How can you calculate the settlement of a shallow foundation?

Shallow foundations are one of the most common types of foundations used in engineering design. They transfer the load of a structure to the soil near the surface, usually within a depth of 3 meters. However, shallow foundations can experience settlement, which is the downward movement of the foundation due to the compression and deformation of the soil. Settlement can affect the stability, performance, and aesthetics of a structure, so it is important to calculate it before and after construction. In this article, you will learn how to calculate the settlement of a shallow foundation using four steps: estimating the soil properties, determining the stress distribution, applying the settlement formula, and checking the settlement criteria.

1 Soil properties

The first step is to estimate the soil properties that affect the settlement of a shallow foundation. These include the unit weight, the shear strength, the modulus of elasticity, and the coefficient of consolidation. You can obtain these properties from soil tests, empirical correlations, or typical values for different soil types. For example, the unit weight of sand is usually between 16 and 20 kN/m3, while the shear strength of clay is usually between 50 and 150 kPa.

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Kehinde Omotayo (R.Eng) I Msc l MNSE I MEng I A.M.ASCE I GT FocusScholarX2

PhD student in Engineering and computational sciences | Research & Teaching Assistant at Tennessee State University
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Determination of soil properties involves a combination of field sampling and laboratory testing. Standard tests include grain size analysis, Atterberg limits, compaction tests, permeability tests, consolidation tests, and shear strength tests, among others. All these tests help engineers and scientists understand soil behavior under different environmental conditions and loads, guiding the design of foundations, and earthworks.

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2 Stress distribution

The second step is to determine the stress distribution under the shallow foundation. This depends on the shape, size, and depth of the foundation, as well as the load intensity and the soil profile. You can use various methods to calculate the stress distribution, such as the Boussinesq theory, the Westergaard theory, or the finite element method. The stress distribution will give you the vertical stress increment at any point below the foundation due to the applied load.

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Kehinde Omotayo (R.Eng) I Msc l MNSE I MEng I A.M.ASCE I GT FocusScholarX2

PhD student in Engineering and computational sciences | Research & Teaching Assistant at Tennessee State University
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Stress distribution in the soil beneath foundations is important for understanding how the loads from structures are transferred to the ground and how this affects settlement and stability. The concept is based on the principle that the soil must safely support the loads applied to it without excessive deformation or failure. Several theories and models used to describe stress distribution in soils include Boussinesq theory and Westergaard theory. Each model and theory is applicable under different conditions and assumptions. Boussinesq's theory is a common theory for elastic soils, and the Westergaard solution for more specific conditions.

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3 Settlement formula

The third step is to apply the settlement formula to calculate the settlement of a shallow foundation. There are different formulas for different types of soils and settlement mechanisms. For example, for elastic settlement in granular soils, you can use the formula: s = qB(1 - v) / E where s is the settlement, q is the average net pressure, B is the width of the foundation, v is the Poisson's ratio, and E is the modulus of elasticity. For consolidation settlement in cohesive soils, you can use the formula: s = CcHlog(p1 / p0) where s is the settlement, Cc is the compression index, H is the thickness of the compressible layer, p1 is the final effective stress, and p0 is the initial effective stress.

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4 Settlement criteria

The fourth step is to check the settlement criteria to evaluate the acceptability of the calculated settlement. The settlement criteria depend on the type of structure, the soil conditions, and the serviceability requirements. For example, for buildings, the allowable settlement is usually between 25 and 50 mm, while for bridges, it is usually between 10 and 20 mm. You should also check the differential settlement, which is the difference in settlement between two points of the foundation. The differential settlement should not exceed a certain limit, such as 1/500 of the span or 1/300 of the height.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Kehinde Omotayo (R.Eng) I Msc l MNSE I MEng I A.M.ASCE I GT FocusScholarX2

PhD student in Engineering and computational sciences | Research & Teaching Assistant at Tennessee State University
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In my experience, determining the settlement of a shallow foundation involves calculating how much a structure will sink or settle into the ground after construction as a result of the weight of the structure itself and any load applied to it. This is important for ensuring the safety and longevity of buildings and structures. Settlement calculations generally consider the immediate settlement of cohesive soils, consolidation settlement of clay (cohesive soils), and the settlement of sandy (cohesionless) soils.

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How can you calculate the settlement of a shallow foundation?

1

2

3

4

5

1 Soil properties

2 Stress distribution

3 Settlement formula

4 Settlement criteria

5 Here’s what else to consider

Engineering Design

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How can you calculate the settlement of a shallow foundation?

1

2

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4

5

1 Soil properties

2 Stress distribution

3 Settlement formula

4 Settlement criteria

5 Here’s what else to consider

Engineering Design

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