FOUNDATIONS. 261 



(pp. 266 et seq.), and then continue the work upwards by means 

 of blocks, or to carry the mass-work up to the level of about 

 one-third tide. 



If the latter course be adopted, it will be found that by 

 working the tide down and up, there will be sufficient time to 

 finish off the work and make it secure before the water reaches 

 it, always supposing that too much is not taken in hand at one 

 time. It may, furthermore, be worked at during neap tides as 

 well as during springs, an advantage which, in some localities, 

 can scarcely be overrated. 



It has been stated that foundations on rock often present 

 considerable difficulties, which, as a matter of course, are much 

 increased when the work is situated below low water. I propose 

 now to give a few examples. 



1. Extreme unevenness of surface. 



Here steep slopes and ridges occur, points project, and gullies 

 and pockets are, it may be, so arranged and shaped that satis- 

 factory work cannot be made, either with ordinary bag- work 

 or with mass- work in frames, until some of the inequalities have 

 been reduced. 



In such cases I, from experience, strongly recommend the 

 making of a model of the rock surface, to a scale of, say, J inch 

 to one foot. No amount of cross-sections, or soundings, or 

 under- water examinations, can convey the same idea of the shape 

 of the bottom, or so well enable a decision, as to how best to 

 deal with it, to be arrived at. 



Mass concrete, deposited from skips or from tripping bags 

 (p. 267), will generally be found useful as a first step towards 

 levelling up such foundations. 



As a means of confining concrete deposited in this manner, 

 where boarding cannot well be used, cement-bags, about three 

 parts filled with sand, will be found serviceable. 1 These can be 

 easily handled and built up by divers, so as to form a compact 

 wall, which, if kept a few feet below low water, will not be 

 disturbed even by a moderate sea. 



Similar bags, filled with soft concrete, may be used in lieu 

 of mass-work in positions where there is not room for larger 

 bags to be deposited, and where there is no risk of their being 

 withdrawn by the sea ; but they do not form such close work 

 as mass-concrete, and they cost more. 



1 The mouths of these should be sewn, not tied. 



