SINKING DEEP FOUNDATIONS FOR ENGINEERING WORKS. 569 



Tlie pueuniatic processes are largely in favour on the Continent 

 of Europe and in America, and illustrate a successful application 

 of scientific knowledge to practical work. It is a pity that 

 physical conditions limit the extent of the application. The 

 effect of highly compressed air on the human system, is that the 

 blood is driven in from the exterior and soft parts of tlie body to 

 the central organs, especially to the brain and spinal cord, causing 

 violent neuralgic pains and sometimes paralysis. 



There is hardly sufficient experience yet of the .system of a 

 double casing witli the weighting material between, employed at 

 the Poughkeepsie, the Hooghly, and tlie Hawkesbury to enable 

 the law to be laid down with regard to them, Ijut the experience, 

 so far gained Ijy the author at tlie latter Ijridge, appears to show 

 that where a great depth has to be reached there must be ample 

 latitude given in the size of the caisson in plan, so as to allow for 

 any divergence that may occur in siuking. Such divergence, 

 trifling in a shallow foundation, may become serious in a deep 

 one, and the enormous weight of the structure renders conti^ol 

 very difficult. Such control may, in the opinion of the author, 

 be given to some extent, in the first place as regards horizontal 

 divergence, by the shape of the lower portion, which should be 

 vertical on the outside. Any outward splay given to this part 

 may intensify a lateral movement, however caused, in the direction 

 of the splay on one side by a wedge-like action, while the similar 

 splay on the opposite side has no counteracting influence. In the 

 second place, to counteract any tilting action, the dredging holes 

 should be sufficient in number, and so placed, that by dredging in 

 any one, a " straightening up " ettect can be produced, that is to 

 say, there should be four at least, so as to control tilting action 

 towards any of the four cardinal points. It must be remembered 

 that in dredging these deep holes, the slightest tilt throws the 

 dredging grab over to that side of the well, at the bottom, which, 

 in order to recover verticality, ought to be avoided, as it tends' to 

 undermine that cutting edge which is already too low. It is 

 different from the case of a foundation of a single large shallow 

 well or cylinder, which can be dredged at any desired point, and 

 that side sunk accordingly. 



In the multiple tube system, as it may be called, on the 

 contrary, each well must act for itself in regulating the descent 

 on its own side, and its position should be so fixed in the design 

 .so as to lead to that result. As these caissons must necessarily 

 be deep in proportion to their area, there is no chance by weight- 

 ing them unevenly to restore their balance, except when the 

 .sinking is just begun. 



