Scamping Tricks and Odd Knowledge Occasionally Practised upon Public Works

by John Newman

"A 9 feet 6 inches screw blade has been used on a 7 feet in diameter cylinder, but that is the largest I have heard of, but then it only projected 2 feet 6 inches beyond the column. Five feet is usually about the largest, and is only used for very soft soils. When more than that size they are unwieldy and very liable to be broken, and if the screws are fixed to a shaft and have to be shipped they are awkward things, and the freight becomes expensive. For hard soil, and that which will not compress nicely, about 2 to 3 feet is large enough for the diameter of the screw, and 3 to 5 feet for soft soils. The pitch of the screw is generally from one-third to one-seventh of the outside diameter of the blade. It varies according to the hardness and softness of the ground and is steeper as it becomes harder. When the pitch is increased the effect of the power applied to screw it is reduced, therefore the steeper or greater the pitch the harder the screwing.

"Piles can be screwed with a small pitch when sufficient power cannot be obtained to make a steep-pitched screw penetrate. Piles with a single turn of the screw, it seems to me, are the best, although the double-threaded screw may be right in soft marshy ground; but the usefulness of a double thread is doubtful, for I believe it breaks up the ground for no good, although some state that the screw threads work in parallel lines, and that a double-threaded screw is steadier; for they say a single-threaded pile is always likely to turn on the outside edge of the blade, and that the double-threaded is not, as it has a lip on both sides.

"Generally the screw has rather more than one entire turn round the pile, and when it is below the ground each side of the blade steadies the other, for the turns range from one to about two. Sometimes the edge of the blade is notched like a saw; but it is a question whether the saw-edge blade will screw into ground that an ordinary blade will not, and until it is proved by experiment it can only be a matter of opinion; but there is one thing to consider, a saw-cut edge blade may to some extent wedge the soil between the teeth; still, I have used them, and they penetrated thin limestone, chalk, and compact gravel seams. Instead of double threads, double points are the thing, and all screw piles should have a point of some kind. For soft ground, a single gimlet, and a double for hard soils, and I have noticed what I call a double gimlet point is best for keeping a pile in the required position, as each point prevents the other departing from a correct line. By points I mean the ends are spread out about 3 to 4 inches on each side of the axis of pile like spiral cutters.

"Unless it is certain the ground is easy and uniform, a pile with a screw having one turn to two turns for bearing purposes, and two, three, or four solid inclined screw-threads projecting about three-quarters of an inch with two end spiral cutters as just named, is my desire, or in addition to the bearing blade a single-turn thread of about 3 to 4 inches projection and the same kind of point; then unless it will screw, none will. They are less trouble when cast in one piece with the pile; but not for transport or shipping, or foreign work generally, because to be able to detach the screws is an advantage in many ways, such as packing, defects, breakages, carriage, and I think the castings are better when the blade is not cast on the pile. It may also happen that a rocky bed is unexpectedly encountered, then the pile is useless with the screw, but might be fixed firmly in Portland cement without the blade in a hole made in the rock. At the top of the screw blade seat in which a pile has to be fixed there should be a wrought-iron ring about half to three-quarters of an inch in thickness, and not less than 2 inches in width, to relieve any strain on the casting. It may be put on hot, so as to cool sufficiently tight but not strain the casting. A firm and even bearing for the pile on the socket seat is important, and it should fit accurately.

"I have heard of screw piles in which the blade was made of two or more separate segments so as to obtain, it was supposed, equal pressure all round, and to ease screwing, but rather fancy they might be inclined to jam the ground, as they would be not unlike a lot of very large round saw teeth. They may be right, but it has to be proved they will screw where a plain blade will not, provided the latter pile has double cutter-points to steady it.

"Give me a screw blade not more than about 2 feet from the points, and not one with a blade 10 feet or so above the points and say from 5 to 6 feet in the ground, for then, should the screw work at all crooked and the pile be not exactly upright at the commencement of screwing, it is no easy task to get it to stand vertically upon applying the power, because such piles are generally long and slender, and shift about until the blade is screwed. They want careful and constant guidance. Of course, the idea of placing the screw a little way down is that when the ground bears as well at that place as at the point, and there is no scour, it is no use putting the bearing blade lower. That is right; but then it always occurs to me to ask what is the use of anything below the bearing level if the foundation be protected from scour, for a thin pile by itself has little lateral strength.

"Of course, you are bound to make out a pile requires a lot of screwing or you will be considered as making too much profit, but always take care to watch how the first pile screws, and measure the distance every few minutes. What the ground is can then be judged, and you will be able to think out things for 'extra' profit. It causes me a lot of consideration sometimes, but after a struggle I generally manage to think rightly for my pocket, and work it all serene. What a beautiful sharpener of one's brain 'extras' are!

"It is not always an experimental pile is screwed so as to judge of the distance the permanent piles should penetrate, and therefore a guess has to be made from the experience of screw piles under the same conditions of screwing and in the same soil. There is a good deal of chance about it, for although the soil may be of the same general character it often varies in hardness; and that is where the bother is, for it makes the 'extras' to be wrong way about for some time. What I do then is to work the oracle, and try to make out the screw blades will be broken or injured for certain if I am compelled to screw them as ordered, and I work on the proverb that equal support is not to be obtained at a uniform depth when the ground varies, which is true; and I state that the resistance is different and offer to screw on, but say am afraid the blade may be broken, and in that how-kind-I-am-to-consider-your-interests sort of way generally manage to obtain a bit 'extra,' or save something that would have been loss, and get the pile measured at once for a making-up length, and really without damaging any one, for if the ground is harder at one place than at another there is no occasion to go so deep, always provided scour is not to be feared. So I am pleased, and it does not hurt them.

"Now for a hint or two on screwing piles. I shall not refer to the columns above the ground, but to the bearing piles below, i.e., the part that has to be screwed into the ground. However, I will just say that upon the top of some of the columns the usual hinged shoes of bearing-blocks should be placed to receive the ends of the girders, and by that means the pressure on the columns will be on the centre of the pile, and allowance be made for expansion and contraction, and that is important.

"Fixed staging is far the best from which to screw piles, but the chances must be considered of its being swept away by floods in a river, or smashed by the sea, and on any exposed coast there may not be time to construct it during the working season, so as to give a sufficient number of days for screwing operations. When a fixed stage cannot be erected, or the work be done from the end of a finished pier, pontoons or rafts are then a makeshift, but care must be taken that they do not break from the moorings. A couple of pontoons well braced together will do with a space between them to screw the pile, but in a steady or shallow river, perhaps making a timber stage upon the shore and floating it out can be done if a centre pile is fixed on the bed of the river to be certain it is in the right position when grounded. The staging must be equally weighted to make it sink, and arrangements made so that it can be floated away at any time if necessary.