Expansive Subgrade: The Silent Threat Beneath Our Pavements

Introduction

Pavement engineers often focus on surface distress, such as cracking, rutting, and roughness, but the actual cause of premature failure usually lies hidden beneath the surface. The structural behaviours tell more reliable stories about the pavement's health and should be prioritised. Among the layers, the subgrade soils and expansive clays pose one of the most significant risks to pavement performance. Their tendency to undergo dramatic volume changes in response to moisture fluctuations introduces stresses that can shorten pavement life and increase maintenance costs.

What Are Expansive Soils?

Expansive soils are fine-grained clays, rich in minerals such as montmorillonite, which can absorb and release significant amounts of water. When dry, they shrink and crack; when exposed to moisture, they swell and generate substantial pressures against any overlying structure.

This shrink–swell cycle is not merely an academic concern; it is a leading cause of uneven pavements, reduced ride quality, and even structural failures in roads, airfields, and building foundations.

Mechanism of Shrink–Swell Behaviour

When expansive soils dry, capillary stresses develop as water evaporates or is drawn down by vegetation or a lowering of the groundwater table. These stresses pull soil particles closer together, reducing volume until the shrinkage limit (SL) is reached, an Atterberg limit beyond which no further shrinkage occurs despite continued drying.

When water infiltrates, clay platelets attract and absorb it between their layers. This absorption generates pore pressures that, if greater than the overlying pavement surcharge, force the soil to expand. Swelling pressures typically range from 100 to 200 kPa (14.5 to 29 psi) but have been documented as high as 1000 kPa (145 psi).

Laboratory criteria, such as the Plasticity Index, Shrinkage Limit, and Colloidal Content, are used to determine the degree of expansion risk in soils, as shown in the table attached.

Laboratory tests provide a way to assess expansion potential. Criteria such as Plasticity Index (PI), Shrinkage Limit (SL), Colloidal Content, and Probable Expansion % (Holtz & Kovacs, 1981) are commonly used. For example, soils with PI > 35 and SL < 11 typically fall into the very high expansion category, warranting special attention in design.

Engineering Impacts on Pavements

The consequences of expansive subgrades are far-reaching:

• Surface Cracking – caused by tensile stresses as the soil contracts and expands.

• Loss of Serviceability – uneven heave leads to roughness, poor ride quality, and higher International Roughness Index (IRI) values.

• Accelerated Deterioration – pavement structures designed without accounting for shrink–swell behaviour may fail prematurely.

• Maintenance Burden – repeated patching and overlays do little to address the root cause.

Mitigation Strategies

Pavement engineers have developed several approaches to reduce the risks posed by expansive soils. The choice of strategy depends on the results of soil classification, site conditions, and project economics.

• Soil Stabilisation with Lime or Cement
  Lime treatment is one of the most effective methods for expansive clays. Lime reduces the Plasticity Index (PI) and permanently alters soil structure through pozzolanic reactions, forming calcium silicate hydrates that increase stiffness and minimise swelling potential. It also improves workability and moisture control during construction. Cement stabilisation may be used in soils with lower clay content, providing additional strength and durability.

• Over-excavation and Replacement
  Highly expansive soils may be removed and replaced with non-expansive fill. While effective, this approach is often more expensive and less sustainable for large projects..

Credit: structville.com

• Moisture Control and Drainage
  Preventing seasonal moisture fluctuations is critical. Surface drainage, subsurface barriers, and proper vegetation management all help maintain consistent soil moisture and reduce the severity of shrink–swell cycles.

• Geosynthetics and Reinforcement
  Geogrids, geotextiles, and other reinforcement methods distribute loads more evenly, reducing the effects of differential soil movements on the pavement structure.

Conclusion

Expansive soils remain one of the most persistent challenges in geotechnical engineering for pavement design. By understanding their behaviour, correctly classifying their expansion potential, and adopting targeted mitigation measures, engineers can design pavements that resist premature failure and deliver sustainable serviceability.