3.2. Guidelines for Hammer 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. So, highest stress across in the pile mainly during it is driving than when compare to during it is service life.

Piles experience different types of stress when being installed. During the driving phase, which is the phase when piles are hammered into the ground, they endure more stress compared to their service life, which is the time they support loads after being installed. This heightened stress during driving comes from handling and driving impacts, necessitating careful consideration during pile design to prevent damage.

Imagine trying to insert a thin metal rod into solid ground. If you pound it too forcefully without care, the rod might bend or break. Similarly, when driving piles, we need to manage the force applied to avoid structural failure.

So, how to control the driving stress? So, the commonly adopted method is, we have to introduce some cushioning material between the pile and the bile hammer so that is a basic thing we can do it. Particularly for the concrete piles as you know, concrete piles are weak in tension and they are more brittle.

To mitigate driving stresses on piles, especially concrete ones, a cushioning material is placed between the pile and the hammer. This cushioning helps absorb and distribute the force from the hammer, preventing the pile from shattering due to the impact. Wood timber is a commonly used cushioning material, and it must be of adequate thickness (at least 10 cm) to be effective.

Think of placing a soft pillow under a heavy object dropped from a height. The pillow absorbs some impact, reducing the chances of damage. Similarly, cushioning between the pile and hammer acts as a protective layer to safeguard against high stress.

So, this is a common setup which you can see to control the driving stress, so why we can see this is your pile and this is your hammer. So, you have two cushions, one is your pile cushion, other one is your hammer cushion. And there is also a H shaped helmet which helps you to distribute the load uniformly over the head of the pile.

A typical arrangement to control driving stresses includes using two types of cushions: a pile cushion and a hammer cushion. Additionally, an H-shaped helmet is utilized to ensure the load from the hammer is evenly distributed across the pile's head, minimizing the chance of stress concentration which could lead to damage.

Imagine stacking books on a table. If you place a heavy book directly on a single thin book, the thin one may buckle. However, if you spread the weight across all books evenly, they're less likely to be damaged. The H-shaped helmet ensures the pile can withstand the impact without failing.

Another important guideline which you should keep in mind to control the driving stress is, the driving stress is will be very high when the impact velocity is high, that depends upon your height of fall.

Driving stress increases with the velocity of impact, which corresponds to how high the hammer is dropped. It is advisable to increase the hammer’s weight rather than raise the height from which it's dropped. A heavier hammer at a lower drop height typically results in a lower impact velocity and reduced stress on the pile.

Consider a basketball being dropped. If you drop it from a very high point, it bounces higher and with more force, which can be problematic for a surface below. Conversely, a heavier basketball dropped from a lower height creates less impact, explaining why we prefer heavier hammers at shorter strokes.

So, if you want to increase the blow energy of your pile, it is preferable to increase the weight of hammer but do not increase the height of fall.

To effectively drive piles and increase blow energy (the energy delivered by the hammer), it's best to utilize a heavier hammer while keeping the drop height low. This strategy minimizes the risk of damage to the pile, especially in cases where the material is brittle, like concrete.

Think of a bowling ball being released down a ramp. A heavier ball, rolling a shorter distance, can knock down pins just as effectively and with less risk of misalignment compared to a lighter ball dropped from a great height.

So, now let us see with how to determine the safe load on the piles? As a piles are likely to be subjected to more amount of stress during driving.

Determining the safe load on piles involves understanding the relationship between hammer energy and soil resistance. A popular method for calculating this is the Engineering News Formula, which relates the weight of the hammer and the height of fall to the allowable safe load on the pile. Energy from the hammer needs to counteract the resistance from the soil for the pile to be driven effectively.

Think of a scenario where you're trying to push a stick through sandy soil. You need to push hard enough to overcome the sand's resistance, which changes depending on how deep you want to go. The formula helps calculate how hard to push (the energy) to move the stick effectively into the sand.

So, now let us see what are all the basic factors which governs the pile hammer selection.

Choosing the right hammer for pile driving is influenced by several factors. These include the type and weight of the pile, the number of piles being driven, desired productivity, soil condition, available equipment, and project constraints such as noise restrictions. Each of these elements determining the suitability of a hammer is critical for efficient pile installation.

Imagine planning to lift a heavy box. You’d consider the weight of the box, the strength of your lifting tool, and the space around you. In the same way, selecting a hammer requires careful consideration of various factors to ensure effective and safe driving of the piles.

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