Soil texture class test

The natural soil consists usually of a mixture of sand, silt and clay. If a soil contains mostly sand and some clay then we would call it clayey sand. There exists an international texture classification that refers to the descriptive names of the soil. In this classification the descriptive names refer to a percentage composition with the following size limitations:

           Sand    2.0 to 0.06 mm diameter
           Silt       0.06 to 0.002 mm diameter
           Clay     smaller than 0.002 mm diameter


The find the percentage distribution of sand, silt and clay a soil sample from the site should be taken.
Fill a container with water and add the soil sample to it.
Shake the container vigorously and leave it standing until the soil particles have settled.
Then measure the layer thickness of each layer as shown in Figure 3. All layers add up to 80 mm. We find the percentage of the different layers by dividing the single layer thickness by the total thickness of all layers (that is 80 mm.). The percentages are shown below.

      
            Figure 3

Percentage of clay   = 25/80  = 31.25 % 
Percentage of silt   = 15/80  = 18.75 % 
Percentage of sand  = 40/80  = 50.00 % 


A textural classification chart is shown in Figure 4 below. To find the soil classification place the percentage figure on the chart as shown. Knowing the percentage distribution of a specific soil any classification can be quickly worked out.


Figure 4

Soil texture class descriptions

Sand
Soil consisting mostly of coarse and fine sand, and containing so little clay that it is loose when dry and not sticky at all when wet. When rubbed it leaves no film on the fingers.

Loamy sand
Consisting mostly of sand, but with sufficient clay to give slight plasticity and cohesion when very moist. Leaves slight film of fine materials on the fingers when rubbed.

Sandy loam
Soil in which the sand fraction is quite obvious, which moulds readily when sufficient moist, but in most cases does not stick appreciably to the fingers. Threads do not form easily.

Loam
Soil in which the fractions are so blended that it moulds readily when sufficient moist, and sticks to the fingers to some extent. It can with difficulty be moulded into threads but will not bend into a small ring.

Silt loam
Soil that is moderately plastic without being very sticky and in which the smooth, soapy feel of the silt is the main feature.

Sandy clay loam
Soils containing sufficient clay to be distinctly sticky when moist, but in which the sand fraction is an obvious feature.

Clay loam
The soils is distinctly sticky when sufficiently moist, and the presence of sand fraction can only be detected with care.

Silty clay loam
This contains quite subordinate amounts of sand, but sufficient silt to confer something of a smooth, soapy feel. It less sticky than silty clay or clay loam.

Silt
Soil in which the smooth, soapy feel of silt is dominant.

Sandy clay
The soil is plastic and sticky when moistened sufficiently, but the sand fraction is still an obvious feature. Clay and sand are dominant, and the intermediate grades of silt and very fine sand are less apparent.

Silty clay
Soil which is composed of almost very fine material, but in which the smooth, soapy feel of the silt fraction modifies some extent the stickiness of the clay.

Clay
The soil is plastic and sticky when moistened sufficiently and gives a polish surface on rubbing. When moist the soil can be rolled into threads, and it is capable of being moulded into any shape and takes clear fingerprints.

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

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Particle size distribution

Table 1
Particle size limits
Type
 Range of particle size 
in millimeter
Cobbles
Coarse gravel
Medium gravel
Fine gravel
Coarse sand
Medium sand
Fine sand
Coarse silt
Medium silt
Fine silt
Clay
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

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More example calculations

Example 1
A soil sample has a porosity of 41%, a moisture content of 15.2% and a specific gravity of 2.65. Determine the degree of saturation and the air void ratio.
We can't directly calculate the degree of saturation or the air void ratio. First we need to find the void ratio because e is needed to find the answers.

The porosity is given, therefore we can find the void ratio.      

      (or 58%)

Air void ratio,     Av = n (1-Sr) = 0.41 (1-0.58) = 0.1722          (or 17.2%)

       (or 17.2%)

Example 2
Determine the dry and bulk density of a soil sample having a porosity of 0.32 and a moisture content of 25%   (Gs = 2.70)

Example 3
Determine the saturated bulk density of the soil in Example 2, assuming no volume change.
If the soil is saturated then Sr = 1.0

Example 4
Determine the void ratio of a saturated soil sample that has a mass of 178 g before drying and 139 g after drying in an oven. Assume the specific gravity of the soil solids to be 2.7.


For a saturated soil Sr = 1
Void ratio, e = m × Gs = 0.28 × 2.7 = 0.756

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