Missouri soil classification is the practice of sorting the ground by measurable physical properties, so its behavior under load, water, and slope can be predicted before anything is built on it. The property under a planned wall or patio is not generic dirt. It behaves in a specific way that depends on how it formed, what it is made of, and how it holds or sheds water.
Three systems describe that behavior, and a sound hardscape design reads all three. The USDA taxonomy and its named soil series, mapped by the Natural Resources Conservation Service, tell you what soil sits on a given parcel; the Unified Soil Classification System and the AASHTO system translate that soil into engineering terms for load and drainage. Across St. Louis, Jefferson, and Franklin counties, the dominant soils differ enough to change how a wall or slab is built.
The three classification systems come first, then a way to find the soil under a specific property, then what each county’s dominant ground demands from a retaining wall or paver base. Soil type is where every one of those decisions starts.
The Three Soil Classification Systems Behind Missouri Hardscape
Missouri soils are described by three systems: the USDA taxonomy and named soil series from the Natural Resources Conservation Service, the Unified Soil Classification System used in engineering, and the AASHTO system used for load-bearing suitability. A hardscape design reads all three.
How Does the USDA System Classify Missouri Soils?
The USDA system classifies soil into named series based on horizons, texture, and drainage, and the Natural Resources Conservation Service maps those series nationwide, so a given site is identified by its series name rather than a code. The taxonomy sorts soil by properties a scientist can observe in a pit and measure in a lab, and each mapped soil gets a series name tied to a place, the way Menfro takes its name from a Missouri town.
Missouri has about 488 recognized soil series, according to the NRCS. One property often sits across more than one mapped unit, so the useful question is which unit falls under the footprint of the wall or patio, not which soil the county has.
Each series description spells out the soil’s texture, depth, drainage class, and slope range. That is what a designer reads to anticipate bearing and drainage. Reading the unit under the footprint, not the county average, tells you what you are building on while there is still time to design for it.
What Is the Unified Soil Classification System (USCS)?
The Unified Soil Classification System, standardized as ASTM D2487-17 (2025), sorts soil into 15 basic groups across three divisions, coarse-grained, fine-grained, and highly organic, using particle size, liquid limit, and plasticity index. It is the engineering language of soil, and its two-letter symbols carry real meaning: GW is well-graded gravel, CL is lean clay, CH is fat clay.
Liquid limit is where a soil starts to flow; plasticity index is the range where it stays moldable. A soil on the boundary between two groups gets a dual symbol, such as CL-ML or GC-GM, the pair you often see on a real geotechnical report. The dual symbol is a signal the material is borderline.
A CH fat clay behind a wall is the case the symbol warns you about: it holds water and swells, and that pressure can climb past what the wall was built to take. Granular fill drains instead, which is why an engineered wall, covered in the design section, calls for imported gravel over on-site clay. Once engineered, the symbol matters more than the series name.
How Does the AASHTO System Rate Soil for Load Support?
The AASHTO system rates soil for load-bearing suitability in seven groups, A-1 through A-7, standardized in AASHTO M 145-91 (2021). The split runs at 35 percent of the material passing the No. 200 sieve, the fine mesh that sorts sand and gravel from silt and clay.
Load support is the question under a driveway, a paver base, or a slab. All of it rests on the subgrade, the native soil beneath.
Soils on the granular side of that line, roughly A-1 through A-3, drain and carry load well. The silt-clay soils above it, A-4 through A-7, hold water and weaken as fines and plasticity climb. The group label is only half the reading.
AASHTO M 145 also assigns a group index, where zero is a good subgrade and 20 or more is very poor. A paver base over an A-7 subgrade with a high group index needs more aggregate and drainage than the same base over A-1, because the weaker soil holds water where the stronger sheds it.
How Do You Identify the Soil on Your Property?
You identify your property’s soil by running the address through the NRCS Web Soil Survey, which returns the soil map units covering that location, each with its drainage class, slope, and shrink-swell rating. The tool is free and public.
It is the same data professionals start from, and the lookup takes about five steps:
- Open the Web Soil Survey at websoilsurvey.nrcs.usda.gov and click Start WSS.
- Enter your address, or your state and county, in the Quick Navigation panel.
- Zoom to the property and draw your Area of Interest with the rectangle or polygon tool.
- Open the Soil Map tab to see the mapped soil units and their names for that area.
- Open the Soil Data Explorer tab to pull drainage, slope, shrink-swell, and depth-to-bedrock for each unit.
A report becomes useful when you read it against the actual project. Drainage class flags whether water will sit behind a wall. Slope and depth-to-bedrock drive footing depth and excavation effort. The shrink-swell rating warns whether the ground will move from season to season under a slab or patio.
A mapped rating is a screening tool, not a substitute for a site boring once the wall carries real load.
