"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.