SINKING DEEP FOUNDATIONS FOR ENGINEERING WORKS. 565 



these wells, sunk close together and filled with 'concrete, have 

 been used for railway bridge foundations ; the concrete in those 

 sunk by the author being made in those days, of one part lime, 

 two of brickdust, and four of broken stone. Such wells were 

 sunk in India, complete including everything, for about sixteen 

 shillings per lineal foot. 



Larger brick wells and fewer in number are more in use latterly ; 

 they are eighteen feet and upwards in diameter, with iron curbs, 

 and have been successfully sunk to over one hundred feet below 

 beds of rivers. A description is given in this paper of a curious 

 operation witnessed by the author at Cawupore, when the remains 

 of the submerged wells of a bridge previously carried away by flood, 

 were blown away under the sanl by guncotton, to make room for 

 the foundations of the present Ganges bridge at that place. 



Iron cylinders sunk and tilled with concrete are then alluded to, 

 as most generally in use out of India, and having the advantage 

 over brick where deep water has to be dealt with, Mr. Barlow 

 combining both systems in the new Tay Bridge, the foundations 

 of which, and the mode of operations, being described in detail. 



The stalnlity of cylindrical piers against overturning by flood or 

 drift wood is adverted to, and instances given of the two cylinders 

 forming a pier being connected together by sol d plating in South 

 African bridges to attain this object. Where the sinking is not 

 guided by piles, tapering out at bottom of the clinders is strongly 

 deprecated, experience amply shewing that this renders true 

 vertical sinking very difficult. 



Where water-bearing strata have to be passed through with 

 obstructions which cannot be removed except by actual manual 

 labour, and where the largeness of the area gives room to work 

 in, the pneumatic process has been generally adopted. While 

 applicable to, and often used in cylinder work, the method is 

 indespensable in inverted caissons. The caisson is like an inverted 

 box with tlie open side downwards, and the masonry is begun on 

 the top, which is made specially strong, and is proceeded with as 

 the caisson sinks into the ground, excavated by the men inside of 

 it. Holes or shafts are left in the masonry for the passage of 

 men and material to and from the caisson, and for the supply of 

 the compressed air wliich is introduced to force out the water and 

 enable the men to work. Each shaft is provided with an airlock, 

 a contrivance for preventing escape of compressed air, which is 

 fully described. When the desired depth is reached, the caisson 

 is filled with concrete and left as part of the permanent work. 



The pneumatic system has its limit at about one hundred feet 

 below water level, as men cannot work properly under greater 

 pressure than is necessary to sustain a column of water of that 

 height. 



In the great Brooklyn Bridge, the caisson is of timber strongly 

 roofed, and in the deeper pier, which is founded seventy-eight feet 



