A caisson foundation, also called a drilled shaft or drilled pier, is a large-diameter hole drilled deep into the ground and filled with reinforced concrete to transfer heavy structural loads to competent soil or bedrock. It functions similarly to a pile but is typically larger in diameter and cast in place rather than driven or screwed.
The drilling process typically uses a temporary steel casing to keep the shaft from collapsing before the concrete is placed, which is later withdrawn or left in place depending on soil conditions.
What Does a Caisson Foundation Involve?
- A large-diameter drilled shaft
- Temporary casing to keep the hole open during drilling
- Reinforcing steel cage placed in the shaft
- Concrete poured to fill the shaft
Why Are Caisson Foundations Important?
Because caissons are cast in place at a large diameter, they can carry substantially heavier loads per support point than typical driven piles, which is why they’re often chosen for heavier structures or where deep, reliable bearing capacity is essential. The drilling process also produces far less vibration and noise than driving piles, making caissons a common choice near existing structures or in urban settings sensitive to construction disturbance.
Their high individual capacity also means a building can often be supported on fewer total foundation elements compared to a pile-based approach, which can simplify the overall foundation layout on constrained sites. In some ground conditions, particularly where groundwater is present, the drilling process may also involve slurry to stabilize the shaft walls in place of or alongside a steel casing.
Caisson Foundation vs. Pile Foundation
A pile foundation is typically driven, screwed, or drilled at a relatively narrow diameter, while a caisson is a larger-diameter drilled and cast-in-place shaft. Caissons generally carry higher individual loads, while piles are often used in greater numbers to achieve similar total capacity.