Lesson 1/Learning Event 3
Learning Event 3
ATTERBERG LIMITS AND CASAGRANDE PLASTICITY CHART
The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80.
Tropical iron-rich laterite soils will generally have a specific gravity of 3.0 or more. Sand particles
composed of quartz have a specific gravity around 2.65. Clays can have values as high as 3.50. Most
minerals of which the solid matter of soil particles is composed will have a specific gravity greater than
2.60. Therefore, values of specific gravity smaller than 2.60 indicate the possible presence of organic
matter.
SOIL MOISTURE CONTENT
The moisture content of a soil mass is often the most important factor affecting the engineering
characteristics of the soil. The water may enter from the surface or it may move through the subsurface
layers either by gravitational pull, capillary action, or hygroscopic action. This moisture, present in
most cases, influences various soils differently, and may have the greatest implication upon the soil's
behavior when subjected to loading.
To define the amount of water present in a soil sample, the term moisture content (symbol w) is used. It
is the proportion of the weight of water to the weight of the solid mineral grains (weight of dry soil)
expressed as a percentage or:
The effects of soil moisture depend to a great extent upon grain size. Coarse-grained soils with larger
voids permit easy drainage of water. They are less susceptible to capillary action. The amount of water
held in these soils is less than in fine-grained soils since the surface area is smaller and excess water will
tend to drain off whenever possible. The fine grains and their small voids retard the movement of water
and also tend to hold the water by surface tension.
Fine-grained clay soils, called cohesive soils, exhibit plasticity within a range of moisture content. Their
properties may vary from essentially liquid to almost brick-hard with different amounts of moisture.
Further, clays are essentially impervious to the passage of free or capillary moisture.
A cohesive soil has considerable strength when air-dried, but has low strength when its moisture content
is high. Such clays soils are composed of fine-grained particles of so-called clay minerals. Clay
particles (flat, platy shapes) are capable of holding a film of absorbed water on their surfaces. Absorbed
water is held by physiochemical forces and has properties substantially different from ordinary or
chemically combined water. The attraction exerted by clay particles for water molecules gives these
materials the property of plasticity.
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