Like any great team, business, or relationship, a strong foundation is the key to long-term success. Evaluating different types of foundations and exploring the strengths and weaknesses of each is a crucial step in planning any project. Through our years of experience in deep foundations, we have seen many foundation types and methods used to support structures.
Deep OR shallow foundations
All structures rely on either shallow or deep foundations for support. Many factors can be used to decide the type of foundation required, but the soils are commonly the most influential. The structure’s size, height, and weight should also be factored in when choosing a foundation method. Shallow foundations are more common and will be sufficient in most situations. Shallow foundations are used when the soil at the surface is dense enough to support the structure. Deep foundations are required when the uppermost layers of earth cannot support the structural loads. A deep foundation transfers structural loads to deeper soils that are denser and less compressible. They can extend beyond 300 feet below the surface elevation but are typically terminated between 20 to 100 feet. Deep foundations, as well as shallow foundations, are created using a variety of methods and systems. We will compare some of the most common foundation systems.
column footing Foundations
Spread footings are a conventional foundation method and are considered shallow foundations. Column footings are a common type of spread footing. Column footings rely on a concrete pad at the base of each load-bearing column to distribute the structural loads into the soil. This foundation is very economical as it requires minimal materials and labor. Column footings are often used in residential applications as the loading is less, allowing the system to provide sufficient support in the majority of situations. They can also be considered for applications where dense soils or rock is present at the surface and structural loads are reasonable.
If a structure is built over varied soil conditions or the building will endure high stress, other foundation solutions may be a better fit. Column foundations are very susceptible to settlement or failure in situations similar to those mentioned above. A compromised building foundation can be very costly to repair and can even cause a building to be uninhabitable.
A mat foundation, also known as a raft foundation, is another shallow foundation type. Mat foundations transfer structural loads into the soil through a structural concrete slab, much like column footings. However, the concrete slab spreads across the entire structure’s footprint when creating a mat foundation. The most significant advantage of a mat foundation is the increased surface area of the concrete slab, allowing more contact with the underlying soils. Increasing the surface area of the concrete allows the foundation to support greater loads and better prevent faults. The increased surface area also allows a mat foundation to perform better when the structure spans multiple soil types.
Nonetheless, like other spread footings, a mat foundation relies on dense and noncompressible soils near the surface for support. While a mat foundation is more versatile and supportive than a column footing, it certainly is not a substitute for a deep foundation. In certain instances, however, mat foundations have been used in addition to a deep foundation to create an economical hybrid foundation solution that can support high loads in poor soils.
helical pile Foundations
Helical piles, an end-bearing deep foundation system, consist of a structural steel shaft with helices added to the shaft to create a screw-like device. Helical piles are also referred to as helical piers or helical anchors. A helical is referred to as a pier when it resists compressive or downward loads and as an anchor when it resists tensile or upward loads. In many cases, a helical unit is designed to withstand both tension and compression loads. Helical piles bypass weak soils and are drilled down to denser, deeper, and more stable soils below the surface. Helical piles are used in various commercial and residential applications across numerous industries. A helical pile can support loads from 5 to 500 tons and be as large as 36″ in diameter.
When a deep foundation is required, helical pile systems are one of the most economical and versatile solutions. Helical piles will not be cost-effective or necessary when a shallow foundation can support the structure. A helical option also may not be best when poor soils extend beyond 100 feet. In this case, a friction pile such as a micropile may be more economical and resist the required loads at a lesser depth.
A micropile consists of a central steel shaft with high-strength concrete encasing the shaft. The steel shaft of the micropile is drilled into the earth, and grout is added using various methods. Grout is a concrete mix made of sand and cement, and its fluid qualities allow it to create a bond between the steel shaft and surrounding soils. A traditional micropile is typically grouted after the steel is installed using pressure or tremie grouting methods. Drilled-in displacement micropiles are grouted during the shaft installation using mechanical grouting. Micropiles rely on the grout-to-soil bond along the pile length to resist loads. This quality makes a micropile a friction pile rather than an end-bearing pile. Micropiles can often achieve higher capacities at lesser depths in loose or poor soil conditions.
While micropiles can appear costly when comparing the cost per linear foot to other deep foundations, they can be very competitive when designed correctly. When high loads are required in poor soils, it has consistently been proven that micropiles can be installed for a lower cost per kip than alternate deep foundation solutions.
After examining these deep and shallow foundations, we hope you better understand common foundation types and the situations in which each is most useful. We recommend consulting a local geotechnical engineer or established foundation installer to learn more about soil types and commonly used foundation methods in your region.
Our in-house Design team is always willing to provide foundation recommendations and assist with designing and providing design support for Deep Foundations. Don’t hesitate to contact our team of cheerful foundation specialists if you have further questions on other foundation methods or which foundation is right for your structure.