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For many years, box springs have been the standard foundation for most mattresses, but with more modern, high-quality mattresses on the market, there are more options available to us. From solid to slatted foundations—understanding which one is right for you can be confusing. To answer all of your foundation-related questions, we have put together this comprehensive guide.
The foundation is the surface your mattress rests on. This base could be a box spring, a wooden slat platform, or a solid base. The mattress foundation has two jobs—to support you as you sleep and ensure the mattress doesn’t sink, slide, or sag.
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If your mattress doesn’t have the right base, it will not support you correctly. Indents can start to form, causing you to sleep in uncomfortable and awkward positions to accommodate the lack of support. When this happens, you can wake up with stiff, sore muscles and joints.
Most mattresses need some type of foundation. The type of foundation you need will largely depend on the kind of mattress you have. Throughout this article, we will breakdown the most common foundation types and the mattresses most suited for each.
In addition to supporting you, a stable base offers some unexpected benefits. Below, we include six ways a mattress foundation can promote better sleep.
Memory foam, latex foam, and hybrid mattresses contour to the body, so your muscles can recover as you sleep. These beds provide the perfect combination of cushioning and support so your spine remains in a safe, neutral position.
Without the right foundation, the mattress will begin to sag in certain places. As this happens, your body will also sink—forcing your spine out of alignment. When the spine is not straight, our bodies have to overcompensate for the lack of support by sleeping in uncomfortable positions. However, a stable support base will allow you to get the most out of your advanced mattress.
Placing your mattress on the right foundation will allow it to support you correctly, but it will also ensure that it does so throughout many years of use.
The average person spends about 33 years of their life in bed—when we are not sleeping, we are either trying to get to sleep or using the mattress for sitting up in bed.
Mattresses need a sturdy support base to accommodate this constant use. The right foundation will save you the inconvenience and expense of having to replace your mattress after only a few years of use.
Allowing your mattress to rest on the ground is never a good idea. When sleeping on the floor, we are more open to cold air, dust, dirt, and humidity. It is also challenging to get out of bed when your bed is resting on the ground.
A mattress foundation can add anywhere from 16 to 23 inches of height—many also come with the ability to adjust the height to your preference. However, it is best not to raise your bed too high, either.
The ADA (Americans with Disabilities Act) recommends beds between 20 to 23 inches high from the floor to the top of the mattress—this will make it easier to climb into bed each night and also allow you to safely get out of bed in the morning.
The floor is typically the dirtiness spot in the house—this is where dirt, hair, dead skin cells, bugs, dust mites, pet hair and dander, and other allergens collect. Moisture and humidity also tend to gather near the floor, which can encourage mold and mildew to form in the mattress.
To ensure these things stay on the floor and not in your bed, you will need to keep your mattress lifted off the ground.
Innerspring mattresses tend to have open space between spring coils where bacteria and dirt can collect. So these beds should remain lifted off the floor.
Because latex foam and memory foam mattresses are so dense, they are more resistant to dust mites and bacteria. However, moisture can cause foam to deteriorate, so these beds should also rest on a raised platform.
We have a number of guides on maintaining a clean mattress:
Staying cool and comfortable is one of the best ways to get adequate sleep. If your mattress rests on the floor, air will not circulate properly throughout the bed—causing you to become overheated during the night.
The right foundation will help release trapped heat and circulate air around the bed. No matter what type of mattress you have, the right foundation can provide much-needed breathability so you can sleep comfortably night after night.
If your mattress rests on the ground, it is likely to slide around as you sleep. Additionally, if it’s on the wrong foundation, it may slip—causing it to sink in the areas where it is not supported.
A good foundation will keep your mattress stable and firmly in place—preserving its lifespan and preventing the discomfort from a mattress sliding around.
Now that you’re familiar with the benefits of a foundation, we will outline the most common types so you can decide which one is right for you.
The terms “bed frame” and “mattress foundation” (or mattress base) are often used interchangeably, but there is a difference.
A bed frame is a decorative portion surrounding the mattress—this includes a headboard, footboard, and side panels (four sides). Typically, the bed frame will be attached to a foundation that the mattress can rest on.
