Raft foundation is a thick concrete slab reinforced with steel which covers the entire contact area of the structure like a thick floor. Sometimes area covered by raft may be greater than the contact area depending on the bearing capacity of the soil underneath. The reinforcing bars runs normal to each other in both top and bottom layers of steel reinforcement.
Sometimes inverted main beams and secondary beams are used to carry column loads that require thicker foundation slab considering economy of the structure. Both beams cast monolithically with raft slab. Now I shall discuss where raft foundations are required. Raft foundation is required where soils have low bearing capacity and have to support heavy structural loads.
Normally structures on marshy land, soft clay and land that are made up of sanitary land fill or other materials (like debris, unconsolidated soil and solid waste etc. where differential settlement is suspected)-require raft foundation. Raft foundations are preferred in the soil that are suspected to subsidence. Subsidence may occur from different sources like change in ground water level due to climatic change specially in case expansive soil or foundation in mining area. In one words, where deep foundation like pile foundation are not economical and feasible and isolated column footing is impracticable due to large footing size or over-lapping of neighbor footing , raft foundation is the economical solution.
Different types of raft foundations are used to meet different geotechnical, structural requirements and to mitigate uncertainties. It is classified based on
• Support condition
• Structural system
Depending on supporting method they are classified in three categories:
• Raft on soil
• Raft on piles
• Buoyancy raft
Depending on structural system they are also classified in three categories:
a. Rafts having uniform thickness of slab, sometimes they may have pedestal
b. Raft with beams & slab system;
c. Framed raft or cellular raft having foundation slab, columns, walls rendering essential rigidity to structure.
Raft slab of uniform thickness is very popular because of simplicity in design & construction. According to ACI committee 336 mat foundation is-
Continuous footing that supports several rows of columns in either direction, covering at least 3/4th of total assembly area of columns (outer parameter), having shape like slab with/without opening or depression.
A linear distribution of soil pressure is assumed when foundation is considered rigid to that extent to limit deformation under loading to very small amount. Rigidity may be obtained from footing rigidity or form superstructure. It is recommended to consider use subgrade reaction for determining distribution of pressure
• When sub-soil condition is such that localized deformation is suspected especially near superimposed loads
• When, 1/2 X (ultimate bearing capacity)> (maximum contact reaction)
So, in rigid footing consideration, uniform distribution of pressure on footing are solved assuming statics. In flexible footing consideration, the computer model is produced with soil spring (coupled/uncoupled); spring properties are defined by above stated modulus of subgrade reaction. Again subgrade reaction should be adjusted or determined by
• Footing size
• Nodal tributary area
• Variation of this modulus with change in depth
• Effective depth.
The soil pressure is determined by
Raft Foundation FAQ’S
- Selection of Raft Foundation READ
- What are Time Dependent Considerations for Combined Footing and Mat Foundation? READ
- What is Parametric Study in Mat Foundation Analysis? READ
- How is Mat Foundation Idealized in Software “SAFE”? READ
- Assumptions and Their Consequences in Designing Mat Foundation With Rigid
Beam Approach READ
- What are the Lateral Loads Considered in Raft Foundation Design? READ
- What are the Analysis Methods for Raft Foundation? READ
- Variables in Pressure Distribution beneath Raft or Mat Foundation READ
- Foundation System of PETRONAS Tower READ
- How are Raft Foundations Used to Reduce Settlement? READ
- What is Raft Foundation? Difference between Raft Foundations and Mat Foundation READ
- Raft Foundation Construction Method READ
when resultant of all forces and geometric center of mat is concurrent. When there has eccentricity of resultant with geometric center, soil pressure is determined by
Once mat dimensions are established, soil reaction under different location of base can be computed.