2.2. Guidelines for High Number of Blows in Non-Cohesive Sand

Now how to make the pile hammer selection with respect to your soil type, as I told you we can classify the soil into different categories. So, based upon there are some standard tests to categorize the soil into different categories. Like once this test is your standard penetration test where you find what is the number of blows needed for the standard penetration.

This chunk discusses how soil types influence pile hammer selection. The Standard Penetration Test (SPT) is a crucial method used to assess soil characteristics by measuring the number of blows required to drive a sampler into the soil. The results help classify the soil into different categories, which are crucial for determining the correct hammer to use for driving piles.

Think of this process like testing the hardness of different types of fruit with a hammer. Just as you might need to hit a soft peach with less force than a hard apple to penetrate, soil testing helps determine how much force is needed for piles based on the soil's hardness.

Basically, you classified into cohesive soil and non-cohesive soil. So, the first three refers to non-cohesive soil and this is a cohesive soil. So, they are further classified into three different categories depending upon a number of blows. So, for 0 to 10 blows, it is very loose to lose sand, for 10 to 30 it is medium non-cohesive sand, for 30 to 50 plus blows it is dense non-cohesive sand as a number of blows increase the densification increases.

Here we see how non-cohesive soils are divided based on the number of blows recorded during the SPT. Very loose sand (0-10 blows), medium non-cohesive sand (10-30 blows), and dense non-cohesive sand (30-50+ blows) indicate increasing density and stability of the soil. As the blow count increases, it suggests that the sand is denser and more compact, which affects how piles should be driven into it.

Picture packing a suitcase. At first, it’s easy to add clothes (very loose sand), but as more clothes are packed in without any organization, it becomes more compact and stuffed (medium to dense sand). The tighter it is packed, the more force you need to add more in (more blows needed). This is similar to how denser soil requires more energy to penetrate with a hammer.

So, one basic guideline we need to keep in mind is for very tough soil conditions, it is preferable always go for heavier hammer, which is nothing but your drop hammer or single acting hammer that is always heavier, single acting hammer for tough soil condition and for the heavier pile. So, for lighter conditions, it is preferable to go for double acting hammer. To the maximum possible try to avoid double acting hammer for the concrete piles.

This segment highlights the selection of hammer type based on soil conditions. Heavier hammers like drop or single acting types are preferred in tough soil conditions to ensure adequate energy transfer for effective pile driving. Conversely, in lighter soil conditions, double acting hammers can be beneficial but should be avoided for concrete piles due to their lower impact energy.

Imagine trying to drive a nail into a piece of hard wood and a soft foam. For the hard wood, you would use a heavy hammer (like a sledgehammer), but for the soft foam, a lighter tapping hammer would suffice. Similarly, the choice of pile hammer depends on the soil density – tougher soil needs a heavier hammer.

So, with this basic guideline let us discuss what are the guidelines available from U.S army corps of engineers. So, for very loose to loose sand, it is given that for concrete pile or the wood pile you can go for double acting. So, I mentioned that for concrete pile it is preferable to go for single acting hammer. But if the soil condition is very loose, in that case it is exceptional, you can also go for double acting hammer even if it is concrete pile.

Detailed guidelines indicate appropriate hammer types based on specific soil conditions. For very loose or loose sand, a double acting hammer may be accepted for wooden or concrete piles, though single acting hammers are generally preferred. This rule highlights that in particular cases of extremely loose soil, deviations from standard practices may still be suitable.

Think of a flexible rule – like using a specific type of shoe for running but deciding to wear something lighter for a track covered in soft grass rather than pavement, which makes a slipper feel more adaptable. Adjustments to hammer choice can make it more effective for unique soil scenarios.

Now as a densification of the sand increases, though the sand is non-cohesive but the densification increases, indicated that increase in number of blows 30 to 50 plus. Now you can see what is the change in the guideline, for wood or the concrete pile you can go for single acting hammer as we discussed earlier but for very high number of blows say 50 plus.

As the density of non-cohesive sand increases (as shown by the increased blow count), the selection of the hammer changes. For sand densities represented by blow counts over 30 to 50+, a single acting hammer is recommended for wood or concrete piles. The reasoning is that greater energy is required to penetrate denser material, thus a hammer with adequate force is essential.

Consider hitting a tennis ball against a wall. The bouncier the ball, the more energy you need to hit it for it to reach the wall with force. Similarly, for denser sand, you need to adapt your hammer choice to ensure effective penetration.

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