Page 87 - RoadCem Manual - PCT BV

coats them. When expandable clays are present their action can help define and form wedge structures. In regions that experience freezing of the ground surface such action may aid in structural development. In some ways this is similar to shrink/swell because the soil is compacted as object is force upward. The compaction form ped faces which, if stabilized may form structural units. However the next freeze/thaw cycle could destroy the original feature.
Compaction generally results in denser soil maybe even structureless.
The soil water chemistry affects structure as well. As with physical influences these affects can be both positive and negative. Flocculation is the bringing of the particles close together and is enhanced by polyvalent cations (Al+++, Ca++, etc.). The flocculated units may form some of the initial building blocks for structural units (This is for example one of the mechanisms of RoadCem activation).
Fe oxides may coat and bridge particles. These oxides may be attached to clay or other particle charges. The oxides may help define a weak subangular blocky structure. Similar to oxides, organic acids will coat and stabilize. Commonly these acids will overwhelm the particles and cement the particles in the horizon together.
The biological influence generally enhances the structure. Microbes and fungus add to structure by their by-products, their mediation of redox reactions (Fe oxides), and binding particles together. This phenomenon is most common in the topsoil but can occur wherever enough carbon is present for food.
The type of structure has a profound impact on how water will move through the soil. A well structured soil will conduct water away faster as the larger voids will have a greater conductivity and may allow for deeper flow.
In Conclusion structure is an important but overlooked aspect of soil evaluation
If possible structure should be viewed in a pit. Structure should be assessed in the field with an eye for how will water flow through the soil. This can best be done by looking at roots and ped faces. The relationship between structure and water movement is complex and differs depending on moisture content.
Examples of different soil structures are given in figure 4.38 and 4.39.
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