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Types of settlement
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Methods of analysis
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Acceptable limits
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Here’s what else to consider
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Settlement analysis is a crucial step in shallow foundation design, as it determines how much the foundation will deform under the applied loads and soil conditions. Excessive settlement can cause structural damage, serviceability problems, and aesthetic issues. In this article, you will learn how to perform a settlement analysis for shallow foundations and what are the acceptable limits according to different codes and standards.
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1 Types of settlement
There are three main types of settlement that affect shallow foundations: immediate or elastic settlement, consolidation settlement, and secondary or creep settlement. Immediate settlement occurs due to the elastic deformation of the soil and the foundation under the applied load. It is usually calculated using elastic theory and soil parameters such as Young's modulus and Poisson's ratio. Consolidation settlement occurs due to the gradual dissipation of excess pore water pressure in the soil under the applied load. It is usually calculated using consolidation theory and soil parameters such as coefficient of consolidation and compression index. Secondary settlement occurs due to the long-term adjustment of the soil structure under the applied load. It is usually calculated using empirical or semi-empirical methods and soil parameters such as secondary compression index and time factor.
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Performing a settlement analysis for shallow foundations involves:1. Geotechnical Investigation: Assess soil properties.2. Foundation Design: Consider load, geometry, and soil.3. Settlement Analysis: Calculate expected settlement, accounting for soil compressibility.4. Load Distribution: Evaluate load transmission.5. Consolidation Analysis: If needed, assess primary and secondary settlements.6. Settlement Limits: Compare calculated settlements to acceptable limits based on project criteria, often considering total, differential, and rate of settlement. Limits depend on structure use and local codes. Consult experienced geotechnical engineers to ensure results' reliability. Mitigation may be necessary if settlements exceed limits.
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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 hom*ogeneous, 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 hom*ogeneous, 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.
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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.
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4 Here’s what else to consider
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