3: Deep foundations (piles, caissons)

Deep Foundations (Piles, Caissons)

When shallow foundations are inadequate due to weak surface soils, high structural loads, or challenging site conditions (like high water tables), deep foundations transfer loads to deeper, competent soil or rock strata. They are essential for skyscrapers, bridges, heavy industrial structures, and sites with poor upper soil. The two primary types are piles and caissons.

Pile Foundations are slender, structural columns installed deep into the ground. They transfer loads either through end-bearing (resting on a firm layer like bedrock), skin friction (shear resistance along the pile shaft against surrounding soil), or a combination. Key types include:

• Driven Piles: Prefabricated from concrete, steel (H-piles, pipe piles), or timber, hammered into the ground using impact or vibration. Installation densifies surrounding soil but can cause noise/vibration and soil heave.
• Bored Piles (Cast-in-Place): A hole is drilled, a reinforcing cage is placed, and concrete is poured. Suitable where driving is disruptive or for large diameters. Requires temporary casing or drilling mud to prevent hole collapse in unstable soils. Includes continuous flight auger (CFA) piles.

Piles often work in groups capped by a pile cap. Efficiency reduces in groups due to overlapping stress zones ("group action"). Design considers individual pile capacity and overall group settlement.

Caisson Foundations are large-diameter, deep foundations constructed by excavating or drilling a shaft, often filled with concrete. They handle extremely high loads and are common for bridge piers and monumental structures. Types include:

• Drilled Caissons (Drilled Shafts): Similar to large bored piles but typically $>750mm$ diameter. Constructed by drilling a hole, often under slurry or casing, placing reinforcement, and concreting. Can have enlarged "belled" bases to increase end-bearing area.
• Open Caissons: Large, open-bottomed boxes sunk through soft ground or water by excavating material from within. The bottom edge cuts into the soil. Used in wet conditions, but dewatering and base sealing are critical.
• Pneumatic Caissons: Employ compressed air to keep water out of the working chamber at the base, allowing excavation in saturated soils. Complex and costly, used for deep foundations below water tables where other methods fail.

Selection depends on soil profile, groundwater, load magnitude, site access, environmental constraints, and cost. Caissons generally offer higher individual capacity than piles but require more complex construction and quality control, especially for water exclusion and base cleaning before concreting.