Soil Description and Classification

1. Types of soils:

Soil Type Particle Size Characteristics

Gravels 10 cm – 2 mmThey are usually visible to the naked eye and have low water retention due to large inter-particle spaces.

Sands 2 mm – 0.060 mmThey are still recognizable with the naked eye and do not form continuous aggregates when mixed with water and readily to separate from it.

Silts 0.060 mm – 0.002 mm They have better water retention than larger sized particles.

Clays Less than 0.002 mm

They form smaller particles than silt with gel-sized particles resulting from chemical changes. They are formed mainly from silicate minerals and are joined by weak covalent bonds that allow water molecules to enter the chains. The resulting structure has a high water retention capacity with small inter-particle spaces.

2. Particle size distribution

Particle size analysis is carried out to determine the percentage by weight of particles within the different size ranges. For particles greater than 0.075 mm the sieving method is used and for particles smaller than 0.075 mm the sedimentation method is used, using a hydrometer.

Sieving method – sample of soil is dried and the particles are disaggregated, and then sifted by shaking through a series of sieves. These normally decrease in size in geometric progression, with a scale factor of 2. The material remaining in each sieve is weighed and the percentage of material that passes through a sieve can be determined from the given initial weight of the sample.

These data can be used to show the soil particle size distribution as a curve by plotting percentage of particles passing through against sieve diameter.

2 coefficients:

a) Coefficient of uniformity, Cu is the ratio between the diameter of the sieve where 60% of the material passes through, and of the sieve where 10% passes through.

If Cu < 5 = the soil has uniform grain sizeIf 5 < Cu < 20 = the soil has slightly uniform grain sizeIf Cu > 20 = the soil is well graded

The more uniform the soil particle size distribution curve, the more uniform the void size will be. This makes the soil less dense and more liable to erosion.

b) The fine particle content is the percentage of soil that passes through ASTM sieve 200 (0.075mm). This percentage indicates the proportion of clay and silt contained in the soil, and is related to potential water retention. The greater the content of fine particles, the more difficult it will be to expel water.

c) Coarse-grained soils have more than 50 % of the particles larger than 0.075 mm, and fine-grained soils have more than 50 % equal to or smaller 0.075 mm.

3. Plasticity

Measuring the soil particle size distribution is the first step in soil classification, but in some soils this is not so clear especially for the case of soil in mixtures of silts, clays and sand.

The water content is the weight of water in the soil divided by weight of dry soil. It normally varies from 5 – 8 % in granular soils, and from 60 – 70% in clayey soils. The weight of water is calculated from the difference between weight of soil sample before and after it is oven-dried for the time required for the water to evaporate.

Atterberg defined 3 limits:

a) Shrinkage limit – transition between solid and semi-solid stateb) Plastic limit – separating semi-solid from plastic statec) Liquid limit – separating plastic from semi-liquid state

Plastic and Liquid Limit are the most commonly used in practice and they are determined from the soil fraction that passes through sieve 40 (0.01 mm).

Plastic limit is determined by kneading dry soil with a little water to form small balls and then rolling them out with the palm of the hand on a smooth surface to a diameter of about 3 mm and length of about 25 – 30 mm. If at this stage the rolls crack into pieces of about 6 mm long, their

water content corresponds to that of the plastic limit. If they do not crack, the rolls are reshaped into a ball and rolled in the hand, until they lose water content and re-rolled, this being repeated until they start cracking.

There are several methods in determination of liquid limit and the percussion method, originally proposed by Casagrande is a simple yet effective method, this method is normally included in the learning syllabus.

The liquid limit can be obtained using percussion method by kneading dry soil with sufficient water to make a suspension with the consistency of yoghurt, and putting this into the Casagrande cup mould. Using a grooving tool, a groove of 2 mm wide at its lowest point is then cut across the center of the mass. The mould is placed on a base and subjected to regular blows, which are counted. The liquid limit is the water content of the sample when the groove closes along a distance of some 12 mm after 25 blows. As it is difficult to achieve this, the water content is determined by interpolation from several samples, in which 12 mm closure has to be achieved with more or less 25 blows. At least 3 tests for the same soil should be made at varying moisture content.

Once Liquid Limit (LL) and Plastic Limit (PL) has been found, a point representing each soil sample can be obtained from the Casagrande Plasticity Chart which shows the ratio between the liquid limit and the plasticity index (PI).

PI represents the moisture content interval for passing from a semi-solid to a semi-liquid state.

From a series of practical studies, Casagrande defined soils with LL > 50 as having high plasticity i.e. they absorb a large amount of water and may experience considerable plastic deformation. Below this value, soils are considered to have low plasticity. Casagrande completed this system of identification with particle size distribution data and developed the widely used Unified Soil Classification System.

The liquidity index, LI is defined by the ratio of the natural water content of the soil sample minus water content at the plastic limit to the index of plasticity. It normally varies between 0 and 1.

LI = (w – PL) / (LL – PL)

In sands, where water retention and plasticity are very low or absent, a comparison of this type is not usually made.

The table shows the properties of fine grain soils

Fine Soils Liquidity Index, LI Water Content, W (%)

Very Soft 1.00 – 0.80 > 55

Soft 0.80 – 0.65 40 – 55

Firm 0.65 – 0.40 25 – 40

Stiff 0.40 – 0.25 15 – 25

Hard < 0.25 < 15