How do you perform a settlement analysis for shallow foundations and what are the acceptable limits?

2 Methods of analysis

When analyzing the settlement of shallow foundations, the method chosen depends on the type of soil, the type of foundation, and the desired accuracy. Terzaghi's method is based on the assumption that the settlement of a shallow foundation on a homogeneous, isotropic, and saturated clay layer is equal to the average consolidation settlement of the soil layer beneath it. This method uses Terzaghi's one-dimensional consolidation equation and a shape factor to consider the foundation geometry. Schmertmann's method is based on the assumption that the settlement of a shallow foundation on a homogeneous, isotropic, and unsaturated sand layer is equal to the sum of immediate and consolidation settlement. This method utilizes a pressure influence factor and a depth influence factor to take into account stress distribution and soil stiffness variation with depth. Lastly, Meyerhof's method relies on the assumption that the settlement of a shallow foundation on a layered soil system is equal to the sum of immediate and consolidation settlement for each soil layer beneath it. It employs a bearing capacity factor and a settlement factor to account for both foundation geometry and soil properties.

3 Acceptable limits

When assessing the acceptability of a shallow foundation settlement, there are no hard-and-fast rules, as it depends on factors such as the type and function of the structure, the sensitivity of the structure to differential settlement, the soil conditions, and the design codes and standards. However, literature and practice offer some general guidelines. For instance, Eurocode 7 suggests that total settlement should not exceed 25 mm for buildings and 40 mm for bridges, while differential settlement should not exceed 1/500 of the span for buildings and 1/800 of the span for bridges. Similarly, American Society of Civil Engineers recommends that total settlement should not exceed 50 mm for residential buildings and 75 mm for industrial buildings, while differential settlement should not exceed 19 mm for residential buildings and 25 mm for industrial buildings. Additionally, Eurocode 7 suggests that angular distortion should not exceed 1/300 for buildings and 1/500 for bridges.

4 Here’s what else to consider

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