Soils are named and classified on the basis of physical and chemical properties in their horizons (layers). “Soil Taxonomy” uses color, texture, structure, and other properties of the surface two meters to key the soil into a classification system to help people use soil information.

Layers of Soil

• The O-Horizon.
• The A-Horizon or Topsoil.
• The E-Horizon.
• The B-Horizon or Subsoil.
• The C-Horizon or Saprolite.
• The R-Horizon.
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• Tensiometers.

What are three main factors used to classify soils? Soils are classified based on climate, plants, and soil composition.

All soils consist mainly of two kinds of material: particles of minerals and rocks, and organic matter. Organic matter is any matter that is or once was living. Soil is likely to have several kinds of rock and mineral particles.

Coarse grained soils are those with more than 50% of the material larger than 0.075mm size. Coarse grained soils are further classified into gravels (G) and sands (S)….(iv) Unified Soil Classification System.

Soil is classified into four types:

• Sandy soil.
• Silt Soil.
• Clay Soil.
• Loamy Soil.

There are five soil horizons: O, A, E, B, and C. (R is used to denote bedrock.) There is no set order for these horizons within a soil. Some soil profiles have an A-C combination, some have an O-E-B, an O-A-B, or just an O.

According to the USCS classification, soil is divided into: coarse grained soil, fine-grained soil, and highly organic soil. The particle size distribution of soil and consistency limits are used in classification of soils.

The classification is done into seven major groups as, A1, A2, A3, A4, A5, A6 and A7. When soil granules of 35% or less than that passes the No.

In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub-groups. The grain-size range is used as the basis for grouping soil particles into boulder, cobble, gravel, sand, silt or clay.

Step-1 [Identifying as Coarse Grained Soil or Fine Grained Soil]

1. Take a representative sample of soil and spread it on a flat surface or palm of the hand.
2. Remove all particles whose sizes are larger than 75 mm.
3. If the soil contains more than 50% of visible particles then the soil is said to be a coarse grained soil.

Unified Soil Classification System

1. Coarse grained soils: In these soils, 50% or more of the total material by weight is larger than 75 micron IS sieve size.
2. Fine grained soils: In these soils, 50% or more of the total material by weight is smaller than 75 micron IS sieve size.
3. Highly organic soils and other miscellaneous soil materials:

The AASHTO system uses both grain-size distribution and Atterberg limits data to assign a group classification and a group index to the soil. The group classification ranges from A-1 (best soils) to A-8 (worst soils). Group index values near 0 indicate good soils, while values of 20 or more indicate very poor soils.

A plasticity chart , based on the values of liquid limit (WL) and plasticity index (IP), is provided in ISSCS to aid classification. The ‘A’ line in this chart is expressed as IP = 0.73 (WL – 20). Depending on the point in the chart, fine soils are divided into clays (C), silts (M), or organic soils (O).

The plasticity index is expressed in percent of the dry weight of the soil sample. It shows the size of the range of the moisture contents at which the soil remains plastic. A high PI indicates an excess of clay or colloids in the soil. Its value is zero whenever the PL is greater or equal to the LL.

The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, a soil may appear in one of four states: solid, semi-solid, plastic and liquid.

The Casagrande plasticity chart has been used for more than 70 years to classify fine-grained soils in accordance with the USCS. The majority are classified in the field as SILT, whereas these same soils are classified by the plasticity chart as CLAY.

Plasticity of soil is its ability to undergo deformation without cracking or fracturing. Engineering Properties:- The main engineering properties of soils are permeability, compressibility and shear strength. Permiability indicates facility with which water can flow through soils.

The vertical line at LL = 50 separates high-plasticity soils from low-plasticity soils. The A-line separates clay from silt. The equation of the A-line is PI = 0.73(LL – 20). The U-line is not used in classification but is an upper boundary of expected results for natural soils.

What is plastic limit of soil? The plastic limit of a soil is the moisture content at which soil begins to behave as a plastic material. At this water content (plastic limit), the soil will crumble when rolled into threads of 3.2mm(1/8in) in diameter.

2.1 The plastic limit of a soil is the moisture content, expressed as a percentage of the weight of the oven-dry soil, at the boundary between the plastic and semisolid states of consistency. It is the moisture content at which a soil will just begin to crumble when rolled into a thread ⅛ in.

Record the difference between the wet and dry weights as the weight of moisture. Calculate the plastic limit wp by dividing the “weight of moisture” by the “dry weight of sample” and multiply by 100. Plasticity Index (Ip) of a soil is the numerical difference between its Liquid Limit and its Plastic Limit.

procedure Mix it thoroughly with distilled water in the evaporating dish till the soil mass becomes plastic enough to be easily molded with fingers. Continue rolling till you get a thread of 3 mm diameter. Continue the process until the thread crumbles when the diameter is 3 mm.

Plastic limit test is one of the laboratory tests used internationally to differentiate or classify soils into groups. When the water content or moisture content of soil increases beyond a limit then the soil starts to behave as liquid.

Atterberg Limits Test Procedure:

1. Liquid Limit is measured by spreading a portion of the soil sample in the brass cup of a liquid limit machine and dividing it using a grooving tool.
2. Plastic Limit is determined by repeatedly remolding a small ball of moist plastic soil and manually rolling it out into a 1/8in thread.

The amount of water is determined by subtracting the dry weight from the initial weight, and the moisture content is then calculated as the amount of water divided by the dry weight or total weight, depending on the reporting method.