When the subgrade on a Missouri site won't support the pavement or building pad above it, there are three common fixes: treat the soil in place with lime, treat it in place with cement, or dig it out and replace it. This guide explains each option in plain terms. It is general information, not a design — on every project, the geotechnical engineer of record and the lab mix design govern what is used, at what rate, and to what depth.
Why subgrade fails
Much of the St. Louis region and Missouri sits on clay soils. Clays with a high plasticity index swell when wet, shrink when dry, and lose strength when saturated. Wet soils can also be too soft to compact, so they fail a proof roll and pump under loaded trucks. The fix has to deal with strength, moisture, or both.
Lime stabilization
Lime (usually quicklime or hydrated lime) is spread on the subgrade and mixed into it. It works on clay in two ways. In the short term, it dries wet soil and changes the clay particles so the soil becomes less plastic, more workable, and easier to compact — often called modification. Over a longer period, lime reacts with the silica and alumina in the clay to form cementing compounds that add strength — stabilization. Because those reactions depend on clay content, lime is typically chosen for high-plasticity clays. Lime specs often include a mellowing period between an initial mix and a final mix so the lime can react before compaction.
Cement stabilization
Portland cement is spread and mixed into the soil with water. As the cement hydrates, it binds soil particles together into a stiffer, stronger layer. Cement does not depend on a reaction with clay minerals, so it is typically chosen for lower-plasticity, silty, or granular soils, where lime has less to react with. Cement begins to set once it is mixed and wetted, so the spec usually limits how long the contractor has to finish mixing, grading, and compacting.
Undercut and replace
Undercutting removes the unsuitable soil to a depth the engineer sets and replaces it with imported fill or aggregate, compacted in lifts. It needs no chemical reaction and no mix design, which makes it simple to specify. The trade-offs are logistics: the spoils need a place to go, the import has to be bought and hauled, every load adds trucks to the site and the roads around it, and open excavations can sit idle after rain. On small or isolated soft spots, undercutting is often the practical answer. On large areas, the trucking adds up.
Modification vs. stabilization
Specs don't always use these words the same way, so it helps to settle the intent early. Modification means treating the soil so it can be worked and compacted — drying a wet site, cutting plasticity, and giving crews a stable platform. It is often a constructability measure and may not be counted in the pavement design. Stabilization means the treated layer is expected to reach a design strength and is counted as part of the pavement or pad structure. The difference drives the application rate, the testing, and the curing requirements, so the plans should say which one is intended.
Pricing the options
Costs depend on the area, the treatment depth, the application rate, haul distances, and the price of import material and disposal, so there is no single answer to which option is cheaper. On larger areas, in-place treatment avoids hauling spoils out and fill in. On small or scattered soft spots, an undercut can be simpler. The reliable way to decide is to price both on the same quantities, which is why we price both so you can compare.
Side-by-side comparison
| Factor | Lime | Cement | Undercut |
|---|---|---|---|
| What it does | Reacts with clay to reduce plasticity, dry wet soil, and build strength over time | Binds soil particles into a stiffer, stronger layer as it hydrates | Removes unsuitable soil and replaces it with imported fill or rock |
| Typical soils | High-plasticity clays | Lower-plasticity, silty, or granular soils | Any soil the engineer rejects for the design |
| Material on the trucks | Lime deliveries | Cement deliveries | Spoils out, import in |
| Spoils generated | None from the treated zone | None from the treated zone | Yes — needs a disposal site |
| Timing items | Mellowing time is often specified between mixing passes | Working time is limited once cement is mixed and wetted | Haul distance and import availability |
| Who sets it | Engineer of record and lab mix design | Engineer of record and lab mix design | Engineer of record (depth and backfill spec) |
When full-depth reclamation applies
The three options above are for soil. When the problem is a failed asphalt parking lot or road — cracking, rutting, and potholes that go down through the base — the fix is often full-depth reclamation (FDR). FDR pulverizes the old asphalt together with the base beneath it, blends in a stabilizing agent chosen by the mix design, and compacts the result into a new base that is then paved. The old pavement is recycled in place instead of being hauled off.
Checklist for the engineer
- Geotech report with borings, moisture contents, and plasticity index (PI) results
- Whether the subgrade is being modified for constructability or stabilized for design strength
- Lab mix design: agent, application rate, and target strength
- Treatment depth and the limits on the plan sheets
- Moisture and compaction requirements
- Testing requirements and who the owner's testing agency is
- Weather and temperature limits for placement and curing
- Whether an undercut alternate should be priced for comparison
Common spec items to coordinate
- Depth
- The treated thickness, measured after mixing. Our MU Walton Stadium work was mixed to a 12-inch depth because that is what the project specified.
- Application rate
- Set by the mix design, usually as a percentage of dry soil weight or pounds per square yard. The contractor spreads to the rate; it doesn't choose it.
- Mellowing time (lime)
- Many lime specs call for an initial mix, a waiting period for the lime to react with the clay, and a final mix. The duration comes from the spec.
- Moisture
- Water is usually added during mixing to reach the moisture range in the spec, both for the chemical reaction and for compaction.
- Compaction
- Density and moisture targets for the treated layer, tested by the owner's testing agency.
- Curing
- Protection of the finished layer — keeping it moist or sealed and limiting traffic — for the period the spec requires.
How Kuesel fits in
Kuesel Excavating Co. self-performs lime stabilization, cement stabilization, full-depth reclamation, and subgrade drying across Missouri and Illinois. We review the geotech report and plans, work from the mix design prepared by the engineer or testing lab, spread and mix to the specified depth, and compact, while quality-control testing is done by the owner's testing agency. On request, we price both stabilization and undercut so you can compare. See our soil stabilization page and our page for civil & geotechnical engineers.
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