How do you select and design foundations for different structures?

1 Shallow foundations

Shallow foundations are the most common type of foundations for low-rise buildings and light structures. They are placed near the surface of the ground, usually within a depth of 3 meters. They can be classified into two categories: spread foundations and mat foundations. Spread foundations are individual footings that support single columns or walls. Mat foundations are continuous slabs that cover the entire area of the structure and distribute the load evenly. Shallow foundations are suitable for soils that have adequate bearing capacity, low settlement, and low compressibility.

2 Deep foundations

Deep foundations are used for high-rise buildings, bridges, and heavy structures that require greater stability and load transfer. They are placed below the surface of the ground, usually at a depth of more than 3 meters. They can be classified into two categories: pile foundations and caisson foundations. Pile foundations are long, slender elements that are driven, cast, or drilled into the soil. They can be made of steel, concrete, or timber. Caisson foundations are large, cylindrical chambers that are excavated and filled with concrete. They can be open, closed, or pneumatic. Deep foundations are suitable for soils that have low bearing capacity, high settlement, or high compressibility.

3 Factors affecting foundation selection

The selection of the type and size of foundation depends on several factors, such as the type and size of the structure, the soil conditions, the environmental impacts, and the economic feasibility. The foundation should match the structural system, the load distribution, and the dimensions of the structure. Additionally, it should consider soil properties such as strength, stiffness, density, water content, and variability. Furthermore, it should minimize disturbance to the natural environment, such as groundwater, vegetation, and wildlife. Finally, it should balance cost and benefit of the project regarding material, labor, equipment, and maintenance costs.

4 Foundation design principles

The design of foundations involves two main aspects: geotechnical design and structural design. Geotechnical design deals with the interaction between the foundation and the soil. It aims to ensure that the foundation can safely support the load of the structure without excessive settlement, shear failure, or uplift. Structural design deals with the internal forces and stresses within the foundation. It aims to ensure that the foundation can resist the bending, shear, torsion, and axial forces without cracking, buckling, or collapsing.

5 Foundation design methods

The methods of foundation design vary depending on the type and complexity of the foundation and the soil. However, some general steps can be followed, such as collecting and analyzing site data, determining the design load, selecting the type and size of foundation, performing geotechnical design, performing structural design, and preparing drawings, specifications, and reports. Specifically, this includes the topography, geology, hydrology, geotechnical investigation results, dead load, live load, wind load, seismic load of the structure and foundation, factors affecting foundation selection and preliminary calculations, bearing capacity, settlement, stability and uplift resistance of the foundation, internal forces and stresses of the reinforcement, connections and details of the foundation. Ultimately, these steps will help document design assumptions, calculations, results and recommendations.

6 Foundation design challenges

Designing foundations is a complex task, involving many uncertainties, assumptions, and limitations that can influence the performance and reliability of the foundation. Variability and unpredictability of soil conditions can cause unexpected settlement, differential settlement, slope instability, liquefaction, or erosion of the foundation. Additionally, complexity and diversity of load combinations can result in dynamic, cyclic, or thermal effects on the foundation. Furthermore, environmental and social impacts may lead to pollution, contamination, or damage to the surrounding environment or community. Lastly, quality and safety issues can result in defects, errors, or accidents during construction, testing, or operation of the foundation.

7 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?