Dred3ing Through Wells

"All materials were rounded off at their face to sledge shape and drawn lengthwise and horizontally over the gravel or sand, the latter being leveled and bedded as solid as it is likely. to be in its natural position. The riveted sheet iron contained twenty-five rivets on a surface of 2.53 by 1.67 = 4.22 square feet; the rivet-heads were half-round and 4a inch in diameter." Notice that for dry materials and also for wet gravel and sand, the frictional resistance at starting is smaller than during motion, which is contrary to the ordinary statement of the laws of friction.

Values from .Practice.

Cast Iron. During the construction of the bridge over the Seine at Orival, a cast-iron cylinder, standing in an extensive and rather uniform bed of gravel, and having ceased to move for thirty-two hours, gave a frictional resistance of nearly 200 lb. per sq. ft.* At a bridge over the Danube near Stadlau, a cylinder sunk 18.75 feet into the soil (the lower 3.75 feet being "solid clay") gave a frictional resistance of 100 lb. per sq. ft.* According to some European experiments, the friction of cast-iron cylinders in sand and river mud was from 400 to.600 lb. per sq. ft. for small depths, and 800 to 1,000 for depths from 20 to 30 feet.f At the first Harlem River Bridge, New York City, the frictional resistance of a cast-iron pile, while the soil around it was still loose, was 528 lb. per sq. ft. of surface; and later 716 lb. per sq. ft. did not move it. From these two experiments, McAlpine, the engineer in charge, concluded that "1,000 lb. per sq. ft. is a safe value for mod erately fine material."$ At the Omaha Bridge, a cast-iron pile sunk 27 feet in sand, with 15 feet of sand on the inside, could not be with drawn with a pressure equivalent to 254 lb. per sq. ft. of surface in

contact with the soil; and after removal of the sand from the inside, it moved with 200 lb. per sq. ft.¶ Wrought Iron. A wrought-iron pile, penetrating 19 feet into coarse sand at the bottom of a river, gave 280 lb. per sq. ft.; another, in gravel, gave 300 to 335 lb. per sq. ft.** Masonry. In the silt on the Clyde, the friction on brick and concrete cylinders was about 3i tons per sq. ft.tt The friction on the brick piers of the Dufferin (India) Bridge, through clay, was 900 lb. per sq. ft.tt Pneumatic Caissons. For data on the frictional resistance of pneumatic caissons, see f 887.

Piles. For data on the frictional resistance of ordinary piles, see 4 781-84, p. 398.

It is difficult to obtain data under this head, since but comparatively few foundations have been put down by this process. Furthermore, since the cost varies so much with the depth of water, strength of current, kind of bottom, danger of floods, requirements of navigation, etc., no such data are valuable unless accompanied by endless details.

For the relative cost of different methods, see Art. 6 of this chapter.

A serious objection to this method of sinking foundations is the possibility of meeting wrecks, logs, or other obstructions, in the underlying materials; but, with the possible exception of the freezing process (see Art. 5 of this chapter), the method by dredging through tubes or wells is the only one that can be applied to depths which much exceed 100 feet—the limit of the pneumatic process.