Soil Composition, Index Properties and Classification

Introduction

Soil is a natural, heterogeneous material formed by the weathering and transportation of rock and, in some environments, by biological activity. Unlike steel or concrete, soil is not manufactured to a uniform specification. Its engineering response depends on mineral composition, particle size, particle arrangement, water content, density, and stress history. These factors control bearing capacity, settlement, permeability, compaction behavior, and shear strength.

For foundation engineering, a soil description such as “sand” or “clay” is not sufficient. Two soils both called clay may have very different plasticity, compressibility, and strength. Consequently, engineers determine measurable index properties and use standardized classification systems to organize soils into groups with broadly similar behavior. This chapter develops the phase relationships, density and unit-weight relationships, particle-size distribution, Atterberg limits, and Unified Soil Classification System (USCS) procedures needed for preliminary foundation assessment.

Learning Objectives

By the end of this chapter, the student should be able to:

1. Represent soil as a three-phase system of solids, water, and air.
2. Calculate water content, density, unit weight, specific gravity, void ratio, porosity, and degree of saturation.
3. Use phase relationships to determine unknown soil quantities.
4. Interpret particle-size distribution curves and calculate coefficients of uniformity and curvature.
5. Explain liquid, plastic, and shrinkage limits and calculate the plasticity index.
6. Classify coarse-grained and fine-grained soils using the USCS and recognize the role of national standards.
7. Apply index properties to practical foundation and earthwork decisions while recognizing their limitations.

1. Soil as a Three-Phase Material

A soil mass consists of solid particles and voids. The voids may contain water, air, or both. Therefore, soil is commonly idealized as a three-phase system:

• Solid phase: mineral particles or organic matter.
• Water phase: water occupying part or all of the void space.
• Air phase: air occupying the remaining void space.

The total volume and total mass are

where V is total volume, V_s is volume of solids, V_v is volume of voids, V_w is volume of water, V_a is volume of air, M is total mass, and M_s and M_w are masses of solids and water. The mass of air is normally neglected in geotechnical calculations because it is very small compared with the masses of solids and water.

A useful phase diagram separates the material into volume and mass columns. The relationships derived from it are valid only when the definitions are applied consistently and units are compatible.

Figure: Idealized three-phase representation of a soil mass.

1.1 Weight and mass

In SI practice, engineers often use unit weight, the weight per unit volume:

where γ is unit weight in kN/m³, W is weight in kN, and V is volume in m³. Mass density is

where ρ is density in Mg/m³ or kg/m³. Since W = Mg, the corresponding relationship is γ = ρg, where g is gravitational acceleration, approximately 9.81 m/s². In geotechnical work, γ_w denotes the unit weight of water, approximately 9.81 kN/m³ at ordinary laboratory temperature.

2. Water Content, Specific Gravity, and Basic Index Properties

2.1 Water content

Water content is the ratio of the mass of water to the mass of oven-dried solids:

It is reported as a percentage. In a laboratory determination, a moist specimen is weighed, oven-dried generally at approximately 105–110 °C for mineral soils, and weighed again. The water mass is the difference between wet and dry masses.

Water content is not the same as degree of saturation. A soil can have a high water content because it contains many voids and still be only partly saturated.

2.2 Specific gravity of soil solids

The specific gravity of soil solids is

where ρ_s and γ_s are the density and unit weight of solids, and ρ_w and γ_w are those of water. G_s is dimensionless. Common mineral soils have G_s values near 2.60–2.75, although iron-rich minerals, volcanic soils, organic soils, and unusual industrial fills may differ substantially. It must be measured or selected carefully rather than assumed automatically.

2.3 Void ratio and porosity

Void ratio is the ratio of void volume to solid volume:

Porosity is the ratio of void volume to total volume:

Porosity is usually expressed as a fraction or percentage. Since V = V_s + V_v,

and

These relationships are geometric identities and do not depend on saturation.

2.4 Degree of saturation and air content

Degree of saturation is the percentage of void space occupied by water:

The air content, sometimes called the percentage of voids occupied by air, is

A soil is dry when S = 0 and fully saturated when S = 100. In practice, “dry” often means sufficiently dry for the selected test, not necessarily an absolute absence of adsorbed water.

