This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
One practical trouble in driving piles, especially those made of soft wood, is that the end of the pile will become crushed or broomed by the action of the heavy hammer. Unless this crushed material is trimmed off the head of the pile, the effect of the hammer is largely lost in striking this cushioned head. This crushed portion of the top of a pile should always be cut off just before the test blows are made to determine the resistance of the pile, since the resistance of a pile indicated by blows upon it, if its end is broomed, will apparently be far greater than the actual resistance of the pile.
Another advantage of the steam pile-driver is that it does not produce such an amount of brooming as is caused by the ordinary pile-driver. Whenever the hammer bounces off the head of the pile, it shows either that the fall is too great or that the pile has already been driven to its limit. Whenever the pile refuses to penetrate appreciably for each blow, it is useless to drive it any further, since added blows can only have the effect of crushing the pile and rendering it useless. It has frequently been discovered that piles which have been hammered after they have been driven to their limit, have become broken and crushed, perhaps several feet underground. In such cases, their supporting power is very much reduced.
Usually about two inches of the head is chamfered off to prevent this bruising and splitting in driving the pile. A steel band 2 to 3 inches wide and 1/2 to 1 inch thick, is often hooped over the head of the pile to assist in keeping it from splitting. These devices have led to the use of a cast-iron cap for the protection of the head of the pile. The cap is made with two tapering recesses, one to fit on the chamfered head of the pile, and in the other is placed a piece of hardwood on which the hammer falls. The cap preserves the head of the pile.