When the soil is not sufficient strong to bear the loads coming from foundation, then we have to attempt some steps to make soil more stronger and stiffer to carry the loads on itself.
The following methods can be used as per suitable on soil condition at site for improving the bearing capacity of soil.

1. By going to more deep
2. By desiccation of soil 
3. By compaction of soil
4. By restricting the soil
5. By replacement of weak soil
6. By use of grout material 
7. By mixing of granular materials
8. By chemical treatment

1. By going to more deep : 
Due to overburden at deep depth, The bearing capacity of soil is more because the soil is more compacted at deeper depth. Hence it shows higher bearing capacity than upper ground. 
This is applicable only on cohessionless soil.
This method can not be used where the soil becomes more wet or moist at depth.
This methods has a limited use because with increasing the depth, cost of foundation and load also would be increased.

2. By desiccation of soil :
The soil which has high water content is showing low bearing capacity. At some extent the presence of water is good for binding the soil particles but more than it, is not good for strength to carry a load.some experiments and studies proved that more than 50% bearing capacity is reduces due to presence of excess water. It can be drained by laying the porous pipes to a gentle slope, over a bed of sand and filling the trenches above the pipes with loose boulders. These trenches subsequently should lead to the nearest well or any water body.

3. By compacting of soil :
When we compact the soil the volume of soil will be decreased and the density of soil will be increased. As it becomes a dense it will be impart more shear strength hence directly it increases the bearing capacity of soil. There are many methods to compact the soil. some of its are given below.

  • By spreading metals, gravel or sand on surface of soil and afterward ramming it well.
  • Using an appropriate roller as per the soil type to move at a specified speed.
  • By drilling the holes to sufficient depth and filling the holes with sand or concrete.

4. By restricting the soil :
Sometimes the sand at the boundaries has low bearing capacity. it will be improved by enclosed the soil with the help of sheet piles. This soil further compacted by using appropriate method.

5. By replacement of weak soil : 
In this method the area of soil which are subjected to foundation are replace by the more stronger soil. It will be filled up by superior material such as sand, stone, gravel or any other hard material. In this methods filling are completed in layers. First the excavation of soil would be 1.5 m deep and It will be laid in every 30 cm layer. and it will be compacted individually.

6. By use of grout material :

This method is applicable for soils where the soil contains presence of pores, voids or cracks etc underneath the foundation. In this method, weak soil bearing strata is hardened by injecting the cement grout under pressure, because it fills up cracks or pores or voids etc. For proper distribution of the cement grout, the ground is bored and perforated pipes are introduced to force the grout. This method is used near the existing structure where the huge crack occur due to settlement near the foundation of building and their will be risk about failure of foundation hence to prevent this phenomena, The crack should be filled with grouting material with pressure.

7. By mixing of granular materials :
In this method certain amount of granular materials such as sand, gravel, crushed stone etc is blended into the weak soil by ramming. Hence the poor soil becomes much stronger and bearing capacity is improved.

8. By chemical treatment :
This method of improving bearing capacity of soil is costly and applied in exceptional cases. In this method, chemical solutions, like silicates of soda and calcium chloride is injected with pressure into the soil. These chemical along with the soil particles form a gel like structure and develop a compact mass.This is called chemical stabilization of soil and used to give additional strength to soft soils at deeper depths.


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