Bearing Capacity of Soil: Definition, Types, Formula, and Calculation Methods

Ever wondered how engineers know whether the ground is strong enough to support a building?

Well, they use something called bearing capacity of soil to determine this. 

You see, not all soils can safely carry the same amount of weight. For instance, while dense gravel or stiff clay soil can support heavy structures, that might not be true for loose sand, soft, waterlogged soil.

Unfortunately, a lot of property owners ignore soil bearing capacity until they run into problems like:

• Uneven foundation settlement
• Cracked walls or floors
• Sinking pavements
• Structural movement
• Costly foundation repairs

As a geotechnical consultant in the Carolinas, I know how understanding the soil conditions can make the difference between a stable structure and years of expensive repairs.

Now, having soil with a lower bearing capacity doesn’t mean your site isn’t suitable for construction. With the right foundation and footing system you can build safely even on challenging soil conditions. 

In this blog, I will explain, including:

• What is bearing capacity of soil
• Why it matters in construction
• The different types of soil bearing capacity
• How engineers calculate and test soil bearing capacity

And much more.

So, let’s get into it.

Key takeaways

• Bearing capacity of soil is the maximum load the ground can safely support.
• Every foundation is designed based on the soil’s bearing capacity.
• Soil type, moisture, density, and footing size all affect bearing capacity.
• Engineers commonly use terms like ultimate bearing capacity and allowable bearing capacity.
• Exceeding the soil’s bearing capacity can lead to settlement or shear failure.
• Soil testing helps determine whether the ground is suitable for construction.
• Even sites with weak soil can often be built on with the right engineering solutions.

So, what is bearing capacity of soil exactly?

To put it simply, the bearing capacity of soil is the maximum amount of weight the ground can safely support without failing or settling excessively. And it is one of the most important things engineers check before designing a foundation.

You see, every structure transfers its weight through the foundation and into the ground below. 

Now, if the soil bearing capacity is high, the load is safely distributed and the structure remains stable.

But if the soil cannot support that load, there can be problems like:

• Excessive settlement
• Foundation movement
• Cracking in walls and slabs
• Uneven floors
• Structural instability

This is why engineers do geotechnical assessments and check how much weight the soil can safely carry so the foundation can be designed accordingly. Here, they conduct field and laboratory testing to determine the soil’s actual bearing capacity.

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Why is soil bearing capacity important in construction?

Soil bearing capacity is important because every structure depends on the ground under it for support.

Whether you’re building a home, an office, a warehouse, or even a retaining wall, the foundation transfers the weight of the structure into the soil. And if the soil isn’t strong enough to handle that load, the foundation might start moving over time.

Now, here’s the thing: The effects of poor soil bearing capacity aren’t always immediate. In fact, a building might appear perfectly stable when construction is complete. 

But as the soil continues settling or becomes weaker due to changing moisture, problems can slowly develop. These can include:

• Uneven foundation settlement
• Cracked walls, floors, and ceilings
• Doors and windows that no longer close properly
• Tilting or sinking structures
• Damage to underground utilities

In severe cases, the soil can even fail suddenly under the applied load. This type of failure is known as shear failure, something known to cause catastrophic structural collapses .

This is why the bearing capacity of soil helps engineers design foundations according to soil conditions and ensure fool-proof foundation stability.

What factors affect the bearing capacity of soil?

Not all soils can support the same amount of weight. In fact, two construction sites located just a few miles apart can have completely different soil bearing capacity because of differences in soil composition, groundwater, and moisture conditions.

Here are some of the biggest factors that affect the bearing strength of soil: 

1. Soil type

The type of soil is one of the biggest factors affecting its bearing capacity.

For instance, dense gravel and well-compacted sand generally have a much higher bearing capacity than soft clay or loose, organic soils. That’s because stronger soils are better at distributing loads without excessive settlement.

On the other hand, weaker soils compress more easily under heavy loads and increase the risk of foundation movement.

2. Moisture content

Water can significantly change how soil behaves.

You see, some soils lose bearing capacity as they become saturated. Clay soils, in particular, become softer after prolonged rainfall or when groundwater levels rise, requiring professional foundation stabilization on clay soil. 

