Piles vs. Piers vs. Anchors – what is the difference? Whether researching deep foundations, earth retention, or foundation underpinning, you have likely seen these terms used interchangeably and without clear distinction. For example, a quick Google search for “helical piles” reveals thousands of results including those for “helical piers” and “helical anchors”. This article will explain the differences and similarities between these commonly used geotechnical terms.
- Key Similarities & Differences
- What are Foundation Piles?
- What are Foundation Piers?
- Piles vs. Piers
- What are Ground Anchors?
- Piles vs. Piers vs. Anchors – Conclusion
Piles vs. Piers vs. Anchors – Key Similarities & Differences
The terms “pile” and “pier” are very similar in that they both refer to deep foundation elements that resist or transfer vertical and/or horizontal loads, whether compression or tension. Whereas, “anchor” refers to a pile or pier only in uplift/tension applications such as retaining wall tiebacks or vertical ground anchors designed to resist overturning forces.
A foundation pile is defined as “a structural column of timber, steel, concrete, etc., installed in the ground to resist or transfer vertical, horizontal or combination loads imposed upon it.” – Glossary of Foundation Terms. Deep Foundation Institute, 1981.
Foundation piles are deep foundation elements that support vertical and horizontal loads, both that of compression and tension. In their vertical application, piles are used for underpinning foundations, providing structural support, and for transferring loads to deeper soil layers. Sheet piling is an example of piles that are driven and interlocked to create a tight wall to resist the horizontal or lateral pressure of adjacent water, earth or other materials.
Foundation piles are typically made of long steel, concrete, or wood sections. They achieve their carrying capacity by end-bearing in dense soil/rock or from the friction created between the side surface of the pile and the surrounding soil. Foundation piles are often supplemented with cement grout to increase skin-friction with the soil. Once in place, the pile can be attached to an existing foundation or used for new construction.
There are many different types of foundation piles, including:
A foundation pier has a few definitions from the Deep Foundations Institute, consider two:
- Category applied to column-like foundations, similar to piles. A pier is generally considered the type of deep foundation which is constructed by placing concrete in a deep excavation large enough to permit manual inspection. – Glossary of Foundation Terms. Deep Foundation Institute (DFI), 1981.
- Pier is also used frequently to indicate heavy masonry column units which are used for basement-level substructural support. – Glossary of Foundation Terms. Deep Foundation Institute(DFI), 1981.
Very similar to piles, piers are also deep foundation elements which are used to provide structural support. Piers, however, will invariably include the use of concrete/masonry and have a larger minimum-diameter than that of piles. There is no specific diameter at which a pile would become a pier. However, the DFI’s definition mentions that the construction of piers would include “a deep excavation large enough [diameter and depth] to permit manual inspection“, such as in the case of drilled shafts, also known as drilled piers, which range from 2′ to 30′ in diameter.
Piles vs. Piers
The terms “piles” vs. “piers” are often used interchangeably in the geotechnical industry. Technically speaking, however, piers are deep foundations of a larger diameter such as drilled shafts/drilled piers, which range in diameter from 2′ to 30′. Piers are also invariably constructed with concrete according to both definitions from the DFI, whereas piles can be made solely with steel, wood, etc. Piles, can be as small as 2″ in diameter and can rightly be called “piles” even at large diameters, as long as they do not use concrete in their construction, at which point they would fit the definition of a pier.
What are Ground Anchors?
Ground anchors, earth anchors, and soil anchors are often used when describing deep foundation elements of similar construction to piles/piers. However, the key difference is that anchors work in tension or uplift applications. Similar to a literal boat anchor that resists the force of the current on the vessel, anchors resist uplift or tension forces.
Read more on designing piles against uplift: https://www.slideruleera.net/PileUpliftDesign.pdf
Anchors can be installed either horizontally or vertically. Horizontally, they are often installed behind retaining walls and referred to as tiebacks. In vertical applications, they are commonly referred to as anchor piles, foundation anchors, ground anchors, earth anchors, or soil anchors.
Tieback Anchors (Horizontal Ground Anchor)
Tieback anchors are commonly used in earth retention applications such as in repairing failing/tilting retaining walls and seawalls. A tieback anchor is installed behind or through structures such as retaining walls. The anchor is driven beyond the area where the soil may slip. Once the anchor is in place and the desired tension capacity is achieved, tieback cables or rods connect the anchor to a waler device on the opposite side of the retaining wall. The lateral load that was posing a threat to the structure is now being transferred to the soil through the tieback anchor. During installation, tieback anchors are often supplemented with neat-cement grout to interface with the soil, adding friction and increasing uplift resistance. Helical tieback anchors are a very effective choice for this application.
Anchor Piles (Vertical Ground Anchors)
Anchors in vertical applications are commonly referred to as, anchor piles, or simply ground anchors, foundation anchors, friction anchors, etc. They can also be used to resist overturning or uplift forces on structures caused by cantilevered construction, earth pressures, wind, and earthquake loading. Resistance is supplemented by the helixes on helical anchors or through grout to soil friction with grouted (uncased) anchors. Once installed, ground anchors are attached to the structure’s foundation.
Can a pile or pier also be an anchor?
Yes. The terms “pile” or “pier” refer to the construction element itself, whereas the term “anchor” simply refers to the application of the pile or pier. Thus a pile or pier can also be an anchor, otherwise known as an “anchor pile”. This term is often applied to the reaction piles driven adjacent to a test-pile which are used to anchor the jacking beam during load testing.
Piles vs. Piers vs. Anchors: Conclusion
In review, are piles, piers, and anchors the same? “Piles” and “piers” are very similar, and are often referred to synonymously, but there are key differences in size and construction materials. The term “anchors” on the other hand simply refers to a pile or pier in a tension or uplift application.