A grading curve is a useful aid to soil description. The geometric properties of a grading curve are called grading characteristics.
To obtain the grading characteristics, three points are located first on the grading curve.
D60 = size at 60% finer by weight
D30 = size at 30% finer by weight
D10 = size at 10% finer by weight
The grading characteristics are then determined as follows:
1. Effective size = D10
2. Uniformity coefficient,
3. Curvature coefficient,
Both Cuand Cc will be 1 for a single-sized soil.
Cu > 5 indicates a well-graded soil, i.e. a soil which has a distribution of particles over a wide size range.
Cc between 1 and 3 also indicates a well-graded soil.
Cu < 3 indicates a uniform soil, i.e. a soil which has a very narrow particle size range.
consistency of soils
The consistency of a fine-grained soil refers to its firmness, and it varies with the water content of the soil.
A gradual increase in water content causes the soil to change from solid to semi-solid to plastic to liquid states. The water contents at which the consistency changes from one state to the other are called consistency limits (or Atterberg limits).
The three limits are known as the shrinkage limit (WS), plastic limit (WP), and liquid limit (WL) as shown. The values of these limits can be obtained from laboratory tests.
Two of these are utilised in the classification of fine soils:
Liquid limit (WL) - change of consistency from plastic to liquid state
Plastic limit (WP) - change of consistency from brittle/crumbly to plastic state
The difference between the liquid limit and the plastic limit is known as the plasticity index (IP), and it is in this range of water content that the soil has a plastic consistency. The consistency of most soils in the field will be plastic or semi-solid.