Soil Composition, Classification and Compaction

Introduction and Context

Soil is a natural, particulate engineering material formed by the weathering and transport of rock and organic matter. Unlike steel or concrete, it is generally heterogeneous, discontinuous and multiphase. Its engineering behaviour depends not only on the mineral particles but also on the proportions and interactions of solids, water and air. Settlement, shear strength, permeability, shrink–swell behaviour and compaction response are therefore controlled by soil composition and structure.

For foundation engineering, an accurate description of soil is the first stage of responsible analysis. Soil classification provides a common language for field investigation and design; index properties permit preliminary prediction of behaviour; particle-size analysis identifies grading; Atterberg limits indicate the activity of fine-grained soils; and compaction establishes a controlled construction state. These procedures do not replace engineering judgement or site-specific testing, but they provide the essential framework for interpretation.

Learning Objectives

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

1. Explain the three-phase nature of soil and calculate phase relationships.

2. Determine and interpret water content, unit weight, specific gravity, void ratio, porosity and degree of saturation.

3. Conduct and interpret sieve and hydrometer particle-size analyses.

4. Explain Atterberg limits and calculate the plasticity index and liquidity index.

5. Classify soils using the Unified Soil Classification System (USCS) and understand the purpose of AASHTO classification.

6. Relate soil fabric and capillarity to engineering behaviour.

7. Explain compaction mechanisms and interpret laboratory moisture–density curves.

8. Select field compaction methods and develop practical quality-control procedures.

9. Recognise the limitations, uncertainty and ethical implications of soil classification and compaction decisions.

1. Soil as a Multiphase Material

1.1 Constituents of soil

An idealised soil mass contains three phases:

• Solid phase: mineral grains and, in some soils, organic matter.
• Water phase: water occupying part or all of the void space.
• Gas phase: usually air occupying the remaining void space.

The total volume is

\[V = V_s + V_v = V_s + V_w + V_a\]

where \(V_s\), \(V_v\), \(V_w\) and \(V_a\) are the volumes of solids, voids, water and air, respectively. The total mass is

\[M = M_s + M_w\]

because the mass of air is normally neglected in geotechnical calculations. The phase relationships are geometric and do not by themselves describe soil strength; they provide the quantitative basis for interpreting behaviour.

1.2 Fundamental index properties

Water content is

\[w = \frac{M_w}{M_s}\times100\%\]

where masses must be measured consistently. Water content may exceed 100 percent in highly compressible organic or clay soils.

The specific gravity of solids is

\[G_s = \frac{\rho_s}{\rho_w}\]

where \(\rho_s\) is the density of soil solids and \(\rho_w\) is the density of water at the reference temperature. Typical values for inorganic mineral soils range from about 2.60 to 2.75, but iron-rich minerals, volcanic materials and organic matter may produce substantially different values.

The void ratio is

\[e = \frac{V_v}{V_s}\]

and porosity is

\[n = \frac{V_v}{V}\times100\% = \frac{e}{1+e}\times100\%\]

Void ratio is dimensionless; porosity is commonly expressed as a percentage. The relation between them follows directly from \(V=V_s+V_v\).

The degree of saturation is

\[S_r = \frac{V_w}{V_v}\times100\%\]

with \(S_r=0\%\) for a completely dry soil and \(S_r=100\%\) for a fully saturated soil. Combining the definitions gives the important relationship

\[S_r e = wG_s\]

when \(w\) is expressed as a decimal. This relationship assumes that the density of water used in defining \(G_s\) is the same reference density used in the calculation.

The bulk or moist unit weight is

\[\gamma = \frac{W}{V}\]

where \(W\) is total weight. The dry unit weight is

\[\gamma_d = \frac{W_s}{V}\]

and, because \(W=W_s+W_w=W_s(1+w)\),

\[\gamma = \gamma_d(1+w)\]

with \(w\) in decimal form. For a saturated soil,

\[\gamma_{sat}=\frac{G_s+e}{1+e}\gamma_w\]

and for a submerged soil the buoyant or submerged unit weight is

\[\gamma'=\gamma_{sat}-\gamma_w\]

where \(\gamma_w\) is the unit weight of water. These expressions assume negligible air mass and use consistent force and volume units.

