Shallow foundations are a type of foundation that are constructed when the soil layer at a shallow depth, usually up to a depth of 1.5 meters, is strong enough to support the structural loads. These foundations are designed to distribute the weight of the structure over a larger area of the soil in order to prevent excessive settlement or heaving.
The depth of shallow foundations is typically less than their width. This design is important because it allows the foundation to spread the load over a wider area, which reduces the stress on the soil. By doing this, the foundation can be made shallower and wider, which can save money and construction time.
The choice to use shallow foundations over other types of foundations is often determined by the soil conditions and the load-bearing capacity of the soil. If the soil is strong enough to support the structure, then shallow foundations can be a cost-effective and efficient option. However, if the soil conditions are poor or the loads are too heavy, then other types of foundations may be necessary to ensure the stability and safety of the structure.
Different Types of Shallow Foundations
The different types of shallow foundation are:
- Strip footing
- Spread or isolated footing
- Combined footing Strap or cantilever footing
- Mat or raft Foundation
1. Strip Footing
When a load-bearing wall needs support, a strip footing is used. Similarly, when there is a row of columns with spread footings that overlap or come close to touching each other, a strip footing is a more cost-effective option compared to providing multiple spread footings in a single line. A strip footing is also commonly referred to as a continuous footing.
2. Spread or Isolated Footing or Individual Footing
A spread footing, also referred to as an isolated footing, pad footing, or individual footing, is a type of foundation used to support a single column. The footing takes the form of a circular, square, or rectangular slab with a consistent thickness. In some cases, the footing may also be stepped or haunched to distribute the load over a wider area. The purpose of the spread footing is to provide a stable base that can support the weight of the column without sinking or shifting. By distributing the load of the column across a larger area, the spread footing can help prevent settlement and ensure the long-term stability of the structure.
3. Combined Footing
When two columns are situated close to each other, to the point that their individual footings would overlap, a combined footing is used to support them. Similarly, if one of the columns is positioned very close to the property line, such that a spread footing would be loaded eccentrically when fully contained within the property line, a combined footing can be used. By joining the footing of the interior column with that of the exterior column, the load is distributed evenly.
There are two types of combined footings that can be used in such situations: rectangular and trapezoidal. These footings are designed to evenly distribute the load of the columns and ensure stability. This type of foundation is commonly used in construction projects where two columns are located close together and individual footings would not be feasible. By utilizing a combined footing, the columns can be properly supported without the need for overlapping individual footings or placing a spread footing eccentrically on the property line.
4. Strap or Cantilever Footing
A strap footing, also known as a cantilever footing, is a type of foundation that comprises two independent footings linked by a structural strap or lever. This connection between the footings enables them to function as a single unit. To ensure rigidity, the strap is designed as a rigid beam. The design of the individual footings is such that their combined line of action intersects with the resultant of the total load.
When the allowable soil pressure is comparatively high and the distance between the columns is significant, a strap footing proves to be a more cost-effective option than a combined footing. This type of footing offers numerous benefits, including improved structural stability and load distribution, which can help prevent structural failure. By connecting the two footings with a strap, the load-carrying capacity of the foundation is enhanced, resulting in a more stable structure.
Overall, a strap footing is an excellent foundation solution for structures that require additional support and stability. Its design allows for the effective distribution of loads, providing a stable base for the building. Additionally, its cost-effectiveness makes it a popular choice for structures that need to be built on a tight budget.
5. Mat or Raft Foundations
Mat or raft foundations are used to support multiple columns and walls under an entire structure or a large portion of it. They consist of a large slab that distributes the weight of the structure over a larger surface area, reducing the pressure on the underlying soil. These types of foundations are necessary when the allowable soil pressure is low, or the columns and walls are located so closely that individual footings would overlap or nearly touch one another.
One of the benefits of using mat foundations is that they can reduce the differential settlements on non-homogeneous soils. These soils have varying properties, which can lead to uneven settling of individual footings. With a mat foundation, the weight of the structure is distributed more evenly, reducing the potential for differential settlements.
Another advantage of mat foundations is their ability to handle a large variation in loads on individual columns. In traditional foundation systems, individual footings are designed to handle specific loads. However, with a mat foundation, the load is distributed over the entire surface area, making it possible to support columns with significantly different loads without requiring additional footings.
Overall, mat foundations are a useful solution for supporting large structures on difficult soils or where there is a need to reduce differential settlements and handle variations in loads.