Plastic & Gravel Soils

Fine-grained cohesive & Coarse-grained non-cohesive Soils

Consistency limits

Liquid limit (LL) is when the soil represents a near liquid state. This occurs at very high water contents when the soil behaves as a viscous liquid in that it flows and will not hold a specific shape. In other words the soil will flow under its own weight.

Plastic limit (PL) is the lowest water content at which the soil exhibits plastic behavior. It is the minimum moisture content at which the soil can be rolled into a thread of 3mm diameter without breaking up.

Shrinkage limit (SL) is the water content at which there is no more volume change in the soil due to reduction in water. In other words the maximum moisture content at which further loss of moisture does not cause a decrease in the volume of the soil.

Plasticity index (PI) is the range of moisture content in which the soil is plastic; the finer the soil the greater is the plasticity index.

  Plasticity index = liquid limit - plastic limit
PI = LL - PL

Figure 5 shows the different stages of a plastic soil and the volume changes due to moisture content.

Figure 5

Particle size distribution

Table 1
Particle size limits
 Range of particle size 
in millimeter
Coarse gravel
Medium gravel
Fine gravel
Coarse sand
Medium sand
Fine sand
Coarse silt
Medium silt
Fine silt
200 - 60
 60 -20
20 - 6
6 - 2
   2 - 0.6
 0.6 - 0.2
  0.2 - 0.06
0.06 - 0.02
  0.02 - 0.006
0.006 - 0.002
Less than 0.002

Soils usually are classified according to their grain size. We have coarse-grained non-cohesive and fine-grained cohesive and organic soils. The grain of the various soils are all different. Table 1 below shows the range of particle sizes.

The distribution of the particles is determined by sieving. The chart shown in Figure 1 is used for the sieve analysis. The percentage of a sample greater than a given size is determined for coarse soils. For fine soils the particle sizes are meaningless. The distinction between silt and clay is made not on the basis of an arbitrary size distinction but on their behaviour in the present of water. The consistency of fine soil varies with the amount of water present.

In the graph shown in Figure 6,
Curve 1
presents a well graded silty sand (high percentage of silt fraction),
Curve 2
a uniform fine sand (with a high percentage of equal grain sizes), and
Curve 3
a well graded sand.

Figure 6