For example, many wooden frames have a slatted foundation that consists of wood planks attached to the two side panels of the frame. A metal frame often has thin metal rods instead of traditional wide slats. Platform bed and panel bed frames are two common types.
However, some bed frames only provide a rim around the perimeter of the mattress—leaving no base for you to place the mattress on.
If you have the proper support base for your mattress, you don’t necessarily need a bed frame. Headboards and footboards are mostly decorative–they don’t support the bed.
If your mattress is resting on the correct foundation, that is all you need; however, if you have a slatted foundation attached to your bed frame, you will not be able to separate the base from the frame—the two must stay together.
For the most part, the frame is optional, but the base the mattress rests on, is not. You will need to have the right base for support and to preserve the life of your mattress.
With a memory foam mattress, it’s best to use a slatted or solid mattress foundation. If you go with a slatted foundation, the slats should be no further than 2.75 inches apart. Any further apart and you’ll need to buy extra slats or a bunkie board to create a more stable surface for your memory foam mattress.
Modern platform beds are another good bed frame for memory foam mattresses. Platform beds are essentially just a more decorative mattress foundation, as they often contain features like built-in headboards or under-bed storage drawers. When you opt for a platform bed, you can skip the extra foundation and save money.
Just remember, the 2.75-inch rule for slat spacing applies to platform beds, too. So you’ll need a platform bed with closely-spaced slats that still allow for plenty of air circulation.
When it comes to the best bed frame material for memory foam mattresses, whether to choose a wooden or metal frame, wood is preferred over metal. A metal bed frame tends to have thin rods instead of thick slats and simply cannot provide the support that a memory foam mattress needs.
Since memory foam beds are so flexible, they also pair great with adjustable bed bases to promote a luxury night’s sleep.
Traditionally, innerspring mattresses needed a box spring underneath to help support body weight. The two layers contained coils that bounced off each other to distribute weight and absorb pressure evenly.
Today, most box springs do not include coils—they are simply a wood or metal box (open on all four sides) covered with cloth. They do not offer much in terms of stability or support, but they can provide a lift.
If you have a foam or hybrid mattress, you should not use a box spring. Because box springs don’t have a solid surface, they do not provide a good foundation for the mattress to rest on—some warranties actually prohibit the use of a box spring.
For mattresses containing foam, we recommend a solid foundation or a bunkie board, which we will discuss further down.
If you prefer an innerspring mattress, these are still typically sold with a box spring. But an innerspring mattress is sturdy enough on its own that it doesn’t require a box spring. However, box springs do provide additional height.
A slatted foundation consists of solid wood planks or metal beams that lay across the bed frame, attaching to the sides of the frame. They can be individual slats, or they may be connected in a series of planks.
A slatted base helps support the mattress above and prevents any sagging or slipping. They are also easy to install, inexpensive, and can be replaced if one breaks. But keep in mind slats do not add height to a bed, only support.
Also, if slats are not secured to the frame correctly, they can slide around and potentially fall—leaving sections of the mattress unsupported.
Although wooden slats are the most common, they may sag or bend over time. When this happens, your mattress can develop indents. New metal slats are durable and won’t bend like some wooden slats.
However, if you opt for wood slats made from solid pine or oak, warping is very rare. Some metal slats are also adjustable, so you can form them to fit the exact width of your bed frame.
These foundations consist of a series of wooden slats connected and placed inside a wood frame, similar to a box spring. They are unique because they feature an adjustable suspension beneath each wooden slat, allowing you to change the bounce and flexibility of the bed.
Pine, Oak, Poplar, Maple, Beech, and Mahogany are all long-lasting, high-quality woods for a bed frame. The slats, however, will usually feature a cheaper, unvarnished wood since they are under the bed and out of sight.
Bed slats too far apart will not support the mattress correctly—this is especially true with memory foam or latex mattresses since these beds can sag between wide gaps.
Typically, it is best to keep slats no more than 2.75 inches apart. This distance will provide adequate support and keep air circulating through the bed.
Most mattresses can rest on a slatted foundation—including memory or latex foam, hybrid, or innerspring beds. If you have an innerspring mattress and prefer to use a box spring for height, just be sure the foundation can support the weight of the box spring and the mattress.
