Soil Bearing Capacity: What Engineers Need to Know

 

What is Bearing Capacity

Bearing capacity is defined as the weight or the load that the soil can carry without failure or excessive deformation. It is an important parameter in geotechnical engineering, which identifies the stability of the structures that are constructed in the ground.

How was Bearing Capacity discovered?

Karl Terzaghi, a renowned civil engineer, developed a comprehensive theory for evaluating the ultimate bearing capacity of shallow foundations in 1943. We've already explored his journey and contributions in a previous blog, so let's dive straight into the concept.

Purpose of Finding Bearing Capacity of Soil

It is important in the determination of the bearing capacity of soil to be able to determine the safety and stability of structures. Trying to make the foundation settle or sink due to the inability of the soil to support the weight of the structure, cracks in the walls and floors, structural damage or even a collapse may occur.

Bearing Capacity of Different Types of Soil

The bearing capacity of soil varies greatly depending on its type, density, and moisture content. Here are some general bearing capacity values for different soil types:

 Clay Soils: 1-5 tons/sq.m (10-50 kPa)

 Sandy Soils: 5-20 tons/sq.m (50-200 kPa)

 Gravelly Soils: 10-50 tons/sq.m (100-500 kPa)

 Rock: 50-1000 tons/sq.m (500-10,000 kPa)

Factors Affecting Bearing Capacity

Bearing capacity of soil can be affected by several factors and they include:

Soil Type and Properties: Density, moisture content and shear strength.

Depth and Shape of foundation: A deeper foundation will be able to cause the load in more stable layers of soil.

Water Table and Drainage: the availability of water may also decrease the soil strength.

Type and Duration of loads: Dynamic loads or long-term loads may lead to settlement of the soil.

Methods for Determining Bearing Capacity

There are several procedures that engineers employ to ascertain the bearing capacity of soil, they include:

Plate Load Test: It is a field test where a plate is loaded on the surface of the ground. 

Standard Penetration Test (SPT): an in-situ test that is commonly employed to determine the soil resistance.

Cone Penetration Test (CPT): A test where a cone tool is used to measure the soil resistance.

Laboratory Tests: Triaxial and unconfined compression tests are also possible.

Famous Structures that Failed due to Low Bearing Capacity

Among the remarkable examples of structures that failed because of low bearing capacity of soil, there are:

Pisa leaning tower: The tilt of the tower is a typical case of soil settlement and bearing capacity failure. The foundation of the tower is just 3 meters deep and the soil under the tower is soft and compressible.

The Transcona Grain Elevator: This grain elevator in Canada had a failure in its bearing capacity in 1913 causing a massive tilt accompanied by damages on the structure. This was blamed on the soft clay soil on the ground under the foundation.

The Mexico City Subway: Mexico City subway has witnessed some cases of soil settlement and bearing capacity failures because of the soft and compressible soil in the city.

The issue with the Leaning Tower of Pisa is that the tower has undergone settlement of the foundation over the years and the ground on which the tower rests cannot bear the load of the building anymore. It is a typical case of a bearing capacity failure.

Calculating Bearing Capacity

The bearing capacity of soil can be calculated using various methods, including:

Terzaghi's Bearing Capacity Equation: a widely used equation that considers soil properties like cohesion, friction angle, and unit weight

Meyerhof's Method: a method that considers the shape and depth of the foundation

Brinch Hansen's Method: A method that accounts for the inclination and eccentricity of the load.

The formula for Terzaghi's bearing capacity equation is:

Qult = c' Nc + q Nq + 0.5γBNγ

Where:

 Qult = ultimate bearing capacity

 c' = effective cohesion

 q = effective overburden pressure

 γ = effective unit weight of the soil

 B = footing width

 Nc, Nq, Nγ = bearing capacity factors

Soil Improvement Techniques

After all we have discussed some of the common factor affecting bearing capacity of soil. Now we will see in detailed some of the techniques that engineers usually do while improving the Soil bearing capacity. We will discussed them one by one:

Compaction

It is common method of soil improvement techniques in which soil is compacted by applying mechanical force (using rollers, vibrators or rammers). By doing so air voids in the soil particles reduces, therefore, increases the soil density, and improves its shear strength. This process makes the soil stable and therefore less prone to settling under loads. It also reduces the permeability (ability to absorb water) of soil.

Soil Stabilization

Soil stabilization is the technique in which weak soil is improved by mixing them with some stabilizing agents such as lime, cement, or fly ash. The process involves selecting the appropriate stabilizer, preparing the soil by assessing its type and moisture content, and mixing the stabilizer with the soil using either dry or wet methods. The mixture is then compacted to increase its density and bearing capacity.

After mixing, the stabilized soil undergoes curing to allow chemical reactions to strengthen the material. Tests like the California Bearing Ratio (CBR) are performed to ensure the desired improvements in soil strength. This technique is commonly applied in foundation work, road construction, and pavement bases to create more durable, stable surfaces.

Grouting

Most of the students does not understand the word grouting. There are various types of grouting depending upon the type of soil (e.g. clay soils may require a different grout than sandy soils). In simple words it is a mixture of cement, water, aggregates and admixtures.

Grouting is one of the ground improvement techniques. It is one of the methods that improves the properties of soil increasing its strength and bearing capacity. Grouting is a widely used technique in geotechnical engineering for various applications, including soil stabilization, foundation repair, and underground construction.

Geotextiles

Geotextiles are permeable fabrics used to stabilize the soil. They are made from natural materials such as polypropylene or polyester. It prevents soil erosion and instability. It allows water to drain through while retaining soil particles. The main function of geotextiles is to provide tensile strength to the soil.

Vibroflotation

Vibroflotation is a ground improvement technique that is used to densify loose soil, granular soil like sand or gravel. This process involves three processes.

A Vibrating probe is inserted into the ground, creating a void. Then aggregate material (sand or gravel) is added to the voids. In the last step, the vibrating probe compacts the aggregates, densifying the surrounding soil.

Dynamic Compaction

Dynamic compaction is a ground improvement technique that involves repeatedly dropping a heavy weight or tamper onto the ground surface to densify the soil. This process compacts the soil, improving its strength and stability, while minimizing settlement and deformation. By enhancing the soil's bearing capacity, dynamic compaction enables it to support heavy loads. The technique involves dropping a heavy weight (typically 5-20 tons) from a height (typically 10-30 meters) in a grid pattern to achieve uniform compaction.

Conclusion

The bearing capacity of soil is a critical parameter in geotechnical engineering, determining the stability of structures built on the ground. By understanding the factors that affect bearing capacity and using various methods to determine it, engineers can design safe and durable foundations that support the weight of structures. It's essential to consider the soil properties, foundation design, and loading conditions to ensure the stability of structures and prevent failures.