"I have seen screw piles screwed into soft ground for fully fifteen feet, and they seemed quite right, and yet when they were loaded they vanished. I have also known them to be twisted about something like a corkscrew, and to be impossible to get down at all when they have reached a hard layer of gravel, and nearly so when they met with a streak of hard stiff clay. Sometimes they are overscrewed, and made to penetrate somehow or other; and I remember once, when they were loaded for testing and were thought to be right, a washout occurred at one place, owing to a mistake in dredging, and the piles, although they screwed, were found to be twisted about into all sorts of shapes, and at the bottom were turned up a trifle and never went down more than a few feet, and while it was thought we were screwing them down we were screwing part of them aside. They were small solid wrought-iron piles. It is well not to forget that sand varies very much; for it is found nearly everywhere, and may be anything from large hard angular deposit that will bind, to little round mites easily blown away, and it is mixed with pretty well everything; and therefore sand is a thing you must be careful with before you take it to be just the thing for watersunk disc piles or screw piles, and you ought to know all about it. Well, assuming that it is right, and the soil will not become jammed in the screw blade, it is always advisable to try whether the sand grains will roll well together and do not wedge; for you want sand, if it is to be nice for pile sinking, just the reverse of sand for mortar or concrete, for that with round grains is the kind to screw in and not that with sharp angular grains, and if it is slimy, so much the better—just the opposite of that for mortar or concrete.
"The soil must be loose, and if it is silty so much the better. Don't undertake to screw piles into hard and compact sand, gravel, stiff clay, or where there are boulders in the ground or streaks or layers of soil of which you hardly know the character. If you do, good-bye to profit from any screwing, and may be to the screw blades, and your fishes must be got out of 'extras' by omitting a length, smashing a screw blade, or short screwing. Be careful to be paid for all piles you have screwed down directly you have done them, and take no maintenance; for I have known a ship drift, or a gale arise, and sweep away the unbraced piles like sticks, and if you are only paid when you have finished screwing a cluster of them, where are you then, and who's which? Suppose you have nearly fixed a cluster of piles, they will say you ought to have braced them at once, and you will be charged for breakages, and not be paid for having screwed them. You may talk as long as you like, and say, How could I get them all braced when the piles must be screwed separately? You will only be told that is your look-out, and that you knew the terms of the contract and must have considered any risk in the prices. So I bar injury from waves or wind, earthquakes and shakes, collisions from vessels or other floating or moving substances; and believe the last to cover all fishes, from sea-serpents, whales, porpoises, and sprats, to balloons, stray air-balls, wreckage, and mermaids; and it gives you a chance of wriggling out of squalls with an I'm-so-sorry-at-your-loss sort of countenance.
"You have to think over the staging. Fixed staging may be out of the question because of the expense; then you must either screw from the finished end of the pier as you proceed with the work, or from a floating stage, but you may not be able to get sufficient power to screw the piles from any moored floating stage. The shore piles of a pier may screw easily, but when you get out in the sea fixed staging may soon be smashed, and in that case you are compelled to do it from the end of the finished portion of the pier. There is a good deal of uncertainty, as you can judge, and you want to well consider whether and how you can get the power cheaply to screw the piles.
"The idea of the screw pile was that it should easily enter the ground and push aside any obstruction in its descent without much disturbance of the soil, with the ultimate object of obtaining, by reason of the screw blade, a strong resistance to upward and downward strain. Well, it is all right if the whole of the blade bears equally upon the soil and the earth is of the same character; but if it is not, the strain upon the screw blade is unequal, and it will sooner or later crack or break; and except in any earth like fine sand or silt and all of one kind, I should be sorry to say that the whole area of the blade does the work as I said before. And here comes in the value of an allowance of extra strength, for you cannot tell how much it has been weakened by corrosion, nor can you inspect, paint, or do anything to the screws when they are down. If I was engineer of an iron pile structure, I should have a few piles screwed at convenient places independently of the pier, but near to it, and have, say, one or two taken up every few years—say every seven or ten—just to have a look to see how matters seemed to be, and have a piece of the iron analysed, and compare it with the original analysis; and I should take care the piles were all the same quality of metal, so that the makers should not get up to fun at the foundry.
"The piles have to bear a heavy twisting strain during screwing; and take my advice, always see that the joint flanges are not light, for when piles break in screwing, they usually fail at the flanges. What I have learned shows me it is a great mistake to have the screws of very large diameter, so as to have few of them; let the blades be small rather than large, and they are best for screwing when of moderate size, and are also likely to be sounder metal. There is not the same risk of breaking them in screwing, and you may be able to screw a small blade when a big one would be smashed, and besides it is as well to have the load distributed as much as possible. A screw pile shaft should not be a thin casting because of the strain upon it in screwing, and it should be thicker on this account than a disc pile, but the latter will not do for any soils except those named before. I have known screw piles to penetrate hard and dense sand, gravel, soft sandy ground, limy gravel, loose silt, limy clay ground something like marl, stiff mud, chalk, clay, marl, and all kinds of water-deposited soil, and in almost every earth except firm rock, but it is not advisable to use them for anything much harder than fine sandy gravel, for the blades must then be strained very much and the pile and screw may be injured. It is not using them rightly, or for the purpose for which they were designed, and another system of foundations should be used except under special circumstances.
"Don't attempt to screw piles into ground having boulders in it. It is always difficult to penetrate, as also is spongy mud and stiff tenacious clay. In any ground harder than loose sand, silty and alluvial soil screwing is not easy, and you cannot say what it will cost to obtain the necessary power to screw. As regards that kind of screwing I always feel so benevolent that I like some one else to do it. Do you understand?"
"Yes; when you know a loss looks more likely than profit."
"If you like to put it that way it is not in me to object. I'm too polite. Saying 'yes' and agreeing with every one, gets you a nice character as an agreeable man, whereas you are a big fraud and a high old liar."
"Parliamentary language, please; no matter what you think."
"All right, then. You know what pure sand is?"
"You mean quite clean angular grains, and hard, too, like broken-up quartz rock?"
"Yes. Well, avoid it for screw piles, for then it is very difficult to screw them to any considerable depth. You can't displace the sand enough. It wedges and binds almost like rock."
"You mean it wedges up, and will not move?"
"That's near enough. Well, avoid clean, sharp, angular sand and shingle gravel as much as you can, and take screwing in dirty sand instead. I mean round-grained dirty sand with some clay upon it, or sandy gravel. What is wanted is something to separate the particles of the soil and act like grease so as to make them roll and not compress and become bound. You can't be too careful about this."
"I will put that down in my note-book so as not to forget it."
"To save bother, be sure to ascertain whether the work is in rough ground; and if you are abroad see that about five per cent. is allowed for breakages of all kinds, or the piles may run short.
"I have seen piles screwed into a kind of clay rock seam, the end of the pile was made like a saw, toothed, in fact, and stiffened from the bottom to the underside of the screw blade with ribs shaped to cut the ground