St. Louis County Soil and What It Means for Hardscape
St. Louis County soil falls into three hardscape-relevant settings: wind-deposited loess over most of the uplands, residual clay over limestone and shale that swells when wet, and deep river alluvium along the Missouri, Mississippi, and Meramec valleys.
- Loess uplands (Menfro): well-drained silt that loses strength when it saturates
- Residual clay over limestone and shale: swells when wet, cracks light footings
- River alluvium (Missouri, Mississippi, Meramec valleys): deep, layered, variable, high water table
Loess Uplands and the Menfro Series
More than half of St. Louis County is rolling upland covered by loess, the wind-blown silt that forms the Menfro series. Menfro is Missouri’s state soil: a deep, well-drained silt loam, a Typic Hapludalfs.
Near the river bluffs the loess comes in two layers. The upper Peoria loess is low in clay, drains fast, and will stand in a near-vertical cut, but it loses strength once it saturates. Beneath it, the higher-clay Roxana loess drains slower, so water moving down collects at the contact between the two and can turn that seam into a slide plane.
A wall or footing cut into loess needs positive drainage and protection from concentrated runoff. Without that, the cut face and the loess contact weaken over time.
Residual and Expansive Clay Over Limestone and Shale
Where the loess thins, residual clay forms in place over the county’s limestone and shale bedrock. This clay swells when wet, and the Missouri Geological Survey, in Engineering Geology Series No. 4, recorded swelling pressures above 3,300 psf in residual clay over limestone. That is enough to crack the footing of a light structure.
The same survey recorded even higher pressures, above 6,000 psf, in clays over shale and along the broad ridgetops, so hardscape over this ground needs drainage, non-expansive backfill, and footing detailing for seasonal movement. Clay behavior at depth is covered in the expansive-clay guide.
River Alluvium and the Florissant Basin
The Missouri and Mississippi valleys hold alluvium well over 100 feet deep, stratified sand, silt, and clay carrying a high water table. Bearing strength changes sharply from one layer to the next. Buried organic pockets carry very low capacity.
The Florissant basin adds compressible lake-bed silt and clay under a loess cap. Any load-bearing hardscape on these deposits depends on a site-specific boring, not a map rating.
Jefferson County Soil and Its Hardscape Implications
Jefferson County soil is dominated by cherty residuum from limestone and dolomite on the uplands and side slopes, with loess-capped ridges and shallow soil over bedrock in the rugged River Hills. Rock, drainage, and slope drive most hardscape decisions here.
Expansive clay is not the county’s dominant material the way it is in St. Louis County. Where clay occurs, it is chert-rich red clay on the upper side slopes, which drains better and moves less than the thick residual clay over limestone and shale to the north.
Cherty Upland Residuum: Goss, Gasconade, and Weingarten
Goss, Gasconade, Menfro, and Weingarten are the major upland soils mapped in the Soil Survey of Jefferson County, Missouri. Goss is very deep and well drained; Gasconade is shallow to very shallow over dolomite bedrock. Weingarten is loess over cherty residuum.
The engineering reality is high coarse-fragment content and shallow depth to rock, which complicates excavation even though the ground drains well. Expansive clay here is a localized side-slope condition, not the uniform ridgetop clay that defines St. Louis County’s uplands, so shrink-swell is a site-by-site check rather than a countywide default.
River Hills and Loess-Capped Ridges
In the River Hills, the survey names Gasconade, Menfro, and Sonsac as the major soils, and the split is by landscape position. Menfro caps the ridgetops in thick loess, while Gasconade and Sonsac hold the cherty, rocky side slopes with limestone outcrop below. The three can sit within a single neighborhood.
A hardscape plan here reads slope and depth-to-bedrock first. A wall on a loess ridgetop and a wall on a rocky side slope a few hundred feet away are not the same build.
Franklin County Soil and Its Hardscape Implications
Franklin County soil splits between two worlds: Ozark-border uplands of cherty, fragipan-bearing residual soils, and the river-bottom floodplains that parallel the county’s streams and the Missouri River along its northern edge. Hardscape design depends heavily on which one a site sits on.
Ozark-Border Upland Associations
The uplands are mapped in two general soil associations, Union-Goss-Gasconade-Peridge and Hobson-Clarksville-Gasconade, both in the Ozark Border. Union carries a fragipan, a dense, cemented subsurface layer that restricts roots and water. That perched water complicates drainage behind a wall.
Clarksville is the other soil to know: a very deep, somewhat excessively drained cherty soil with 35 to 70 percent rock fragments in its control section. Between Union’s perched water and Clarksville’s heavy chert, the recurring hardscape problems here are excavation difficulty and drainage.
River-Bottom Floodplains
Along the county’s river bottoms, the Franklin County survey maps the Haymond-Pope association, a silty to sandy alluvial soil. Haymond is a very deep, well-drained floodplain silt loam, but the bottoms as a whole carry a high water table, seasonal flooding, and layered materials of uneven strength. The ground is young and variable.