Because slatted foundations don’t add height, only support, if you have a memory or latex foam mattress, or a hybrid, it is best to pair a slatted bed with a bunkie board. Bunkie boards provide both support and height without the use of a box spring.
A solid mattress foundation, also called a platform base, is typically made of a series of wood beams covered in durable material—so there is no space between slats. The beams then fit inside a supportive, solid wood platform with a center support beam.
Solid foundations are designed to replace traditional box springs and provide a supportive base for more advanced memory foam, latex foam, and hybrid mattresses. These bases are typically 8.5 to 10 inches and lift most mattress thicknesses to a safe and comfortable height.
Many foundations also come with adjustable legs to accommodate your preferred height. If you enjoy a low profile look, you can lower it as far down as 4 to 6 inches. For extra storage space, you can raise the foundation further with a set of bed risers.
Solid foundations are easy to put together and require no tools to assemble. You can pair them with most bed frames (headboard, footboard, and two side panels). However, these foundations should not be placed on top of a slatted foundation.
To pair a solid base with a bed frame, you would simply remove the slats and replace them with your new solid foundation (with the solid foundation resting on the floor).
If you have a memory foam, latex foam, or hybrid mattress, a platform bed frame provides a sturdy support system for these beds. Since slats can’t be more than 2.75 inches apart for foam beds, it’s sometimes easier to opt for a solid foundation rather than worrying about the distance between each slat. These foundations ensure weight is evenly distributed, and the entire mattress is protected.
Amerisleep’s Platform Bed features solid wood side rails, center support beams, and slats. The slats are oversized for minimal support gaps, and the side rails keep the mattress firmly in place. This system will ensure no soft stops, sagging, or indents form on the mattress and that it keeps supporting you throughout years of use.
Our frame sits at a comfortable 13.5-inches high and pairs well with most standard size mattresses. This frame locks together in minutes and requires no tools for assembly.
An adjustable foundation, sometimes referred to as an adjustable bed frame or power base, is a hinged mattress foundation that allows you to customize support. These beds come with head and leg articulation, so you can raise or lower the top and bottom portion of the bed to achieve an ideal sleep position.
Hospitals have relied on adjustable beds for many years to help patients find relief from certain conditions and rest more comfortably. Modern adjustable bases have more high-tech features, but they provide sleepers the same customizable comfortable and health benefits as hospital beds.
When the head is elevated, airways open up and breathing becomes more comfortable, so you experience less snoring and obstructive breathing issues due to sleep apnea. This slight angle can also help alleviate symptoms related to nasal congestion and acid reflux.
Raising the knees reduces pressure on the lumbar disc, which can relieve back pain and sciatic nerve pain—allowing you to sleep soundly and wake with less pain.
Our Amerisleep Adjustable Bed+ offers this same customizable sleep position plus a host of other advanced features—including full-body massagers, a wireless remote with preset options, under-bed lighting, and wall glide technology.
Most adjustable beds move away from the wall when raised, moving you further away from your nightstand and bedside lamp. The Amerisleep Adjustable Bed+ comes with wall glide technology to prevent this. This technology keeps the bed snug to the wall even when fully upright, so you never have to stretch to reach your essentials.
You can raise the head and foot of the mattress with the wireless remote, saving up to three favorite positions. Adjustments can also be made with a smartphone app or through your Amazon Alexa or Google Home. The USB ports conveniently located on the base’s sides also make it easy to charge and access your devices when in bed.
Since stress can often keep us from falling asleep at night, we built our base with dual full-body massagers to help you gently relax before bed. The under-bed lighting illuminates a path around the bed, so it’s safe for you to get up during the night to use the restroom.
You can pair the base with most upholstered bed frames, setting the bed height at 9 inches, 12 inches or 15 inches as needed to match your frame.
Our Amerisleep Adjustable Bed+ provides a truly luxurious sleep experience and allows you to find the perfect sleep position for your needs. When purchasing our adjustable base, we offer 30 percent off the base. This also includes in-home setup and a 10-year warranty on the base. For a complete sleep experience,
A bunkie board is a flat barrier, usually about 1 to 3 inches thick and made of wood, plywood, or particleboard. Although initially used to support bunk beds, they have recently become a popular replacement for box springs.
