What Is Soil Improvement and Stabilization?

Soil improvement is the process of increasing engineering performance by altering the physical and chemical properties of the existing soil.

The term improvement describes interventions made to raise the bearing capacity of the soil in the short term and generally provides the temporary strength required for site operations.

Stabilization, on the other hand, aims for a permanent, long-term increase in strength and forms a structural layer beneath the pavement. The TS EN 14227-11 standard classifies these two types of application separately for lime-treated soils and defines their mechanical performance criteria. Stabilization methods are addressed in three main groups: mechanical (compaction, granular fill, geotextile), chemical (lime, cement, fly ash, bitumen) and physical methods (freezing, drainage, electro-osmosis).

What Is Soil Improvement and Stabilization?

Key Problems Encountered in Problematic Soils

High-plasticity clays, silts, organic soils and expansive clayey soils are the most frequently encountered problem groups in the field.

The common feature of these soils is significant volume change linked to water content, low CBR values (generally in the 1-3% band) and a high plasticity index (PI>25).

Expansive clays cause cracks in the pavement, local settlements in road surfaces and differential settlements at foundation level under seasonal changes in water content. From a structural engineering standpoint, five main problems frequently come to the fore in problematic soils: Low bearing capacity and insufficient CBR value; High total settlement with the risk of differential settlement; Swell-shrink behavior and volume change.

Key Problems Encountered in Problematic Soils

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Methods Used in Soil Stabilization

Mechanical stabilization aims to reach an optimum grain distribution by proportionally mixing materials of different granulometry.

Limestone-based aggregate is widely used in these mixtures as sub-base, base and drainage material.

Chemical stabilization, on the other hand, alters the chemical structure by adding binders to the soil in certain proportions and imparts long-term strength. Stabilization with lime stands out as the most economical and most common solution, especially for clayey and silty soils. While cement is preferred in low-plasticity soils (PI<10), lime or a lime-cement combination gives more suitable results for high-plasticity clays.

The Role of Lime-Based Solutions in Soil Stabilization

Lime is an engineering-proven binder with over 100 years of application history in the improvement of clayey soils.

Lime added to the soil triggers a two-stage reaction chain: short-term cation exchange and flocculation, followed by a long-term pozzolanic reaction.

During cation exchange, the Na⁺ and K⁺ ions on the surface of clay minerals are replaced by Ca²⁺; this mechanism causes a rapid drop in the plasticity index, an increase in workability and the soil to flocculate into a more friable structure. The pozzolanic reaction develops over weeks and months, forming calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels that provide permanent strength. Quicklime (CaO) is the most heavily preferred product in soil stabilization.

The Role of Lime-Based Solutions in Soil Stabilization

Technical Points to Consider in Application

Preliminary assessment is critical for the correct result.

The Eades-Grim test is the reference method for determining the minimum lime dosage the soil will require: lime is added until the 1-hour pH value of the soil-lime-water mixture reaches 12.4.

In soils with an organic matter ratio above 2%, the pozzolanic reaction weakens, so the organic matter content and sulfate amount must be measured before fieldwork. In the presence of high sulfate (>0.3%), the risk of ettringite formation and delayed swelling arises; in this case, a different binder strategy should be considered instead of lime. Mixing homogeneity, curing time and application weather conditions are the other factors that determine success.

Technical Points to Consider in Application

Sectoral Approach and Good Practice as of 2026

As of 2026, soil stabilization applications in Europe and Türkiye are being reassessed along both the carbon-footprint-reduction and circular-economy axes.

The "Code of Good Practice – Soil Treatment with Lime" document published by EuLA emphasizes that soil treatment with lime offers a clear carbon advantage over cement-based alternatives.

The same approach is followed under TS EN 14227-11 and BS EN 14227-11, and lime-fly ash dual binder systems are increasingly preferred in field applications. In different segments such as motorways, urban arterials, railway infrastructure, wind turbine foundation access roads, airport aprons and logistics warehouse sites, lime-based solutions are used either alone or together with limestone aggregate.

Sectoral Approach and Good Practice as of 2026

Frequently Asked Questions

Stabilization is necessary if the existing soil's bearing capacity is insufficient for the project loads, if the CBR value is low (1-3% band), if the plasticity index is high (PI>25) or if the soil shows a swell-shrink tendency. The decision is made based on preliminary survey results in road, airport, railway and industrial site projects.

Improvement aims to facilitate site operations by raising the soil's short-term bearing capacity. Stabilization, on the other hand, is an application that provides a long-term, permanent increase in strength and forms a structural layer. TS EN 14227-11 defines these two categories separately and specifies their performance criteria.

Lime stabilization gives the best results especially in high-plasticity clays, silty clays and mixed soils with a high clay content. Lime is not suitable for clean sands with a plasticity index below 10; cement or mechanical stabilization is preferred for such soils. The efficiency is higher in soils with a low organic matter ratio.

Quicklime provides rapid drying and a permanent strength increase in clayey soils with high water content. By reacting with water, CaO chemically binds approximately 0.32 kg of water per kilogram and produces exothermic heat. It also significantly increases CBR and compressive strength by triggering cation exchange and the pozzolanic reaction.

The typical dosage varies between 2-8% of the dry weight of the clay. In field applications, a minimum of 1.5% is accepted as the lower limit. The exact dosage is determined by the Eades-Grim pH test and CBR tests according to soil type, plasticity index and organic matter ratio. The target performance is generally achieved at a pH level of 12.4.

Because quicklime (CaO) contains approximately 25% more active calcium per unit mass, it gives more economical and faster results in clays with high water content. Hydrated lime, on the other hand, is preferred in projects near residential areas that require more controlled hydration and dust-free application. Both products use the same chemical mechanism.

With the correct dosage and curing, the CBR value can rise from the 2-3% band to the 15-30% range. Even higher values are observed in some clayey soils after 28 days of curing. The increase is directly dependent on the lime ratio, the type of clay mineral, the compaction quality and the curing time.

After lime spreading, a 24-72 hour pre-hydration period is allowed, followed by a second mixing and compaction. The pozzolanic reaction develops over weeks and provides a notable strength increase at the end of 28 days of curing. During curing, the surface must be protected against sudden drying and kept off-traffic.

In soils with a sulfate content above 0.3%, the risk of ettringite formation and delayed swelling arises with lime and water. Chemical analysis before fieldwork is mandatory in such soils; if necessary, a sulfate-resistant binder or alternative stabilization methods should be preferred. Laboratory tests clearly determine the risk.

Limestone aggregate is laid as a sub-base and base layer over the lime-stabilized subgrade. With an appropriate grain distribution, the aggregate supports drainage, limits capillary water rise and increases the load distribution capacity of the pavement. In this way, chemical improvement with lime and mechanical reinforcement with aggregate work together.