214. Cushing Pile Foundation. A combination of steel, con crete, and wooden piles is known as the Cusliing pile foundation. A cluster of piles is driven so that it may be surrounded by a wrought iron or steel cylinder, which is placed over them, and which is sunk into the soil 6ntil it is below any chance of scouring action on the part of any current of water. The space between the piles and the cylinder is then surrounded with concrete. Although the piles are subject to decay above the water line, yet they arc so thoroughly surrounded with concrete that the decay is probably very slow. The steel outer casing is likewise subject to deterioration, but the strength of the whole combination is but little dependent on the steel. If such foundAions are sunk at the ends of the two trusses of a bridge, and are suitably cross-braced, they form a very inexpensive and yet effect ive pier for the end of a truss bridge of moderate span. The end of such a bridge can be connected with the shore bank by means of light girders, and by this means the cost of a comparatively expensive masonry abutment may be avoided.
215. Cost. In comparing the cost of timber piles and concrete or reinforced-concrete piles, the former are found to be much cheaper per linear foot than the latter. As already stated, however, there are many cases where the economy of the concrete pile as compared with the wooden pile is worth considering.' In general, the require ments of the work to be done should be carefully noted before the type of pile is The cost of wooden piles varies, depending on the size and length of the piles, and on the section of the country in which the piles can be bought. Usually piles can be bought of lumber dealers at 10 to 20 cents per linear foot for all ordinary lengths; but very long piles will cost more. The cost of driving piles is variable, ranging from 2 or 3 cents to 12 or 15 cents per linear foot. A great many piles have been driven for which the contract price ranged from 20 cents to 30 cents per linear foot of pile driven. The length of the pile driven is the full length of the pile left in the work after cutting off the pile at the desired level of the cap.
The contract price for concrete piles, about 16 inches in diameter and 25 to 30 feet long, is approximately S1.00 per linear foot. When a price of S1.00 per linear foot is given for a pile of this size and length, the price will generally be somewhat re duced for a longer pile of the same diameter. Concrete piles have
been driven for 70 cents per linear foot, and perhaps less; and again, they have cost much more than the approximate price of $1.00 per linear foot.
216. Piles for the Charles River Dam. The first piles driven for the Cambridge (Mass.) conduit of the Charles River dam were on the Cambridge shore. On'January 1, 1907, 9,969 piles had been driven in the Boston and Cambridge cofferdams, amounting to 297,000 linear feet. Under the lock, the average length of the piles, after being cut off, was 29 feet; and under the sluices, 31 feet 4 inches. The specifications called for piles to be winter-cut from straight, live trees, not less than 10 inches in diameter at the butt when cut off in the work, and not less than inches in diameter at the small end.
The safe load assumed for the lock foundations was 12 tons per pile, and for the sluices 7 tons per pile.
The Engineering News formula was used in determining the bear ing power of the piles. The piles under the lock walls were driven very close together; and as a result, many of them rose during the driving of adjacent piles, and it was ,necessary to redrive these piles.* 217. Pile Foundation for Sca=Wall at Annapolis. The piles for constructing the new sea-wall at Annapolis, 1.11d., ranged in length from 70 feet to 110 feet. On the outer end of the break water, piles 70 feet to S5 feet were used. These piles were in one length, single sticks. Toward the inner end of the breakwater, lengths of 100 feet to 110 feet were required. Single sticks of this length could not be secured, and it was therefore necessary to re sort to splicing (see Fig. 50). After a trial of several methods, it was found that a splice made by means of a 10-inch wrought-iron pipe was most satisfactory. When the top of the first pile had been driven to within three feet of the water, it was trimmed down to 10 inches in diameter. On this end was placed a Jiece of 10-inch wrought-iron pipe 10 inches long. The lower end of the top pile was trimmed the same as the top of the first pile, and, when raised by the leads, was fitted into the pipe and driven until the required penetration was reached. The piles were cut off 41 feet below the surface of the water, by a circular saw mounted on a vertical shaft.t