Drilled shafts, also called caissons, are large-diameter holes drilled deep into the ground and filled with reinforced concrete. This foundation system is used for buildings requiring heavy support in poor soil conditions or where ground improvement is needed. Holes are drilled using specialized drilling equipment; diameters typically range from 2 to 10 feet. Drilling depth can exceed 100 feet if needed to reach competent soil or rock. The drilling process requires continuous inspection to identify soil layers and verify bearing capacity. Soil samples are extracted and tested to determine appropriate bearing capacity. Casing may be used to support the hole walls in unstable soils. Temporary casing is removed after drilling. In clay soils, drilling can proceed without casing if soil is stable. In loose sands or silts, casing or slurry support is required. Drilling fluid (bentonite slurry) may be used to maintain hole wall stability and carry cuttings to the surface. At the bottom of the hole, the soil must be properly cleaned and inspected. Competent soil or rock must be exposed at the bottom to develop bearing capacity. Reinforcement cages are lowered into the hole and positioned. Concrete is then placed, displacing drilling fluid as the concrete rises. Proper concrete placement prevents segregation and maintains quality. After concrete cures, the shaft provides excellent bearing capacity and extends through poor soil layers to competent material. Large-diameter shafts support heavy columns directly. Load testing verifies the shaft provides adequate capacity.