In fat clays, having angle of internal friction Ø = 0, the depth z of tension cracks is found to be equal 2c/y, where c is the unit cohesion an y is the unit weight of the soil. These cracks result in loss of support beneath the footings, resulting in high settlements. Some expansive and shrinkable soils stick to the footing base and
pull the footing down when they shrink. Thís results in horizontal cracks in the walls and oher flexible units of the structrure.
Black cotton soils and other expansive soils are dangerous due to their shrinkage and swelling characteristics. In addition to this, these soils have very poor bearing capacity, ranging from 5 t/m2 to 10 t/m2. In designing footings on these solis, the following points should be keept in mind:
1. The safe bearing capacity should be properly determined, taking into account Ihe effect of susained loading. The long term effect of loading results in slow consolidation. In absence of tests, the bearing capacity of these soils may be limited to 5 to 10 t/m2.
2. The foundation should be taken at Ieast 50 cm lower than the depth of moisture movement. This depth should also be much more than depth of tension cracks.
3. Where this soil occurs only in top layer, and where the thickness of this layer does fol exceed 1 to 1.5 m, the entire layer of black cotton soil (or other expansive soil) should be removed, and the foundation should be laid on non-shrinkable non- expansive soil.
4. Where the depth of clay layer is large, the foundation or footing should be prevented from coming in contact with the soil. This can be done by excavating wider and deeper foundation trench and interposing layer of sand/mooram around and beneath the footing.
5. Where the soil is highly expansive, it is very essential to have minimum contact between the soil and the footing. This can be best achieved by transmitting the loads through deep piles or piers and by supporting wall loads on capping beams which are kept some distance (5 to 15 cm) above the ground surface, to permit free expansion of the soil.
6. Where the bearing capacity of soil is poor, or soil is very soft, the bed of the foundation trench should be made firm or hard by ramming mooram and ballast mio it.
7. The foundations should be constructed during dry season. Also suilable plinth protection around the external wall should be made on the ground surface, with its slope away from the wall, so that moisture does not penetrate the foundation during rainy season.
Types of foundation in black cotton soils.
Foundation in black cotton soils may be of the following types:
1. Strip or pad foundatíon. For medium loads, strip foundation (for walls) and pad foundation (for columns) may be provided, along with special design features discussed above. Fig. 3.31 shows some typical section of shallow footings sutable for black cotton and other expansive soils....
2. Pier foundation Fig. 3.32 shows a typical pier foundation for a wall carrying heavy loads. Piers are dug at regular interval and filled with cement concrete. The piers may rest on good bearing strata. These piers are be connected by....
3. Under-reamed pile foundation. An under-reamed pile is a pile of shallow depth (1 to 6 m) having one bulb a its lower end. If this bulb is taken or provided at a level lower than the critical depth of moisture movement in expansive solis, the foundation will be....
FIG. 3.30 FORMATION OF TENSION CRACKS IN EXPANSIVE SOIL.