2.5 Fundamental phase relationships

Combining the definitions of water content, specific gravity, void ratio, and saturation gives

when w is expressed as a decimal. Thus,

and

These equations assume consistent definitions and negligible air mass. For a fully saturated soil, S = 1, so

The dry unit weight is

The moist or bulk unit weight is

For a saturated soil, the saturated unit weight is

The submerged or buoyant unit weight is

The saturated and submerged expressions apply when the voids are completely filled with water and the soil is below the relevant water table. They should not be used for partially saturated soil.

3. Worked Phase-Relationship Example

A moist soil specimen has a total volume of 0.010 m³, total mass of 18.5 kg, water content of 12%, and G_s = 2.70. Determine its dry unit weight, void ratio, degree of saturation, and bulk unit weight. Take γ_w = 9.81 kN/m³.

The dry mass is

M_s = M/(1 + w) = 18.5/(1.12) = 16.52 kg.

The dry density is 16.52/0.010 = 1652 kg/m³, or 1.652 Mg/m³. Therefore,

Using the dry unit-weight relationship,

The degree of saturation is

Finally,

The results are physically plausible: the soil is partly saturated, and its bulk unit weight exceeds its dry unit weight.

4. Particle-Size Distribution

Particle-size distribution (PSD) describes the relative amounts of particles of different sizes. It is important because particle size and gradation influence permeability, drainage, compaction, frost susceptibility, and the behavior of mixtures containing fines.

4.1 Sieve analysis

Sieve analysis is generally used for particles retained on the 0.075 mm sieve in common USCS practice. A dried sample is passed through a stack of sieves. The mass retained on each sieve is measured, cumulative mass retained is calculated, and percentage passing is plotted against particle size on a semilogarithmic graph.

The percentage passing a sieve is

The mass balance must be checked: the sum of masses retained and material collected in the pan should approximately equal the original dry mass. Significant loss indicates a testing or recording problem.

4.2 Hydrometer or sedimentation analysis

Particles smaller than the sieve range are commonly analyzed by sedimentation. The method relies on the tendency of particles to settle at different velocities in a fluid. Under idealized conditions, Stokes’ law gives

where v is settling velocity, D is equivalent particle diameter, μ is dynamic viscosity, and the particles are assumed to be approximately spherical, sufficiently dilute, and settling individually in a fluid of known temperature and viscosity. Natural clay particles are not spherical, so the reported diameter is an equivalent sedimentation diameter rather than a direct physical measurement.

4.3 Gradation descriptors

The particle diameter corresponding to x% passing is denoted D_x. Two common descriptors are

where C_u is the coefficient of uniformity and C_c is the coefficient of curvature. A high C_u generally indicates a broad range of particle sizes; a low C_u indicates a more uniform soil. For USCS classification, a clean gravel is well graded when C_u ≥ 4 and 1 ≤ C_c ≤ 3; a clean sand is well graded when C_u ≥ 6 and 1 ≤ C_c ≤ 3. These criteria are classification rules, not universal laws of soil behavior.

A well-graded soil has a relatively continuous range of sizes and can achieve a low void ratio through particle packing. A poorly graded soil may be uniformly graded or gap graded. Gap grading means that one or more intermediate size ranges are deficient.

5. Atterberg Limits and Plasticity

Fine-grained soils can change consistency as water content changes. Albert Atterberg’s consistency limits provide standardized boundaries between these states:

• Liquid limit (w_L): water content at the boundary between liquid and plastic behavior.
• Plastic limit (w_P): water content at the boundary between plastic and semisolid behavior.
• Shrinkage limit (w_S): water content below which further drying does not produce further reduction in volume under the specified test conditions.

The plasticity index is

where all water contents are expressed in percentage points. A nonplastic soil has no measurable plastic range, and I_P is reported as NP rather than as a negative number.

The liquidity index is

for a soil with measurable plasticity. It compares the in situ water content with the plasticity range. The consistency index is

and, when both are defined, I_L + I_C = 1. These indices are descriptive and should not be treated as direct substitutes for strength or compressibility tests.

The liquid limit may be obtained by a cup method or a cone penetration method, depending on the governing standard. The plastic limit is determined by rolling soil threads until they reach a specified small diameter and begin to crumble. Because operator technique, mineralogy, preparation, and standard-specific procedures affect results, the test method and national standard must always be reported.

5.1 Plasticity chart

The USCS plasticity chart plots liquid limit on the horizontal axis and plasticity index on the vertical axis. The A-line is

for the relevant range of the chart. Points above the A-line are generally associated with clays, while points below it are generally associated with silts and organic soils. The U-line, used as an upper-bound check for natural soils, is

The chart is an empirical classification tool. Borderline cases require careful application of the standard, laboratory review, and engineering judgment.

