Soil Types Around the House

Why this matters

Every pad, post, trench and footing you set is resting on whatever the ground happens to be at that spot, and the ground around a house is rarely what the soil map says. A generator pad that tilts two inches in a year, a deck post that sinks, a trench that slumps in overnight, a condenser that strains its own line set: most of these trace back to someone treating dirt as dirt. Six materials cover nearly everything a residential crew digs into, and each one behaves differently under load, under water and under frost.

This card is about how soil behaves as a foundation and a drainage medium. It is not the OSHA trench classification, which sorts soil into Stable Rock and Types A, B and C for protective-system purposes under different rules; that lives in How Soil Is Classified and What Each Class Permits, and a trench is classified by that card's method, never by the descriptions here.

The five materials in plain terms

Clay is made of particles too fine to see, and they are flat plates that cling to each other and to water. Wet clay is sticky and moldable; dry clay is hard and cracks into blocks. It drains very slowly, so water that reaches it stays. It bears load reasonably while dry and loses strength as it gets wetter. Some clays swell and shrink dramatically with moisture, which is its own subject in Expansive Soils and the Cracks They Cause.

Silt sits between clay and sand. It feels smooth and floury, like talc, and does not stick to your fingers the way clay does. It holds water but lets it move upward by capillary action, which is what makes it the most frost-susceptible of the five: a silt with a water supply below it grows ice lenses and heaves. Wet silt turns to something close to soup under vibration, and it erodes easily.

Sand is gritty and you can see the grains. It drains fast, does not heave, and bears load well when it is confined, which is why sand under a footing is usually good news. Unconfined it has no strength of its own, so a sand trench wall stands only as long as it is damp and then runs.

Gravel behaves like coarser sand and is the best natural bearing and draining material a residential crew normally meets.

Fill is not a soil type but it is the most important category on this card. It is anything placed by people: the backfill against a foundation wall, the material that leveled a lot, old construction debris, topsoil spread over a buried slab. Fill can be excellent when it was placed in thin layers and compacted under engineering control. Around an existing house it almost never was, and you cannot tell by looking.

Rock bears more than anything else, but it brings its own problems: it is slow to dig, it can sit under one end of a footing and not the other, and water that soaks down through soil runs along the top of it.

What each one means for the work

The frost column names susceptibility, not depth. How deep frost goes and how deep a footing must sit is the subject of Frost Depth and What It Sets.

The numbers the code assumes

The IRC, as adopted by your jurisdiction, gives presumptive load-bearing values that a footing may be designed to when no soil test is required. In Table R401.4.1 of the 2021 edition they run, in pounds per square foot: crystalline bedrock 12,000; sedimentary and foliated rock 4,000; sandy gravel and gravel 3,000; sand, silty sand, clayey sand, silty gravel and clayey gravel 2,000; and clay, sandy clay, silty clay, clayey silt, silt and sandy silt 1,500. The code also lets the building official require a soil test where expansive, compressible or shifting soils are likely, and the IRC's footing section (R403.1) puts footings on undisturbed natural soil or engineered fill. Backfill against a foundation is neither.

Recent editions build their deck footing tables on the same bearing values, so a deck footing sized from the table is already making a soil assumption. Know which one.

Reading the soil at the hole, not at the map

The USDA's Web Soil Survey maps soil series across most of the country and tells you what was there before the house. It is a good first look at drainage class and depth to water. It does not know about the eight feet of backfill along the foundation or the fill that leveled the back yard.

At the hole, three quick field reads sort the native soils:

• Feel it wet. Rub a moist pinch between thumb and finger. Gritty is sand. Smooth and floury is silt. Sticky and slick is clay.
• Roll it. Squeeze moist soil out between thumb and forefinger into a ribbon. Clay forms a long ribbon that holds together; silt breaks off short; sand will not ribbon at all.
• Settle it. Shake a handful in a clear jar of water and set it down. Sand drops in seconds, silt over an hour or so, clay can stay cloudy for a day or more. The layer thicknesses give a rough mix.

