Classification of Soils - Civil Engineering SSC JE (Technical) - Civil Engineering

Soil classification systems - an overview

Soils are classified by engineers to describe their engineering behaviour, choose appropriate tests, select construction methods and estimate design parameters for foundations, pavements and earthworks. The important classification systems used in practice are:

• USCS - Unified Soil Classification System
• ISSCS - Indian Standard Soil Classification System (closely related to USCS with some local conventions)
• AASHTO - American Association of State Highway and Transportation Officials classification (earlier known as HRB system)
• Textural classification - based on relative proportions of sand, silt and clay; not recommended for engineering design and not used in ISSCS/USCS for structural purposes

Basic division of soils

Soils are broadly divided into three categories based on particle-size distribution and organic content:

• Coarse-grained soils: More than 50% by weight of the material is larger than 75 μm (No. 200 IS sieve). Coarse-grained soils are typically composed of gravel (G) and sand (S).
• Fine-grained soils: More than 50% by weight of the material passes the 75 μm sieve. Fine-grained soils are primarily silt (M) and clay (C), and their engineering behaviour is strongly influenced by Atterberg limits (liquid limit and plasticity index).
• Highly organic and miscellaneous soils: Soils containing a large proportion of fibrous organic matter (e.g., peat, Pt) or substantial non-soil materials (shells, cinders) that influence compressibility, strength and permeability are placed in a separate division.

AASHTO classification (HRB)

The AASHTO classification is used widely for highway subgrade evaluation. It groups soils into categories A-1 to A-7, with a special group A-8 for peat or muck. Classification uses both particle-size composition and plasticity characteristics.

The Group Index (GI) is an empirical value used to indicate the quality of soil as a subgrade material; higher GI implies poorer subgrade material for highways.

In this expression the terms a, b, c and d are defined as follows:

• a = part of percentage passing 75 μm sieve which is greater than 35% and not exceeding 75% (expressed as a positive whole number; range 1-40)
• b = part of percentage passing 75 μm sieve which is greater than 15% and less than 55% (range 1-40)
• c = part of liquid limit (WL) greater than 40% and less than 60% (range 1-20)
• d = part of plasticity index (PI) greater than 10% and less than 30% (range 1-20)

Using the maximum values of these parameters (a = 40, b = 40, c = 20, d = 20) gives

GImax = 0.2 × 40 + 0.01 × 40 × 20 + 0.005 × 20 × 40 = 20

Thus the Group Index varies from 0 to 20. If the computed value from the formula is negative, it is reported as zero. Greater GI indicates less desirable soil for highway construction.

Indian Standard Soil Classification System (ISSCS)

The Indian Standard Soil Classification System (ISSCS) divides soils into three broad divisions using the 75 μm (No. 200) sieve:

• Coarse-grained soils - when 50% or more of the total material by weight is retained on the 75 μm sieve.
• Fine-grained soils - when more than 50% of the total material passes the 75 μm sieve.
• Peat and highly organic soils - soils with large percentage of organic matter and decomposed vegetation (classified as Pt).

ISSCS recognises 18 primary groups: 8 groups of coarse-grained soils, 9 groups of fine-grained soils and one group for peat.

Prefix and subgroup notation in ISSCS

USCS (Unified Soil Classification System) - essentials

The USCS is widely used in geotechnical engineering. Symbols are formed from a combination of letters denoting grain-size and plasticity. Common group symbols are:

• G - gravel
• S - sand
• M - silt (i.e., non-plastic or low plasticity fine soils)
• C - clay (plastic fine soils)
• O - organic
• Pt - peat

For coarse-grained soils the grading descriptor W (well-graded) or P (poorly graded) is added, and the presence of significant fines (passing 0.075 mm) is noted by appending -M or -C. For example GW = well-graded gravel, SP = poorly graded sand, SM = sand with silt, CL = clay of low to medium plasticity, CH = clay of high plasticity, ML = silt of low plasticity.

Classification procedure (practical steps)

• Obtain particle-size distribution (sieve and hydrometer/settling analysis) to find percentage retained/passing 75 μm (No. 200) sieve.
• If more than 50% is retained on 75 μm, classify as coarse-grained. Use grain-size distribution and gradation coefficients to distinguish gravel and sand and mark gradation (W or P).
• If more than 50% passes 75 μm, classify as fine-grained. Determine Atterberg limits (liquid limit WL and plasticity index PI) and use these with the A-line on the plasticity chart to separate clays and silts and to assign low or high plasticity groups (e.g., CL, CH, ML, MH).
• Check for organic content or peat and assign O or Pt where applicable.

Gradation parameters (common indices)

• Coefficient of uniformity Cu = D60 / D10 (D60 and D10 are particle sizes from grain-size curve corresponding to 60% and 10% finer respectively). Cu indicates the range of particle sizes.
• Coefficient of curvature Cc = (D30)² / (D60 × D10) and gives shape of the grading curve.
• Typical criteria used to call coarse material well-graded are Cu > 4 for gravels and Cu > 6 for sands together with 1 ≤ Cc ≤ 3. (These are standard practical criteria used in USCS classification.)

Plasticity chart and the A-line

The plasticity chart plots Plasticity Index (IP or PI) on the vertical axis and Liquid Limit (WL) on the horizontal axis. The A-line separates clays (above the line) from silts (below the line). The A-line relation is commonly expressed as:

IP = 0.73 (WL - 20)

Soils plotting above the A-line are generally classified as clays (C) and those below as silts (M), with further subdivisions into low and high plasticity (L or H) depending on the liquid limit.

Division of soil fractions on the basis of grain sizes

Common engineering ranges for particle sizes are given below; these are used to describe individual fractions (note that national standards may express the same limits slightly differently):

• Gravel: particles larger than 4.75 mm (retained on 4.75 mm IS sieve)
• Sand: particles between 4.75 mm and 0.075 mm (pass 4.75 mm but retained on 0.075 mm sieve)
• Silt: particles between 0.075 mm and 0.002 mm
• Clay: particles smaller than 0.002 mm

Engineering significance and applications

• Choice of foundation: Coarse-grained, well-graded gravels and sands generally provide higher bearing capacity and lower settlement than fine-grained clays and peats.
• Compressibility and settlement: Clays (especially high-plasticity clays) and organic soils show larger primary and secondary settlement; these require special attention in foundation design.
• Pavement subgrade: AASHTO group and Group Index are used to assess suitability of subgrade materials for highway pavement design; lower GI is preferred.
• Permeability and drainage: Coarse soils are more permeable and drain faster; fine soils are less permeable and can trap pore water affecting effective stress and shear strength.
• Construction practices: Classification informs compaction control, choice of stabilisation (lime, cement), and methods for handling problematic soils (e.g., peats and highly plastic clays).

Summary

Classification groups (USCS, ISSCS and AASHTO) provide a concise notation to communicate key soil properties: particle-size distribution, gradation, plasticity, and organic content. For engineering design, always base classification on laboratory tests (sieve analysis, hydrometer, Atterberg limits) and use classification symbols together with index properties (Cu, Cc, WL, PI, GI) to select suitable construction and foundation solutions.