6. Soil Classification Using USCS

The USCS classifies soils according to grain-size distribution and plasticity. It is widely used in geotechnical reports, although project specifications may require another system such as ASTM, AASHTO, BS, IS, or a national adaptation. The symbols should never be transferred between systems without checking definitions and boundary criteria.

6.1 Primary divisions

A soil is coarse-grained when more than 50% of the total sample is retained on the No. 200 sieve (0.075 mm). It is fine-grained when 50% or more passes the No. 200 sieve. Organic soils and peat receive special consideration.

For coarse-grained soils:

• Gravel (G): the larger coarse fraction predominates.
• Sand (S): the smaller coarse fraction predominates.

The amount and plasticity of fines determine whether a coarse soil receives a clean, silty, or clayey dual symbol.

For fine-grained soils:

• M: inorganic silt.
• C: inorganic clay.
• O: organic silt or clay.
• L: low plasticity, generally w_L < 50%.
• H: high plasticity, generally w_L ≥ 50%.

Peat is represented by Pt and is identified primarily by its highly organic character, dark color, fibrous structure, and very high compressibility rather than by ordinary mineral-soil plasticity criteria.

6.2 Coarse-grained classification procedure

A practical procedure is:

1. Determine the percentage passing the No. 200 sieve.
2. If more than half of the coarse fraction is gravel-sized, use G; otherwise use S.
3. For soils with little fines, evaluate gradation using C_u and C_c. Use GW or SW when the relevant well-graded criteria are satisfied; otherwise use GP or SP.
4. For soils with appreciable fines, determine whether the fines plot above or below the A-line and use M or C accordingly.
5. Where the fines content lies in a borderline range, use a dual symbol such as GM-GC or SM-SC as required by the applicable standard.

Exact fines-content boundaries and dual-symbol conventions should be checked against the adopted standard because national versions can vary in presentation or detail.

6.3 Fine-grained classification procedure

For a soil with 50% or more passing the No. 200 sieve:

1. Determine liquid limit and plastic limit.
2. Calculate I_P.
3. Plot the point (w_L, I_P) on the plasticity chart.
4. Identify whether it lies above or below the A-line.
5. Apply the low- or high-plasticity division at w_L = 50%.
6. Check for organic characteristics and whether the soil is borderline.

For example, a soil with 62% passing the No. 200 sieve, w_L = 42%, and w_P = 20% has I_P = 22%. The A-line value is 0.73(42 − 20) = 16.06%. Since 22% is above the A-line and w_L is below 50%, the inorganic classification is CL, assuming no organic or borderline condition applies.

6.4 Relation to national standards

USCS is not the only accepted system. AASHTO classification is frequently used for highway and pavement materials; BS 5930, IS 1498, and other national standards contain related but not identical terminology and procedures. A professional report should state:

• the classification system and edition;
• the sieve sizes and test methods;
• whether percentages are based on total or fraction-specific mass;
• the treatment of organic soils and borderline symbols; and
• any deviations from the standard.

Classification is a communication framework, not a complete design analysis. A soil class cannot by itself establish allowable bearing pressure, settlement, liquefaction resistance, or slope stability.

7. Integrated Worked Classification Example

A soil sample has the following results: 68% passes the No. 200 sieve, liquid limit is 58%, and plastic limit is 31%. Determine its preliminary USCS classification.

First,

The soil is fine-grained because more than 50% passes the No. 200 sieve. The A-line value is

The measured plasticity index, 27%, is slightly below the A-line value. Thus, the point lies on the silt side of the chart. Because w_L exceeds 50%, the preliminary symbol is MH, provided the material is inorganic and not subject to a borderline rule. If organic behavior is identified, the classification must be reconsidered as an organic soil.

This example illustrates why both liquid limit and plastic limit are necessary. A liquid limit alone cannot distinguish the principal fine-grained groups.

8. Applied Case: Preliminary Foundation Assessment for a Small Building

Background

This illustrative case concerns a proposed two-storey masonry building on a gently sloping site. A preliminary investigation obtains a disturbed sample from 1.5 m depth. The project team needs to decide whether the near-surface material is likely to provide uniform support and whether additional investigation is necessary.

Problem and data

Laboratory results indicate 72% passing the No. 200 sieve, w_L = 46%, w_P = 24%, G_s = 2.71, natural water content w = 31%, and dry unit weight γ_d = 15.8 kN/m³.

