Concrete foundation types and footing design field guide

What a foundation does and the type families

A foundation transfers the weight of a building into the ground, spreading the load over enough soil that the structure does not sink, tip, or crack as the soil takes it. Everything above the foundation, the columns, the walls, the floors, the snow on the roof, the racks on the floor, ends up as a load that has to land somewhere the earth can hold it. The foundation makes the handoff from structure to soil, and it has to do it without settling more than the building can tolerate.

Four things pick the type: the load, the soil, the frost, and the structure. A light house on firm ground and a heavy warehouse column on soft clay are not the same problem, and they do not get the same foundation. The two big families are shallow and deep. Shallow foundations, the spread footing, the strip footing, the mat, bear near the surface on soil that is good enough close to grade. Deep foundations, piles and drilled piers, reach down through weak soil to a stronger layer below.

This guide stays on the foundation and the footing. The slab on grade that often sits inside the foundation walls, and the reinforcing steel that ties the whole thing together, each get full treatment in the related slab and rebar material, and they cross this one constantly.

The soil is the foundation, and the soils report is where it starts

The soil under a footing is what actually carries the building. The concrete spreads the load out, but the dirt holds it up, and the number that governs everything is the soil's bearing capacity, the pressure the ground can take before it fails or settles too much. It is given in pounds per square foot, and it varies a lot: soft clay might be good for 1,000 to 1,500 psf, a firm sandy soil for 2,000 to 3,000 psf, dense gravel or rock for far more. A footing sized for 3,000 psf soil poured on ground that is only good for 1,500 is undersized by half, and nobody can see it.

That number does not come from a chart or a guess. On anything structural it comes from a geotechnical investigation, the soils report, where a geotechnical engineer bores the site, tests the soil, and sets the allowable bearing pressure along with the expected settlement and any special concerns. Skip the soils report on a real structure and you are designing the foundation on hope.

Two things matter as much as the bearing number: how much the soil will settle under load, and whether the soil swells and shrinks with moisture. A weak or expansive soil can carry the static load fine and still wreck the building through movement. Push the geotech for anything that matters structurally, and let that report drive the foundation, not the other way around.

What is the difference between a shallow and deep foundation?

A shallow foundation bears near the surface and spreads the load sideways into soil that is strong enough close to grade. A deep foundation reaches down through weak or compressible soil to carry the load to a stronger stratum or to bedrock well below. The split comes down to where the competent bearing soil is.

When the good soil is at or near the surface, a shallow foundation is the cheaper and simpler answer, and most buildings get one. Spread footings, strip footings, and mats are all shallow. They work by area: make the bearing surface big enough that the pressure on the soil stays under the allowable, and the structure stands.

When the surface soil is too weak, too soft, or too compressible to carry the load without settling, you go deep. Piles driven into the ground and drilled piers bored down to a firm layer carry the load past the bad soil to soil that can take it. Deep foundations cost more and bring rigs and specialty crews, so the rule is plain: stay shallow if the soil near the surface lets you, go deep when it does not. The soils report tells you which case you are in.

What is a spread footing?

A spread footing, also called an isolated or pad footing, is a pad of concrete under a column that spreads the column's point load out over enough soil to keep the bearing pressure within the allowable. It is the most common foundation element on framed buildings. A column lands a big load on a small base, and the footing under it, usually square, sometimes rectangular, takes that concentrated load and fans it out into the ground over a much larger area.

The size comes straight from the load and the soil. Divide the service load on the column by the allowable bearing pressure and you get the plan area the footing needs. A 60,000 lb column load on soil good for 3,000 psf wants 20 square feet of bearing, so about a 4 ft 6 in square footing. That is the geotechnical side, sized on service loads, because the allowable pressure already carries its own safety factor.

The footing also has to be thick enough and reinforced enough to handle the bending and the shear as it spreads that load, which is the structural side, run on factored loads under ACI 318. Two checks, two load levels, one footing. The engineer runs both. The size on the drawing is the answer to both at once.

The strip footing under a wall

A strip footing, also called a continuous or wall footing, is a long, continuous run of concrete under a wall or a line of closely spaced columns. Where a spread footing handles a point load, a strip footing handles a line load, the weight of a wall spread along its whole length. It is wider than the wall it carries so the load fans out into the soil on both sides.

