What is Soil Bearing Capacity?
Capacity of soil to support the loads
applied to the ground.
The bearing capacity of soil is the
maximum average contact pressure
between the foundation and the soil
which should not produce shear failure
in the soil.
Why is Soil Bearing Capacity required?
The stability of foundation depends on:
1. The bearing capacity of soil
2. The settlement of soil beneath the foundation.
Soil behaves in a complex manner when
loaded, so it is important to know about it’s
Improving Soil Bearing Capacity
Sometimes, the safe bearing pressure of soil is so low that
the dimensions of the footings work out to be very large and
uneconomical. In such a case, it becomes essential to
improve the safe bearing pressure, which can be done by the
following methods :
I. Increasing depth of foundation
II. Compacting the soil
III. Draining the soil
IV. Confining the soil
VI. Chemical treatment
Increasing depth of foundation
At deeper depths, the over burden pressure on soil is higher;
hence the soil is more compacted at deeper depth. As a
result it shows higher bearing capacity.
This is applicable only for cohesionless soils.
This method is not applicable if the subsoil material grows
wetter as depth increase.
This method has a limited use because with increase in
depth, the weight and cost of foundation also increases.
Compacting the soil
If we compact soil using appropriate method, then there will be increase
in its density and shear strength. As a result the bearing capacity of soil
also increases. Few methods of compacting soil of them are mentioned
Compaction by surcharge load: The ground can be surcharged with
heavy loads from aggregates or other materials to speed up the
settlement and improve bearing capacity of soil
Using sand piles: This method is very useful in sandy soil or soft soils.
Hollow pipes are driven in the ground, at close interval. This results in the
compaction of soil enclosed between the adjacent pipes. These pipes are
then gradually removed, filling and ramming sand in the hole, resulting in
the formation of sand piles.
Vibration: Heavy vibratory rollers and compactors may
compact a layer of granular soils to a depth of 1 to 3 m.
Using an appropriate roller as per the soil type to move at
a specified speed.
Vibro-flotation: It is a combination of vibration and jetting. A heavy
cylinder, known as vibroflot is inserted in the ground (soil) while the
cylinder vibrates, a water jet on the tip of the vibroflot supplies a large
amount of water under pressure. As the vibroflot sinks, clean sand is
added into a crater that develops on the surface.
Draining the soil
It is a weIl known fact that presence of water decreases the bearing
power of soil, specially when it is saturated.
Reason for it is low shearing strength of soil in presence of excess water
Drainage of the soil results in:
decrease in the voids ratio
improvement of bearing capacity of soil
Confining the soil
Sometimes the safe bearing pressure of the soil is low because of
settlements resulting due to the lateral movement of loose granular soil.
Such a tendency of lateral movement can be checked by confining the
soil, outside the perimeter of foundation area, by driving sheet piles, thus
forming an enclosure and confining the soil.
This method is useful in loose gravels and fissured rocky strata.
Bores holes in sufficient numbers are driven in the ground and cement
grout is forced through these under pressure.
The cracks, voids and fissures of the strata are thus filled with the grout,
resulting in the increase in the bearing value.
This method of improving bearing capacity of soil is costly and applied in
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.