Bearing Capacity of Soil
The soil is generally used as a substratum on which the the foundation of the structure and hence the structure too rests. The soil below the foundation should be strong enough to withstand the load of the entire structure. The load of foundation is compressive, but shear force also enacts doe to lateral forces on soil. This strength for resisting the compressive stress is called ‘Bearing Capacity of Soil’.
The bearing capacity plays a vital role in civil engineering aspects such as :
1. Foundation Design :
Foundation is the most important part of any structure. All the load of the structure comes on the foundation. Further the foundation transmits the load in the soil at the bottom of the foundation. Thus the area of soil at base of the foundation decides the intensity of load per unit area. Every soil has its own capacity to bear the intensity of load per unit area. This capacity is referred as to bearing capacity of soil.
While designing the foundation care is take that the load of the structure is less than the bearing capacity of the soil. Thus bearing capacity is important factor while designing the foundation.
2. Compaction and Consolidation of Earth :
In different civil engineering works the properties of the soil are always enhanced or modified according to the conditions and requirements of the structure. The natural process of soil to change its property is called as consolidation and when the property of the soil is modified by humans for their purpose it is called as compaction.
In both the process the density of soil always changes, which results in increase in shear strength and bearing capacity of soil and decrease in the permeability.
3. Earth and Gravity Dams :
The construction of the Earthen dam is entirely done by soil and hence it is very much important to know the details of the soil by which it is going to be constructed and on which it is going to be constructed.
In case of gravity dam due to the uplift pressure below the dam and the load intensity of the dam coming on the soil below the base of the foundation it is mandatory to have full knowledge of the bearing capacity of the soil.
4. Retaining Walls :
The design of retaining wall involves mobilisation of soil strength under different conditions of limiting equilibrium. Thus active, passive and neutral earth pressure are also considered.
These are called as lateral pressure and the bearing capacity is vertical compressive stress. Also in Rankine’s theory of earth pressure bearing capacity of the soil is one of the most important factor to be considered.
5. Tunnels :
Tunnels are constructed by penetrating the soil from one end to other. Rock subgrade, stability of open cuts, docks and harbour construction, in short all the civil engineering works where excavation is done and use of soil as substratum for supporting the load is required, bearing capacity of soil plays a vital role.
Different Types of Bearing Capacity.
1. Ultimate Bearing Capacity :
Ultimate bearing capacity that any soil can withstand without the rupture in shear or without the excessive settlement of the structure over it is called as ultimate bearing capacity of soil.
2. Safe Bearing Capacity :
The maximum pressure which the soil can resist without any failure is called as the safe bearing capacity of the soil. It is the value of bearing capacity used in the design of foundations. This value is obtained by dividing with ultimate bearing capacity with suitable factor of safety. Generally the factor of safety is kept between 3 to 5.
3. Allowable Bearing Capacity :
It is net load intensity, including the surcharge on the foundation at which neither the soil fails due to shear rupture nor there is excessive settlement of the foundation. But this bearing pressure is not considered while designing. Generally for uncertainty and unknown factors always a sufficient factor of safety is used.