10/02/2026
WHAT ARE FOUNDATIONS?
Foundations are structural elements that transfer the loads of a building safely to the underlying soil or rock that has adequate bearing capacity and acceptable settlement characteristics. Their primary function is to provide stability and prevent excessive settlement or failure of the structure.
Foundations are broadly classified into two main categories:
1. Shallow Foundations
Shallow foundations are used when the structural loads are relatively small and the surface soils have sufficient bearing capacity. Examples include:
Strip foundations
Pad (isolated) foundations
Raft (mat) foundations
2. Deep Foundations
Deep foundations are required when surface soils cannot safely support the structural loads. In such cases, loads are transferred to deeper, stronger soil layers or rock strata. Examples include:
Pile foundations
Caisson foundations
WHAT IS A FLAT SLAB RAFT FOUNDATION?
A flat slab raft foundation (also known as a mat or raft foundation) consists of a uniformly thick reinforced concrete slab, usually 150 mm thick or more, covering a large area—often the entire footprint of the building.
This type of foundation is suitable when:
Column loads are relatively uniform
Columns are evenly spaced
Soil bearing capacity is low but fairly uniform
The raft distributes the structural loads over a wide area, reducing soil pressure and differential settlement.
To resist bending moments caused by soil pressure and column loads, two layers of reinforcement are provided:
One mesh at the top of the slab
One mesh at the bottom of the slab
CONSTRUCTION PROCEDURE OF A RAFT FOUNDATION
Topsoil is excavated and removed.
A blinding concrete layer (50–75 mm thick) is placed to create a clean, level base.
After the blinding sets, a damp-proof membrane (DPM) is laid to prevent rising damp.
Reinforcement is fixed according to the design.
Concrete is poured, compacted, and cured properly.
FORM OF SOIL PRESSURE IN A RAFT FOUNDATION
The behaviour of a raft foundation is similar to a grid of strip foundations. Soil pressure is:
Higher under column locations, where loads are concentrated
Lower in intermediate areas between columns
When beams (stiffeners) are incorporated into the raft, they help distribute soil pressure more uniformly and reduce differential settlement.
A solid raft foundation, consisting of a single continuous slab, is the simplest form. It is easy to construct, with straightforward formwork and reinforcement detailing.
DETAIL OF RAFT FOUNDATION REINFORCEMENT
Reinforcement in a solid raft foundation can be divided into three main categories:
Slab reinforcement
Slab free-edge reinforcement
Punching shear reinforcement (where required)
SLAB REINFORCEMENT AND FREE-EDGE REINFORCEMENT
Two reinforcement meshes are provided:
One at the bottom fibres
One at the top fibres
Areas around columns experience the highest stresses and therefore require additional or stronger reinforcement.
Free edges of the slab are reinforced using hairpin bars or specially shaped wire mesh to control cracking and edge failure.
FLAT SLAB RAFT FOUNDATION WITH PUNCHING SHEAR REINFORCEMENT
Punching shear reinforcement is required when:
The raft slab is relatively thin
Column loads are high
This reinforcement may consist of:
Stirrup cages
Bundled bent reinforcement bars
Proprietary industrial shear reinforcement systems
It is provided around column locations to prevent punching shear failure.
FLAT SLAB RAFT FOUNDATION WITH HIDDEN BEAMS
In some designs, hidden (concealed) beams are introduced along column lines within the slab thickness. These beams:
Act as stiffeners
Reduce slab deflection
Help control punching shear at column locations
Stirrups used in hidden beams may be:
Two-legged
Four-legged
More than four-legged, depending on structural demand
