268 HARBOUR CONSTRUCTION. 



but bags of any convenient size may be adopted. A straight-edge (a 

 light section Vignole's rail makes a good one) rests upon the upper 

 edges of the side planks, and spans the distance between them. 



" The divers commence the foundation at one end, and work 

 backwards, so as not to disturb the newly deposited concrete. The 

 concrete is merely pressed down by the flat hand to the required 

 level, and gently struck off by the straight-edge. In this way a 

 perfect surface is formed, which, at a depth of 5 feet or 6 feet below 

 low water, is undisturbed by small waves. 



" In the course of a few hours the concrete becomes sufficiently 

 hard to resist a moderate sea, and after three days it is hard enough 

 to bed blocks upon. 



" It is considered essential to success that the concrete used in 

 forming foundations as described should be thoroughly well mixed, 

 enough water being used to form it into a not too stiff slimy paste, 

 which should be put into place as soon after mixing as possible. 



" The system is applicable to the levelling of bag-work or rubble 

 foundations, as well as rock ; but in the case of rubble foundations, 

 it is necessary to cover the entire area with jute sacking, in order to 

 prevent the concrete finding its way down amongst the stones." 



Concrete mass-work within frames, also bag- work, have 

 already been described (pp. 183 et seq.\ so, although largely used 

 in foundation work, they need not be further noticed here. 



Where a rock bottom is fairly level, small irregularities are 

 sometimes adjusted by means of a layer of broken stone similar 

 to road metal. In order to prevent its escape, small bags of soft 



concrete are sometimes placed 



smaii Ba SS of Broken stone along the margins of it, as 



^Concrete. / 



shown in Fig. 88. These 

 i-'^r , iji i j j- A i 



''' R 10 c K ' y should be placed immediately 



FIG. ss. before the setting of the blocks, 



in order that the blocks may, 



by their weight, bed themselves well down upon the metal, and 

 squeeze the concrete in the bags down to the level required. 

 The concrete in these bags should be extra good, and free from 

 large stones. 



After blocks have been so bedded, the broken stone should, 

 if possible, be grouted with neat Portland cement grout, or with 

 grout composed of Portland and Medina cements in equal pro- 

 portions, introduced through the lewis-holes, or through holes 

 specially made for the purpose, by means of pipes extending to 



