Page 63 - RoadCem Manual - PCT BV

FIELD DESCRIPTION OF SOILS
Understanding and interpreting soils is an iterative process that begins with a soil description and leads towards an assessment of the soils suitability for a particular job. The factors to consider are: profile description, wetness conditions, and restrictive horizons, aerobic conditions, internal vs. external drainage, etc.. Evaluation of the soil is just one component of fully assessing a site.
Where do we begin to describe a soil? Just like the recipe for woodchuck stew starts with “first find a woodchuck hole” so with soil description we must first find a hole or soil to describe. Some examples of different soils and soil profiles are given in figures 4.12 and 4.13.
Once the hole is dug decisions need to be made as to what do you describe. The level of detail of this description will be related to the proposed soil use, however a soil description should include most if not all of the following; horizon, depth, color, texture, features, consistence, structure, pores, roots, and reaction. Each component of the description will aid in the overall interpretation. However, depending on the intended use some will be more important than others. For our purposes the important factors are: color, texture and structure and consistency parameters such as plasticity, stickyness and penetration resistance.
SOIL COLOUR
Colour is the first soil property to consider. It is perhaps the most obvious and easily determined soil characteristic.
Important characteristics can be inferred from soil colour. Well drained soils have uniform bright colours. Soils with a fluctuating water table have a mottled pattern of gray, yellow, and/or orange colors. Organic matter darkens the soil and is typically associated with surface layers. Organic matter will mask all other colouring agents. Iron (Fe) is the primary coloring agent in the subsoil. The orange brown colors associated with well drained soils are the result of Fe oxide stains coating individual particles. Manganese (Mn) is common in some soils resulting in a very dark black or purplish black color. Several other soil minerals have distinct colors, thus making their identification straight forward. For example, glauconite is green, Quartz has various colors but is often white or gray, feldspars range from pale buff to white, micas may be white, brownish black, or golden, and kaolinite appears gray to white.
Colour determination can be quite subjective if just a verbal description is used. In general each person will perceive color differently, thus there is a need to standardize it.
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