These foundations fit between your mattress and the surface it sits on, to create a stable, even surface. You can use a bunkie board with most bed frames—metal bed frames, platform beds, box springs, or a slatted foundation.
Foam and hybrid mattresses must be placed on a solid surface or on slats no more than 2.75 inches apart. If your bed frame doesn’t have a solid base or the slats are further apart, you will need a flat base to place your mattress on. That’s where a bunkie board can help.
A bunkie board can be used on top of any bed frame or foundation to create a stable surface for your mattress. If you already have a solid foundation, you can use a bunkie board to add a bit more height if needed.
When considering what foundation is best for you, you may also want to consider the size mattress you have. So your mattress fits correctly on the base, you’ll want to know its measurements before purchasing your new foundation. Most foundations on the market today follow these standard measurements. Below, we outline common US mattress sizes and what type of space these beds often require.
76 inches by 80 inches
A king size mattress is excellent for couples who are looking for more space. Kings offer enough room to accommodate both you and your partner’s needs, but they do require a larger bedroom to comfortably fit. Since the standard US bedroom is 11 by 12 feet, a king bed leaves little room around the bed for movement or additional furniture. A master bedroom may be more suited for this size mattress.
72 inches by 84 inches
California king mattresses (72 inches by 84 inches) offer more length than a standard king size mattress. These beds are 4 inches longer, but 4 inches narrower—making them a great option for anyone over 6 feet tall. If you plan to place a Cal king mattress on your new foundation, just be sure it can accommodate the additional length.
60 inches by 80 inches
Queen size mattresses are by far the most popular because they work well for most couples and fit comfortably inside most 11 by 12-foot bedrooms. A queen size bed will also leave you plenty of additional floor space and room to maneuver around the bed.
54 inches by 75 inches
Full size mattresses, also referred to as “double beds,” are ideal for growing teens who are no longer comfortable in their twin bed. These beds offer single sleepers more space to stretch out but may be too confining for couples. Full size beds also fit comfortably in most bedrooms.
38 inches by 75 inches
Twin size mattresses are a popular choice for kids and toddlers who have outgrown their cribs. These mattresses offer enough room to accommodate them as they grow, but won’t take up valuable floor space in their bedroom. With a 11 by 12-foot bedroom, a twin bed leaves plenty of open space around the bed.
The type of mattress you have will ultimately decide the best kind of base for you. As an overview, we have included a list of the five most common mattress types and the foundations that work best for each.
Other than mattress type, you’ll want to keep a few more points in mind as you shop for your new foundation. Below, we offer a shopping checklist to make this process as easy as possible.
Before purchasing a foundation, be sure you understand your mattress warranty. You can void your warranty if you place your mattress on a foundation that is not approved by the manufacturer.
Some bed frames, those with a head and footboard, often take up quite a bit of space. Meanwhile, solid bases, platform beds, and floating bed frames are more streamlined.
Before you commit to a foundation, consider how much space you would like your bed to take up. If you have a smaller space with lower ceilings, you may want to opt for a base without these additions.
You get in and out of bed every day, and you want this process to be as comfortable as possible. So, it is helpful to decide your preferred height before buying.
You don’t want a foundation that is too high and will require you to use a step to climb into bed, but you also don’t want something too low that will force you to squat to get out of bed. Standard 8.5-inch height is usually comfortable for most people.
Some frames come with a weight limit, expressing how much they can support before the frame becomes unsafe. Before you bring a foundation home, be sure it is safe for your needs.
A box spring, a mattress, an optional mattress topper, plus body weight may be too much for some frames. If you plan to use a box spring, you will want to double-check weight restrictions.
Most high-quality foundations are easy to install and require no additional tools. Although cheaper options exist, these often require difficult assembly and may not be worth your time or money. Instead, opt for something you can put together quickly and easily, or look for in-home installation.
Proper support, cleanliness, and breathability, are just a few of the benefits that come with finding the right foundation. Your mattress was designed to provide superior comfort and support, but this will be difficult to find if it’s not resting on the correct base. With so many stylish options to choose from, you are sure to find a base that fits your bedroom decor and also helps you find a perfect night’s sleep.
