"All right, then. You know what pure sand is?"
"You mean quite clean angular grains, and hard, too, like broken-up quartz rock?"
"Yes. Well, avoid it for screw piles, for then it is very difficult to screw them to any considerable depth. You can't displace the sand enough. It wedges and binds almost like rock."
"You mean it wedges up, and will not move?"
"That's near enough. Well, avoid clean, sharp, angular sand and shingle gravel as much as you can, and take screwing in dirty sand instead. I mean round-grained dirty sand with some clay upon it, or sandy gravel. What is wanted is something to separate the particles of the soil and act like grease so as to make them roll and not compress and become bound. You can't be too careful about this."
"I will put that down in my note-book so as not to forget it."
"To save bother, be sure to ascertain whether the work is in rough ground; and if you are abroad see that about five per cent. is allowed for breakages of all kinds, or the piles may run short.
"I have seen piles screwed into a kind of clay rock seam, the end of the pile was made like a saw, toothed, in fact, and stiffened from the bottom to the underside of the screw blade with ribs shaped to cut the ground as the pile was turned, and I doubt if they could have been screwed without. They seemed to steady the pile; but care must be taken when there is a projecting end and it is tapered to a less diameter than the pile shaft, as generally is the case, that the axis is true, or the pile will not screw vertically.
"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."