4.3. Pile Hammer Mechanics and Selection

So, everyone knows particularly the precast piles or likely to be subjected to more amount of stress while driving it. They are subjected to more amount of handling stresses as well as when you drive the pile into the ground they are subjected to more amount of driving stresses. That is why all the stresses should be taken into account when you design your pile.

Precast piles face significant stresses during driving, which includes both handling and driving stresses. When transferring these stresses into the design, engineers must consider that the highest stress occurs during driving, not the service life.

Imagine a raw egg being dropped onto a hard surface. The moment of impact causes the most stress compared to it simply sitting on a table. Similarly, the driving of piles needs careful planning to avoid 'shattering' them, much like the egg.

So, how to control the driving stress? The commonly adopted method is, we have to introduce some cushioning material between the pile and the pile hammer. A commonly used cushion is wood timber cushion so you have to choose a sufficient thickness depending upon the length of the pile needed.

To manage the high stresses during pile driving, cushioning materials are placed between the pile and the hammer. A typical choice is wood timber, chosen for its ability to absorb impact. The thickness of the cushion should not be less than 10 centimeters to ensure sufficient protection for the pile.

Think about using a pillow to soften a fall—if you fall onto a hard surface without anything, it hurts. Using a pillow (the cushion) makes the impact much gentler. In construction, we use the cushion to safeguard concrete piles from high impact.

Another important guideline to control the driving stress is that the driving stress will be very high when the impact velocity is high. The blow energy is the product of W into H, where W is the weight of the hammer and H is the height of fall.

Driving stress increases with the velocity of the hammer during impact. Engineers can influence this by either increasing the weight of the hammer (W) or controlling the height from which it falls (H). It's more beneficial to increase the weight rather than the height to minimize stress on the pile.

Consider a dart thrown at a dartboard. If you throw it hard (high impact velocity), it may stick well but also risk damaging the board (like high stresses on a pile). If you throw a heavier dart but gently, it will still hit the mark without causing much damage.

Now let us see how to determine the safe load on the piles. As piles are subject to more stress during driving, we need to determine what is the safe load allowable on the pile that is very important.

It's crucial to calculate the safe load on piles to prevent failure during driving. Engineers often use established formulas like the Engineering News Formula to derive safe weight limits and driving energy required to embed the piles adequately into the ground.

Just as a bridge must be able to hold a certain weight to ensure safety, piles must also have calculated safe limits. If too many cars (weight) drive over the bridge, it could collapse. Similarly, overload on piles can lead to structural failure.

So, we have come to the end of this lecture, let me now summarize what we have discussed so far. We have discussed that pile hammer selection should depend on the type, size, and weight of the pile.

Choosing the right hammer is critical. A general rule of thumb is that the hammer's weight should be at least equal to the weight of the pile. However, for heavier concrete piles, hammers weighing one-third of the pile's weight may suffice. Additionally, soil type and project conditions must be factored into hammer selection.

Consider finding the right tool for your home project. If you're trying to drive a nail into a wall, using a hammer that’s too light won’t work effectively, but using a heavy sledgehammer could damage the wall. The same principle applies to selecting pile hammers for effective pile driving.

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