The principle is the same as the spread footing, just in one direction. The required width is the line load per foot divided by the allowable bearing pressure. A wall carrying 4,000 lb per foot on 2,000 psf soil wants a 2 ft wide footing. The footing projects out past each face of the wall, and that projection does the spreading, so it gets the reinforcing that handles the bending as it cantilevers out from under the wall.

Strip footings carry foundation walls, bearing walls, and the perimeter of most houses. On a residential foundation the perimeter strip footing under the foundation wall is the workhorse, sized for the wall load and set below the frost line, which is the next thing that drives how deep it goes.

The mat or raft foundation for heavy loads and weak soil

A mat foundation, also called a raft, is one large reinforced concrete slab that carries the whole building and spreads its entire load over the full footprint. Instead of separate footings under each column, the mat ties everything together on one continuous bearing surface. It shows up in two situations: when the loads are so heavy or the soil so weak that individual footings would have to be huge, and when you want the foundation to ride out uneven soil as one rigid unit.

The rule of thumb engineers use: when spread footings would cover more than about half the building footprint, a mat is usually the more economical and better-behaved choice. At that point you are nearly pouring a slab anyway, and the mat spreads the pressure out further, cuts the bearing pressure on any one spot, and bridges soft pockets that would settle individual footings unevenly.

Mats carry high-rises, heavy industrial equipment, and buildings on poor soil, and they are common on expansive clay because spreading the load evenly reduces the differential movement that tears a building apart. A mat is a structural slab in its own right, designed for bending across the whole footprint, and it overlaps the heavy slab-on-grade work in the related slab material. The engineer sizes the thickness and the two-way reinforcement to the loads and the soil.

How deep should a footing be?

A footing has to bear on soil that can carry it, and in cold climates it has to sit below the frost line, the depth to which the ground freezes in winter. Those two conditions, plus a code minimum, set the depth. The IBC and IRC require footings to bear below the local frost depth and at least 12 in below undisturbed ground, but the controlling number on most jobs is the frost line, and it is regional.

Here is why frost depth matters. When water in the soil freezes it expands, and frost-susceptible soil under a footing can lift, which is frost heave. Heave does not lift evenly, so it cracks walls, shears anchor bolts, and racks doors out of square over a few winters. Set the footing bottom below the frost line and the soil under it never freezes, so it never heaves. The frost line runs from a foot or so in the warm south to 4 ft, 5 ft, or more across the northern states, and the adopted local code sets the figure you build to.

There is one approved way around digging that deep. A frost-protected shallow foundation, the FPSF, uses rigid foam insulation around the perimeter to hold heat in the ground and keep the soil under a shallow footing above freezing, so on a heated building the footing can sit as shallow as about 12 in. The IRC covers the FPSF with its own tables and figures. It is a cold-climate detail that has to be built exactly as drawn, so confirm the depth, the frost line, and any FPSF design against the soils report and the adopted code, not a rule of thumb.

How is a footing sized and designed?

A footing is designed in two passes, and they use different loads. The first pass sizes the bearing area: take the service load and divide by the allowable soil bearing pressure from the soils report, and that gives the plan size, the width of a strip or the dimensions of a pad. Service loads, because the allowable pressure already has the safety factor baked in.

The second pass sizes the concrete, the thickness and the reinforcement. As the footing spreads the load, it acts like an upside-down cantilever, bending up at the edges and trying to punch through under the column. The thickness has to beat two kinds of shear, one-way beam shear and two-way punching shear around the column, and the steel near the bottom has to carry the bending as the footing fans the load out. This pass runs on factored loads under ACI 318.

The projection, how far the footing sticks out past the column or wall, creates the bending the steel resists, so the wider you spread, the harder the thickness and the steel have to work. There is no single answer that fits every footing, and the numbers here are the framework, not a design. Footing sizing, thickness, and reinforcement belong to the structural engineer working from the soils report and the code. Build it to the stamped drawing.

P / w (service load)

The unfactored column load in pounds, or wall load in pounds per foot, the footing carries

q-allow

Allowable soil bearing pressure in psf, from the geotechnical report, with safety factor included

A / B

Required bearing area of a pad footing, or required width of a strip footing

Reinforcement and dowels in the footing

Most structural footings carry a mat of reinforcing bars near the bottom, running both ways on a spread footing and along the length on a strip footing. The steel sits low because that is where the concrete goes into tension as the footing bends and spreads the load. Cover protects it: a footing cast against earth gets 3 in of cover to the steel, the largest cover value in the code, because the bar is against raw soil and has to be kept back from it. Bars laid in the mud at the bottom of the trench have no cover and do nothing, so they ride on supports set on something solid.