Then look for the signs of fill: brick and concrete fragments, a sharp color change at a uniform depth, layers that do not match the native profile, a buried topsoil horizon (dark, organic) under lighter material. Any of those means the presumptive values above do not apply to that spot.

Where the fill always is

The single most common soil mistake around a house is setting equipment on the backfill wedge. The foundation was dug wider than the wall, the wall was built, and the gap was pushed back in, often in one lift with no compaction. That wedge keeps settling for years and settles most when it gets wet.

        grade
  ------+                 pad set here sits
        |\                entirely on backfill
  base- | \
  ment  |  \   backfill wedge:
  wall  |   \  dumped in, rarely
        |    \ compacted, settles
        |     \ for years
        |      \
  ------+-------\   undisturbed
  footing        \  native soil
  <------- overdig, width varies ------->

The overdig width varies with the depth of the basement and the soil, but a pad or post within a few feet of a basement wall is often on backfill even when the surface looks settled. A slab-on-grade house has a much narrower wedge; a deep basement in soft soil has a wide one.

Worked example: sizing a deck post footing on clay

A handyman crew is replacing a rotted deck post and footing. The deck plan, from a designer, puts 2,000 pounds on this post (illustrative; your load comes from the design or the code's tables, never from a guess). The old footing was a 12 inch round pier.

Step 1, read the soil. The hole is sticky gray-brown soil that ribbons two inches before breaking. That is clay, so the presumptive value is 1,500 psf. The hole is 14 feet from the house, outside any backfill, and there are no debris fragments or buried topsoil.

Step 2, check the old footing. A 12 inch round has an area of 3.14 times 0.5 ft times 0.5 ft, which is 0.785 sq ft. The bearing pressure is 2,000 lb divided by 0.785 sq ft, about 2,550 psf. That is 2,550 divided by 1,500, roughly 1.7 times what clay is presumed to carry. The old footing was undersized for this soil, which is a likely reason the post sank.

Step 3, size the new one. The area needed is 2,000 lb divided by 1,500 psf, which is 1.33 sq ft. For a round footing, the diameter is the square root of 1.33 times 4 divided by 3.14, which is the square root of 1.70, about 1.30 ft, or 15.6 inches. They round up to a 16 inch round.

What changes the answer. On sand at 2,000 psf, the same 2,000 lb needs 1.0 sq ft, a 13.5 inch round, so a 14 inch form. On sandy gravel at 3,000 psf it needs 0.67 sq ft, an 11 inch round, and the original 12 inch footing would have been adequate. Nothing about the post or the deck changed across those three cases; the soil alone moved the footing from adequate to 1.7 times overloaded.

What breaks it entirely. If the hole had turned up brick fragments at 18 inches, none of those numbers apply, because fill has no presumptive value. The crew either digs through it to native soil (and confirms the depth still clears frost) or stops and asks for a design. Pouring a bigger footing on fill only spreads the settlement over a larger area.

The hazards in the work itself. Before digging any hole, the utility locate request goes in through your state's one-call system and the marks are respected, with hand digging inside the tolerance zone around any mark. If a gas line is struck anyway, everyone moves away from it, no engine is started and no phone is used beside it, and the call to the gas utility and 911 is made from a safe distance. Cutting the old concrete pier with a saw generates respirable crystalline silica, so it is cut wet or with a shroud and vacuum, in line with 29 CFR 1926.1153 for construction work, rather than dry.

How to verify the call

The record should say what the soil was and how you know: "clay by ribbon test at 30 inches, native, no fill observed," not "good soil." It should say which bearing value you used and where it came from, and the footing size it produced. Twelve months later, a post that has not moved is the real verification, and a post that has moved sends the next tech straight to the note to see whether the soil read or the load was the weak link.

References

• International Residential Code, Table R401.4.1 presumptive load-bearing values and Section R403.1 footings, 2021 edition, as adopted by your jurisdiction
• USDA Natural Resources Conservation Service, Web Soil Survey
• 29 CFR 1926.1153, respirable crystalline silica in construction
• See related: How Soil Is Classified and What Each Class Permits
• See related: Frost Depth and What It Sets