278 



HARBOUR CONSTRUCTION. 



is often experienced at and near the point where the mound 

 meets the rock. At this point unequal settlement frequently 

 takes place to such an extent as to badly crack the work. This 

 may, to some extent, be met by forming a vertical joint at that 

 point, right across and from top to bottom of the superstructure, 

 the object being to localize, as far as possible, the disturbance 

 caused by the settlement. When settlement has ceased, the 

 joint may be filled up and made good by grouting. 



Bond in breakwaters and in works of a like kind differs from 

 that in most other structures, inasmuch as it is seldom required 

 to distribute weight, or to tie the work together longitudinally. 

 Indeed, it is sometimes contended that bond of any kind in a 

 breakwater is an evil, seeing that it tends to prevent the struc- 

 ture from freely accommodating itself to settlement, in the event 

 of such taking place. 



In arranging the bond of a sea-work, vertical movement may, 



however, be provided for, and it should, no doubt, be kept 



prominently in view, while at the same time the withdrawal of 



any individual block should be rendered as difficult as possible. 



A breakwater composed of large monolithic blocks, of the full 



width of the structure, 

 bonded to each other by ver- 

 tical grooves and tongues, as 

 in Fig. 89, would, so far as 

 bond is concerned, be an 

 ideal one ; but in practice 

 such a mode of construction is not always possible or expedient. 

 Attention has already been directed (pp. 181, 182, 244) to the 

 mischief which may result from the "dry bedding" of large 

 blocks laid in horizontal courses, but it may not be out of place 

 to again refer to the subject here. 



It was pointed out that when blocks are merely laid one 

 upon another in horizontal courses, without any bed of cement 

 between them, unless the surfaces are perfectly true, and the 

 blocks laid in exactly the same plane, they cannot bear evenly 

 upon each other. Hence, as the weight of superposed blocks is 

 applied, those underneath are subjected to transverse strains; 

 and if these should be intensified by leverage, as would be the 

 case if long longitudinal bonding-blocks or "stretchers" were 

 introduced, the blocks, which possess practically no elasticity, 

 would in all probability be broken. 



PLAN. 



