Efficiency of pile group is always: Correct answer is option 'D'
Efficiency of a pile group is defined as the ratio of the ultimate load-carrying capacity of the pile group to the sum of the ultimate load-carrying capacities of the individual piles in the group. The efficiency can be calculated using the formula:

Efficiency = (Ultimate load capacity of pile group) / (Sum of ultimate load capacities of individual piles)

The correct answer to this question is option 'D', which states that the efficiency of a pile group is always correct. This means that the efficiency of a pile group is always equal to or greater than 1.

Explanation:
Pile groups are commonly used in foundation design to distribute the load from a structure to the underlying soil. The load-carrying capacity of a pile group is influenced by various factors such as pile spacing, pile arrangement, pile diameter, soil properties, and load distribution.

When piles are grouped together, they interact with each other and with the surrounding soil. This interaction can lead to a higher load-carrying capacity compared to individual piles. As a result, the efficiency of a pile group is generally greater than 1.

Factors influencing pile group efficiency:
1. Pile spacing: The spacing between piles in a group affects their interaction and load-sharing behavior. Closer pile spacing generally leads to higher efficiency as the piles can distribute the load more effectively.

2. Pile arrangement: The arrangement of piles in a group, such as a square, rectangular, or circular pattern, can influence the efficiency. Different arrangements distribute the load differently among the piles and can result in varying efficiencies.

3. Pile diameter: The diameter of the piles in a group also plays a role in determining the efficiency. Larger diameter piles generally have higher load-carrying capacities and can contribute more to the overall efficiency of the group.

4. Soil properties: The properties of the underlying soil, such as its strength and stiffness, affect the load transfer mechanism and the behavior of the pile group. Soils with higher bearing capacity can enhance the efficiency of the group.

Conclusion:
In summary, the efficiency of a pile group is always equal to or greater than 1. This is because the interaction between the piles and the soil in a group can increase the load-carrying capacity. The efficiency is influenced by factors such as pile spacing, arrangement, diameter, and soil properties. Designers consider these factors to optimize the efficiency of pile groups in foundation design.