Generally, foundation systems are divided into shallow and deep foundations. Shallow foundations are almost always cast against the 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 design option than deep foundations.
Difference between footings and foundations
The terms footings and foundations are common words used in civil engineering, but still, some people do not recognize the distinction between the two terminologies. Foundation is a general term used to denote a part of a structure that transfers the load from the superstructure to the supporting ground. It can be either classified as a shallow or deep foundation. On the other hand, footing is a part of a foundation in contact with the ground. Typically, footings are associated with shallow foundations only. Foundation may include footings, piles, piers, or caissons, while footing is only made up of a reinforced concrete slab. In simple words, all footings are considered foundations, but not all foundations are footings.
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 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 between these two foundation systems include 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 deep strata (ranging from 20-65 meters or 60-200 feet).
Shallow foundation construction is cheaper as it requires less labor, equipment, and materials. As aforementioned, relatively little excavation and labor are 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 the earth. Soil is much harder to excavate, and soil pressure increases 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. A 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 in Figure 1.
Figure 1: Types of Foundation Systems
Types of shallow foundations
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 comes from columns. Each footing supports its 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, the safe bearing capacity, and excessive soil settlement.
Figure 2: Isolated Footing
Wall footings, also known as strip footings, 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 broader footing area, the more it spreads the gravity force from the wall to reduce the chances of settlement. This is especially useful when supporting load-bearing walls, as they would support not just the dead loads of the structure but also design loads. 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
Like isolated footings, the combined footing is constructed when columns carry structural loads. This is used when two or more columns are so close to each other that their isolated footing overlap. 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, which makes the isolated footing eccentrically loaded when kept entirely into the property line. Combined footing may be rectangular, trapezoidal, or tee-shaped in the 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 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 commonly connect two footings that support columns that carry 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 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 constructed where the entire basement floor slab acts as the foundation. Mat foundation is chosen when the building is supported by weak soil. Thus building loads are spread over an extremely large area. This prevents differential settlement that would be prevalent with isolated footings. This is most suitable and economical to use when the building footprint is relatively small or if columns are close together, limiting material cost. Conversely, mat foundations are not desirable to construct when the groundwater is located above the bearing surface of the soil.
Figure 6: Mat or Raft Foundation
Types of deep foundations
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 in more rapid work progress.
- Cast-in-place concrete piles can support large and tall structures like skyscrapers, where a shallow foundation would not suffice.
- Driven piles can also be used in locations where it is not advisable to drill holes due to pressurized groundwater 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
- Planning and equipment is essential for proper handling and driving of piles into the ground
- The heaving of the soil or an already driven pile may pop up when a pile is driven into the soil with low or poor drainage qualities.
- The driving of piles generates vibration, affecting 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 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 are 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