As discussed earlier this type of foundation is used when the bearing capacity is low and total area of spread footing exceeds 50% of the total plinth area.
Mat foundation consists of rows of columns built monolithic with a continuous slab covering the entire foundation area, with or without depressions or openings.
A true mat is a flat concrete slab with uniform thickness throughout the entire area. This type is most suitable where the column spacing is fairly small and uniform and the column loads relatively small. For large column loads a portion of the slab under the column may be thickened. If bending stresses become large, thickened bands may be used along the column lines in both directions. Under extremely heavy column loads, two-way grid structure made of cellular construction may be used. Basement walls are also sometimes used as ribs or as deep beams.
The choice of mat type depends on one or more of the following factors:
- For fairly small loading and uniform column spacing and the supporting soil is not very much compressible a flat concrete slab with uniform thickness of mat may be provided.
- In order to provide adequate strength against shear and negative bending moment for heavy loaded columns the slab is thickened.
- For unequal column loading and wide spaced columns beam and slab type of raft is more economical.
- For heavy structures, cellular rafts or rigid frames may be adopted.
Example 15.1
A residential building is to be constructed on a sandy soil with a safe bearing capacity of 1.65 kg/cm2 and the angle of shearing resistance (angle of repose) is of 30°, and the unit weight of the soil 1580 kg/m3. The thickness of wall is 30 cm. The total load transmitted is 11500 kg per metre length of the wall.
Solution:
Depth of foundation can be found using Rankine’s formula
Width of footing,
From practical consideration:
Hence B = 90 cm is adopted.
Load on wall/metre length = 11500 kg
Assuming 10% of load–Self Weight = 1150 kg
Total load on the soil = 11500 + 1150
= 12650 kg/m length
Pressure on soil 
This is less than SBC of the soil, hence satisfied.
Using 1:4:8 concrete the modulus of rupture f = 2.45 kg/cm2 and taking offset of concrete = 15 cm,
As the structure is lightly loaded a bed thickness of 15 cm is considered. Design features are shown in Fig. 15.23.
Figure 15.23
Example 15.2
Design an isolated footing to carry a brick-pillar of 300 mm square. The load transmitted at the top of footing is 140 kN. The bearing capacity of the soil at the location is 150 kN/m2. The unit weight of soil is 18.5 kN/m3. The angle of repose, 
is 
. The base concrete is of 1: 3: 6 mix plain concrete.
Solution:
Load on the footing = 140 kN
Self weight (Taking 10% of the load) = 
∴ Total load on the soil = 140 + 14 = 154 kN
Considering a square base,
Let the side be adopted as 110 cm. Considering an offset of 5 cm,
Width of bottom-most course of pillar footing = 700
∴ 
Which is less than the SBC of the soil, hence OK.
Using 1:3:6 plain cement concrete the modulus of rupture, f = 350 kN/cm2
A minimum thickness of 15 cm is adopted
Depth of foundation, 
A minimum depth of 80 cm may be adopted (Fig. 15.24).
Figure 15.24
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