Piles are often times combined into groups, by connecting them with pile caps at their top. When grouped, piles act differently than individually. This is called group action. The piles in groups must not be too close to each other, because each pile takes support from its surrounding soil. If the piles are too close, the same soil area must now support more than one pile, which will decreases soil support dedicated to each pile. Therefore, there must be a minimum spacing between piles in pile groups. This spacing amount is dependent on the soil type, and the pile diameter. The larger the pile diameter, the more soil area it will require around it to support itself, which means greater space is required between piles.
In figure 6a, on the left you can see the effect zone of one pile to the surrounding soil, in terms of stress, which is called the stress bulb. On the right, you see two piles near each other, with overlapping stress bulbs. Overlapping means, we do not have enough space between piles. These two bulbs should not overlap, so that all areas of soil should only contribute to only one pile and not more.
In figure 6b, you can see the stress bulb of a pile group. As seen here, the carrying capacity of a pile group can be more than the sum of the individual carrying capacities of single piles. In this particular case, the spacing between piles could be increased to avoid intersection zones, but if that is done too much, the piles will be too far apart and the group action will disappear as below:
In the next post of this series, we will discuss “Types of Piles”