3. Soil density and compaction

Generally speaking, denser soils can support heavier loads. That’s because when soil is properly compacted and soil particles packed closer together, the ground becomes stronger and more stable.

Loose or poorly compacted soil, on the other hand, contains more empty spaces between particles, making it more susceptible to settlement.

4. Depth of the foundation

The depth at which a foundation is placed also influences its bearing capacity.

For instance, in many cases, deeper soil layers are denser and more stable than the material found near the surface. Here, engineers are able to improve the overall stability of the structure with deep foundations construction where they extend foundations to stronger soil. 

Pro tip: The ideal foundation depth depends on your site’s specific soil conditions. And it should always be determined through a proper geotechnical investigation.

5. Groundwater conditions

Groundwater doesn’t just affect moisture, it can also influence how much load the soil can safely support.

How?

High groundwater levels reduce the effective strength of certain soils, particularly sands and silts, making them less capable of carrying heavy loads.

This is one reason engineers check groundwater conditions as part of a geotechnical investigation.

6. Size and shape of the footing

The foundation itself also affects how loads are transferred into the ground.

For instance, a larger footing spreads the weight over a greater area, reducing the pressure on the soil below.

That’s why engineers carefully consider footing dimensions during the design process rather than using the same foundation size for every project.

In short, here are the main factors affecting soil bearing capacity:

What are the different types of bearing capacity?

If you’ve been reading up about bearing capacity, you’ve probably come across terms like ultimate bearing capacity, allowable bearing capacity, and safe bearing capacity of soil.

At first, these terms can sound confusing. But they’re all simply different ways engineers describe how much load the ground can support.

Let me break them down one by one.

1. Ultimate bearing capacity

The ultimate bearing capacity is the maximum load the soil can withstand before it fails.

In other words, it’s the point at which the ground can no longer safely support the foundation. So, once this limit is exceeded, the soil can experience excessive settlement or even shear failure, causing the foundation to lose stability or even collapse.

2. Allowable bearing capacity

The allowable bearing capacity is the maximum load that can be safely applied to the soil during normal conditions.

To determine this value, engineers take the ultimate bearing capacity and apply a factor of safety. This creates a safety margin that accounts for uncertainties in soil conditions, construction, and loading.

Simply put, the allowable bearing capacity is the value engineers use when designing foundations. It helps ensure the soil remains stable throughout the life of the structure without experiencing excessive settlement.

3. Safe bearing capacity of soil

You’ll often hear the terms safe bearing capacity of soil and allowable bearing capacity used interchangeably. 

And in many construction projects, they refer to essentially the same idea. Both describe the load that the soil can safely support without risking failure or unacceptable settlement.

In short, here are the main types of soil bearing capacity:

What is shear failure in soil?

Earlier, I mentioned that one of the biggest risks of overloading the ground is shear failure.

But what exactly does that mean?

To put it simply, shear failure happens when the soil can no longer resist the load placed on it.

Instead of staying in place, the soil begins to move or deform beneath the foundation. And as this happens, the ground loses its ability to properly support the structure. This can result in foundation settlement, tilting foundations, or even structural collapses.

Engineers classify shear failure into three main types, namely:

1. General shear failure

This type of failure usually occurs in strong, dense soils with a high bearing capacity.

Here, the soil suddenly fails along well-defined surfaces beneath the foundation. And since the failure develops quickly, it can cause noticeable ground movement and major foundation settlement.

2. Local shear failure

Local shear failure occurs in soils with medium strength.

Here, instead of failing suddenly, the soil compresses gradually before eventually reaching its limit.

As a result, the foundation can experience slower settlement rather than an abrupt collapse.

This type of failure is generally less dramatic than general shear failure, but it can still lead to long-term structural problems if not addressed.

3. Punching shear failure

Punching shear failure is common in soft or loose soils. Here, the foundation essentially pushes downward into the ground, rather than spreading the load outward. 

In these situations, there will be very little visible ground movement around the foundation, even though the structure continues settling downward.

In short, here are the three types of shear failure:

How is the bearing capacity of soil calculated?

