Raft
Foundation – Design Requirements and Applicability
A raft foundation consists of a raft of reinforced concrete
under the whole of a building. This type of foundation is described as a raft
in the sense that the concrete raft is cast on the surface of the ground which
supports it, as water does a raft, and the foundation is not fixed by
foundations carried down into the subsoil.
When Raft
Foundations Are Used?
- 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
- It may be used for buildings on compressible ground such as very soft clay, alluvial deposits and compressible fill material where strip, pad or pile foundations would not provide a stable foundation without excessive excavation.
- The reinforced concrete raft is designed to transmit the whole load of the building from the raft to the ground where the small spread loads will cause little if any appreciable settlement. Raft Footings are normally used to distribute gravity loads when soil conditions do not support point loads.
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| Rectangular Raft Foundation |
Mat
or raft foundation is classified as a shallow foundation since its depth is
considerably smaller than deep foundation, but its depth is greater than other
types of shallow foundation. It is useful in controlling the differential
settlement and transferring the loads not exceeding the bearing capacity of the
soil due to integral action of the raft foundation.
Mat
foundations are an ideal type of foundation in several conditions for instance
low bearing capacity soil, spread footing cover about 70% of the structure,
high structure loads, soft pockets or cavities of in the soil to unknown extent
raft, and highly compressible soil and extents to a great depth.
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| Circular Raft Foundation |
There
are certain requirements that need to be considered during the of raft
foundation in order to produce adequate design such as minimum depth of 500 mm,
required excavation depth of approximately 2.5m, and rebar cover of 50mm.
Raft Foundation Design Requirements
1.) Calculation
methods for mat foundation design shall be based on the latest version of
applicable codes.
2.) It
shall not be placed on the topsoil.
3.) As
per IS 1080, a minimum depth of 500 mm shall be used for mat foundation. This
is required to ensure that the soil has a safe bearing capacity which is
assumed in the design.
4.) The
depth of mat foundation must satisfy shear requirements.
5.) A
uniform thickness can be used for raft foundation if columns are equally spaced
and the loads are not very heavy.
6.) According
to ACI 318-14, British standard; Eurocode 7; and IS 456; a minimum cover of 50
mm is required for mat foundation.
7.) The
above reinforcement cover may be increased based on harmful chemicals and
minerals in the soil and fluctuations of water table when it is very near to
the foundation.
8.) Mat
foundation should be placed below the level which would not be influenced by
the seasonal change of weather to cause swelling and shrinking of the soil.
Added to that, frost also may endanger the foundation, if placed at a very
shallow depth.
9.) When
mat foundation is constructed on sand, the minimum depth of foundation is
around 2.5 m below the surrounding ground surface. if a smaller depth is
considered, the edges of the raft settle appreciably more than the interior due
to lack of confinement of the sand.
10.) However,
British standards specify a minimum depth of 0.6m below the surrounding ground
surface.
11.) When
raft foundation is founded on sand, differential settlement governs the design
but this is determined by the strength and stiffness of the raft structure and
is very difficult to assess.
12.) Accurate
estimations of all types of loads, moments, and forces are needed for the
present as well as for future expansion. This is crucial because once the
construction of the foundation is completed and settles well into the soil, it
would be difficult to strengthen it in future.
13.) Foundation
structures should be able to sustain the applied loads, moments, forces, and
induced reactions without exceeding the safe bearing capacity of the soil.
The
settlement of the structure should be as uniform as possible and it should be
within the tolerable limits.
14.) Mat
foundation should provide adequate safety for maintaining the stability of
structure due to either overturning and/or sliding
15.) Foundation
structures undergo soil-structure interaction. Therefore, the behavior of foundation
structures depends on the properties of structural materials and soil. That is
why soil investigation is needed to specify the properties of soil, strata-wise
and its settlement criteria.
16.) Rafts
constructed on saturated clay have to be examined for both bearing capacity and
settlement because either may control the design.
17.) The
weight of the raft is not considered in the structural design because it is
assumed to be carried directly by the subsoil.
18.) The
raft may be ribbed where the column spacing is irregular or for economy in
using a relatively thin slab over most of the area. Alternatively,
rafts may be thickened at the column locations for economy and depth should be
made sufficient to resist shear.
Applicability of Raft (Mat) Foundations
- High structure loads
- Structures and equipment sensitive to differential settlement
- Watertight construction under basements below groundwater table
- Highly compressible soil and extends to a great depth
- Low bearing capacity soil
- Spread footing cover about 70% of the structure
- For structures like chimneys, silos, tanks, large machines
- Soft pockets or cavities of in the Soil to unknown extent raft
Advantages of Raft Foundations:-
- It may be able to overcome differential settlement problems for the raft acts as a unit. Load incurred by raft foundation will be transferred to the underlying soil by reinforced concrete continuous slab by covering the entire site structure.
- Raft foundation can across the low soil bearing capacity and will distribute the loads on wide area. It spreads the loads over a larger area when the construction takes place on the soft or loose soils with low bearing capacity.
- Reduce differential settlements as the concrete slab resists differential movements between the loading positions.
- To carry loads which are too heavy to be supported by a shallow foundation. The loads are to be transferred to deeper, stronger and less compressible strata or over a larger depth of the foundation soil as in foundations of tall buildings.
- To carry horizontal loads as in bridge abutments or retaining walls and also to increase the stability of tall buildings. Inclined piles are also used to carry inclined loads with horizontal force components.
- To withstand uplift forces in foundations as in expensive soils and floating foundations
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