FOUNDATIONS. 271 



considerably thicker. It extends on either side of the piers for a 

 distance of about 50 feet, and on the seaward side it is protected, 

 near the bases of the piers, by wave-breakers of concrete blocks, 

 which also give additional stability to the piers themselves. 



When scour took place along the edge of the rubble, as was 

 invariably the case soon after it was deposited, the rubble sub- 

 sided as the scour progressed, and formed a rough rubble slope, 

 which effectually encased and protected the sand underlying 

 the piers. 



At Tynemouth, where the breakwaters are also of the 

 vertical type and founded upon sand, the same system was 

 adopted. The rubble was, however, deposited over a greater 

 width than at Ymuiden, and the blocks, weighing 36 tons each, 

 which were deposited along the seaward faces of the breakwaters, 

 merely formed aprons, as they were not carried up high enough 

 to constitute wave-breakers. 



With the object of avoiding or reducing the excavating 

 action of waves playing against a vertical face, some engineers 

 favour a curved profile for breakwaters, such profile being either 

 the arc of a circle, a cycloidal curve, or, indeed, almost any other 

 which fancy may suggest. It has, however, been found in 

 practice that all such curved profiles throw the water higher 

 than a straight, vertical face, and that they rather increase than 

 diminish the scour at the toe of the wall, notwithstanding that 

 they may meet the beach-line almost tangentially. They are, 

 moreover, very difficult to construct under water, and they are 

 weak structurally, presenting, as it were, the intrados of an arch 

 to the wave-stroke. In cases where walls with curved profiles 

 have been built, it has generally been found necessary to protect 

 their toes by throwing down large rubble stones, unless they 

 happen to have been founded upon rock. 



Sometimes, when sand overlies more stable ground, it is re- 

 moved by means of a sand-pump, or in some other convenient 

 way, the breakwater being founded upon the firmer bed thus 

 laid bare. In some instances caissons have been sunk, and the 

 work built inside of them. 



Mud contrasts unfavourably with sand, because of its extreme 

 readiness to yield when weight is applied. Hence the systems 

 of dealing with sand foundations, as just described, are not 

 always applicable to a mud bottom. 



The covering of rubble would sink into the mud in much the 



