Scamping Tricks and Odd Knowledge Occasionally Practised upon Public Works. Newman John PhilipЧитать онлайн книгу.
the screw is right. Some slip will generally take place, therefore note at first how much it is, and consider whether it will not churn up the ground, for if the screw blade turns on nearly the same lines, the bite will be gradually destroyed, and then it may be very difficult to obtain a fresh hold of the ground, and the pile will most probably not screw vertically, and the screw blade is liable to be injured and may become worn away considerably.
"Piles can be screwed by means of men, horses, oxen, and machines. Man-power can be used anywhere, machines in most places, but horses and oxen only on land when the piles are screwed on a foreshore or between tides; of course all live power works at the end of the capstan bars. Once I had the option of screwing by horses or oxen, and chose oxen. Another man had horses. I made more profit than he did, and the piles screwed easier than his. I did not let him come near me when screwing; but if you have the choice, use oxen in preference to horses. Of course, I am speaking of those countries where they are used to the yoke."
"Why?"
"Because they do not stop at any time or back like horses, not even when the resistance of the pile becomes too great without more power, but continue to pull, and therefore backward motion of the pile is prevented. The oxen were yoked to two cross-arms attached to the end of the lever.
"There are several machines for screwing piles worked by steam or other power, and when the ground is not easy to penetrate, and a large number of piles have to be screwed, their cost will be saved in the regularity, quickness, and ease in screwing, and in stiff soil by machine power I have known them screwed at the rate of 4 to 6 inches per minute. Of course, it is a special machine, and not easily sold when not further wanted except at a much less price than has been paid for it, and that has to be considered. There are several different methods of screwing piles from a fixed stage; for instance, suppose a pile of sufficient length and with the screw attached is brought to the site by barge or otherwise, the capstan head is then fixed, and the pile swung vertically over the pitch by sling-chains fastened to temporary eye-bolts passing through the bolt-holes in the flanges or otherwise, and is moved either by a jib crane, a derrick upon a raft, or some such hoisting apparatus; it is lowered into its place between the guide-piles or steadied by sling-chains or other means, then the capstan bars are put into the sockets of the capstan head, which should be at equal distances apart, and the pile is ready for screwing after it is known that it is vertical.
"Where circumstances did not allow of room for capstan bars of sufficient length for men to walk round, I have screwed piles by ropes, but it will only do when the soil is easy to penetrate. The way we worked was something like this, we had two endless ropes passing round the ends of short capstan bars and round two double purchase crabs placed upon opposite sides of the pile, about six or eight men worked at each crab, four or five winding, and two or three hauling in the slack, one rope being passed through a sufficiently deep upper slot in the capstan bar end so that it did not slip, also one in the lower slot same end. Both the taut and the slack ends of the lower and upper ropes were attached each to its own crab. A man must be stationed at the end of the capstan bars to put the slack ends of the taut and slack ropes into the slots. One rope gives the capstan half a turn when it is taut, and then it falls out of its slot and is slack, and so with the other rope, but it is not easy to keep the two ends of the rope equally tight, and the power obtained is not great and may not be sufficient. It is a kind of makeshift."
"How do you fix the capstan head to the pile shaft?"
"In many different ways. Sometimes it is keyed on or clamped tightly to the top of the pile length by steel wedges, also placed upon the pile length and fixed by temporary bolts passing through the top flanges of the pile length, and also by fixing a temporary ribbed pile into the capstan head, and by connecting it with the permanent pile by bolts or slots, and so wedging is not wanted and it can be raised and lowered. Another way is, two of the internal sides of the pile at top are cast flat for a foot or so down into which the capstan head fits, and the inside diameter is lessened for an inch or two to prevent the capstan head slipping down, but it generally can't do that, even without the narrowing of the pile for that object.
"As the capstan is subject to great wear and tear and sudden strain, it should be strong, for if it breaks the work is stopped. Wrought-iron capstan heads are used, but cast-iron are perhaps better. Sometimes the capstan sockets are made to fit the ends of rails, if rails instead of timber are used for the capstan bars, but rail bars are rather heavy and are not nice to handle. The capstan socket is generally made to receive from eight to ten or more radial lever arms, and the lengths of the bars are anything from 5 to 40 feet, but the latter is rather too long as it is very difficult to control the strain and the bar usually bends and springs. The best working lengths are from about 8 to 20 feet, if the staging is so large. The best height for the capstan bars above the floor stage is from 3 feet 6 inches to 4 feet 6 inches. The capstan bars have to be lifted and again fixed as the pile penetrates, or a temporary pile of different length has to be fixed in it, unless the capstan head can be slipped up and down on a ribbed pile, hence you may want a platform you can raise or lower easily when required. If you use double-headed rails of the same section top and bottom for the bars, you can have them bent up a little near the capstan head, and when you start, the bent end is lowest, and then the bars can be reversed and so the work proceeds.
"Put the men, horses, or oxen in the most natural position for exerting their full strength or a loss of power will result, and therefore it will cost more to screw the piles.
"Should there be gantry staging on the site, the piles can be pitched from a traverser, or by means of an ordinary crab winch. They can also be screwed from the permanent structure by means of a projecting stage temporarily fixed to it, and of a length sufficient to reach the next span. The pile is run forward upon rollers and placed in the right position. Then it is screwed on the endless rope system previously described, or by passing the rope round a deep groove in the capstan bar ends, and the rope is held tightly by being placed round a smaller grooved pulley fixed about a hundred feet or so back towards the shore. The men haul the endless rope and so the screwing is done. The worst of screwing by endless ropes this way is that the pile very probably may be pulled over towards the source of power as it comes from one direction, therefore, support is required on the side of the pile to prevent this tendency. The circumference of the ropes used varied from 4½ to 6 inches, but I have used a 10 inch rope. Small ropes are generally relatively stronger than large ones. Stretch a rope well before using, as it yields, especially hemp ropes. The distance between the point at which the power is applied, and the ground should be as little as possible. In firm sand, when the power has been more than about 20 to 25 feet above the ground, it is often very difficult to screw piles by ordinary means to more than a small depth, as two places in the pile are wanted from which to apply the screwing force, and both as low down as convenient; but in screwing from a second stage care should be taken that the pile shaft is not bent, for it may then be strained like a girder and not merely as a column, also when much power to screw is required it is not easy to avoid pulling them out of the vertical. Always screw them steadily and prevent jerking. Any obstruction, such as a boulder, tends to displace a pile, and loosens the ground around it. In soft soils it may be possible to pull piles upright by pushing aside an obstruction if the pile is given a turn or two after meeting it and before pulling; but it must be carefully done, or the pile may be smashed, and it is only safe to pull it over in easy soils and when much force is not required."
"How much power is generally wanted for screwing?"
"That is not so easily answered as asked. It varies very much, and, of course, depends upon the kind of soil and the size and pitch of the screw. Ten men may be sufficient and a single stage, but two stages may be necessary should the pile be 50 or 60 feet in length, and then not far from one hundred men. An engineer told me the force generally required for piles of usual sizes under ordinary screwing circumstances varies from about 8 to 10 tons to as much as 50 tons, and usually from about 10 to 25 tons, and, of course, the number of men to screw in proportion.
"Ordinary piles and screws have gone down 21 feet in sand in eight hours, and by steam machinery in clay at the rate of 6 inches per minute, and also, to my loss only about 1 foot in a day—and then it is time to stop altogether, should many piles hold like that. To compare what has been done with what has to be done is misleading