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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:
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:
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."