Yes, your bed’s foundation can impact your bed’s feel, airflow, and longevity. Slatted bases are more breathable than solid ones, but solid foundations are better at firming up a mattress. Adjustable bed frames can change your entire sleep experience by letting you change your sleeping angle.
It’s best to use a foundation that’s compatible with your mattress to ensure your bed doesn’t sag or feel uncomfortable. A foundation made with durable, high-quality materials is important as well; if your mattress foundation sags or breaks down, your mattress can too.
So long as the bed’s slats are appropriately spaced and secured to the bed frame, your mattress should be safe on a slatted foundation or platform bed. If the bed frame’s slats are further than 2 to 2.75 inches apart, your mattress is at risk of dipping between the gaps and sagging. Unsecured slats can slide out of place and leave your mattress unsupported.
No, you cannot place a mattress directly on a bed frame because bed frames are mostly decorative. While they may have one or two slats, the slats are there to support a foundation, not your mattress. Placing your mattress straight onto a bed frame causes your mattress to sag and voids your mattress’s warranty. The only exception to this rule are platform beds but even those will sometimes require a bunkie board for extra support.
While the floor is a sturdy and supportive surface, it’s also cold and dirty. Placing your mattress directly on the floor also hinders your bed’s breathability, meaning your mattress is more likely to trap heat, sweat, dust, and other bacteria. It’s also easier for bugs and other pests to reach your bed when on the floor.
We recommend placing your mattress on a foundation to keep it off of the ground.
We recommend replacing your foundation when you get a new mattress. Most mattress foundations wear down faster than mattresses, so if your mattress is showing signs of wear and needing to be replaced, then it’s safe to say your foundation also needs to be swapped with a new one. If you’re working on a tighter budget and don’t want to buy both a foundation and a mattress, look for bundle deals or brands offering freebies with a mattress purchase. Sometimes, you can score a free foundation with your new mattress. Alternatively, you can give an old foundation new life by pairing it with a bunkie board.
Finding the right bed frame for your new mattress isn’t all that complicated—start by looking at what your mattress requires and then narrow down your options by budget and personal preference. The most important thing about choosing a bed frame is guaranteeing it’s compatible with your mattress, so as long as you keep compatibility the primary focus, your search should be easy. Plus, many brands sell mattress foundations built to pair with their beds; that way, you know what you’re buying is perfect for your new mattress.
[Note that this article is a transcript of the video embedded above.]
When we bought our house several years ago, we fell in love with every part of it except one: the foundation. At 75 years old, we knew these old piers were just about finished holding this old house up. This year we finally bit the bullet to have them replaced. Any homeowner who’s had foundation work done can commiserate with us on the cost and disruption of a project like this. But homes aren’t the only structures with foundations. It is both a gravitational necessity and a source of job stability to structural and geotechnical engineers that all construction - great and small - sits upon the ground. And the ways in which we accomplish such a seemingly unexceptional feat are full of fascinating and unexpected details. I’m Grady and this is Practical Engineering. Today, we’re talking about foundations.
There’s really just one rule for structural and geotechnical engineers designing foundations: when you put something on the ground, it should not move. That seems like a pretty straightforward directive. You can put a lot of stuff on the ground and have it stay there. For example, several years ago I optimistically stacked these pavers behind my shed with the false hope that I would use them in a landscaping project someday, but their most likely future is to sit here in this shady purgatory for all of eternity. Unfortunately, buildings and other structures are a little different. Mainly, they are large enough that one part could move relative to the other parts, a phenomenon we call differential movement. When you move one piece of anyTHING relative to the rest of it, you introduce stress. And if that stress is greater than the inherent strength of the thing, that thing will pull itself apart. It happens all the time, all around the world, including right here in my own house. When one of these piers settles or heaves more than the others, all the stuff it supports tries to move too. But doorframes, drywall, and ceramic tile work much better and last much longer when the surrounding structure stays put.
There are many kinds of foundations used for the various structures in our built environment, but before we dive into how they work, I think it will be helpful to first talk about what they’re up against, or actually down against. Of course, buildings are heavy, and one of the most important jobs of a foundation is to evenly distribute that weight into the subsurface as downward pressure. Soil isn’t infinitely strong against vertical loads. It can fail just like any other component of a structural system. When the forces are high enough to shear through soil particles, we call it a bearing failure. The soil directly below the load is forced downward, pushing the rest of the soil to either side, eventually bulging up around the edges.