Dowels are the other half. These are vertical bars that stick up out of the footing to tie it to whatever lands on top, a foundation wall, a column, a pier. The dowels lap with the wall or column steel so the load path is continuous from the structure down through the footing into the soil, and so the connection transfers moment and shear instead of just sitting loose. A column footing with no dowels is two separate pieces of concrete stacked on each other.

The placement, cover, laps, and supports are an inspection in themselves, covered in full in the related rebar material. The short version for the footing: steel low, on chairs, with its 3 in of cover, and dowels tied in position so they do not float when the concrete comes in.

The foundation wall, the stem wall, and the keyway

On top of a strip footing sits the foundation wall, the vertical wall that carries the building up out of the ground to where the framing or the slab starts. It is either poured concrete or concrete masonry units, CMU block, and it does two jobs: carry the wall and floor loads down to the footing, and on a basement or deep crawlspace hold back the soil pressure pushing in from outside. A short foundation wall that only lifts the structure out of the dirt is often called a stem wall.

The wall and the footing are usually poured separately, footing first, then the wall on top, so there is a cold joint between them. That joint has to transfer shear and resist the soil pushing the wall sideways off the footing. Two details handle it. A keyway, a continuous notch troweled or formed into the top of the wet footing, gives the wall a key to sit into so it cannot slide. Vertical dowels set in the footing lap into the wall steel and tie the two together. Many jobs use both, or dowels in place of a keyway on engineered walls.

Get that connection wrong on a basement wall and the wall can slide or rotate at the base under backfill load, which is one of the classic foundation failures. The wall reinforcement, the dowel size, and whether a keyway is required come off the structural drawings. Poured or block, the wall is only as good as its tie to the footing under it.

Slab, crawlspace, or basement: which foundation for a house?

Houses get one of three foundation types, and the choice runs on climate, soil, water table, site slope, and budget. A slab-on-grade pours the floor and the foundation as concrete at ground level, with a thickened edge or a perimeter footing around it. A crawlspace lifts the house on a perimeter stem wall and interior piers, leaving an accessible space a few feet high under the floor. A full basement extends the foundation walls down far enough to make a usable story below grade.

Each has a lean. Slab-on-grade is the cheapest and fastest, common in warm climates with a high water table or shallow frost, but it buries the plumbing in the slab and gives no under-floor access. A crawlspace suits sloping lots and mild-to-cold climates, keeps the wood framing up off the ground, and leaves the mechanicals reachable, but it has to be drained and vented or it grows moisture problems. A basement makes sense where the frost line is already deep, because once you are digging 4 ft for frost, going down a few more feet for a basement is cheap floor area, which is why basements are common in the north and rare in the south.

The slab-on-grade case is its own subject, the subgrade, the thickness, the vapor retarder, the joints, covered in depth in the related slab material. Whichever type, the footing under it still has to bear on good soil below the frost line.

The monolithic slab versus the separate footing and stem wall

On a slab-on-grade house there are two ways to build the edge where the slab meets the ground, and they are worth keeping straight. A monolithic, or thickened-edge, slab pours the perimeter footing and the floor slab together in one placement: the edge of the slab drops into a thickened beam that acts as the footing, and the whole thing comes out of one pour. A floating slab, by contrast, pours a separate footing and stem wall first, then casts the floor slab inside it as its own piece.

The monolithic pour is faster and cheaper, one form, one pour, and it is common in warm climates where the frost line is shallow enough that a thickened edge a foot or so deep clears it. Where the frost line is deep, the thickened edge cannot reach it economically, so you go to a separate footing and stem wall, or to a frost-protected detail with the insulation.

The thickened edge has to be built the way it is drawn, dug to a clean bottom on undisturbed or compacted soil, with its reinforcing held up off the trench bottom, or it settles and cracks along the edge. The detailing of the slab itself, the integral footing, the reinforcement, and the joints, carries over into the related slab-on-grade material.

Deep foundations: piles, drilled piers, and helical piers

When the soil near the surface cannot carry the load, the foundation goes deep, reaching past the bad soil to a stronger layer. There are a few ways to get down there, and the soil and the load pick the method.

