Lime Stabilization of Soil: Process, Lime Rates, Limits

Lime stabilization of soil is one of the oldest tricks for building pavement on clay. Mixing a few percent of lime into a sticky, plastic clay changes it chemically, first into a crumbly soil that compacts well and then, over weeks and months, into a stiffer layer that carries load and resists water. Highway agencies, airports and large site developers use it to avoid hauling out bad soil. Here is how it works and how a job is built.

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The short version

Best for: fine-grained clay soils. The National Lime Association (NLA) manual suggests at least 25 percent passing the No. 200 sieve and a plasticity index (PI) above 10.
Typical rates: 3 to 6 percent lime by dry soil weight for subgrade stabilization, 2 to 4 percent for clayey base materials, and 1 to 4 percent for drying or short-term modification, per the NLA manual. A lab mix design sets the real number.
Forms of lime: quicklime (calcium oxide), hydrated lime (calcium hydroxide) or a slurry of hydrated lime in water.
Watch for: sulfates and organics in the soil, cold weather and poor curing.

What lime does to clay

The NLA describes three effects that overlap in time:

Drying. Quicklime reacts with water in the soil and gives off heat, which drives off more moisture. A wet site can become workable within hours.
Modification. Calcium ions crowd onto the surface of the clay particles, which then clump together. The soil turns friable and granular, and its plasticity drops sharply. It also swells less.
Stabilization. With enough lime and the right clay minerals, the high pH lets lime react with silica and alumina in the clay to form cementing compounds. This pozzolanic reaction builds strength slowly and keeps going for a long time.

Modification alone can be temporary. Stabilization is what gives the layer structural value in a pavement design.

Quicklime, hydrated lime or slurry

Dry quicklime is more concentrated. The NLA notes about 3 percent quicklime equals about 4 percent hydrated lime when it fully hydrates. It is the best drying agent, but it needs water equal to about 32 percent of its own weight to hydrate, plus whatever evaporates.
Dry hydrated lime spreads quickly but is a fine powder, so dust can rule it out near homes and roads.
Lime slurry is dust-free and spreads evenly, but it goes on more slowly, needs more equipment and is no help for drying a wet site.

How a lime stabilization job is built

Test the soil. A lab runs plasticity, sulfate and organic checks, finds a lime content (often starting from a pH-based test such as ASTM D6276), and checks strength.
Scarify and pulverize. The soil is ripped and broken up to the treatment depth, often with a rotary mixer.
Spread the lime at the design rate by bulk spreader truck or slurry tanker.
First mixing and water. The lime is blended in and water added to bring the soil above optimum moisture.
Mellow. The mixed soil sits, typically 1 to 7 days, so the lime can soften clods. Heavy clays are often given 24 to 48 hours.
Final mixing. The layer is remixed until clods are broken down to the size the spec requires.
Compact. The NLA guide calls for at least 95 percent of standard Proctor (AASHTO T 99) maximum density, matched to the lift depth with padfoot and smooth rollers. See our Proctor test guide.
Cure. Keep the surface moist, or seal it with an asphalt prime coat, until the next layer goes on.

The NLA manual also says air temperature should be at least 40 °F in the shade and rising for stabilization work, because the strength-building reaction slows sharply in cold soil.

Sulfates and other limits

Sulfates: calcium, alumina, water and soluble sulfates can form expansive minerals that heave a finished layer. The NLA manual treats soils under 3,000 ppm (0.3 percent) as unlikely to cause problems. Higher levels call for special steps such as extra water and a longer mellowing period.
Organics: soils with more than about 1 percent organic material may need extra lime or a different approach.
Low-clay soils: sandy or silty soils without much clay do not react much on their own. Cement, or lime combined with fly ash, is often the better fit. Our cement-treated base page covers that route.
Safety: lime is caustic. Crews need eye, skin and breathing protection, and quicklime must stay dry until it is meant to react.

How much lime a job needs

Lime is specified as a percent of dry soil weight, so you need the treated volume and the soil's dry unit weight from the lab.

Example (made-up numbers): treating 1,000 square yards (9,000 square feet) to 8 inches deep is 6,000 cubic feet of soil. If the lab reports a dry unit weight of 105 pounds per cubic foot, that is 630,000 pounds of dry soil. At 5 percent lime, the job needs 31,500 pounds, or about 15.75 tons, before any allowance for spreading losses.

Lime under a driveway?

On a single home driveway, lime stabilization is rare because the equipment and testing cost more than the job. The usual answers on clay are a thicker stone base, fabric, and fixing drainage, or digging out the worst spots. Our guides to paving on clay soil and undercutting soft spots cover those. Lime makes more sense on a long private road, a large lot or a subdivision street, and it also shows up in full depth reclamation of old roads.

Questions people ask

What does lime do to clay soil?

Calcium from the lime swaps onto the clay particles and makes them clump into a crumbly, less plastic soil that is easier to work and compact. Over time, the lime also reacts with silica and alumina in the clay to form cementing compounds that add lasting strength.

How much lime is used for soil stabilization?

The National Lime Association's construction manual says subgrade stabilization generally takes lime equal to 3 to 6 percent of the dry soil weight, and base stabilization 2 to 4 percent. The real rate should come from a lab mix design for the soil on that job.

Which soils are good candidates for lime?

Fine-grained clays. The NLA manual points to soils with at least 25 percent passing the No. 200 sieve and a plasticity index above 10. Sandy, low-plasticity soils usually respond better to cement or a lime and fly ash blend.

What is the mellowing period?

A wait between the first and final mixing, typically 1 to 7 days, that lets the lime start breaking down clay clods. Heavy clays are often mellowed 24 to 48 hours. Low-plasticity soils may not need one.

Can sulfates cause problems with lime-treated soil?

Yes. Sulfates can react with calcium, alumina and water to form expansive minerals that heave the layer. The NLA manual says levels below 3,000 ppm are unlikely to cause trouble, and higher levels need special procedures.

Is quicklime or hydrated lime better?

Both work. Quicklime is more concentrated, about 3 percent quicklime equals 4 percent hydrated lime, and it dries wet soil fast, but it needs plenty of water and care in handling. Hydrated lime is easier to mix but dusty when spread dry. Slurry avoids dust.

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