SINKING DEEP FOUNDATIONS FOR ENGINEEKING WORKS. 5G7 



by fifty-four feet by thirteen feet deep, divided into three compart- 

 ments by vertical partitions. These partitions, besides strengtlien- 

 ing the caisson, atlbrded shelter to the men during blasting, as the 

 bottom was rock of greatly inclined surface. 



One of the most difficult feats in tliis kind of work was the 

 sinking of a wooden caisson foundation for a lightliouse in Dela- 

 Avare Bay, an exposed position open to the full fury of the 

 Atlantic. It was towed out with part of the cast-iron shaft upon 

 it, and then further loaded and sunk. The compressed air in this 

 case was also made use of to force out the sand up through 

 vertical pipes. 



Beyond the pneumatic limit, about one hundred feet under 

 water level, all excavation must be done by dredgers, and special 

 care is required, in the design, that no contingency sliall arise at 

 the bottom, which has to be dealt with by manual labour, diving 

 operations being impossible. At these depths also the skin friction, 

 unimportant in a cylinder of moderate depth, becomes so great 

 that special arrangements for overcoming it must be provided. 



There are four well-known railway bridges, two now in course 

 of construction, and two completed, in which these difficulties had 

 to be met. The Benares Bridge over the Ganges, the Pough- 

 keepsie over the Hudson River, the Hawkesbury Bridge in New 

 South Wales, and the Jubilee Bridge over the Hooghly in Bengal. 



In the Benares Bridge the principal piers are sunk to a depth 

 of one hundred and forty feet below water level, and are formed 

 of oval brick wells sixty-five feet by twenty-eight feet, and as 

 they had to be begun in water, the bottom lengths are cased in 

 iron. Each is divided into three compartments in which the 

 dredging is carried on. 



In the Poughkeepsie and Hawkesbury foundations somewhat 

 identical principles are adopted, namely, caissons divided into 

 dredging and loading compartments. 



In the American work the caisson is of timber one hundred 

 feet by sixty feet by one hundred and twenty-five feet deep, 

 divided vertically into forty cells, the sinking, which is to about one 

 hundred and thii"ty feet below water, being effected by filling in 

 some of them and excavating in others. 



In the Australian bridge the caisson is oval and is of steel and 

 iron forty-eiglit feet by twenty feet, splaying out at bottom two 

 feet more all round, and it has three dredging wells in line at the 

 centre, parallel with its length, and splaying out to meet the outer 

 .skin and each other at the bottom in a cutting edge. Between the 

 wells and the outer skin,.whicli are strongly stayed together, the 

 space is filled with concrete as the structure sinks. The greatest 

 depth, which is also believed to be the greatest ever reached in a 

 bridge foundation, is one hundred and sixty-one feet below water 

 level. 