Driven piles are precast concrete, steel, or timber members hammered into the ground until they hit refusal or develop their capacity through friction along the shaft. They suit soft soils where a drilled hole would cave, and a pile rig drives them in groups that get capped and tied together. Drilled piers, also called caissons or drilled shafts, are made by boring a hole down to firm soil or rock and filling it with reinforced concrete, so the pier bears at the bottom and through side friction. They are a common answer for new construction on bad soil because a big drilled pier can carry a heavy column straight down to a competent stratum. Helical piers are steel shafts with screw-like plates that get turned into the ground hydraulically, going in fast with small equipment, which makes them a frequent pick for repairs, additions, and lighter structures on poor soil.

Deep foundations carry the load by end bearing on the firm layer, by skin friction along the shaft, or by both. They cost more than spreading the load near the surface and they bring specialty crews and rigs, so they are used when the soil leaves no shallow option. The type, the depth, the capacity, and the layout are a geotechnical and structural design, full stop. This is the overview, not the engineering.

The grade beam spanning between piers

A grade beam is a reinforced concrete beam that runs along the ground and spans between deep foundation points, piers, piles, or pile caps, carrying a wall or a line of load across to them. On a deep foundation the building loads do not bear on the soil between the piers, so the grade beam picks up the wall or column line and hands it to the piers, spanning over the weak soil rather than resting on it.

It looks like a strip footing but it works the opposite way. A strip footing bears down on the soil along its whole length. A grade beam is often designed to span clear between supports and carry little or no soil bearing in between, so its reinforcement is sized for bending across the span like any beam, top and bottom steel, not just a bottom mat. On expansive soil the grade beam is sometimes cast over a void form so the swelling clay can lift without loading the beam.

Grade beams also tie a row of piers or piles together so they act as a line instead of isolated points. Whether a footing under a wall is a bearing strip footing or a spanning grade beam changes how it is reinforced, so read the drawing, do not assume.

Excavating to bearing and protecting the footing bottom

The footing is only as good as the soil it sits on, and the excavation is where that soil gets exposed or ruined. You dig down to the bearing elevation the design calls for, which means firm, undisturbed soil below the frost line, or the compacted fill the geotech specified. Undisturbed is the word that matters. The natural soil that has been in place for ages is predictable. Soil that has been dug up and dumped back loose is not, and a footing on uncompacted backfill settles.

A few things wreck a good bearing surface fast. Over-digging past grade and backfilling loose to bring it back up, which puts the footing on fill instead of native soil. Letting the open trench take rain so the bottom turns to mud. Leaving it open long enough that the soil dries, cracks, or freezes. The fix for a soft or wet bottom is often a mud mat or seal slab, a thin layer of lean concrete poured over the bearing soil to give a clean, firm working surface, protect it from weather, and keep the reinforcing up out of the dirt.

Pour onto the bearing the design assumed, not whatever the trench looks like the morning of the pour. If the bottom is disturbed, wet, or frozen, it gets fixed before concrete, because once the footing is on it nobody can see what it is sitting on. Site earthwork and compaction are a discipline of their own, covered by topic in the related earthwork material.

Foundation drainage and waterproofing

Water is the enemy of a below-grade foundation, and the footing is where the drainage starts. A footing drain, a perforated pipe in washed stone run along the outside of the footing, collects the water that reaches the base of the wall and carries it away to daylight or a sump before it builds up. Without it, water ponds against the foundation wall, raises the pressure pushing the wall in, and finds its way through any crack into the basement or crawlspace.

The wall itself gets treated against water on any habitable below-grade space. Damp-proofing is a coating that slows moisture; waterproofing is a membrane or coating that holds back liquid water under head pressure, and a basement that has to stay dry needs the waterproofing, not just the damp-proofing. Grade the soil to slope away from the building, keep the downspouts discharging away from the wall, and the drainage system has far less to handle.

The classic failure is a footing drain that was never installed, got crushed during backfill, or drains to nowhere, and the symptom is a wet basement and hydrostatic pressure cracking the wall years later. Below-grade waterproofing is its own subject, covered by topic in the related waterproofing material. For the foundation, the rule is plain: drain the footing and waterproof the wall before you backfill, because you cannot get to either one after.

What does the inspector check before a footing pour?

Before a structural footing pours, the bearing soil gets verified, and on anything engineered that verification is the geotechnical engineer's or the inspector's call, not the crew's. The question they answer is whether the soil at the bottom of the excavation actually matches the bearing capacity the footing was designed for. A footing sized for 3,000 psf soil poured on a bottom that is only good for 1,500 is a failure waiting on the calendar, and the only time to catch it is with the trench open.

