Page 86 - RoadCem Manual - PCT BV

elongated. Columnar structure is similar to prismatic but the unit tops are frequently rounded and bleached due to the high salt content.
STRUCTURELESS SOILS
Structureless soils are broken into 2 official groups:
1. Single Grain, Massive and Massive.
2. Rock Controlled Fabric.
Single grain refers to sands whereas massive refers to any soil that does not break apart into any predictable and repeatable type or shape. Massive rock controlled structure is used for soil developed from saprolite. Unlike simple massive structure, rock controlled fabric has a preferred orientation of the minerals. The material may easily break into the individual mineral grains.
Size is broken into 5 groups. The actual size ranges vary depending on the type of structure. Note that platy refers to thickness and columnar and prismatic refer to diameter. (Figure 4.38)
Grade refers to how well expressed or how stable the structure is. There are 4 groups of structure grade (0-3). All structureless soils have a grade of 0. The others range from 1 to 3. (Figure 4.39).
• Structure less are soils where no discrete units observable in place or in hand sample.
• Weak structured soils have units that are barely observable in place or in a hand sample.
• Moderate structured soils have well-formed units that are evident in place or in a hand sample.
• Strong structured soils have units that are distinct in place (undisturbed soil), and separate cleanly when disturbed.
• Compound structure is described when smaller structural units held together to form larger units. For example a soil may be described as having “Moderate coarse prismatic structure parting to strong medium sub angular blocky structure.” This means that sub angular blocky is the primary structure and prismatic is the secondary structure.
The formation of structure in soils has not received the level of scrutiny proportional to its importance in road construction. Structure formation is related to physical, chemical and biological processes at
work in
• • • • • •
the soil. The physical processes that affect structure are:
Alleviations/eluviations.
Freeze/thaw.
Compaction.
Disruption (mechanical, natural i.e. slope movement) Wet/dry
Shrink/swell.
Eluviated zones generally have weaker structure or are structureless where as the Illuviated zones show stronger structures as the illuviated material will help define the structures units by coating and stabilizing peds.
Even if the material does not have a high amount of expandable clay minerals, desiccation will cause cracks. These cracks may become stabilized as clay, oxides or organic matter move through and/or
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