Bearing capacity changes sharply with depth. As in the St. Louis County valleys, a load-bearing structure here is a boring-first decision, and flood exposure shapes everything above the footing.
Loess vs Clay vs Alluvium: Hardscape Behavior Compared
Loess drains fast and stands in steep cuts but loses strength when saturated; residual clay swells and shrinks with moisture and can crack light footings; river alluvium is layered, weak in organic pockets, and variable with depth. Each demands a different drainage and footing response.
The table lines up the three Missouri parent materials against the attributes that decide how a wall, patio, or slab has to be built.
| Parent material | Drainage | Strength when wet | Shrink-swell (LEP class) | Bearing and variability | Dominant failure mode | Primary design response |
|---|---|---|---|---|---|---|
| Loess (Menfro) | Well drained, fast | Loses strength on saturation | Low (LEP under 3) | Moderate, fairly predictable | Cut-face weakening | Positive drainage, runoff protection |
| Residual clay | Poorly drained, holds water | Softens, stays weak | Moderate to very high (LEP 3 to 9+, per NRCS NSSH Part 618) | Variable with moisture | Seasonal footing movement | Non-expansive backfill, drainage, movement-tolerant footings |
| River alluvium | Water-table dependent | Weak in organic pockets | Low | Highly variable with depth | Differential settlement | Site-specific boring |
How Soil Class Drives Retaining Wall and Paver Design
Soil classification sets the design lateral load a wall must resist, the backfill and drainage it needs, and how deep the footing goes, because the building code ties each of these directly to the soil’s USCS class and its shrink-swell behavior.
Design Lateral Load by Backfill Class
A wall resists what sits behind it. The building code sets a minimum design lateral soil load by USCS class, with the material classified per ASTM D2487 (IBC Section 1610 and Table 1610.1). Clean granular backfill pushes on a wall far less than clay, which is why the backfill you choose changes the wall you have to engineer.
| Backfill (USCS class) | Design lateral soil load, active pressure |
|---|---|
| Well-graded or poorly graded clean gravel (GW, GP) | 30 psf per foot of depth |
| Clayey gravel (GC) | 45 psf per foot of depth |
| High-plasticity fat clay (CH) | Unsuitable as backfill |
Choosing imported granular fill over on-site clay lowers the active pressure the wall is engineered against, and it drains.
Drainage and Shrink-Swell as Design Inputs
Drainage is not an add-on to a retaining wall; the code treats it as structural. A wall must be built to hold the full hydrostatic pressure of undrained backfill unless a drainage system is installed, and the design pressure must go up where soils are expansive (IBC Section 1610). Trapped water is often the largest load a wall sees.
Shrink-swell is rated by linear extensibility percent, running from low under 3 percent to very high at 9 or more. Anything moderate or above can damage walls, slabs, and pavement, so on expansive ground drainage and backfill selection are the design, not an upgrade.
Footing Depth and Frost
A footing sits below the frost line, or freezing soil lifts it. In the St. Louis metro that line is set at 30 inches, under the Code of the City of St. Louis, Table R301.2(1). An exterior footing must reach below it and at least 12 inches into undisturbed ground (IRC Section R403.1.4).
Some outlying jurisdictions require deeper. On the region’s loess and clay, frost depth is only the starting number. Combined with shrink-swell and drainage, it usually sets a real footing depth deeper than the frost minimum alone.
When Does Your Soil Require Engineered Wall Plans?
A retaining wall over 4 feet from the bottom of the footing to the top requires a permit and engineered design, and a wall of any height needs one if it carries a surcharge, which on the region’s expansive and variable soils often means a stamped design and a soil investigation.
The 4-Foot and Surcharge Thresholds
A retaining wall is permit-exempt only if it stands no more than 4 feet from the bottom of the footing to the top and carries no surcharge (IRC Section R105.2). Because the buried footing counts in that measurement, a wall standing three feet above grade can already cross the line. The exposed height is not the number that matters.
A surcharge removes the exemption at any height. A slope, a driveway, a pool, or a structure above the wall all count as surcharge, and on sloped hardscape that clause is what usually triggers a permit. Note the distinction: R105.2 governs the permit, while whether the wall needs a stamped engineered design is a separate question.
When Soil Forces a Stamped Design and a Boring
On the metro’s expansive clays, deep alluvium, and fragipan uplands, the sound path is a soil boring plus a stamped design from a licensed professional engineer. The practice of engineering, defined at §327.181 RSMo, puts a professional engineer in responsible charge of any design of a structure that can affect public safety.
Practicing without a license is a class A misdemeanor (§327.076 RSMo). A wall over 4 feet, a wall on a surcharge, or a wall on high-shrink-swell or variable ground is the point to bring in engineering rather than build to a generic detail.
The building code lets a designer use the table’s default lateral loads or a site-specific geotechnical investigation, and on the region’s variable soils those defaults are often too coarse to trust.
