The Name of the Game -- Heavier Ram - Shorter Stroke

Concrete piles cracking while driving in either soft silt or very hard ground? The solution is the same for both problems: Use a thicker cushion block and a hammer with a heavier ram and shorter stroke. The solution and others to related problems, comes from Prof. T. J. Hirsch of Texas A&M's Texas Transportation Institute, who led a research project in the problems of cracking and spalling concrete piles. TTI is the official research agency for the state highway department. His reasoning is this: The compression wave sent down a pile by the hammer blow. rebounds back up the pile when it reaches the end - as a tension wave if the pile tip is in a soft medium that offers little resistance, and as a compression wave if the pile tip is lodged against something very firm. If the pile is long and the stress waves short, the return wave will meet the compression wave of the next hammer blow, and the two can add up to a resultant force that is destructive. A heavier ram has a longer impact time on the head of the pile. and thus produces a longer compression wave. Thicker cushioning also stretches out the impact time. Professor Hirsch sums up the fundamentals of good pile design and pile driving as follows:

1. Use adequate cushioning materials between the pile driver's steel helmet or cap and the concrete pile head. For piles under 50 ft., only 3 or 4 in. of wood cushioning material (such a green oak, gum. pine, or fir wood plywood) may be adequate if there is reasonably good point soil resistance. More wood cushioning, 6 to 8 in. or more, may be required when driving longer piles in very soft soil. The wood cushioning material should be placed on top of the pile with the grain horizontal and inspected to see that it is in good condition. When it begins to become highly compressed, charred, or burned, it should be replaced. Some specifications require a new cushion on every pile. If driving is extremely hard, the cushion may have to be replaced several times during driving of a single pile. Adequate cushioning is usually a very economical way to control driving stresses.
2. Driving stresses can be reduced by using a heavy ram with a low impact velocity (short stroke) to obtain the desired driving energy rather than a light ram with a high impact velocity (large stroke). Driving stresses are proportional to the ram impact velocity.
3. Reduce the ram velocity or stroke during early driving when light soil resistance is encountered. Anticipate soft driving, or at the first sign of easy driving reduce the ram velocity or stroke to avoid critical tensile stresses. This is very effective when driving long piles through very soft soil.
4. If predrilling or jetting is permitted in placing the piles, be sure that the pile point is well seated with reasonable resistance at the point before full driving energy is used. Driving and jetting should not be done simultaneously.
5. Be sure that the pile driving helmet or cap fits loosely around pile top so that the pile may rotate slightly without binding within the driving head to prevent torsional stress. The helmet should be centered on pile head so eccentric load will not be applied.
6. The pile should be straight and not cambered because of uneven prestress or poor concrete placement during casting. High flexural stresses may result during driving of a crooked pile.
7. The top of the pile must be square or perpendicular to the longitudinal axis of the pile. Eccentricity concentrates stress.
8. Cut the ends of prestressing or reinforcing steel flush with the end of the pile head to prevent their direct loading by the ram stroke.
9. Use adequate spiral rdnforcing at the pile head and tip to reduce the tendency of the pile to split or spall.
10. Use adequate amount of prestress in prestressed piles or reinforcement in ordinary precast piles to resist reflected tensile stresses.
11. Chamfer the top and bottom edges and corners of the pile to reduce the tendency of the concrete to spall.

Originally appeared in Hirsch, T.J. (1966) Construction Methods and Equipment, New York: McGraw-Hill.

The whole subject of "heavy mass-low striking velocity" is one that Vulcan dealt with throughout the entire history of the air/steam hammer line. The debate has survived the company: one competitor took space in his first online newsletter to make the following commentary on this very tip:

That was written in 1966. At the time, the air hammer was a popular tool. Then the diesel hammer took over the world, and the mighty air hammer manufacturers are a tiny fraction of what they were.

While there's no doubt diesel hammers have been succesful, the physics that Dr. Hirsch describes are still correct, but must be understood in proper context.

When steam hammers were first introduced, one of the major changes that took place was the relationship between ram weight and hammer energy. Drop hammers were inherently long-stroke hammers, and the pile top damage to the wood piles could be considerable. Shortening the stroke and increasing the ram weight reduced the peak force in the pile, and thus the driving stresses in the pile. This became more critical when concrete piles became popular, as controlling the tension cracking phenomenon Hirsch describes is critical.

Diesel hammers, to some extent, reversed the trend by reducing the ram weight and lengthening the stroke. This made for a lighter hammer, and one that didn't require the external power source. So how to control the pile stresses? One way is to decrease the cushion stiffness, which decreases the natural frequency and increases the impact period. To some extent, the effect of shortening the stroke can be replicated in this way, but not entirely.

It's also noteworthy that steel piling are more resistant to driving stresses. So high velocity impact is not as critical with these piles as it is with concrete piles. Vulcan recognised this fact in the development of the 560, first produced around the time this tip was issued.

However, for the driving of concrete piles, especially larger size and capacity piles, it's still best to use a hammer with a low impact velocity and a heavy ram to limit the pile stresses, be that hammer an air/steam or hydraulic hammer.

For a more contemporary treatment of the subject, click here for the 2008 paper "The effect of ram mass on pile stresses and pile penetration."