ALL you need to know about " Bearing Capacity of Soil "

BEARING CAPACITY OF SOIL

After the completion of this article, you'll be able to answer about 🠖

1. Definition of bearing capacity of the soil.
2. Common types of bearing capacity and their definition
3. Types of test used to determine the bearing capacity.
4. How to increase the bearing capacity of the soil?

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What is Bearing Capacity of Soil?

Bearing capacity of the soil can be defined as "The maximum load per unit area of soil which the soil can resist without showing any failure".

In simple words bearing capacity of the soil is the load carrying capacity of the soil.

When loaded, the behaviour of soil becomes very complex. The soil tends to deform which further leads to failure.

Soils having high bearing capacity is hard to show any significant amount of deformation. Whereas Soils having low bearing capacity will deform under low contact pressure.

This character of soil is the backbone of foundation design.

Now you might know the reason.

Its very simple that designing any type of foundation, the load carrying capacity of the soil must be required.

Gross load coming from the structure is taken care by soil below the foundation.

Now you may have an idea about the importance of knowing the physical parameters of soil, especially the bearing capacity of the soil.

The angle of friction, bulk density, water content, relative density, consistency, level of the water table, cohesion are the factors which on which the bearing capacity of the soil depends.

Well, one more thing you should that the factors on which soil capacity depends are different for granular and cohesive soil.

The value of Cohesion and angle of friction is null for granular and cohesive soil respectively.

Here I have given the definition of basic terminology about the bearing capacity of the soil.

Types of bearing capacity of the soil

 Some important terminology regarding the bearing capacity of the soil discussed below.

1. Ultimate bearing capacity/Gross bearing capacity-

This can be defined as the gross pressure or ultimate pressure at which shear failure of soil occurs.

2. Net ultimate bearing capacity-

Net ultimate bearing capacity = (The ultimate bearing capacity - The stress due to the weight of footing)

Df= Depth of foundation, = unit weight of soil

3. Net safe bearing capacity-

The amount of net pressure that can be safely applied to the soil is known as net safe bearing capacity.

Mathematically it is the ratio of net soil pressure to the factor of safety.

Net safe bearing capacity, (qns )= qnu/ F

This type of bearing capacity is only considered for shear failure.

4. Gross safe bearing capacity-

Gross safe bearing ratio = (net safe bearing capacity + overburden pressure)

Where overburden pressure = (unit.wt of soil*depth of foundation)

Gross safe bearing capacity (qs) = qu/F

5. Allowable bearing pressure

Allowable bearing pressure referred as the maximum pressure that soil can take without causing any settlement or shear failure.

These are some of the most common terms for studying about Bearing capacity of the soil.

Okay, you have already learned the basic concept of bearing capacity and the types of bearing capacity.

Now, Let's move ahead.

Methods of determining the bearing capacity of the soil

The most common and widely used tests to determine the bearing capacity of the soil are listed below-

1. Presumptive analysis
2. Plate bearing test
3. Standard penetration test
4. Centrifuge test

1. Presumptive analysis

It's unfair to call it a test. Because in this case you're actually not finding the bearing capacity of soil by performing any test.

What you have to do is that just determine the type of soil( Such as granular, clay, cohesive etc).

From the experiment and experience, standard code IS 1904(1978 edition) has recommended us a table by which we can determine the bearing capacity.

But one major con of this analysis is that it won't give you the actual or ideal result. 

The chart is given below-

IMAGE SOURCE- NPTEL

2. Plate bearing test

To implement the plate bearing test we should ensure that the soil surface is undisturbed, planar and free from any crumbs.

The surface should be trimmed manually.

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In this test, loading is to be done in 3 cycles. Its necessary to measure the settlement caused by loading each time.

The time interval between each loading is about 15 minutes. Measurement of settlement should be taken 15 minutes before and after the load increment.

If sufficient load is available, test continoue unless a well defined failure occurs.

If you want to know more details about the test to determine the bearing capacity of the soil, please go through this link.

3. Standard penetration test

The standard penetration test[SPT] is required to determine the ultimate bearing capacity of cohesionless soil.

From this test, we obtain the average SPT number. And by using the value of we can easily determine the value of ɸ(Angke of friction).

After getting values of N, ɸ, the width of footing, depth of footing we can evaluate the net ultimate bearing capacity of the soil by using the following formula-

For strip footing, 

For circular footing, 

As you can observe that performing SPT test to find out the bearing capacity is pretty simple. 

But the question is, Is it fully reliable?

Ans is no. 

The relation between Avg SPT value and angle of friction(ɸ) is very poor. 

Thus by using the values N and ɸ, we won't get the appropriate result.

In fact, it is suggested to avoid the SPT test for evaluating the bearing capacity of fine-grained cohesive soil.

Centrifuge test

A centrifuge is nothing but a geotechnical tool by which we can test the soil sample.

It will not only help us to determine the bearing capacity but also helps us to find out stress distribution, Behaviour of foundation under seismic load, settlement of the foundation, etc.

From the load-settlement curve bearing capacity and stiffness of foundation can be determined.

Centrifuge testing is a type of model test where the physical condition is replicated with a prototype comparable to the situation what might exist.

These are the list of some useful test that you'll need to know about regarding bearing capacity of the soil.

Let's recap.

Till now what we have learnt-

• What is bearing capacity of soil?-Introduction?
• Types of bearing capacity- With definition.
• Types of test to determine the bearing capacity of the soil.

Okay, back to the topic.

Now our last topic but not the least is, How to improve bearing capacity of soil?

How to increase bearing capacity of soil?

There is no particular method to increase the bearing capacity of the soil. It is very much possible that one trick that works for site 1, won't work for site 2.

Here most of the useful methods are described as follows-

1. Depth of foundation

 Sometimes the uppermost layer of soil is loose and less compacted hence have the less bearing capacity.

In this case, the depth of foundation can be increased to transfer the total load to the bedrock soil.

Pile foundation may be adopted to reach the bedrock soil (ex- Burj-Khalifa).

2. Draining process

As we know water content influences directly the bearing capacity of the soil.

 If the water table lies at the level of foundation then it is highly recommended to drain out the water as much as possible.

Sandy soil can be drained easily by using gravity pipe.

3. Grouting

Grouting can be defined as the injection of slurry material (called grout) into the soil for the purpose of improving the soil.

Cementitious (neat cement, cement with fly ash, etc.) and chemical grouts are used as the grouting material.

Methods of grouting-

1. Compaction Grouting
2. permeation Grouting
3. intrusion Grouting
4. jet Grouting

Jet grouting- In this method grouting is to be done under high pressure and high velocity. It is a method of soil stabilization.

By hardening of grouted fluid into the soil, soil stabilization is done.

4. Compaction/consolidation

Compaction is nothing but a process to reduce the void ratio. The more compact soil means more bearing capacity.

Soil compaction is a very popular method by which we can not only reduce the void ratio but also enhances the bearing capacity and shear parameters of soil.

Vibratory roller, sheepsfoot roller, vibration can be used for soil compaction.

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Now you have completed this complete guide on bearing capacity of the soil.

I hope you get all your answer. If you still feel any topics we left to cover, please comment below.

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