Even if the subsurface doesn’t full-on shear, it can still settle. This happens when the particles are compressed more closely together, and it usually takes place over a longer period of time. (I have a post all about settlement that you can check out after this.) So, job number 1 of a foundation is to distribute the downward force of a structure over a large enough area to reduce the bearing pressure and avoid shear failures or excessive settlement.
Structural loads don’t just come from gravity. Wind can exert tremendous and rapidly-fluctuating pressure on a large structure pushing it horizontally and even creating uplift like the wing of an airplane. Earthquakes also create loads on structures, shifting and shaking them with very little warning. Just like the normal weight of a structure, these loads must also be resisted by a foundation to prevent it from lifting or sliding along the ground. That’s job number 2.
Speaking of the ground, it’s not the most hospitable place for many building materials. It has bugs, like termites, that can eat away at wooden members over time, reducing their strength. It also has moisture that can lead to mold and rot. My house was built in the 1940s on top of cedar piers. This is a wood species that is naturally resistant to bugs and fungi, but not completely immune to them. So, job number 3 of a foundation is to resist the effects of long-term degradation and decay that come from our tiny biological neighbors.
Another problem with the ground is that soil isn’t really as static as we think. Freezing isn’t usually a problem for me in central Texas, but many places in the world see temperatures that rise and fall below the freezing point of water tens or hundreds of times per year. We all know water expands when it freezes, and it can do so with prodigious force. When this happens to subsurface water below a structure, it can behave like a jack to lift it up. Over time, these cycles of freeze and thaw can slowly shift or raise parts of a structure more than others, creating issues. Similarly, some kinds of soil expand when exposed to moisture. I also have a post on this phenomenon, so you have two to read after this one. Expansive clay soil can create the same type of damage as cycles of freeze and thaw by subtly moving a structure in small amounts with each cycle of wet and dry. So job number 4 of a foundation is to reach a deep enough layer that can’t freeze or that doesn’t experience major fluctuations in moisture content to avoid these problems that come with water in the subgrade below a structure.
Job number 5 isn’t necessarily applicable to most buildings, but there are many types of structures (like bridges and retaining walls) that are regularly subject to flowing water. Over time (or sometimes over the course of a single flood), that water can create erosion, undermining the structure. Many foundations are specifically designed to combat erosion, either with hard armoring or by simply being installed so deep into the earth that they can’t be undermined by quickly flowing water.
Job number 6 really applies to all of engineering: foundations have to be cost effective. Could the contractor who built my house in the 1940s have driven twice as many piers, each one to three times the depth? Of course it can be done, but (with some minor maintenance and repairs), this one lasted 75 years before needing to be replaced. With the median length of homeownership somewhere between 5 and 15 years, few people would be willing to pay more for a house with 500 years of remaining life in the foundation than they would for one with 30. I could have paid this contractor to build me a foundation that will last hundreds of years... but I didn’t. Engineering is a job of balancing constraints, and many of the decisions in foundation engineering come down to the question of “How can we achieve all of the first 5 jobs I mentioned without overdoing it and wasting a bunch of money in the process?” Let’s look at a few ways.
Foundations are generally divided into two classes: deep and shallow. Most buildings with only a few stories, including nearly all homes, are built on shallow foundations. That means they transfer the structure’s weight to the surface of the earth (or just below it). Maybe the most basic of these is how my house was originally built. They cut down cedar trees, hammered those logs into the ground as piles, layed wooden beams across the top of those piers, and then built the rest of the house atop the beams. Pier and beam foundations are pretty common, at least in my neck of the woods, and they have an added benefit of creating a crawlspace below the structure in which utilities like plumbing, drains, and electric lines can be installed and maintained. However, all these individual, unconnected points of contact with the earth leave quite a bit of room for differential movement.