Now that you understand the different types of bearing capacity, you might be wondering how engineers actually determine how much weight the ground can support.

Well, engineers use a combination of soil testing and engineering calculations.

One of the most widely used methods for estimating the ultimate bearing capacity of shallow foundations is Terzaghi’s Bearing Capacity Equation. 

This equation was developed by geotechnical engineer Karl Terzaghi and it considers several factors that influence how the soil behaves under a load. These include:

• The soil’s cohesion
• The soil’s internal friction
• The weight of the soil
• The depth of the foundation
• The width of the footing

How do engineers determine soil bearing capacity?

Before designing a foundation, geotechnical engineers perform site investigations to better understand the soil beneath the proposed structure.

Depending on your project, this can involve several different tests, including:

1. Standard Penetration Test (SPT)

The standard penetration test, or SPT, is one of the most common field tests used during geotechnical investigations. It measures how resistant the soil is to penetration, helping engineers estimate its strength and bearing capacity.

2. Cone Penetration Test (CPT)

In a cone penetration test, a cone-shaped probe is pushed into the ground while measuring the soil’s resistance. This provides detailed information about the different soil layers beneath the site and is commonly used on larger projects.

3. Plate Load Test

A plate load test directly measures how the ground responds when a known load is applied to its surface.

Although it’s not performed on every project, it can provide valuable information about how the soil will behave under actual foundation loads.

4. Laboratory soil testing

Engineers may also collect soil samples and send them to a laboratory for further testing.

These tests help determine important soil properties such as moisture content, density, shear strength, and other characteristics that influence foundation design.

In short, here are some common methods used to determine soil bearing capacity:

Can you build safely on soil with low bearing capacity?

Yes, absolutely.

Finding soil with a lower bearing capacity of soil doesn’t automatically mean construction has to stop. In fact, many projects are built on challenging soil conditions every year.

What’s really important here is understanding the soil before construction begins.

Once engineers know how the soil behaves, they can recommend foundation systems or ground improvement methods that help the structure perform safely over the long term.

Bonus: Here are some common ways weak soils are improved for construction

You might also want to read: How Much Does A Structural Engineer Inspection Cost?

Don’t Let Site Conditions Delay Your Project

From geotechnical reports to environmental assessments, and more, our ACI-certified experts deliver the data and insights you need to stay on schedule.

Contact Us

To sum up

The bearing capacity of soil may not be something most property owners think about when planning a construction project. But then, every successful structure depends on the ground that is able to safely support its weight.

The soil’s bearing capacity helps engineers design foundations that reduce the risk of excessive settlement, structural movement, and long-term foundation problems.

Got more questions or need professional soil testing services?

You can get in touch with us.

At NewTech Engineering, we provide geotechnical investigations and soil testing services backed by decades of hands-on experience throughout the Carolinas.

Our engineers help property owners, developers, and contractors understand site conditions before construction begins, so they can make informed decisions and avoid costly problems later.

FAQs about bearing capacity of soil

What is bearing capacity of soil?

The bearing capacity of soil is the maximum load the ground can safely support without excessive settlement or failure. It helps engineers design foundations that can safely transfer structural loads into the ground.

Why is soil bearing capacity important?

Soil bearing capacity determines whether the ground can safely support a structure. If the soil isn’t strong enough, foundations may settle unevenly, crack, or experience long-term structural movement.

What is the difference between ultimate and allowable bearing capacity?

The ultimate bearing capacity is the maximum load the soil can support before failure. The allowable bearing capacity is a lower, safer value that engineers use when designing foundations.

What factors affect the bearing capacity of soil?

Several factors influence soil bearing capacity, including soil type, moisture content, soil density, groundwater conditions, foundation depth, and footing size.

What is shear failure in soil?

Shear failure occurs when the soil can no longer resist the load applied by a foundation. Depending on the soil conditions, it can occur as general shear failure, local shear failure, or punching shear failure.

Can soil bearing capacity be improved?

Yes. Engineers can improve weak soil using methods such as compaction, soil replacement, lime or cement stabilization, improved drainage, or by designing larger or deeper foundations.