246 HARBOUR CONSTRUCTION. 



For the guidance of those in charge of the barges, the position 

 for the deposit of the stone on the line of the breakwater is 

 indicated by means of leading marks on the land, or on the 

 finished portion of the breakwater, and also by means of buoys 

 afloat. 



In order to prevent undue irregularity in the surface of a 

 mound upon which a superstructure has to be built, care is 

 necessary in depositing the stone ; otherwise the cost of levelling 

 it, which must be done by means of divers, will be unduly large. 



When it is intended to place a superstructure upon a mound, 

 the latter should be extended well in advance of the rest of the 

 work, so as to give it as long a time as possible to consolidate. 

 In this way the amount of settlement, after the superstructure 

 has been built, will be reduced to a minimum. 



An interval of at least one year should be allowed to elapse 

 from the deposit of the rubble to the commencement of building 

 operations ; and if this period can be extended to two or three 

 years, so much the better. 



When barges are employed, it is evident the mound may be 

 extended as far in advance of the superstructure as is desired, 

 without incurring any such additional cost and risk as a corre- 

 sponding extension of staging would involve. When, however, 

 a mound is to be raised above the level of high water, or, indeed, 

 to such a level as would interfere with the barges passing freely 

 over it, they can only be employed if at all to supplement the 

 deposit of the rubble by other means. 



By the use of barges, where applicable, the risks appertaining 

 to staging are avoided ; but others, scarcely less formidable, are 

 often incurred, and the prosecution of the work is greatly de- 

 pendent upon the state of the weather. 



Barges used for depositing rubble are generally constructed 

 with hopper bottoms, through which the rubble is discharged ; 

 but when a mound is to carry a superstructure founded below 

 low water, the last two or three feet of its height should be 

 brought up by selected stone thrown overboard by hand. In 

 this way the surface of the mound may be formed to a fairly 

 level and regular plane whereby the labour and cost of preparing 

 the foundations for the superstructure by means of divers will 

 be considerably reduced. 



Steam-barges, as illustrated on Fig. 64, are preferable to 

 ordinary barges towed by tugs, bacause they are better under 