Another basic type of shallow foundation is the strip footing, which generally consists of a ribbon or strip of concrete upon which walls can sit. In some cases the floor is isolated from the walls and sits directly on concrete slab atop the subgrade, but strip footings can also support floor joists, making room for a crawlspace below. For sites with strong soils, this is a great option because it’s simple and cheap, but if the subgrade soils are poor, strip footings can still allow differential movement because all the walls aren’t rigidly connected together. In that case, it makes sense to use a raft foundation - a completely solid concrete slab that extends across the entire structure. Raft foundations are typically concrete slabs placed directly on the ground (usually with some thickened areas to provide extra rigidity). They distribute the loads across a larger area, reducing the pressure on the subgrade, and they can accommodate some movement of the ground without transferring the movement into a structure, essentially riding the waves of the earth like a raft on the ocean (hence the name). However, they don’t have a crawlspace which makes plumbing repairs much more challenging.
One issue with all shallow foundations is that you still need to install them below the frost line - that is the maximum depth to which water in the soil might freeze during the harshest part of the winter - in order to avoid frost heaving. In some parts of the contiguous United States, the frost line can be upwards of 8 feet or nearly two-and-a-half meters. If you’re going to dig that deep to install a foundation anyway, you might as well just add an extra floor to your structure below the ground. That’s usually called a basement, and it can be considered a building’s foundation (although the walls are usually constructed on a raft or strip footings as described above).
As a structure’s size increases, so do the loads it imposes on the ground, and eventually it becomes infeasible to rely only on soils near the surface of the earth. Tall buildings, elevated roadways, bridges, and coastal structures often rely on deep foundations for support. This is especially true when the soils at the surface are not as firm as the layers farther below the ground. Deep foundations almost always rely on piles, which are vertical structural elements that are driven or drilled into the earth, often down to a stronger layer of soil or bedrock, and there are way more types than I could ever cover in a single video. Piles not only transfer loads at the bottom (called end bearing), but they can also be supported along their length through a phenomenon called skin friction. This makes it possible for a foundation to resist much more significant loads - whether downward, upward or horizontal - within a given footprint of a structure.
One of the benefits of driven piles is that you install them in somewhat the same way that they’ll be loaded in their final configuration. There’s some efficiency there because you can just stop pushing the pile into the ground once it’s able to resist the design loads. There’s a problem with this though. Let me show you what I mean. This hydraulic press has more than enough power to push this steel rod into the ground. And at first, it does just that. But eventually, it reaches a point where the weight of the press is less than the bearing capacity of the pile, and it just lifts itself up. Easy… (you might think). Just add more weight. But consider that these piles might be designed to support the weight of an entire structure. It’s not feasible to bring in or build some massive weight just to react against to drive a pile into the ground. Instead, we usually use hammers, which can deliver significantly more force to drive a pile with only a relatively small weight.
The problem with hammered piles is that the dynamic loading they undergo during installation is different from the static loading they see once in service. In other words, buildings don’t usually hammer on their foundations. For example, if a pile can withstand the force of a 5-ton weight dropped from 16 feet or 5 meters without moving, what’s the equivalent static load it can withstand? That turns out to be a pretty complicated question, and even though there are published equivalencies between static and dynamic loads, their accuracy can vary widely depending on soil conditions. That’s especially true for long piles where the pressure wave generated by a hammer might not even travel fast enough to load the entire member at the same moment in time. Static tests are more reliable, but also much more expensive because you either have to bring in a ton (or thousands of tons) of weight to put on top, or you have to build additional piles with a beam across them to give the test rig something to react against.
One interesting solution to this problem is called statnamic testing of piles. In this method, a mass is accelerated upward using explosives, creating an equal and opposite force on the pile to be tested. It’s kind of like a reverse hammer, except unlike a hammer where the force on the pile lasts only for a few milliseconds, the duration of loading in a statnamic test is often upwards of 100 or 200 milliseconds. That makes it much more similar to a static force on the pile without having to bring in tons and tons of weight or build expensive reaction piers just to conduct a test.
I’m only scratching the surface (or subsurface) of a topic that fills hundreds of engineering textbooks and the careers of thousands of contractors and engineers. If all the earth was solid rock, life would be a lot simpler, but maybe a lot less interesting too. If there are topics in foundations that you’d like to learn more about, add a comment or send me an email, and I’ll try to address it in a future post , but I hope this one gives you some appreciation of those innocuous bits of structural and geotechnical engineering below our feet.
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