VARIOUS TYPES OF BREAKWATERS. 187 



in moderate depths, say to 10 or 15 feet below low water. A 

 type section of this class of breakwater will be found in Fig. 

 35, p. 192. 



Although in principle the system is a good one, there have 

 been many failures in works where it has been adopted; but 

 they may nearly all be attributed either to too small a proportion 

 of cement having been used in the concrete, or to the use of 

 cement of inferior quality, with which I couple cement which 

 has not been properly aerated or slaked. 



Every endeavour should be made to ensure the faces of the 

 work being as good and impermeable as possible. 



Large pieces of clean stone may, from motives of economy, be 

 incorporated in mass concrete if desired. These should not be 

 allowed to touch each other, but should be kept at least 9 inches 

 or 1 foot apart. They should only be deposited where there is a 

 sufficient mass of concrete to ensure not only a good bed and 

 bond, but also a partial submergence in it. 



In under- water work, experience seems to indicate that the 

 proportion of stone incorporated in this manner should not exceed 

 one-sixth of the entire mass ; some engineers consider it un- 

 desirable to use any. Above water, as much as one-third or even 

 more may be used. 



For under-water mass-work many engineers use concrete 

 as rich as 1 part of cement to 3 or 4 parts of shingle. In 

 some positions this is no doubt desirable ; in others it might 

 appear somewhat extravagant. It has, however, been demon- 

 strated in several works that mass-work, even above water, made 

 of concrete composed of 8 or 9 parts of shingle to 1 part of 

 Portland cement has not sufficient strength for sea-work, it 

 having, in numerous instances, been eroded and disintegrated, 

 in a very short space of time, by the impact of the waves and 

 the chemical action of the sea- water. 



Monolithic work should be executed in sections of convenient 

 length, say from 15 feet to 20 feet, each section terminating with 

 a vertical joint. If this precaution be not taken, irregular cracks 

 will make their appearance and break up the work in an 

 unsightly, if not in an injurious, manner. 



Mr. B. B. Stoney, M. Inst. C.E., has suggested a mode of 

 forming breakwaters by constructing very large blocks of 

 concrete say 300 tons in weight in some sheltered position, 

 commanded by powerful floating shears, by means of which 