Analysis

The plasticity index is 22%. The A-line value is 0.73(46 − 20) = 18.98%, so the material plots above the A-line. With w_L below 50%, its preliminary USCS symbol is CL, assuming inorganic composition.

The void ratio is

The degree of saturation is

A calculated saturation greater than 1 is physically impossible. This inconsistency is valuable: it indicates that at least one reported value, sampling condition, assumed specific gravity, or unit conversion is incorrect. The team must not simply accept the result. Possible explanations include an incorrect dry unit weight, a water content reported on a different basis, sample disturbance, or a mismatch between the specimen used for each test.

Approach and solution

The laboratory is asked to verify specimen identities, oven-dry mass calculations, volume measurement, and units. A second sample is tested, and groundwater observations are added to the site investigation. The foundation designer then uses verified parameters together with consolidation and shear-strength tests rather than relying solely on the CL designation.

Outcome and learning points

The principal outcome is not a bearing-capacity number but an improved decision: the soil is preliminarily a low-plasticity clay, yet the inconsistent phase relationship prevents confident interpretation. Index properties are powerful quality-control tools. They can reveal impossible conditions before the results are used in foundation design.

9. Engineering Significance, Limitations, and Best Practice

Index properties support engineering judgment in several ways. Water content and degree of saturation help indicate whether excavation faces may soften or whether compaction conditions are favorable. Dry unit weight and void ratio help evaluate compaction and potential compressibility. PSD informs drainage and filter compatibility. Atterberg limits indicate the likely sensitivity of fine soils to changes in moisture, but not their complete stress–strain response.

Important limitations include:

• Soil is spatially variable; one sample cannot represent an entire site.
• Disturbed samples may not preserve in situ structure or water condition.
• Classification symbols do not directly provide strength, settlement, or permeability.
• Organic matter, mica, expansive minerals, cementation, and salinity can produce behavior not captured by simple classification.
• Test results depend on sample preparation, equipment, operator technique, and standard-specific procedures.
• Phase calculations may become misleading when aggregates, gas, saline water, or unusual solids are present.

Best practice requires chain-of-custody documentation, representative sampling, calibrated equipment, clear identification of test standards, duplicate or repeat testing where results are questionable, and independent checks of mass balances and physical plausibility. When index results conflict with field observations, the conflict should be investigated rather than concealed.

Digital spreadsheets and laboratory information systems can reduce arithmetic errors, automatically generate PSD curves, and flag impossible values such as S > 100%. Machine-learning tools may assist with pattern recognition in large site databases, but they should not replace sampling, standard laboratory testing, engineering judgment, or accountability by a qualified professional. Data provenance and version control are essential when automated classification is used.

Chapter Summary

Soil is modeled as a three-phase system consisting of solids, water, and air. The phase relationships provide the basis for calculating water content, specific gravity, void ratio, porosity, degree of saturation, density, and unit weight. Central relationships include n = e/(1 + e), Se = wG_s, γ_d = G_sγ_w/(1 + e), and γ = γ_d(1 + w). These equations require consistent units and physically plausible input data.

Particle-size distribution is obtained principally by sieve and sedimentation methods. D_10, D_30, and D_60 support calculation of C_u and C_c, which help distinguish well-graded from poorly graded coarse soils. Atterberg limits describe changes in fine-soil consistency; the plasticity index is I_P = w_L − w_P. The USCS combines grain-size distribution, fines content, liquid limit, and plasticity-chart position to produce soil-group symbols such as GW, SP, CL, and MH.

Classification improves communication and supports preliminary decisions, but it is not a substitute for site investigation or design testing. Professional practice requires the selected national standard to be stated, test quality to be checked, and index results to be interpreted in conjunction with field conditions, strength, compressibility, permeability, and groundwater information.

Key Terms

Three-phase system: Idealized soil model containing solids, water, and air.

Water content: Mass of water divided by mass of dry solids.

Specific gravity: Ratio of soil-solid density to water density.

Void ratio: Volume of voids divided by volume of solids.

Porosity: Volume of voids divided by total volume.

Degree of saturation: Percentage of void space occupied by water.

Unit weight: Weight of soil per unit total volume.

Dry unit weight: Weight of soil solids per unit total volume.

Bulk unit weight: Weight of moist soil per unit total volume.

Saturated unit weight: Unit weight when all voids are filled with water.

Submerged unit weight: Effective unit weight of saturated soil below water.

Particle-size distribution: Proportions of particles within specified size ranges.

