To address the question of which types of piles are typically used in areas with expansive soils, let's break down the different options provided and understand the concepts:

• Friction Piles: These piles transfer load to the soil primarily through skin friction along the length of the pile. They are generally used where top layers of soil are weak, and require deeper foundation. However, they are not specifically designed for expansive soils.
• Under-reamed Piles: These piles are specially designed for areas with expansive soils. They have an enlarged base or bulb created at the bottom of the pile, which provides stability against soil movement due to swelling and shrinkage. The enlarged base helps anchor the pile beneath the active zone of expansive soil.
• Screw Piles: These are installed by screwing into the ground, providing a helix-like bearing. They are used for rapid installation and minimal disturbance but are not particularly used for expansive soils.
• Sheet Piles: These piles are used to retain soil and water. They are typically used for retaining walls and waterfront structures rather than dealing with challenges posed by expansive soils.

The correct type of pile for expansive soils is Under-reamed Piles. This choice is due to their enlarged base which provides additional support and minimizes the effects of soil movement.

Conclusion: Among the given options, Under-reamed piles are the most appropriate choice for expansive soils due to their design, which accommodates the soil's tendency to swell and shrink. This makes option Under-reamed piles the correct answer.