Bearing Capacity of Soil

The bearing capacity of soil is the maximum load per unit area which the soil or material in foundation, may be rock or soil, will support without displacement. Very often, a structure fails by unequal settlement or differential settlement.

The allowable bearing capacity or the safe bearing capacity of a soil is obtained by dividing the ultimate bearing capacity by a certain factor of safety and is used in the design of foundation. It is suggested that a factor of safety of 2 for buildings in ordinary construction and a factor of safety of 2.5 or 3 for heavy constructions be adopted.

Bearing capacity of various types of soil

Methods for improving the bearing capacity of soil

It happens sometimes that the required safe bearing capacity of the soil is not available at shallow depth or it is so low that the dimensions of the footings work out to be very large and uneconomical. Therefore, on such circumstances, depending on the site conditions it becomes necessary to improve the safe bearing capacity.

1. By increasing the depth of foundation: In most of the cases, the bearing capacity increases with the depth due to the confining weight of the overlying material. This method is not economical because the cost of construction increases with the depth and the load on the foundation increases with increase in depth. This method should not be used on silts where the subsoil material grows wetter as the depth increases.
2. By draining the soils: The presence of water decreases the bearing capacity of the soil. The studies show that around 50 per cent of bearing capacity is lost in sandy soils due to the presence of excess water. Suitable drains should, therefore, be provided in the foundation channel to drain off the excess water.
3. By compacting the soil: The compaction of soils results in increase in density and strength and, hence, the bearing capacity. Better compaction is achieved in two ways. (1) By hand packing the rubble boulders or spreading broken stone gravel or sand and thereafter ramming well in the bed of trenches. (2) By driving piles either of wood or concrete or driving and withdrawing the piles and filling the holes with sand and concrete.
4. By confining the soil: The movement of soil under the action of load can be prevented by confining the ground by the use of sheet piles. These confined soils can be further compacted for better strength. This method is especially useful for sand soils underlying shallow foundations.
5. By increasing the width of the foundation: By increasing the width of the foundation the bearing area increases and, hence, the intensity of pressure decreases. This method has limited use, since the width of the foundation cannot be increased indefinitely.
6. By replacing the poor soils: The poor soil is first removed and then the gap is filled by superior materials such as sand, rubble stone, gravel or other hard materials. First the foundation trenches are excavated to a depth of 1.5 m, then filled in stages of 30 cm by hard material and finally rammed.
7. By grouting: In poor soil bearing strata, sufficient number of boreholes are driven. Then the cement grout is injected under pressure, because it scales off any cracks or pores or fissures which otherwise reduce the bearing capacity of the soil. This method is employed for materials having pores, fissures or cracks underneath the foundation.
8. By chemical treatment: The chemicals like silicates of soda and calcium chloride with soil particles form a gel-like structure and develop into a compact mass. This is called chemical stabilization and is used to impart additional strength to soft soils at deeper depth. However, the chemicals are added in traces only, but even then it has proved to be costly and, hence, is adopted in exceptional cases.
9. By using geotextiles: This is a method of reinforcing weak soils to improve their bearing capacity. Coir geotextiles are found to be very useful in this context.