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SCREW PILES.

General Consideration‌—‌Manipulation for "extra" profit.

"You want to know my experiences with screw piles first."

"Yes."

"They do very well when the water is not deep and the ground loose sand, silty sand, or sandy fine gravel, and nothing else; and I prefer disc piles for sand, provided the water power can be easily obtained.

"The whole area of a screw blade is often taken as bearing support; but I doubt if it should be, for it is not a bared foundation‌—‌that is, one you can see and know the character of, as in a cylinder pier, for instance; but some appear to assume it is, and then claim that a lot of metal is saved and the same or more bearing obtained. The screw blade may always be right and it may not be, and no one positively knows; because no one can see whether it is down straight, turned, or broken, but the difference between the actual and the breaking strain comes to the rescue.

"Still, it is no certainty that the screw blade is resting upon the same soil, and even if it does it may not receive the load in a vertical line, and may be strained more upon one side than another. And how about the rusting of the blade, for it is thin, and seldom more than half an inch at the ends and two and a half at the pile shaft, and nearly all surface? In a cylinder pier the hearting is placed on the bared ground, and you know it is there, and it cannot rust, that's certain. I don't see the good of iron rings above a few feet higher than highest flood level, for after the hearting is set, if it be of Portland cement concrete, you can give it a coating of nearly neat Portland cement. However, we are talking about screw piles.

"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 as the pile was turned, and I doubt if they could have been screwed without. They seemed to steady the pile; but care must be taken when there is a projecting end and it is tapered to a less diameter than the pile shaft, as generally is the case, that the axis is true, or the pile will not screw vertically.

"Once I had to screw a few wrought-iron unpointed piles with a small screw blade made of angle iron fixed inside as well as the large screw blade outside. The outside blade was about 4 feet in diameter, and of half-inch plate, the inside blade projected about 3½ inches, and both blades had the same pitch; but the engineer, after having tried a few, discontinued having an inside screw, and said he thought it even arrested progress, because it interfered with the internal excavation. The experience we had with them was against their use, and they seemed to make the screwing harder, and no one was able to discover any advantage in them, although they did all they knew to flatter the novelty.

"Now a word as to cast or wrought-iron for screw piles. The question of relative corrosion can be decided at some scientific institution, and there will be hot fighting over that between the cast-iron and the wrought-iron partisans. I merely refer to screwing cast or wrought-iron screw piles into the ground. As regards the blade of the screw, it should be as stiff as possible, and therefore cast-iron is better than wrought-iron, also cheaper; and although a cast-iron screw will break easily, a wrought-iron blade will buckle and bend and give. To me, cast-iron blades seem somewhat easier to screw, if they are good clean castings. I have screwed wrought-iron piles or columns when they have been fixed to cast-iron screws, but in any case when the piles must be long, to have them of cast-iron is my wish. Solid wrought-iron piles can be obtained of a long length, but the price increases, and when they are long and of small diameter, as they must be, they are difficult to screw in a desired direction."

"What do you think of solid piles as against hollow ones?"

"Well, I heard a discussion between two engineers about it, and they agreed that solid piles only do for little or medium heights, and I asked one to write a line or two for my guidance, and this is what he dashed off. Read it."

"No. Read it to me."

"Well, it runs:‌—‌'In designing solid piles it should be remembered that the strength of solid round columns to resist torsion, torsional strength (he means strength against twisting strain) is as the cubes of their diameters, therefore a solid round bar 4 inches in diameter will bear eight times the torsional strain of a bar 2 inches, the lengths being the same.'"

"How's that?"

"Why, 2 × 2 × 2 = 8, 4 × 4 × 4 = 64, and 64/8 = 8."

"I understand."

"In the case of hollow columns, the exterior diameter must be cubed, and the cube of the interior diameter deducted from it when the relative values of different-sized columns can be compared. For transmitting motion, and here torsional stiffness is referred to, the resistance of shafts of equal stiffness is proportional to the fourth power of their diameters. A 2-inch shaft will transmit 16 times the force which would be transmitted by a 1-inch shaft without being twisted through a greater angle. When the height of the pile is considerable the diameter should be relatively larger, in order that the metal may not be subject to severe torsional strain. So don't forget the piles should be of large and not small diameter, or you may have trouble in screwing them."

"You remember old Bill Marr?"

"Rather, who did the iron pilework on the Shore Railway. I should think I did, for old Spoil'em, we called him, and I were in 'Co.' together more than once."

"Oh! you were, were you?"

