"Wait a minute. All right. I understand now; you lessen the chances of failure and the extent of it when it occurs by having a little less to do with goods that are made of material no one seems to knows everything about."
"Now you have it. Shake fins. Glad we have worked on to the right road again, as it looked like a collision just now."
CHAPTER III.
SCREW PILES.
Details.
"Now for some details.
"Solid piles are usually from 4 to 8 inches in diameter, and hollow cast-iron from 10 to 30 inches, and generally 10 to 20 inches. Avoid any cast-iron screw piles that are less than half an inch in thickness. When they are from 1/12th to 1/18th of the diameter is perhaps the best, according as their length is little or great; but of course they have to be of a thickness that will stand the load, and what is the best foundry practice should not be forgotten.
"Now as to the blade of the screw. If of wrought-iron, which seems to me the wrong material for that purpose, it should not be less than half an inch in thickness; if of cast-iron, as usually is the case, the thickness of the blade of the screw at the pile shaft should be about 1·25 to 1·50 that of the column, and at the edge not less than half an inch, and it should taper equally on both sides, and care be taken that the metal is the very best and so cast as to ensure uniformity and strength.
"All sizes of screws from twice to six times the diameter of the pile when hollow I have screwed, but the best are from 2 to 1 to 3 to 1, and when they are more than 4 to 1 it is to be feared they will break before they can be made to penetrate far enough to say nothing about. Solid piles with screws four to seven times the diameter of pile I have also fixed, and 5 to 1 to 6 to 1 is quite large enough; but the kind of ground and the depth to which they must be got down should govern the size and the pitch. The greatest depth, apart from imagination for measurement, to which I have ever screwed a pile is about 25 feet. Without special tackle I have made a 2 feet in diameter screw penetrate hard clay, dense sand, and other hard soil from 8 to as much as 17 feet; but then 10 to 15 feet is deep enough, for there is such a thing as overscrewing. A 3 to 4 feet in diameter screw I have fixed all depths from 10 to 20 feet in ordinary sand, clay, and sandy gravel. A 4 feet to as large as a 5 feet screw, which great size should only be used for soft soils, from 15 to 25 feet, and the most usual depth is about 15 feet, and hardly ever above 20 feet.