Group efficiency of Pile

The group efficiency of a pile refers to the ratio of the load-carrying capacity of a pile group to the sum of the individual capacities of the piles in that group. It is used to assess how effectively the group of piles can support the structure compared to the combined capacity of individual piles working independently.

Mathematical Formula

Where:

• : Group efficiency of the pile group

• : Total load-carrying capacity of the pile group

• : Load-carrying capacity of a single pile

• : Number of piles in the group

Factors Affecting Pile Group Efficiency

1. Pile Spacing:

• Closely spaced piles may lead to interaction between the piles, reducing efficiency due to overlapping stress zones in the soil.

• Optimal spacing (typically 2.5–4 times the pile diameter) reduces interference.

2. Soil Type:

• In cohesive soils (like clay), efficiency may be less than 100% due to group interaction.

• In sandy soils, efficiency can sometimes exceed 100% because of increased confinement.

3. Pile Configuration:

• The arrangement (square, triangular, or rectangular) influences stress distribution and efficiency.

4. Pile Length:

• Short piles may exhibit more group interaction, while longer piles reduce interference.

5. Load Transfer Mechanism:

• The contribution of end bearing and skin friction impacts group efficiency.

6. Installation Effects:

• Methods like driving or boring piles can disturb surrounding soil, affecting the group’s performance.

Empirical Methods to Determine Efficiency

1. Efficiency by Converse-Labarre Formula:

Here, is the number of piles.

2. Efficiency by Feld’s Rule:

Where is the pile spacing and is the pile diameter.

Practical Implications

• Efficiency < 100%: Indicates interaction effects and reduced capacity.

• Efficiency = 100%: Indicates independent performance of piles.

• Efficiency > 100%: Typically occurs in sandy soils where the pile group benefits from increased confinement.

Understanding pile group efficiency is essential for optimizing foundation design and ensuring the stability and cost-effectiveness of the structure.