"Yes. Well, there is not much to be got that way unless it is soft ground for a good depth and the piles are long and the range of tide considerable, then you may pick an odd plum now and again by a bit of useful forgetfulness. I mean this way:‌—‌By using an odd making-up length or two instead of the right length, and getting it fixed on the quiet just as the tide is rising, then you have a nice peaceful few hours in which to get the joint well covered and down before next low water; but it wants some management to keep the coast clear, and you can't do very much at it‌—‌still little fish are sweet. One day I was nearly caught at the game of 'extra' profit, and as we had only just begun, of course at the shore end, it would have been awkward for me if I had been found out, and I might have been ordered change of air and scene by the engineer. It happened like this, the piles had been going down very easily, and acting up to the principle of making hay while the sun shines, I had a couple of short lengths put on six of them. We were screwing them in triangles, so one I got to right length, and two did not find the same home, because they could not, not being long enough. I dodged the lengths so that the joints were all right for the bracing above low water. Now the road was clear, so I ordered a new length to be put on all of them before the tide turned, and that each of them was to be down 3 feet or so before the tide began falling to allow them to set, and told them that then they were to proceed as before. Now, I consider the chap that first went in for making up lengths was born right and with an eye to business and nicked profits. We were working two triangles of screw piles I thought lovely, and said, innocent-like, to my ganger, 'Get the joint of each one down say 3 feet below low-water mark so as to protect it, for no joint is so strong as the solid pile, and then you can screw them down till all the tops are level and right for the bracing.' Of course they said nothing, and I am sure never thought anything or wanted to do, too much trouble. It is not my place to teach them, either."

"No, certainly not; there you are right."

"Well, somehow or other, the ground turned hard, or we got into a streak of compact gravel. I did not trouble further about the piles after I had given orders, as the tide had started rising and the joints were well covered. It was rather an up and down shore. I felt certain in a few hours none of them could be seen except by divers, so I had a bit of business on shore which took me nearly two hours before I got back on the work. My ganger said, 'I am glad you have come back, because they stick; I have tried to get the lot down, but not one has screwed in more than a foot.' That was not exactly what I wanted, and said, 'Why, the long ones went down easily?' 'Yes,' said my ganger, 'but they were at the point of the triangle, and these others are all on one line or nearly so, and have struck hard ground.' I will cut it short, although it got exciting, for it was a race between screwing and, I might say, banging them in, and the tide that was going down; and I was clocking and measuring, and hot and cold, according as the race went, as I thought they would find me out; but I was left pretty well alone, as they cared much more about inspecting the piles than knowing how they were screwed down, besides the engineer was very busy with a lot of groynes and ticklish work improving the harbour channel. However, we just managed; but it made me feverish, and I expect the blades, if they could be seen, are not exactly as when they left the foundry; but there, there is a good deal in pilework that has to be taken on trust, it is not like a foundation you can see and walk upon if so minded. Still, screw piles are all right for some soils, but I like disc piles better for sand, those that sink by water-pressure I mean. I don't think there is the same fear of the disc being broken as there is in the case of the screw, and the sinking is so easy and soft that no parts get strained as in screwing, but the ground must be soft, or there may be a bother.

"After this shave from being bowled out, I always took care to dodge in a short one, now and then, when I knew the ground must be right, and I never got scared again. It was lucky, too, that a good many of the lengths varied, as on most jobs they are all the same, except the making-up lengths, and then down they all have to go unless a whole length can be left out when a seam of hard soil is reached, and that is not often the case, and there is not much chance of a bit of 'extras' that way on the quiet. I have known the game of 'extra' profit carried to breaking off a screw blade purposely, but I draw the line before I come to that."

"Do you? I should not have thought it, as you don't mind cutting off the heads of timber piles, so you have promised to tell me."

"That is a different material and consequently requires different treatment. You understand? Let me also tell you, I once heard a big Westminster engineer say, 'Timber we understand, iron we know a good deal about, and steel also; but we have plenty to find out yet both in the manufacture and use of nearly all metals,' or something like that, he said.

"I acted up to that; and always say to myself, We understand timber, and know how to treat it‌—‌and so I don't mind cutting it, as I know what I am about with it, although I represent unskilled more than skilled labour. Metals are different goods, and it wants skilled labour to tackle them nicely."

"There you are right."

"Yes, different goods. So, following the lead of the engineer, I leave the iron piles as delivered, as we have yet something to learn about the metal; and things that I don't know much about I avoid as much as possible, and consequently there are good grounds for getting in some short lengths as occasion offers, just to have as little to do with the material that you don't know much about and that is a bit mysterious in its behaviour. So I lessen the handling of it, and shorten the lengths, and so increase the odds against the chance of it not turning out as one thought it would; and I ought to be thanked for it, I consider. You look a bit puzzled. I tell you, you are getting thick, and want fresh pointing up to sharpen you. Listen to me. Now, suppose you buy a dozen eggs, and you think and know, on the average, at the price two are bad; you take one away and find it's bad, then you have 11 to 1 odds as against 12 to 2 or 6 to 1, and there can't be so much chance of another bad one turning up so quickly. If you don't understand my meaning I can't make you. There may or may not be a different application of explaining the egg business, but mine is what I mean you to take, and I don't intend to bother about any one else. You are younger than I thought you were, or your brain is all of a tangle."

"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."