Worked example: phase relationships

A soil sample has \(G_s=2.70\), water content \(w=18\%\), and dry unit weight \(\gamma_d=17.0\,\text{kN/m}^3\). Taking \(\gamma_w=9.81\,\text{kN/m}^3\), determine its void ratio, porosity, degree of saturation and moist unit weight.

First,

\[e=\frac{G_s\gamma_w}{\gamma_d}-1

=\frac{2.70(9.81)}{17.0}-1=0.558\]

Thus,

\[n=\frac{0.558}{1.558}=0.358\approx35.8\%\]

Using \(w=0.18\),

\[S_r=\frac{wG_s}{e}=\frac{0.18(2.70)}{0.558}=0.871\approx87.1\%\]

Finally,

\[\gamma=17.0(1+0.18)=20.06\,\text{kN/m}^3\]

The sample is therefore moist but nearly saturated, not fully saturated. Treating it as saturated without evidence would introduce error in seepage, effective stress and unit-weight calculations.

2. Particle Size and Soil Fabric

2.1 Particle-size distribution

Particle-size distribution (PSD) describes the percentage by mass of soil particles smaller than selected sizes. It is obtained by sieve analysis for coarser particles and by sedimentation analysis, commonly hydrometer testing, for fine particles.

In sieve analysis, a dried, representative sample is passed through a stack of sieves. The mass retained on each sieve is measured, and cumulative percentage passing is plotted against particle diameter on a logarithmic horizontal scale. The curve assists in identifying grading and estimating characteristic diameters such as \(D_{10}\), \(D_{30}\) and \(D_{60}\), where the subscript denotes the percentage finer.

For coarse-grained soils, two coefficients are frequently used:

\[C_u=\frac{D_{60}}{D_{10}}\]

\[C_c=\frac{D_{30}^2}{D_{10}D_{60}}\]

These coefficients are meaningful only when the relevant diameters can be read reliably from a continuous grading curve. In USCS practice, a gravel is generally considered well graded when \(C_u\ge4\) and \(1\le C_c\le3\); a sand is generally well graded when \(C_u\ge6\) and \(1\le C_c\le3\), provided fines criteria are also satisfied.

Hydrometer analysis is based on sedimentation. Stokes’ law relates settling velocity to particle diameter for idealised spherical particles settling individually in a quiescent fluid:

\[v=\frac{(G_s-1)\gamma_wD^2}{18\mu}\]

where \(v\) is settling velocity, \(D\) is equivalent particle diameter, and \(\mu\) is dynamic viscosity. In practice, hydrometer calculations require corrections for temperature, meniscus, dispersing agent and specific gravity. Clay particles are not spherical, so the reported diameter is an equivalent hydrodynamic diameter rather than a direct geometric measurement.

2.2 Soil fabric and structure

Fabric refers to the arrangement, orientation and contacts of particles. Coarse-grained soils commonly exhibit single-grain fabric, with interparticle force chains controlling stiffness and strength. Fine-grained soils may have dispersed, flocculated, honeycomb or laminated arrangements. Mineralogy, pore-fluid chemistry and depositional history influence fabric.

Fabric explains why soils with similar PSD can behave differently. A flocculated clay may have high permeability and considerable apparent strength in an undisturbed state, while remoulding can destroy particle associations. Sensitive clays may lose substantial strength when disturbed. Cementation, fissuring and bonding also mean that an index classification may not represent intact field behaviour.

3. Consistency and Atterberg Limits

Fine-grained soils change consistency as water content changes. The conventional limits are:

• Liquid limit (\(w_L\)): boundary between liquid and plastic states.
• Plastic limit (\(w_P\)): boundary between plastic and semisolid states.
• Shrinkage limit (\(w_S\)): water content below which further drying produces little or no volume reduction.

The plasticity index is

\[I_P=w_L-w_P\]

and measures the range of water contents over which the soil behaves plastically. It is an index, not a direct measure of shear strength or compressibility.

The liquidity index is

\[I_L=\frac{w-w_P}{I_P}\]

where \(w\) is the natural water content. Values near or above 1 indicate a soil at or beyond its liquid-limit water content; negative values indicate a water content below the plastic limit. The consistency index is

\[I_C=\frac{w_L-w}{I_P}=1-I_L\]

These indices are useful for preliminary interpretation but should not be used without considering mineralogy, sensitivity, structure and stress history.

