CHAPTER XVI. 



SETTLEMENT AND BOND. 



Settlement can eeldom be altogether avoided Usually greatest in breakwaters of 

 mound type Increased by weight of superstructure Unequal settlement 

 Weight of heavy setting-machines injurious to new work Settlement of break- 

 waters at Holyhead, Alderney, Colombo, Mormugao, and Manora (Kurrachee) 

 Bond Objections to Vertical bond Evils attending "dry bedding" of 

 blocks Objections to " stretchers " Dowels Notching Tabling Dove- 

 tailing Absence of bond in Manora breakwater Sloping coursed work accom- 

 modates itself to settlement Bond of blockwork in Tynemouth breakwaters. 



Settlement. Excepting in the case of breakwaters built upon 

 a rock foundation, more or less settlement can seldom be avoided. 

 It is usually greatest in those breakwaters which are composed, 

 either wholly or in part, of rubble mounds, seeing that in them 

 it is produced not only by the yielding of the ground upon which 

 they stand, but also by the consolidation and shrinkage of the 

 mound itself. When a mound rests upon an unyielding bed, 

 settlement still takes place, but, as a matter of course, it is 

 then due to the latter cause only. 



In constructing composite breakwaters with rubble mound 

 bases, if sufficient time be allowed to elapse between the deposit 

 of the rubble and the building of the superstructure upon it, the 

 mound will, in a measure, be consolidated by wave-action, and 

 will still further gain stability by collecting sand and other drift 

 material. Experience, however, shows that, no matter how long 

 a mound may have been exposed to these consolidating influ- 

 ences, the application of weight almost invariably causes further 

 settlement to take place. It therefore follows that, however 

 stable a mound may have apparently become, further subsidence 

 is sure to take place when it is called upon to support the weight 

 of a superstructure. 



Moreover, when a rubble mound is stopped at the level of, 