Coefficient of uniformity: Ratio D_60/D_10.

Coefficient of curvature: Ratio D_30²/(D_10D_60).

Liquid limit: Boundary water content between liquid and plastic states.

Plastic limit: Boundary water content between plastic and semisolid states.

Plasticity index: Difference between liquid and plastic limits.

USCS: Unified Soil Classification System.

A-line: Empirical line separating clay-like and silt-like behavior on the plasticity chart.

Index property: Simple measurable property used to identify or classify soil.

Review Questions

5 Short Answer Questions

1. Define void ratio and porosity, and state the relationship between them.
2. Why is the mass of air normally neglected in soil phase calculations?
3. What does a high coefficient of uniformity generally indicate?
4. Define liquid limit, plastic limit, and plasticity index.
5. Why must a classification standard be identified in a geotechnical report?

5 Long Answer Questions

1. Explain the three-phase soil model and derive the relationship Se = wG_s.
2. Describe the laboratory procedures and engineering uses of sieve and sedimentation analyses.
3. Explain the USCS procedure for classifying a fine-grained soil using the plasticity chart.
4. Discuss the differences among dry, bulk, saturated, and submerged unit weights, including their fields of application.
5. Evaluate the limitations of using soil classification as the sole basis for foundation design.

3 Analytical / Application Questions

1. A soil has G_s = 2.68, w = 18%, and e = 0.72. Calculate its degree of saturation and state whether the result is physically plausible.
2. A coarse soil has D_10 = 0.18 mm, D_30 = 0.42 mm, and D_60 = 1.08 mm. Calculate C_u and C_c, then assess whether it satisfies the USCS gradation criteria for a well-graded sand.
3. A laboratory report gives w = 28%, G_s = 2.70, and γ_d = 14.0 kN/m³. Calculate e and S. If S exceeds 100%, identify at least two possible causes and propose quality-control checks.

Multiple Choice Questions with Answers

Question 1

Which expression defines void ratio?

A. V_v/V_s

B. V_v/V

C. V_w/V_v

D. M_w/M_s

Correct Answer: A. V_v/V_s

Question 2

A soil has e = 0.60 and n is required. Which value is correct?

A. 0.375

B. 0.600

C. 0.625

D. 1.600

Correct Answer: C. 0.625

Question 3

For a soil with w = 0.10, G_s = 2.70, and e = 0.54, the degree of saturation is approximately:

A. 27%

B. 50%

C. 74%

D. 100%

Correct Answer: B. 50%

Question 4

The coefficient of uniformity is defined as:

A. D_10/D_60

B. D_30²/(D_10D_60)

C. D_60/D_10

D. D_60/D_30

Correct Answer: C. D_60/D_10

Question 5

A soil has w_L = 55% and w_P = 25%. Its plasticity index is:

A. 30 percentage points

B. 80 percentage points

C. 0.45

D. 2.20

Correct Answer: A. 30 percentage points

Question 6

In the USCS, a soil is fine-grained when:

A. More than 50% is retained on the No. 4 sieve

B. 50% or more passes the No. 200 sieve

C. Its liquid limit exceeds 50%

D. Its plasticity index is greater than 20%

Correct Answer: B. 50% or more passes the No. 200 sieve

Question 7

For an inorganic fine-grained soil with w_L = 42% and I_P = 22%, the point is generally:

A. Below the A-line and high plasticity

B. Above the A-line and low plasticity

C. Above the A-line and high plasticity

D. On the U-line and nonplastic

Correct Answer: B. Above the A-line and low plasticity

Question 8

Which unit weight is used to represent the apparent weight of saturated soil below the water table?

A. Dry unit weight

B. Bulk unit weight

C. Saturated unit weight

D. Submerged unit weight

Correct Answer: D. Submerged unit weight

Question 9

If a phase calculation gives S = 123%, the best engineering response is to:

A. Report the soil as supersaturated

B. Set S equal to 123% without comment

C. Check the data, units, and specimen identity

D. Assume the soil is dry

Correct Answer: C. Check the data, units, and specimen identity

Question 10

For a clean sand, the USCS well-graded criterion includes:

A. C_u ≥ 6 and 1 ≤ C_c ≤ 3

B. C_u ≥ 4 and C_c = 0

C. C_u ≤ 6 and C_c ≥ 3

D. w_L ≥ 50% and I_P > 20%

Correct Answer: A. C_u ≥ 6 and 1 ≤ C_c ≤ 3