What is a raft foundation?
Also read : Pile foundations Types and advantages
Raft foundation/Mat foundation is a solid slab placed at a designed depth spreading over the entire area of the structure. Raft foundations consist of columns and shear walls for transferring loads coming on the structure to the ground. These types of foundations are mainly used when the bearing capacity of the soil is low and becomes difficult for individual footings to negotiate the loads. The raft foundation helps to transfer the entire load of the structure to a larger area.
Principle of raft foundation
The raft foundation transfers the total loads coming from the structure to the entire area of the structure. They can reduce stress levels in soil compared to other types of foundations used in civil construction. This stress distribution mechanism makes raft foundations unique from other foundations.
Soil Stress Calculation
stress = total load coming on the structure + self weight of raft/ Area of raft foundation
Consider a total load is 300 T and a foundation size
Size : 20 m x 10 m
Stress on the soil = 300/200 = 1.5 t/sqm
Same structure supported with 8 individual footing
Size : 2m x 2 m
Total area = 8 x 4 = 32 sqm
Stress on soil = 300/32 = 9.375 t/sqm
This shows that same load we are getting stresses of 1.5 T/sqm for raft and 9.375 T/sqm for individual foundations.
As the contact area of raft is more the load is distributed over a larger area and hence stresses coming on the soil is very less.
Factors influencing the choice of raft foundation
Raft foundations are usually preferred over other foundations when the following conditions are encountered.
a) When the bearing capacity of the soil is very low for designing an individual footing and execution of deep foundations like pile foundation turns out to be costly.
b) When the bearing capacity of soil is less and stresses induced to the soil has to be be reduced.
c) The columns, shear wall etc are coming so close to each other and chances of individual footings over lapping each other.
d) Any other type of foundations may cover more than 50% of the total ground area below the structure.
e) When there is a possibility of unequal settlement.
f) Preferred for complex equipment foundations.
g) Used when the proposed structure have basements
Raft foundations are suitable for basement constructions where the foundation slabs can receive direct live loads according to the utility of the building. Sites where the soil conditions are very poor and access for large excavation machinery are restricted, raft foundations prove to be a better option as the excavations can be done with the help of light excavators.
TYPES OF RAFT FOUNDATIONS
- Solid Slab Raft Foundation
- Slab Beam Raft Foundation
- Piled Raft foundation
- Cellular Raft Foundation
- Balancing or Floating raft foundation
Solid slab raft foundation
In this type of raft foundation, the columns and walls are equally distanced and the load distribution is also equal. These types of raft foundations are designed as slabs with uniform thickness and are known as solid slab raft foundations. The reinforcement for these types of foundations comprises a bottom layer and top
There are four different types of Solid slab raft foundations
a) Flat raft mat
b) Wide toe raft
This type of raft foundation is used when it is required to economise the structure. A full-size solid slab mat foundation may not be required to negotiate the loads coming on the structure. In that case, a heavily reinforced toe is provided on both sides as shown in the figure which can handle the loads
c) Blanket raft foundation
Blanket rafts are used when the surface may have unequal settlements or having nonuniform strata. In that case, the surface is compacted stone blankets will be laid as shown in the figure. The stone blankets along with the raft shoes negotiate the load coming on the structure.
d) Slip plane rafts
This type of foundation bears a fully compacted sand bed beneath the raft. The sand bed size shall be kept a little bit larger than the raft size for the transfer of loads. The sides of the foundation can be filled with any compressible material.
Slab beam type raft foundation
When the loads are unequally distributed and the foundation is vulnerable to distortions. Beams included with the slabs act as stiffeners. The reinforcement of the raft constitutes two layers of meshes, one at the bottom and one at the top. The beams can provide extra stiffness and prevent distortions.
Piled raft foundation
In this type of Mat Foundation, rafts are supported by pile foundations as shown in the figure. These types of foundations are used when the loads coming on the structure are extremely high, the soil bearing capacity is very low and the water table is very high. Piled raft foundations are well suitable for high rise buildings, heavy industrial structures like high rise RCC chimneys, silos, storage tanks which usually rests on single foundation elements. They are not used for common residential applications due to their high costs. Piled raft foundations eliminate the idea of designing a very heavy raft foundation or a very conservative pile foundation with larger depths. In place of that, they opt for a combination of an optimised raft foundation and pile foundation design capable of sharing the loads. The raft foundation floats over the pile foundation.
Mostly used in structures like chimneys, silos , bunkers, over head storage tanks etc where even a marginal settlement of soil may cause the structure to fail.
Cellular raft foundation
A cellular raft consists of an arrangement of two-way foundation beams with a solid slab resting on the ground below and a suspended slab at the top surface. The upper and lower slabs are jointed by intermediate beams converting the foundation to an I beam structure.
For covering top slab precast sofits can be used. The top slab casting is done using precast sofits. or other forms of permanent form work or sacrificial form work or filled with lightweight inﬁll blocks.
Mostly used in areas subjected to severe mining activity and having poor soil bearing capacities. Huge bending moments have to be resisted by the foundations. In those cases, cellular rafts are a preferred option. Cellular rafts are used when an increased bearing capacity is achieved on removing overburdens. Soil uplift pressure can be controlled by using cellular rafts.
BALANCING OR FLOATING RAFT FOUNDATION
Balancing rafts or floating foundations are used in areas where the bearing capacity of the soil is very poor and has to maintain the soil settlements within the acceptable range. The floating foundation works on the principle that the total weight of the soil along with water removed from the excavated area shall be equal to the weight of the proposed structure.
ADVANTAGES OF RAFT FOUNDATION
Raft foundations are are a safe and economical solution comparing with other shallow and deep foundation types.
a) Raft foundations are preferred in areas with poor soil bearing capacity, uneven settlement, and the presence of mixed soil types. These foundations achieves the load-bearing capacity through the distribution of stresses to a larger area.
b) In dense urban areas accesses to the sites are mostly restricted and mobilisation of heavy machinery for foundation construction with other methods is not possible.Raft foundations can be executed using very light machinery due to their less heights.
c) To restrict the settlement limits within the codal provisions the raft foundations provide a much-needed option for designers compared to other isolated foundations.
d) The designers can opt for higher settlement values when comparing with normal foundations while doing the design process. The raft foundation avoids unequal settlement
e) Raft foundations is a very flexible design option which can be customised as per the soil conditions and workability.
f) Raft foundation execution is easier than individual footings. This in turn can increase the speed of project.
DISADVANTAGES OF RAFT FOUNDATION
When the soil conditions is very poor most of the times raft design is not considered economical. Complex raft foundations consume a huge quantity of concrete and steel, requires precise professional/technical supervision and workmanship. This in turn makes the structure more expensive than any other alternative foundations.
The soil below the foundation, mainly near the edges has to be preserved