What are the types of building foundations used in Design?
Generally, foundation system are divided into two categories, shallow and deep foundations. Shallow foundations are almost always cast against earth. The site is excavated to relatively shallow depths, underneath the ground elevation. They are easier to construct, cheaper, and therefore usually a more popular option in design compared to deep foundations.
Deep Foundations vs. Shallow Foundations
Deep foundations are more commonly found on sites where the soil conditions are unfavorable. For example, most marine projects will use deep foundations because they are much more stable than implementing shallow foundations. This is because deep foundations will travel much deeper into the earth, past the poor soil conditions, usually landing on harder rock-type soil that is more stable.
The main differences of these two foundation systems includes cost, depth of the bearing soil, load transfer method, and design capacities. Shallow foundations are used primarily when the load will be transferred into a bearing soil located at a shallow depth (as little as 1 meter or 3 feet). Deep foundations are used when the load is transferred into a deep strata (ranging from 20-65 meters or 60-200 feet).
Shallow foundation construction is the cheaper option as it requires less labor, equipment, and materials. As aforementioned, there is relatively little excavation and labor required to dig and form the shallow foundations.
The process for constructing a deep foundation is more complex and costly. It requires heavier equipment, skilled labor, and proper time management. Deep foundations can be driven into the ground or cast against earth. Soil is much harder to excavate and soil pressure gets higher as you go deeper.
Shallow foundations rely primarily on end bearing on the soil. Reinforcement in shallow foundations helps resist overturning and bending of the foundation. The use of deep foundation provides lateral support, resists uplift, and supports larger loads. It relies on both end bearing and skin friction. Different types of shallow and deep foundations are shown on Figure 1.
Figure 1: Types of Foundation Systems
Isolated footings, also known as spread or pad footings, are the simplest and most common type of foundation. They are typically used when the load transfer into the soil from the structure is coming from columns. Each footing supports its own column that it takes the load from and spreads it to the soil it’s bearing on. Isolated footings are almost always square or rectangular. This makes them easier to analyze and construct. Dimensions of the footing are estimated based on the loads from the column, and the safe bearing capacity and excessive settlement of the soil.
Figure 2: Isolated Footing
Wall footings, also known as strip footings, are used to support the weight from load-bearing and non-structural walls to transmit and distribute the loads across an area of the soil in which the soil has an adequate load-bearing capacity. Similar to isolated footings, the wider area of the footing spreads out the gravity force from the wall to reduce the chances of settlement. This is especially useful when supporting load bearing walls, as they would be supporting not just the dead loads of the structure, but design loads as well. Wall footings are also cast with plain or reinforced concrete and sometimes are precast before being brought to the site. Economical wall footings can be constructed provided that the imposed loads are minimum and the soil beneath the footing has good soil conditions.
Figure 3: Wall Footing
Similar to isolated footings, combined footing is constructed when structural loads are carried by columns. This is used when two or more columns are so close to each other that their isolated footing overlap each other. Construction of combined footings may be more economical when the footing materials (concrete) are cheaper than the labor to form two separate footings. Combined footing can also be provided when the column is close to the property line that it makes the isolated footing eccentrically loaded when kept entirely into the property line. Combined footing may be rectangular, trapezoidal, or tee-shaped in plan, depending on the size and location of the columns supported by the footing.
Figure 4: Combined Footing
Strap footings, also known as cantilever footings, are basically two isolated footings connected with a strap beam. Strap footings are used when the distance between the isolated footing is large enough that when a combined footing is used, the width of the footing becomes narrow and causes high bending moments. Specifically, strap beams are commonly used to connect two footings that are supporting columns part of a moment frame that will see substantial lateral forces. The strap beam will help reduce the effects of the lateral load in the same direction that it is running and will not put any additional vertical gravity pressure onto the soil.
Figure 5: Strap Foundation
As the name implies, a mat foundation, also known as raft foundation, is a type of foundation which is spread entirely across the area of the building supporting heavy loads from columns or walls, similar to a slab on grade. It is most often used when basements are to be constructed where the entire basement floor slab acts as the foundation. Mat foundation is is chosen when the building is to be supported by weak soil, thus building loads are spread over an extremely large area. This prevents differential settlement that would be prevalent with the use of isolated footings. This is most suitable and economical to use when the building footprint is rather small or if columns are close together, which will limit material cost. Conversely, mat foundations are not desirable to construct when the ground water is located above the bearing surface of the soil.
Figure 6: Mat or Raft Foundation
Advantages of using pile foundations:
- Piles can be precast into any required specification or design requirement in a controlled environment.
- Precast piles are shipped to the site and immediately able for installation, thus resulting to a more rapid work progress.
- Cast-in-place concrete piles can be used to support large and tall structures like skyscrapers, where a shallow foundation would not suffice.
- Driven piles can also be used on locations where it is not advisable to drill holes due to pressurized ground water tables.
- Pile foundations can be used in locations where soil conditions make other foundation types impossible.
Disadvantages of using pile foundations:
- Concrete piles need to be reinforced adequately to sustain the stresses if driven into the ground
- Advance planning and equipment is essential for proper handling and driving of piles into the ground
- Heaving of the soil may occur or an already driven pile may pop-up when a pile is driven into a soil with low or poor drainage qualities.
- Driving of piles generates vibration, which can affect the integrity of adjacent structures.
Figure 7: Pile Foundations and Pile Cap
Pier (Caisson) Foundation
Pier or Caisson foundations are similar to a single pile foundation, but with a larger “pile” column diameter. Caisson foundations are also installed differently. Unlike the pile foundation, pier foundations are constructed by excavating or dredging the soil beneath the ground and filling it with concrete and steel reinforcement. Caissons can also be drilled into the bedrock or rest on a soil strata but a “belled” cross-section is required to spread the load over a wider area (as shown in Figure 8). Because of the presence of water, pier foundations rely on end bearing to resist the superstructure loads, contrary to the pile foundation which transfers loads through end bearing and skin friction. Typically, pile foundations are installed when there is no firm strata at a reachable depth and pier foundations are often used when the top layer of soil consists of decomposed rocks or stiff clay.
Figure 8: Pier or Caisson Foundation with Pile Cap