Under-reamed pile foundation is most suitable for seasonal moisture change type of condition.

Explanation:
Under-reamed pile foundation is a type of pile foundation in which the pile base is enlarged by forming a bulb or bell-shaped enlargement at the bottom. This enlargement increases the bearing capacity of the pile and improves its resistance against vertical and lateral loads.

Reasons for suitability:

1. Seasonal moisture change:
Under-reamed pile foundation is particularly suitable for areas where the moisture content in the soil varies significantly with seasonal changes. These variations can cause the soil to expand or contract, leading to differential settlement and potential damage to the structure. The under-reamed bulb acts as a counterbalance to the expansive or contractive forces exerted by the soil, reducing the risk of differential settlement.

2. Increased bearing capacity:
The bulb-shaped enlargement at the bottom of the pile significantly increases the bearing capacity of the pile. It provides a larger contact area with the soil, allowing the load to be distributed over a larger area. This is especially beneficial in soil conditions with lower bearing capacity, such as cohesive soils.

3. Resistance against lateral loads:
Under-reamed pile foundations also offer improved resistance against lateral loads. The bulb acts as a stabilizing element, increasing the lateral stiffness of the pile and reducing the deflection under horizontal forces. This is particularly important in areas prone to high wind or seismic activities.

4. Cost-effective:
Under-reamed pile foundations can be a cost-effective solution in areas with seasonal moisture change conditions. By preventing differential settlement and reducing the risk of structural damage, they can minimize the need for costly repairs and maintenance in the long run.

In conclusion, under-reamed pile foundations are most suitable for areas with seasonal moisture change conditions due to their ability to counteract differential settlement, increased bearing capacity, improved resistance against lateral loads, and cost-effectiveness.