For Superior & Quality Craftmenship in Raft Foundations in Gauteng - ROCKHARD CONSTRUCTIONS
How Do Raft Foundations Work?
A raft foundation spreads the weight of the building over the whole ground floor area of that building. The raft is laid on a hardcore, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.
Raft Foundations are built is this following steps:
The soil removed down to correct depth
The foundation bed is then compacted by ramming
Lay reinforcement on spacers over the foundation bed
Pour the concrete over the reinforcement
The foundation may stiffened by ribs or beams built in during construction which will add extra strength and rigidity.
When Raft Foundations Are Used?
A raft foundation is usually preferred under a number of circumstances:
it is used for large loads, which is why they are so common in commercial building which tend to be much larger, and therefore heavier, than domestic homes
The soil has a low bearing capacity so the weight of the building needs to be spread out over a large area to create a stable foundation
The ratio of individual footings to total floor space is high. Typically if the footings would cover over half of the construction area then raft foundation would be used
If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used
Advantages and Disadvantages of Raft Foundations
Raft foundations tend to be cheaper and quicker to use than traditional footings. There are a number of reasons why this is the case:
The foundation and floor slab is combined, which saves time and materials
Less excavation is required
Other reasons that make raft foundations preferable to footings are due to their engineering benefits. They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.
Related to this is that raft foundations can reduce differential settlement, where settlement occurs at different rates across the ground surface of the building, which reduces cracking and other more serious problems.
The main disadvantage is that they can prone to edge erosion if they are not treated properly. They are not effective is the load of the building is going to be focused on a single point, although this is rare in domestic construction, so this isn't generally of concern.
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.