Soil improvement and stabilization

Areas of Expertise (1)

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Soil treatment comprises two clearly distinct concepts:

  • improvement = operation with a virtually instantaneous effect, which improves the geotechnical characteristics (bearing capacity, penetration resistance, compactibility) of a soil by admixing it with a binder in a proportion ranging between 1 to 3 %, while the nature of the soil remains the same. This operation renders the soil workable with conventional earthmoving equipment, but does not guarantee durability with respect to  the action of water and frost;
  • stabilization = operation with a medium or long-term effect, which substantially increases the mechanical characteristics of a soil by admixing it with a binder in a proportion ranging between 4 to 8 %, so as to give the material final durability with respect to the action of water and frost. This effect shows itself among other things in gradual hardening of the mixture in the first few weeks and months after compaction. It can be exploited in the structural design of certain pavements, but does not affect earthworks.

In carrying out earthworks or preparing formation levels for road pavements, the soil in place, which is often water-sensitive and too wet, can be improved with a suitable binder to improve the trafficability of the construction site, increase the compactibilility of the soil, or give the soil immediate bearing capacity which is preserved or even increases with time.

In constructing subbase layers, stabilization makes it possible to improve the mechanical characteristics of soil in place in the medium and long terms and to render it durably stable under the action of water and frost.

The standardized binders with which some experience has been gained in Belgium are lime, cement, combinations of lime and cementitious binders, and cementitious road binders.

The type of binder and the rate of spread are determined with the object of the treatment (improvement or stabilization), the type of soil, moisture content on site, and possibly other requirements (frost and water resistance) in view. Soil type identification is crucial. It is based on grading, methylene blue value, plasticity index, and natural moisture content.

A mix design study is strongly recommended to assess the effectiveness of the treatment and the optimum rate of spread of the treating agent for the moisture contents to be expected on site.

When improving soils for embankments and formation levels, the performance criteria are based on the on-site bearing ratio (IPI) and the CBR value, respectively.

When stabilizing subbase layers, the performance criteria (bearing capacity, laying of the layer, resistance to immersion, frost resistance) are based on measurements of CBR/IPI, compressive strength (after sixty days), or splitting tensile strength (after sixty days).

A preliminary mix design study alone does not guarantee the effectiveness of the treatment. Rules must be observed during the implementation on site as well.

Compaction is checked within the time limits set in the tender specifications.