The steam, or pneumatic, hammer has basically replaced the drop hammer. This hammer (fig. 8-18) consists of a cylinder that contains a steam-driven or air-driven RAM. The ram consists of a PISTON equipped with a STRIKING HEAD. The hammer is rested on the butt or head of the pile for driving.

With a SINGLE-ACTION steam, or pneumatic, hammer, the power drive serves only to lift the ram; the downward blow of the ram results from the force of gravity only. In a DOUBLE-ACTION hammer, the ram is both lifted and driven downward by the power drive. A double-action hammer weighs from 5,000 to 14,000 pounds and a single-action hammer weighs about 10,000 pounds.

The blow of the double-action hammer is lighter, but more rapid than that of the single-action hammer. The double-action hammer generally drives lightweight or average weight piles into soils of average density; its rapid blows tend to keep the pile in motion and thereby

Figure 8-18. - Steam, or pneumatic, pile hammers.

reduces the resistance of inertia and friction. For heavy piles in hard or dense soil, however, the resistance from inertia and friction, together with the rapid, high-velocity blows of the double-action hammer, tend to damage the butt or head of the pile.

The single-action hammer generally drives heavy piles into hard or dense soil; its heavy ram, striking at lower velocity, allows more energy to be transferred into the motion of the pile, reducing impact and damage to the butt or head of the pile.

The diesel pile hammer shown in figure 8-19 is the most common hammer used in the NCF. This hammer

Figure 8-19. - Diesel pile hammer.