Raftfundament

Raftfundament, or raft foundation, is a large reinforced concrete slab that rests on the ground to support a building or structure by distributing its loads over a wide area. It is designed to transfer vertical loads from columns, walls, and other structural elements to the underlying soil more evenly than individual footings.

This type of foundation is typically used when the soil bearing capacity is low, variable, or prone to differential settlement, or when a building has many columns or an irregular layout. By tying together the loads from the structure, a raft fundament reduces differential settlement and can be more economical than multiple isolated footings on weak soils.

Variants of raft foundations include a monolithic raft that is cast together with the superstructure, a stiffened raft with internal beams or ribs to increase rigidity, and a piled raft that combines piles in weaker zones with a raft slab to improve load transfer. The raft commonly covers most or all of the building footprint and may incorporate edge beams or punched reinforcement around columns.

Design considerations for a raft fundament include soil stratigraphy, anticipated load paths, water table, frost effects, and temperature movements. The reinforcement typically consists of a grid or mesh with additional reinforcement around columns and along edges. In some cases, a piled raft is used when partial deep foundations are needed to control settlements.

Construction involves site preparation, excavation, formwork, placement of reinforcement, waterproofing, concrete pouring, and curing, followed by quality control and testing. Raft foundations offer advantages in uniform load distribution and suitability for poor soils but require careful design and workmanship to manage costs and long-term performance.