Scamping Tricks and Odd Knowledge Occasionally Practised upon Public Works. Newman John PhilipЧитать онлайн книгу.
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.
"Piles can also be fixed in a medium depth of water by ordinary gantries, but if they are in the sea the road on the staging should be kept from 12 to 15 feet above high water on an open sea coast or the inclined struts and ties and rail tops as well are very likely to be destroyed, and it is also advisable to construct the flooring of the stage so that it can be easily taken away in case of storms. The stage piles also require to be well stiffened by struts, transoms, diagonals, and capping sills. I have screwed piles from a floor that has been suspended from staging by chains and ropes to the height wanted, and when lowered it was fixed temporarily and as many guides as possible were made for the piles. Perhaps as good a way as any is to fix, say four guide piles having a space between them a shade larger than the outside dimensions of the screw blade and braced to the rest of the stage, and after the screw is in position and ready for screwing in the ground, place, say a couple of frames, one at top and one as low as possible between the guide piles, about an eighth of an inch more than the outside dimension of the pile shaft, for then the pile is kept in its right position as it is screwed. The guide frames should be at about every 10 or 15 feet of the height above the ground, and at some point between the capstan level and the ground. Should it be a tidal river, fix guide booms if a properly made iron frame cannot be placed, and remember the more a pile is guided the easier it is to screw, and especially so at the start.
"The size and strength of the staging must be regulated according to the power available for screwing the piles, but the length of the lever arms and the capstan bars require a space in which to revolve, from, say, 35 to 60 feet square. No timber stage is immovable, for the wood yields. It is well to have two floors in a stage if it does not cost too much, and there is plenty of tackle and a lot of screwing to do; say, one fixed above high-water level and the other about half tide in order to obtain double power, and sufficient power to screw the piles cannot sometimes be otherwise secured. A word about floating stages. With them it is not easy to make a pile screw vertically unless the ground is uniform, and should a pile meet a boulder it will most probably be forced out of position. According to the power required—which really means the nature of the ground, as the harder the soil the harder the screwing—the form of the pile and the depth to which it has to be screwed, so must be the size and strength of the raft, pontoon, or lighter, and the moorings must hold it tightly. In some places a screw cannot be fixed from a floating stage, for the water may nearly always be too disturbed, and the pontoons may sway too much, for in all cases men, horses, or bullocks must have a steady footing, and screwing machinery also requires a firm base. Unless the moorings are very secure the platform will be unsteady. Its level should be as little above the water as practicable for work, so as to keep the point of resistance and that at which the screwing power is applied as near together as possible, and the lower the pontoon the less it rolls. It does not matter much what craft is used so long as it is broad and steady and not high, as a platform or deck must be made upon it in any case. To do any good with floating stages the power required should be little, and the ground soft and uniform, for sufficient force to screw may not be obtainable from a floating body, and in hard soil it may only be possible to screw piles a little way down and not to a sufficient depth for the load they will have to bear.
"Of course, vertical pile screwing is the easiest, and to try to screw them at a greater angle than 63°, or about ½ to 1, is unadvisable, and may not succeed, and even if they do it is too steep to be nice. 1 in 10 to 1 in 20 for raking piles is enough; for if they have to carry girder ends, the more the batter the greater the strain on the pile, and the same during screwing.
"Sometimes in loose soil it is difficult to start screwing, and then a good plan is to cast some clay or solid earth round the pitch; it steadies the pile and will probably make it bite properly, or a heavy weight placed on the pile may make it catch hold of the ground; if not, a few blows from a ram may do it. As a hollow pile penetrates, the core requires to be removed, so as to help it to descend. If it is not large enough for boys to get inside, scoops and tackle can be used. Water forced down makes sand boil round the screw blade, and when the pile is empty the unbalanced head of water outside relieves the pile and the screw blade from some of the surface friction. If water pressure cannot be used, the water inside the pile should be removed either by pumps or buckets so as to help to loosen the ground.
"Piles do not generally screw to the full pitch, but when a pile descends more than the pitch at the last turn, it can be considered the weight of the pile is too great for the ground. The slip usually increases according to the yielding or plastic nature of the soil, and the depth to which the pile is screwed. When water reaches such soils the slip is increased, but not perceptibly in sand and loose grained soils. Suppose the full pitch is 9 inches. The slip may be anything from about 1 inch to as much as 4 inches. By watching the way in which the screw penetrates, and whether it descends about the same distance each turn, or regularly decreases, it can be judged whether the