The check looks at the bottom of the excavation first: is it undisturbed native soil or properly compacted fill, is it firm under a probe or a proof-roll, is it free of water, mud, frost, loose spoil, and organic material. On a job with a soils report the geotech often inspects and approves the bearing personally before any concrete is ordered, sometimes with a hand penetrometer or density tests on compacted fill. Then the reinforcing and dowels get checked, the size, the cover, the supports, and the dowel position, which is the rebar inspection covered in the related material.

Treat the bearing approval as a hold point. The pour does not go until the soil is verified and signed off, because a footing on bad soil is invisible the moment the concrete covers it, and the building tells you about it later, on its own schedule.

Settlement, heave, and how foundations fail

Foundations fail two ways: they go down, or they go up. Settlement is the foundation sinking as the soil compresses under load. A little uniform settlement is normal and the building rides it fine. The damage comes from differential settlement, when one part of the foundation sinks more than another, because the building cannot bend, so it cracks. The crack runs diagonally up from the corner of a window or a door, the part that moved pulls away from the part that did not, and the floors go out of level.

Heave is the opposite, the foundation lifting, usually from expansive soil swelling with moisture or from frost heave under a footing set too shallow. Heave cracks look like settlement cracks at a glance, so the direction of movement has to be read from the building, not guessed.

The causes track back to the soil and the design every time: a bearing capacity that was overestimated or never tested, footings on uncompacted fill, soft pockets the design did not catch, expansive clay that was not addressed, a footing above the frost line, or water that softened the bearing or swelled the soil. The concrete is rarely the culprit. A foundation that cracks is usually telling you something about the dirt under it or the water around it, which is why the geotech and the drainage matter as much as the rebar.

Expansive clay and other problem soils

Expansive soil is clay that swells when it takes on water and shrinks when it dries, and it moves enough to wreck a foundation that was not built for it. The static load is not the problem; the building could sit on the clay all day. The problem is the volume change, which lifts the foundation where the clay got wet and drops it where the clay dried, producing the differential movement that cracks walls and racks the structure. It is one of the most common causes of foundation damage in clay regions, and it does its work over wet and dry seasons, not all at once.

There is no single fix, and the right answer is a geotechnical design decision. Common approaches: spread the load with a stiff mat so the foundation rides the movement as one unit, reach below the active clay zone with drilled piers or piles to bear on soil that does not move, cast grade beams over void forms so the clay can swell without loading them, or treat and control the moisture in the soil so it stays stable. Keeping water away from the foundation, good drainage, gutters, grading, controlled irrigation, matters more on expansive soil than almost anywhere else, because the moisture swing is what drives the movement.

Other problem soils, soft organic ground, collapsible soils, uncontrolled fill, each bring their own failure mode and their own fix. This is the geotech's call, every time. The soils report names the problem and the foundation type that handles it, and that is not a place to improvise.

Heavy equipment pads and data center mat foundations

Some foundations carry loads that dwarf a normal building, and they get governed by the concentrated load and the dynamic load, not the average weight. A generator, a chiller, a transformer, or a big rotating machine sits on an equipment pad sized for its static weight plus the forces it makes running, the vibration, the torque, the dynamic reactions, which can far exceed what the catalog weight suggests. A vibrating machine on an undersized or poorly isolated pad shakes itself and everything near it, so these pads are often a structural and sometimes a dynamic design, not a slab you eyeball.

Data centers push the same problem across a whole floor. Rows of equipment racks land thousands of pounds each on small footprints, and the dynamic load of rolling a loaded rack across the floor during a build is what often sizes the foundation, not the racks at rest. Many data halls run a structural mat or a heavily designed slab at grade to take those concentrated loads, and the foundation under it has to carry the whole thing into the soil without differential settlement that would knock equipment out of level.

The lesson carries to any heavy point load: a mezzanine column, a battery rack, a press, a tank set down full. Name the load, including the install move and the running forces, before the foundation is designed. The static weight on the spec sheet is rarely the load that governs. The heavy slab and mat side of this overlaps the related slab-on-grade material.

Underpinning and foundation repair

When a foundation has already settled or failed, the fix is underpinning, extending the existing foundation down to soil that can carry it, or transferring the load to new deep elements. You do not pour a new footing under a building by digging the whole thing out at once. You work in small sections, supporting the structure as you go, so the building never loses support along its length.