The Casagrande plasticity chart plots \(I_P\) against \(w_L\). The A-line approximately separates clays from silts and organic soils:

\[I_P=0.73(w_L-20)\]

for the relevant range of liquid limits. The chart is an empirical classification tool, not a fundamental constitutive law. Soils plotting above the A-line are generally clay-like; those below are generally silt-like, subject to classification rules and organic-soil identification.

4. Soil Classification Systems

4.1 Unified Soil Classification System

The USCS first separates soils according to the percentage passing the No. 200 sieve, approximately 0.075 mm:

• Coarse-grained soils have more than 50 percent retained on the No. 200 sieve.
• Fine-grained soils have 50 percent or more passing the No. 200 sieve.
• Highly organic soils are identified separately as peat or organic soil where appropriate.

Coarse soils are classified as gravel (G) or sand (S), depending on whether the predominant coarse fraction is retained on the No. 4 sieve (approximately 4.75 mm) or passes it. A second letter indicates grading or fines character: W for well graded, P for poorly graded, M for silt, and C for clay. Examples include GW, SP, GM and SC.

For fine-grained soils, the principal symbols include ML and MH for inorganic silts of low and high plasticity, CL and CH for inorganic clays of low and high plasticity, and OL and OH for organic silts and clays. The boundary between low and high liquid limit is conventionally \(w_L=50\). Borderline symbols, such as CL-ML, may be appropriate where results lie close to classification boundaries.

USCS is valuable because it links a concise symbol to broad expected behaviour, but it does not provide design parameters. Two CL soils may have very different undrained strengths, compressibility or permeability because of fabric and stress history.

4.2 AASHTO classification

The AASHTO system was developed primarily for highway subgrade evaluation. It uses grain-size percentages and liquid limit and plasticity index to place soils in groups A-1 through A-7, with the group index providing an additional indication of subgrade quality. AASHTO and USCS symbols should not be interchanged: they serve different purposes and use different boundaries.

Worked classification example

A soil contains 62 percent passing the No. 200 sieve. Its liquid limit is 58 percent, plastic limit is 27 percent, and natural water content is 44 percent. The plasticity index is

\[I_P=58-27=31\%\]

The A-line value is

\[I_{P,A}=0.73(58-20)=27.74\%\]

Since the measured plasticity index is above the A-line and the liquid limit exceeds 50 percent, the USCS group is CH, an inorganic clay of high plasticity, assuming no evidence of organic behaviour. The liquidity index is

\[I_L=\frac{44-27}{31}=0.55\]

so the natural water content lies approximately midway between the plastic and liquid limits.

5. Capillarity and Water in Soil

Capillarity arises because surface tension and adhesion cause water to rise or be retained in small pores. In an ideal cylindrical tube, capillary rise is approximated by

\[h_c=\frac{4T\cos\theta}{\gamma_w d}\]

where \(T\) is surface tension, \(\theta\) is the contact angle, \(d\) is tube diameter and \(\gamma_w\) is water unit weight. This ideal equation is illustrative; natural soils have irregular, interconnected pores, so the actual capillary fringe is not a sharp, uniform height.

Fine-grained soils have smaller pores and can generate larger capillary suction, although their permeability is low and water movement is slow. Capillary suction can increase apparent cohesion in unsaturated soils. This apparent strength may disappear when the soil becomes saturated, which is important for slopes, pavements and shallow foundations.

Effective stress provides the central framework for saturated soil:

\[\sigma'=\sigma-u\]

where \(\sigma'\) is effective stress, \(\sigma\) total stress and \(u\) pore-water pressure. The equation applies under the conventional Terzaghi effective-stress idealisation for saturated soil and appropriate stress components. In unsaturated soil, suction contributes to behaviour, but a simple single-pore-pressure equation may be insufficient; suction stress or two-stress-variable frameworks may be required.

6. Compaction Theory

6.1 Compaction versus consolidation

Compaction is the densification of soil by mechanical energy, primarily through expulsion of air. Water is not normally expelled rapidly enough to be the principal mechanism. Consolidation is time-dependent volume reduction, generally through drainage and dissipation of excess pore-water pressure under sustained loading. Confusing these processes can lead to incorrect predictions of settlement and field performance.

