Methods Of Improving Soil Bearing Capacity

- VIBHANSHU SINGHB130491CE

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 soil2. The settlement of soil beneath the foundation.

Soil behaves in a complex manner when loaded, so it is important to know about it’s bearing capacity.

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 foundationII. Compacting the soil III. Draining the soil IV. Confining the soil V. GroutingVI. 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 below:

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.

Grouting 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.

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.