Bearing Capacity of Soil: Foundation Design Insights

The bearing capacity of soil is a crucial aspect to consider when planning and designing any construction project. Understanding the soil’s ability to support weight will help you make informed decisions on the type and size of the structures to be built.

bearing capacity of soil

With a thorough grasp on this topic, you can ensure the stability and safety of your building project.

Understanding Bearing Capacity of Soil

The bearing capacity of soil is the maximum load per unit area that the soil can support. This is a critical factor in designing structures, such as buildings and bridges, that rely on the ground for support.

When engineers and architects design structures, they must consider the soil’s ability to withstand the weight of the structure and its contents to ensure stability and safety of the structure. If the soil cannot support the applied load, it may lead to settlement, deformation, or even collapse of the structure. Furthermore, the soil should be able to accommodate additional loads that may be exerted on the structure during its lifetime, such as heavy equipment or increased occupancy.

In order to ensure a stable foundation, engineers must analyze the soil’s characteristics, including density, moisture content, and shear strength. These factors can have a significant impact on the bearing capacity and must be considered during the design process. By doing so, structural failures can be avoided, and the overall integrity of the structure can be ensured.

Factors Affecting Bearing Capacity of Soil

Type of Soil

The type of soil plays a crucial role in determining its bearing capacity. Granular soils like sands and gravels usually have higher bearing capacities due to their better interlocking and frictional properties. On the other hand, cohesive soils like clays possess lower bearing capacities because they are more susceptible to consolidation and settlement when subjected to loading.

Load Type

The nature and distribution of the load applied on the soil can affect its bearing capacity. Uniformly distributed loads tend to increase the overall stability of the soil, while concentrated loads can cause uneven stress distribution and lead to localized settlements. It’s essential to consider the load type while determining the soil’s bearing capacity to ensure a safe foundation design for structures.

Ground Water Level

The presence of groundwater can substantially impact the bearing capacity of the soil. High groundwater levels can increase the pore water pressure in the soil, resulting in decreased effective stress and subsequently, reduced bearing capacity. It is crucial to assess the ground water level during the soil investigation process to make necessary design adjustments for proper foundation performance.

Methods of Determining Soil Bearing Capacity

Geotechnical engineers typically use a combination of in-situ tests and laboratory tests to determine the bearing capacity of the soil for a specific engineering project.

In-Situ Tests

In-situ tests are performed directly on-site to determine the soil’s bearing capacity. One popular method is the Standard Penetration Test (SPT). During SPT, a device is driven into the soil, and the number of blows required to reach a specific depth is recorded.

Another widely used in-situ test is the Cone Penetration Test (CPT). In this test, a cone-shaped device is pushed into the soil while continuously measuring resistance. High resistance indicates a higher bearing capacity. Static and dynamic plate load tests are also effective in determining the soil’s bearing capacity.

The diagram below illustrates the SPT and CPT methods.

Standard Penetration Test and Cone Penetration Test

Laboratory Tests

Laboratory tests involve taking soil samples from the site and testing them in a controlled environment. One common laboratory test is the Proctor Compaction Test, which determines the optimal moisture content and maximum dry density of soil. The test can provide information on the degree of soil density, which is one of the factors that affect soil bearing capacity.

Another important laboratory test is the Triaxial Shear Test. In this test, a cylindrical soil sample is subjected to different combinations of axial and lateral pressures. The resulting data is used to calculate several vital parameters, like cohesion and angle of internal friction, which play a significant role in determining the soil’s bearing capacity.

Finally, there’s the Unconfined Compression Test. This method involves subjecting a cylindrical soil sample to axial compression without any lateral confinement. The test provides valuable data on the soil’s strength and compressibility.

Bearing Capacity Theories

Several theories and methods have been developed to calculate or estimate soil bearing capacity. Here are some of the commonly used theories.

Terzaghi’s Theory

Terzaghi’s bearing capacity theory is widely regarded as the foundation for other bearing capacity theories. This is because Terzaghi was the first to present a comprehensive theory for evaluating the ultimate bearing capacity of rough shallow foundations.

bearing capacity of rough shallow foundations

His theory is based on several key assumptions, including that the foundation is continuous, shallow, and that the effect of soil above the bottom of the foundation can be replaced by an equivalent surcharge. In addition, the shearing resistance of this soil along the failure surfaces is neglected, and the failure surface of the soil is assumed to be similar to general shear failure.

Terzaghi’s equation is used to calculate the ultimate bearing capacity of the soil, taking into account various factors such as the soil’s shear strength and the width and depth of the foundation. For a continuous or strip footing, Terzaghi’s equation can be written as follows:

Terzaghi's equation

Where:

  • qu = ultimate bearing capacity of the soil [kN/m2]
  • c’ = cohesion of the soil [kN/m2]
  • q = overburden pressure [kN/m2]
  • γ = soil unit weight [kN/m3]
  • B = width of the footing [m]
  • Nc, Nq, Nγ = bearing capacity factors [unitless]

The bearing capacity factors are functions of the soil friction angle, as shown in the table below.

For square footing, Terzaghi’s equation can be modified as follows:

Terzaghi's equation for square footing

For circular footing:

Terzaghi's equation for circular footing

Where:

  • B = diameter of the footing [m]

The equations above calculate the soil’s maximum weight capacity, known as the ultimate bearing capacity. If the foundation’s bearing stress exceeds this capacity, it can lead to shear failure. To avoid this, we must design the foundation for a lower bearing capacity, known as the net allowable bearing capacity, which includes the factor of safety and considers the surcharge at foundation level, as shown in the following formula:

soil's maximum weight capacity

Where:

  • qall = net allowable bearing capacity [kN/m2]
  • q = surcharge at foundation level [kN/m2]
  • FS = factor of safety [unitless]
  • Df = depth of foundation [m]

While Terzaghi’s theory has since been refined and expanded upon by other researchers, it remains an important foundation (pun intended!) for understanding the behavior of shallow foundations and the factors that influence their ultimate bearing capacity.

Meyerhof’s Theory

Terzagi’s equations have several shortcomings, including not being applicable to rectangular foundations with a length-to-width ratio between 0 and 1, not considering shearing resistance along the failure surface in soil above the foundation, and not accounting for the inclination of the load on the foundation.

To address these issues, Meyerhof proposed an alternative form of the general bearing capacity equation, as follows:

bearing capacity equation

Where:

  • Fcs, Fqs, Fγs= shape factors [unitless]
  • Fcd, Fqd, Fγd = depth factors [unitless]
  • Fci, Fqi, Fγi = inclination factors [unitless]

The shape factor accounts for the shape of the footing, which is primarily dependent on its width-to-length ratio. On the other hand, the depth factor accounts for the depth of the footing, which is primarily dependent on its depth-to-width ratio. Lastly, the inclination factor accounts for the inclination of the load on the foundation with respect to the vertical. For a vertical load, the inclination factors are equal to zero.

All of these factors are affected by the soil friction angle.

Meyerhof’s Theory is just one of the well-known modifications of Terzaghi’s Theory, which considers the angle of internal friction, cohesion, and the depth of the soil.

Aside from Meyerhof’s Theory, there are other modifications to Terzaghi’s Theory that take into account other factors that can affect soil bearing capacity. For instance, soil compressibility is an important consideration since it can significantly impact the ability of the soil to support a load. In addition, the eccentricity of loading is also another factor that can affect the soil’s bearing capacity, as it can cause the load to be unevenly distributed across the soil.

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