How can you calculate the settlement of a spread footing? (2024)

The first step to calculate the settlement of a spread footing is to determine the properties of the soil. You will need to know the unit weight, the compressibility, and the modulus of elasticity of the soil. These properties can be obtained from laboratory tests, field tests, or empirical correlations. For example, you can use the standard penetration test (SPT) to measure the resistance of the soil to penetration, and then use a chart or a formula to estimate the modulus of elasticity. You can also use the consolidation test to measure the compressibility of the soil, which is related to the amount of water that can be squeezed out of the soil under pressure.

The second step to calculate the settlement of a spread footing is to determine the stress distribution under the footing. The stress distribution is the variation of the vertical stress in the soil due to the load of the footing. You can use the Boussinesq equation to calculate the stress distribution for a point load on a semi-infinite elastic medium. However, this equation is not very accurate for shallow foundations, because it neglects the effect of the soil surface and the shape of the footing. Therefore, you can use some correction factors or charts to modify the Boussinesq equation for different types of footings and soil conditions.

The third step to calculate the settlement of a spread footing is to determine the immediate settlement. The immediate settlement is the settlement that occurs as soon as the load is applied, and it is mainly due to the elastic deformation of the soil. You can use the following formula to calculate the immediate settlement: s_i = qB/Es where s_i is the immediate settlement, q is the net bearing pressure, B is the width of the footing, and Es is the modulus of elasticity of the soil. Note that this formula assumes a uniform stress distribution under the footing, which is not always true. Therefore, you can use some factors or charts to account for the non-uniformity of the stress distribution.

The fourth step to calculate the settlement of a spread footing is to determine the consolidation settlement. The consolidation settlement is the settlement that occurs over time, and it is mainly due to the dissipation of excess pore water pressure in the soil. You can use the following formula to calculate the consolidation settlement: s_c = CcHlog((p_o + p')/(p_o + p)) where s_c is the consolidation settlement, Cc is the compression index, H is the thickness of the compressible soil layer, p_o is the initial effective overburden pressure, p' is the increase in effective overburden pressure due to the footing load, and p is the preconsolidation pressure. Note that this formula assumes a one-dimensional consolidation, which is not always true. Therefore, you can use some factors or charts to account for the three-dimensional consolidation.

The fifth step to calculate the settlement of a spread footing is to determine the total settlement. The total settlement is the sum of the immediate settlement and the consolidation settlement. You can use the following formula to calculate the total settlement: s_t = s_i + s_c where s_t is the total settlement, s_i is the immediate settlement, and s_c is the consolidation settlement. Note that this formula assumes that there is no secondary compression or creep settlement, which are negligible for most soils. However, if the soil is highly organic or sensitive, you may need to consider the secondary compression or creep settlement as well.

The final step to calculate the settlement of a spread footing is to compare the total settlement with the allowable settlement. The allowable settlement is the maximum settlement that can be tolerated by the structure without compromising its functionality or safety. The allowable settlement depends on the type and use of the structure, the soil conditions, and the aesthetic requirements. For example, a residential building may have a lower allowable settlement than an industrial warehouse, and a clayey soil may have a higher allowable settlement than a sandy soil. You can use some guidelines or codes to determine the allowable settlement for different cases. If the total settlement is less than or equal to the allowable settlement, then the footing is acceptable. If the total settlement is greater than the allowable settlement, then you may need to redesign the footing or improve the soil.

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?

Civil Engineering Design

+ Follow

Thanks for your feedback

Your feedback is private. Like or react to bring the conversation to your network.