overlying in one unbroken tough sheet rock totally un- 

 reliable, and full of irregular cavities and passages of a honey- 

 comb nature. 



A plain rectangular section of wall was adopted, as, under 

 the circumstances, offering the greatest advantages and least 

 objections. The width varied from 15 feet to 20 and 26 feet, 

 and slightly exceeded 42 feet 6 inches at the deepest part. 

 The body of the wall was constructed of be'ton, deposited in 

 situ, up to within 7 feet 6 inches of the top. The superstructure 

 consisted of inside and outside facings of masonry in 2 feet 

 6 inch header-and-stretcher courses, backed with common rubble 

 masonry covered with flagging 9 inches thick. 



Owing to the fact that the wall was to be founded on a 

 compressible material, which also dipped towards deep water 

 at an angle of from 2 to 10 degrees, it was evident that settle- 

 ments, causing vertical cracks and fissures, would take place, 

 and it was desirable to ascertain, if possible, when such fissures 

 would be likely to occur. 



A careful survey and contouring of the upper and lower 

 surface of the mud bottom, showing the irregularities, mud, 

 and the protuberances and depressions of the rock surface, 

 rendered the location of the cracks almost a certainty, and 

 they were then provided for by vertical joints across the wall 

 at intervals varying from 40 to 90 feet in length. 



To check the passage of water through the vertical joints, 



and to connect the adjoining ends of contiguous blocks of beton, 



so as to prevent them moving upon each other laterally, a 



rectangular post 18 inches by 24 inches was inserted vertically 



1 M.P.LCM, vol. xxxii. p. 65. 



