Scamping Tricks and Odd Knowledge Occasionally Practised upon Public Works. Newman John PhilipЧитать онлайн книгу.
they are otherwise the power required, cost, and rate of screwing will all be different. I have screwed a 6-inch pile with a 2-feet one-turn screw into 20 feet of ordinary sand with an applied power of 30 tons as calculated by an engineer from measurements and the force of men applied at the capstan bars. There is the surface friction on the screw blade and the pile shaft in the ground, the cutting of the earth by the edge of the blade and the points, and the loss of power from torsion and that applied compared with the effective force, slip, friction, &c., to consider; and the relative surface of the blades, width, and thickness of the cutting edge and the pitch—for a steep pitch means harder screwing. By using capstan bars and men at them, instead of ropes at the ends of the arms worked by crabs, you will find about one-fifth more power is gained, or rather is not lost. Of course, place the men as near to the end of the capstan bar as convenient for work. My lecture is finished, and I am parched."
CHAPTER IV.
IRON PILES.
Arrangement—Driving—Sinking by Water-jet.
"Tell me what you have learned about iron pile fixing, same as you have promised me you will about timber piles."
"Very well. Here goes, then; first a word as to iron piles generally.
"Although a group of piles when properly strutted, tied, and braced have plenty of stiffness, if you have to deal with them singly they are never stiff, but they can be made steadier when getting them down by having two large pieces of wood with a half hole in them, something like the shape of the old village stocks, and by putting or lowering it at low water until it is bedded in the ground. It must be weighted though, so as to prevent it floating. It acts like a waling, and is useful when the ground is treacherous, and provided it is level.
"From watching the behaviour of piles when doing repairs and at other times, I think it wants a lot of careful arrangement to be sure the load is acting equally on the whole group, or, as may be intended, on say a few piles, and straight down the centre of each pile, for it makes a lot of difference to the strain on them, and it is not easy to make them all take the load at once as wished. It wants a good deal of attention, and the piles are not unlike a pair of horses that are not matched and don't work together properly—kind of now me, now you business. Before finishing reference to driving and screwing, let me say all the parts should be properly fitted together at the works and numbered so that the putting up on the ground is easier and in order to be certain all the bolt-holes agree; and it is well to have the lengths interchangeable and all the same, except the making-up pieces, and all bolt-holes as well as the flanges should fit in every respect.
"When columns rest on a masonry, brickwork, or concrete base the piles ought to have a ring or base-plate right round them to hold them tightly together. It lessens the pier being shaken, and saves the side pushing of the holding-down bolts. I heard an engineer say the weight of the pier above their ends should be not less than about four times any force that might tend to lift them. The anchor-plates should be well bedded upon a solid mass or the strain upon the pier may go in one direction, and that the one not wanted. Don't be afraid of bracing and strutting piles, the more of it the better. I don't think much of a single turn of a screw blade a few feet below the ground for taking a load, although some good for steadying purposes generally, because the bed may become scoured out below the blade and then the screw is no use. Therefore the depth of possible scour ought to be positively known before relying upon the blade for permanent support. A lot can be said as to the grouping of piles, whether in triangles or in rows. In a triangle, although the load upon the foundations is spread over a larger area, it does not give as much lateral strength as when the piles are placed in one row, and taking everything into consideration I think if I had six piles to put down I should not place one at the top of a triangle, two lower down, and three at the base, but have two parallel rows of three piles; besides it lessens the length of the struts and the bracing, and that is something, but, of course, each case requires to be treated in a special way, and I have noticed when doing repairs that if there are six piles fixed thus, ∴ in a triangle, the wind and other force acts principally upon the bracing between the parallel rows, and the pile at the point does not do much towards keeping the others in the right place; anyhow the bracing there does not seem to hold as tightly as it does between the parallel rows, and I have had to watch groups of them in storms, and when the sea has been high, and that is my opinion."
"Now, as to fixing iron piles."
"When the ends have to be placed in rock, which has sometimes to be done in shore pieces, 'jumping' the holes in more than about 2 feet of water is to be avoided, for if the water is not still the holes become filled with sand and drift, and you must not take the jumper out but keep on continuously making the hole. It is ticklish business, because sometimes the rock grinds the jumper, and then the wings and point wear away. Occasionally they have to be worked inside a cylinder by ropes, rods, and gearing fixed in it, the cylinder being movable and held from the end of the part of the pier that is finished, but where the water is deep the ends must be put in the rock in Portland cement by divers.
"I have driven a good many iron piles with a ram, but you have to be careful, no matter whether the soil is sand, gravel, clay, or silt. I like a copper ring on the head of the iron pile and a good long timber 'dolly,' not less than 4 or 5 feet in length, and then the ram does not burst the top. When the ground is hard the best way is to make a hole by jumpers of about 3 inches less diameter than the pile to be fixed, and in chalk soil it is doubtful whether they will go down right unless that is done; perhaps they won't drive at all, or a lot of them will be broken. I have used a ram weighing from 1 to 1½ ton for an 8 to a 10 inch pile and about a 3-feet fall, and never more than 4 feet, unless you want to deal with some old metal merchant that will give a good price for the scrap, and it does not matter how many get broken, or it is a positive advantage to break a certain quantity out of every lot, so as to have a big price for such difficult driving, and get 'extras' that way."
"I understand, no breakages deducted."
"That's it. I have driven them at the rate of fully 6 inches a minute for a few feet. They often rebound, so I had a boy with a lever, the end of it being clinched to the pile. Directly the ram fell, he gave the pile from quarter to half a turn for the first 4 or 5 feet of driving, and they scarcely rebounded at all; and he earned his wages, for I considered fully one pile extra was got down out of about every ten by the turning movement. The points require to be regulated according to the ground. From 1½ to twice the diameter or width for the length of the point is about right, but if it is made too sharp it may break. Iron piles that have to be driven are seldom more than 12 inches in width, and the thickness of the metal is generally from one-ninth to one-twelfth of the diameter. I heard an engineer say, I think it was Mr. Cubitt, experiments showed that a T-shaped cast-iron pile about 30 feet in length, should have the top of the T two and a quarter times the length of the upright part, and the thickness a twelfth of the top. Of course, the length of the pile must be considered. I doubt if you can get equally sound metal throughout when the thickness is much more than 2½ inches. From ¾ to 1½ inch is best, and piles I have broken up always seemed more even throughout about those thicknesses; but there, I suppose it is all a question of care in casting and proper machinery.
"One thing, don't drive any piles from a floating stage on the sea if you can help it, it will make you pay for the privilege; besides I have known some places where the sea was always so disturbed it could not be done, even if the moorings were as tight as you dare make them. Driven iron piles are not much seen now, and Portland cement concrete seems the fashion, and no doubt it is better. Still, iron piles can be driven in deep water without much trouble from it, and one might combine the two nicely—the iron to act as a shield to the concrete while depositing it, and give it time to set without disturbance and preserve the face."
"Have you sunk any disc piles?"
"Yes, they are all right for fine sand and silt, but you must be careful the discs are the same in form and dimensions upon all sides, or a pile will almost certainly tilt and sink crookedly. I was busy on the Lancashire