MECHANICS AND USEFUL ARTS. 43 



EXTRACT FROM THE REPORT OF COL. W. A. ROEBLING, CHIEF 

 ENGINEER OF THE N. Y. BRIDGE CO. 



A boring made in 1867 showed gneiss rock at a depth of 96 feet 

 below high water. The strata penetrated consisted in the first 

 place of surface filling through alternate layers of hard pan and 

 boulders of trap embedded in sand and clay. Below 50 and 60 

 feet depth the material was so compact that the bore hole stood 

 without tubing for weeks. No necessity existed, therefore, for 

 going clown to rock; a depth of about 50 feet would suffice. But 

 the great desideratum to be attained was a uniform character of 

 the soil over the whole space of the foundation whatever the 

 depth might be. It is well known that the drift formation of 

 Long Island presents a great variety of strata in comparatively 

 short diagonal distances. Within a hundred or two feet on either 

 side of this foundation, there is no bottom, so to speak, and piles 

 are driven a great depth into mud ; whereas in the centre of our 

 foundation the depth of water was only a few feet ; the existing 

 ferry slip had been blasted out at a great expense, and to drive 

 an iron-shod pile even 2 feet into that material was the work of 

 hours. This hard material, however, occupied only a part of the 

 foundation, which comprises an area of 17,000 square feet. One- 

 third of the area towards the east was much softer in character ; 

 to meet the requirements of the case a heav}^ solid timber founda- 

 tion was decided upon, of sufficient thickness to act as a beam, 

 and having the requisite mass to insure a uniform settling. The 

 importance of a uniform foundation becomes evident at a glance 

 when we consider the size of the tower, weighing 35,000 tons, 

 with a height of 300 feet above the foundation upon which the 

 permanent pressure is 4 tons per square foot. In addition, the 

 buoyancy of the timber enables us to dispense with the screws 

 ordinarily used in towing a caisson. 



In regard to durability, it is well known that timber immerse d 

 in salt water is imperishable, and to protect it against worms it is 

 merely necessary to sink it beneath the river bed. It at once 

 suggested itself to make the timber platform as far as possible a 

 part of the caisson. This has been done by making the roof of 

 the caisson a solid mass of timber, of 15 feet in thickness. The 

 object and purposes of a caisson in sinking a pneumatic founda- 

 tion is too well known to need any description here ; it is merely 

 a (.living-bell on a vast scale. It may well be said that, since the 

 unparalleled achievement of Captain Eads, at St. Louis, the word 

 caisson has become a household word among American engi- 

 neers. 



The caisson of the East River Bridge is a large inverted vessel 

 or pan, resting bottom upwards, with strong sides. Into this air is 

 forced under a sufficient pressure to drive out the water. Entrance 

 is had to the large working-chamber, thus formed underneath, 

 through suitable shafts and air-locks. The material is taken out 

 through water-shafts, open above and below, and 2 supply-shafts 

 send down the material subsequently needed for filling up the air- 

 chamber. 



