Einfluß und Wirkungsweise von CaO bei der Belastung von Aggregaten

Summary

The structure stability of clay soils is often weak because of unfavourable interactions between soil water and soil matrix. Loading of soils with a high clay content is accompanied by the development of positive pore water pressures, which further reduce structural stability. Hence, stabilizing measures of clay material must prevent the appearance of positive pore water pressures by changing the interaction between soil water and matrix.

Stability of clay aggregates from a pseudogley-pelosol soil from Lias (52% < 2 μm) and from pure clays (bentonite, kaolinite) was investigated in confined compression tests. The surfaces of the aggregates were treated with CaO in amounts of 5% related to dry soil. Treated and untreated aggregates were stored at a water suction of p = − 7 cm WS and free air-entry for 1 day to 30 weeks. The compression tests were carried out in measuring compaction and pore water pressure in relation to load and time after CaO-treatment.

As a result of Urning, appearance of positive water pressures and compaction were reduced. In some cases, even negative water pressures occured. The stabilizing effect of liming first increased with time after CaO-treatment and then decreased again. Maximum stabilization (minimum compaction) coincided with minimum pore water pressures. The relation between pore water pressures and stability was more distinct with samples from pseudogley-pelosol soils than from pure clays. The stability effect itself depended on the material under investigation and on the conditions after liming (water and air supply). Maximum stabilization occured before CaCO³-formation was completed.

In these experiments, no new minerals except CaCO₃ were formed. Therefore, the stabilizing effect of liming clayey materials in this case is based on mechanisms connected with properties of CaCO₃, respectively its formation. These mechanisms are:

• Hydration of CaO: the formation of Ca(OH)₂ is water consuming and dries up the aggregates. As a result, water pressures are lowered and stability is improved.

• Particle-size-effect of the CaC03-crust on the edges of the aggregates: this is a more mechanical effect which prevents a direct contact of clay particles with their enveloping water molecules and enables the water to drain between the CaCO₃-particles under conditions of loading. This is another way to reduce the development of positive water pressures.

Hence, both mechanisms change the unfavourable interactions of soil water and soil matrix under loading and stabilize the soil structure.