Civil foundation structures are essential components in construction, providing crucial support and stability for buildings, bridges, and other structures. There are different civil foundation structures, each tailored to specific soil conditions, load-bearing capacities, and structural needs. It is essential that architects, engineers, and construction professionals involved in the planning and execution of building projects understand the different types of civil foundation structures. This blog delves into the various civil foundation structures, their characteristics, and their suitability for different construction applications.
What is a foundation in civil engineering?
Foundational structures are vital components of any civil engineering project. They distribute the weight and load of the structure across a large area of soil. Engineers must be careful while designing and planning the weight distribution of the foundation to ensure it doesn’t exceed the load-bearing capacity of the soil and that the structure’s settlement remains within acceptable limits. Essentially, the foundation acts as the supportive base upon which a building rests, while the stable ground beneath it is known as the foundation bed.
The foundation is classified on the basis of its depth-to-width ratio. It can be broadly classified into:
- Shallow foundation
- Deep foundation
Types of foundation
What is a shallow foundation?
A shallow foundation is a foundation that transfers the structural load to the earth close to the surface. It has a shallow depth (approximately 1.5 m) and is generally less than the width.
We can classify shallow foundations further into
Strip footing
Strip footing, or continuous footing, is a versatile and economical solution for load-bearing walls and closely spaced columns. It efficiently distributes the weight of the structure over a larger area, ensuring stability and longevity.
For load-bearing walls, a strip footing provides continuous support along the entire length of the wall, ensuring stability and preventing uneven settlement. For closely spaced columns, a strip footing is more economical than individual spread footings.
Spread Footing
Spread footing (isolated footing or individual footing) is designed to bear the load of a single column. The footing is usually a square, circular, or rectangular slab with uniform thickness. Occasionally, they may feature a step or haunches to distribute the load across a larger area.
Combined Footing
Civil engineers use combined footings when two columns are close enough that their individual footings overlap. This type of footing is also employed when the property boundary is so close to one column that placing a spread footing entirely within the property line would result in an uneven weight distribution. By combining the footing with that of an interior column, engineers can evenly distribute the load. A combined footing may be rectangular or trapezoidal in shape.
Strap or Cantilever Footing
A strap footing, also known as a cantilever footing, comprises two separate footings linked by a structural strap or lever. This strap interconnects the two footings, causing them to function as a cohesive unit. The strap is engineered as a sturdy and inflexible beam. The individual footings are designed in such a way that their combined line of action aligns with the resultant of the entire load.
Mat or Raft Foundation
A mat or raft foundation is an extensive slab that provides support to numerous columns and walls across the entire structure or a significant portion of it. We use a mat foundation when the permissible soil pressure is limited or when columns and walls are in such close proximity that individual footings would either overlap or come very close to each other. Mat foundations prove beneficial in minimizing differential settlements, especially in non-uniform soils or situations with considerable variations in loads on individual columns.
What is a deep foundation?
We position a deep foundation at a considerable depth beneath the ground surface to facilitate the transfer of structural loads to the deeper layers of the earth. Typically, the depth-to-width ratio for such foundations exceeds 4 to 5.
While the construction of deep foundations is more intricate and costly compared to shallow foundations, it becomes a preferred choice when contending with unfavourable soil conditions near the surface, handling substantial design loads, and addressing site constraints.
Pile foundations
A pile foundation transfers a substantial load from the structure to the solid-rock stratum located below the ground surface and is used when shallow foundations are not safe. It is used to counteract the uplift of the structure caused by lateral forces like earthquakes and winds. Pile foundation can be further subdivided into-
- Sheet piles
- Load-bearing piles
- End-bearing piles
- Friction piles
- Soil compactor piles
Pier Foundation
A pier is an underground structure designed to bear heavier loads beyond the capacity of shallow foundations. Typically less deep than piles, pier foundations find common use in multi-story buildings. We eliminate the need for individual load tests on single piers by determining the base area based on the overall foundation plan strategy Consequently, pier foundations are more prevalent in constrained conditions.
Caison Foundation
A caisson foundation involves the placement of a pre-fabricated hollow cylinder into the soil to a specified depth, subsequently filled with concrete to form a foundation, commonly employed for bridge piers. Caissons are particularly susceptible to construction methods and require specialized construction knowledge.
Various types of caisson foundations include:
- Box Caissons.
- Floating Caissons.
- Pneumatic Caissons.
- Open Caissons.
- Sheeted Caissons.
- Excavated Caissons.
Functions of Foundation in Construction
The construction industry categorize the primary functions of the foundation as follows:
- Overall Lateral Stability: The foundation plays a crucial role in providing overall lateral stability to the structure.
- Level Surface for Substructure: Foundations are responsible for creating a level surface for the construction of the substructure.
- Even Load Distribution: Foundations ensure the even distribution of loads, preventing uneven stress on the structure.
- Safe Bearing Capacity: The foundation reduces load intensity to stay within the safe bearing capacity of the soil.
- Resistance to Soil Movement: Foundations resist and prevent the effects of soil movement.
- Solving Scouring and Undermining Issues: Foundation construction addresses problems related to scouring and undermining.
Comparison of different foundation methods
| Foundation Type | Description | When to Choose | Advantages | Disadvantages |
| Strip foundation | A continuous footing that runs along the length of a wall or row of columns | Small to medium-sized structures with good soil condition. | Economical and easy to construct. | Not suitable for heavy loads or soft soil conditions. |
| Combined footing | A footing that supports two or more columns. | Closely spaced columns with good soil conditions. | Economical and efficient load distribution. | Requires more complex design considerations. |
| Cantilever footing | A footing that is extended beyond the edge of the wall or column it supports. | Soft soil conditions or heavy loads over a small area. | Suitable for difficult soil conditions. | Requires more concrete and steel reinforcement. |
| Raft foundation | A large, thick footing that covers the entire area of a building | Soft or unstable soil. | Uniform load distribution. | Requires a large amount of concrete and steel reinforcement. |
| Pile foundation | A foundation that uses piles to transfer the weight of a building to a stable layer of soil or rock deep below the surface. | Soft or unstable soil, or heavy loads. | Suitable for very heavy loads or poor soil conditions. | Requires specialized equipment and expertise for installation. |
| Pier foundation | A foundation that uses piers to transfer the weight of a building to a stable layer of soil or rock deep below the surface. | Soft or unstable soil, or heavy loads. | Suitable for heavy loads or difficult soil conditions. | Requires specialized equipment and expertise for installation. |
| Caisson foundation | A foundation that uses caissons to transfer the weight of a building to a stable layer of soil or rock deep below the surface | Soft or unstable soil, or very heavy loads. | Suitable for extremely heavy loads or very poor soil conditions. | Requires specialized equipment and expertise for installation. |
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