The common methods mirror the foundation types. Pier and beam underpinning drives or jacks piers, often helical piers or push piers, down to a firm layer and transfers the load to them, which is the usual repair for a settling house. Mass concrete underpinning digs a sequence of pits in a strict pattern beneath the footing and fills them with concrete to deepen it to better soil. Each method is an engineered repair, and the sequence and the pattern matter as much as the hardware, because cutting too much support at once during the work is how a repair becomes a collapse.

Underpinning is a specialty, designed off an investigation of why the foundation moved in the first place. Fix the cause, the water, the soil, the drainage, or the new movement comes right back. This is the overview, not the repair design.

What to document

A foundation is buried the moment it is backfilled, and almost everything that determines whether it lasts is now out of sight under the building. The record is what answers the question years out when a crack opens or a floor goes out of level and someone asks whether the foundation was built to the design.

Capture it footing by footing and area by area. Record the foundation type, the design bearing capacity and its source, the bearing soil and how it was verified, the footing size and elevation, the depth relative to the frost line, the reinforcement and dowels and their cover, the concrete strength and mix, the drainage and waterproofing installed, and the design load the element carries. If anything changed in the field, record what changed and who approved it, because the next person needs to know the foundation as built, not as drawn.

Common mistakes

• Setting a footing above the frost line in a cold climate, so frost heave lifts and cracks it.
• Undersizing the footing for the actual soil bearing capacity, so it overloads the soil and settles.
• Pouring on disturbed, uncompacted, wet, or frozen bearing soil instead of firm undisturbed ground.
• Building a structural foundation with no soils report, sizing the footing on a guessed bearing value.
• Leaving out the dowels or the reinforcement, so the wall, column, and footing are not tied together.
• Ignoring expansive or problem soil and pouring a conventional footing that the moving clay then wrecks.
• Skipping the footing drain and the wall waterproofing, so water builds up and cracks the wall.
• Laying footing steel in the mud with no cover instead of up on supports.
• Reading a spanning grade beam as a bearing strip footing, or the reverse, and reinforcing it wrong.

Field checklist

Standards and references

The structural engineer and the geotechnical engineer govern, and the soils report governs the soil. Everything below is the framework those designs are built on, and where the project specification or the geotech report is stricter, it wins. The numbers here are common values to understand the why, not values to design to.

The building code carries the foundation provisions: the IBC in Chapter 18 for soils and foundations on commercial and larger work, and the IRC in Chapter 4 for one- and two-family dwellings, including the footing depth, the frost-protection rules, and the frost-protected shallow foundation tables and figures. Those provisions set the minimum footing depth, the requirement to bear below the frost line, and the prescriptive sizes for residential footings. The exact section numbers move between editions, so confirm them against the edition the jurisdiction has adopted and any local amendments.

ACI 318, the structural concrete code, carries the design of the footing concrete itself, the shear, the flexure, the reinforcement, and the dowels and load transfer at the column. ACI 301 is the specification for structural concrete the project often references. The allowable bearing capacity, the settlement, the frost depth, and the foundation type for the soil are geotechnical determinations from the soils report, and they are not a place for a rule of thumb. Name the standard that controls the point, lean on the geotech and the engineer for the soil and the sizing, and let the contract documents and the adopted code override anything in this guide.

Units, terms, and conversions

Foundation work mixes the soils report, the structural drawings, and the spec, so the same idea reads a few different ways depending on which page you are on.

Soil bearing capacity is in pounds per square foot, psf, with kPa in metric sources. Footing loads come as pounds, or kips for thousands of pounds, on a column, and as pounds per foot for a wall line. Footing size and depth read in inches and feet here, millimeters and meters elsewhere. Concrete strength is psi, given as f-prime-c, with MPa in metric. Frost depth is a local figure in inches or feet set by the climate and the adopted code.

Bearing capacity

The pressure the soil can carry before it fails or settles too much, in psf

Spread footing

A pad under a column that spreads a point load into the soil; also isolated or pad footing

Strip footing

A continuous footing under a wall that spreads a line load; also a wall or continuous footing

Mat / raft

One large slab carrying the whole building, for heavy loads or weak soil

Frost line / frost depth

The depth the ground freezes; footings bear below it or are frost-protected

FPSF

Frost-protected shallow foundation, insulated so a shallow footing does not heave

Stem wall

The foundation wall on the footing that lifts the structure out of the ground

Grade beam

A beam spanning between piers or piles, carrying load over weak soil

Deep foundation

Piles or drilled piers carrying load through weak soil to a firm layer

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