Shallow foundations are crucial for transferring structural loads to near-surface soils. They come in various types, including spread footings, strip footings, mat foundations, combined footings, and foundations. Each type has unique characteristics suited for different soil conditions and building needs.
Choosing the right shallow foundation involves considering soil properties, site constraints, structural loads, and local regulations. The selected foundation must efficiently distribute loads, resist , and accommodate future modifications while balancing cost and constructability factors.
Shallow Foundation Types
Common Shallow Foundation Varieties
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Mat foundations more challenging to modify for localized changes
Load Distribution and Soil Interaction
Strip footings excel at distributing loads along walls
Spread footings efficient for isolated column loads
Mat foundations distribute loads evenly across large areas
Beneficial in areas with potential for soil settlement or heave
Slab-on-grade foundations integrate well with soil for uniform support
May be problematic in expansive soils without proper design
Combined footings help manage loads between closely spaced columns
Reduce differential settlement concerns in challenging soil conditions
Key Terms to Review (19)
ACI 318: ACI 318 is a standard developed by the American Concrete Institute that provides guidelines and requirements for designing concrete structures. It covers the minimum design and construction requirements for reinforced concrete buildings, including structural integrity, safety factors, and material specifications, ensuring that structures can withstand various loads and environmental conditions.
Backfilling: Backfilling is the process of refilling an excavated area with soil or other materials after construction work has been completed. This practice is essential for ensuring stability and support for shallow foundations, as it helps restore the ground surface and improves load distribution. Proper backfilling techniques can prevent settlement, erosion, and other issues that may arise after the foundation is laid.
Bearing Capacity: Bearing capacity is the ability of soil to support the loads applied to it without experiencing failure or excessive settlement. This concept is crucial in determining the suitability of different foundation types, ensuring that structures can be built safely and sustainably, taking into account various factors like soil conditions and load distributions.
Cohesive soil: Cohesive soil is a type of fine-grained soil that exhibits strong inter-particle attraction, primarily due to its clay content, which allows it to retain shape and resist deformation when subjected to external forces. This soil is significant in various engineering applications due to its unique properties, such as high plasticity and compressibility, influencing factors like drainage, load-bearing capacity, and stability in construction projects.
Combined footing: A combined footing is a type of shallow foundation that supports more than one column or wall and distributes their loads over a larger area of soil. This design helps in preventing excessive settlement and ensures stability when the columns are close together or when one column bears significantly more load than the others. Combined footings are particularly useful in situations where individual footings might overlap or where the soil has low bearing capacity.
Cone Penetration Test: The cone penetration test (CPT) is a sophisticated in-situ testing method used to determine the geotechnical properties of soil. It involves pushing a cone-shaped probe into the ground at a constant rate and measuring the resistance of the soil against the cone. This test provides valuable information about soil stratigraphy, strength, and compressibility, which are essential for foundation design and other engineering applications.
Eurocode 7: Eurocode 7 is a European standard that provides guidelines for the geotechnical design of structures, ensuring safety and reliability in construction. This code plays a crucial role in defining the principles for assessing ground conditions, designing foundations, and ensuring stability, especially concerning shallow foundations, which are essential for supporting various types of structures. It emphasizes the need for comprehensive site investigation and risk assessment to ensure that the foundations can adequately support the loads imposed by the structure.
Excavation: Excavation refers to the process of removing earth or rock from a site to create space for construction or to expose underground features. This activity is crucial in construction projects, particularly for shallow foundations, as it prepares the ground by ensuring that the base is stable and suitable for the load-bearing requirements of a structure.
Factor of Safety: The factor of safety is a measure used in engineering to provide a safety margin in design, ensuring that structures can withstand loads greater than the maximum expected load. It is defined as the ratio of the strength of a material or system to the actual applied load, indicating how much stronger a system is than what it needs to be for safe operation. This concept is crucial in various engineering fields, including geotechnical engineering, where it plays a vital role in assessing the stability of structures and soil conditions.
Granular soil: Granular soil refers to a type of soil that is composed primarily of larger particles, such as sand and gravel, which have little to no cohesion. This type of soil is essential for various geotechnical applications because of its drainage properties and the ability to compact effectively under load. Its characteristics play a critical role in seepage analysis, foundation design, bearing capacity, and stabilization techniques.
Load Distribution: Load distribution refers to how the weight or forces applied to a structure are spread out across its foundation or support systems. Understanding load distribution is essential for ensuring stability and safety, as it influences the design and effectiveness of various foundation types and reinforcement techniques. Proper load distribution helps in preventing localized failures and ensures that the underlying soil can adequately support the structures built upon it.
Mat foundation: A mat foundation, also known as a raft foundation, is a large concrete slab that supports multiple columns and walls of a structure, distributing the load over a wide area. This type of foundation is typically used when soil conditions are weak or when loads from the building are significant, allowing it to help minimize settlement by spreading the weight more evenly. Its design can be crucial in ensuring stability and safety for structures built on challenging sites.
Overturning moment: An overturning moment is a rotational force that tends to cause a structure, such as a retaining wall or foundation, to tip over about its base. This moment is typically generated by lateral loads, like soil pressure or water pressure, acting on the structure. The balance between overturning moments and stabilizing moments is critical for ensuring the structural integrity and stability of foundations and retaining walls.
Settlement: Settlement refers to the gradual downward movement or sinking of the ground surface due to various factors, such as load from structures, changes in moisture content, and soil compaction. Understanding settlement is crucial in geotechnical science because it affects the stability and safety of structures, particularly when considering the soil's behavior under different loading conditions and construction methods.
Site conditions: Site conditions refer to the physical characteristics of a location where construction is planned, including soil type, groundwater levels, and environmental factors. Understanding site conditions is crucial because they directly affect the design and performance of foundations, both shallow and deep. Factors such as soil bearing capacity, settlement potential, and presence of fill materials play a significant role in determining which foundation type is most suitable for a specific site.
Slab-on-grade: A slab-on-grade is a type of foundation where a thick concrete slab is poured directly onto the ground, providing a flat surface for structures like homes or commercial buildings. This design allows for efficient construction and can effectively distribute loads from the structure above. The slab is typically reinforced with steel to enhance its strength and durability, making it a popular choice in areas with stable soil conditions.
Spread footing: A spread footing is a type of shallow foundation that distributes the load of a structure over a larger area to reduce the pressure on the underlying soil. This foundational approach helps in providing stability and support to buildings, especially in areas with weak soil conditions. By increasing the base area, spread footings can minimize the risk of excessive settlement and structural failure, making them crucial in the design and construction process.
Standard Penetration Test: The Standard Penetration Test (SPT) is a widely used in-situ testing method for assessing the geotechnical properties of soil. It involves driving a split barrel sampler into the ground at the bottom of a borehole using a hammer to measure resistance. The results help in determining soil strength, stratification, and index properties, which are crucial for evaluating the suitability of sites for shallow foundations and other structures.
Strip footing: A strip footing is a type of shallow foundation that is designed to support continuous loads along a line, typically used for walls or rows of columns. This foundation type spreads the load from the structure over a larger area of soil, reducing pressure and minimizing the risk of settlement. It’s particularly effective in providing stability for structures with long, linear loads.