FOUNDATIONS. 273 



best overcome by adopting the timber-framed type of break- 

 water. 



Gravel requires much the same treatment as sand. 



Clays must be dealt with according to their character, some 

 being almost as soft and treacherous as mud, while others are 

 sufficiently hard and compact to found directly upon. In all 

 cases, however, protection from scour should be kept prominently 

 in view. 



Sufficient has now been said to indicate the general principles 

 upon which foundations on the several kinds of natural ground 

 should be treated, so we will devote the remainder of this chapter 

 to describing the founding of breakwater superstructures upon 

 rubble mounds. 



Founding upon Rubble. During the time that a mound upon 

 which it is intended to erect a superstructure is being deposited, 

 and especially as it approaches the level at which the super- 

 structure will be founded, accurate soundings should be taken 

 at close intervals, in order to prevent the rubble being carried 

 up too high. 



In the last chapter, it was mentioned that the surface of 

 a mound, say the last 2 or 3 feet, is best formed by stones of 

 moderate size thrown by hand from boats or barges, to the 

 extent which soundings may show to be necessary. 



Almost the whole of the rubble forming the bases of the 

 Ymuiden and Mormugao breakwaters was deposited in this way, 

 it having been found cheaper and more satisfactory than using 

 hopper-barges. The same system was adopted for roughly 

 levelling the surface of the mound at Colombo. 



The usual method of levelling up inequalities, and forming 

 a true bed for the reception of blocks, whether these are to be 

 laid in horizontal courses or on the sloping system, is, after 

 having roughly levelled the rubble, to deposit a layer of small 

 broken stone or masons' chippings, which the divers pack and 

 arrange to the level required. If this is carefully done, it forms 

 a very good bed for the first course of blocks to rest upon. 



The small stone should be confined, as soon as possible after 

 the placing of the blocks upon it, by depositing additional 

 rubble along both faces of the breakwater, and, if necessary, 

 temporarily across the scar-end also, the marginal bags of con- 

 crete illustrated on p. 268 being dispensed with. 



When a mound is not sufficiently compact to warrant a 