Compaction improves dry density, shear strength, bearing capacity and often reduces compressibility and permeability. Excessive or inappropriate compaction can, however, damage particles, create pore-pressure problems, or produce an undesirable fabric.

6.2 Laboratory moisture–density relationship

In a standard or modified Proctor test, soil is compacted at several water contents using a specified mould, rammer, number of layers and energy. The resulting dry unit weights are plotted against water content. The curve normally rises to a maximum dry unit weight and then declines. The peak is the maximum dry unit weight \(\gamma_{d,max}\), and its corresponding water content is the optimum moisture content (OMC).

For a given compactive effort, adding water initially improves lubrication and particle rearrangement. Beyond OMC, water occupies increasing void space and prevents further reduction of air voids; dry density decreases. The OMC and maximum dry density depend on soil type, gradation, compactive energy, drainage and test procedure.

The theoretical zero-air-voids or saturation line is

\[\gamma_{d,ZAV}=\frac{G_s\gamma_w}{1+wG_s}\]

which corresponds to \(S_r=100\%\). It is a limiting line: a laboratory curve should not lie persistently above it if measurements and assumptions are correct. The line is not normally attained because some air remains trapped during compaction.

The relative compaction is

\[R_c=\frac{\gamma_{d,field}}{\gamma_{d,max,lab}}\times100\%\]

where the laboratory reference must correspond to the specified test method. A requirement such as 95 percent relative compaction is incomplete unless the reference test and acceptance zone are stated.

6.3 Compaction mechanisms and equipment

Vibratory rollers are effective for clean sands and gravels because vibration reduces interparticle friction and promotes rearrangement. Smooth-wheel rollers provide surface kneading and are useful for granular materials and finishing. Sheep-foot or pad-foot rollers generate kneading and high contact pressures, making them suitable for cohesive soils. Pneumatic-tired rollers apply repeated kneading and are versatile for mixed materials. Rammers and plate compactors are useful in confined areas, but their effectiveness depends strongly on lift thickness and moisture.

Compaction is controlled by soil type, lift thickness, number of passes, equipment contact pressure, vibration settings and moisture conditioning. More passes do not necessarily compensate for a lift that is too thick or soil that is excessively wet.

Worked field-control example

A laboratory Proctor test gives \(\gamma_{d,max}=18.6\,\text{kN/m}^3\). A field test gives moist unit weight \(\gamma=20.0\,\text{kN/m}^3\) and water content \(w=9.0\%\). The field dry unit weight is

\[\gamma_{d,field}=\frac{20.0}{1.09}=18.35\,\text{kN/m}^3\]

Therefore,

\[R_c=\frac{18.35}{18.6}\times100=98.7\%\]

If the project specification requires at least 95 percent, the point satisfies the density criterion. Acceptance should still consider moisture limits, location, lift identification and whether the correct laboratory reference was used.

7. Field Quality Control and Quality Assurance

A defensible compaction programme begins with material approval and a laboratory reference curve. Field controls should include:

1. Material verification: confirm source, gradation, plasticity and presence of deleterious or organic material.

2. Moisture conditioning: add water uniformly or aerate wet soil; allow adequate mixing and mellowing where appropriate.

3. Lift control: place soil in layers compatible with the equipment and target density.

4. Equipment trials: establish practical passes, speed, vibration and moisture range using a test section.

5. Field testing: determine in-place density and water content by approved methods, such as sand-cone, rubber-balloon, drive-cylinder or nuclear gauge methods, subject to material suitability and regulatory requirements.

6. Documentation: record chainage, elevation, material source, lift number, test location, density, water content, equipment and corrective action.

7. Nonconformance management: identify, isolate and rework failed areas rather than averaging failures away.

Nuclear gauges provide rapid results but require calibration, trained operators, radiation controls and verification. Sand-cone methods can be reliable but are slower and difficult in coarse, soft or highly irregular materials. Sampling bias is a major concern: isolated tests cannot establish uniformity across an entire embankment.

Quality assurance differs from quality control. Quality control is the contractor’s operational testing and process control; quality assurance is the independent verification that specified procedures and performance requirements have been met. Both are necessary for trustworthy earthwork records.

8. Illustrative Case: Foundation Pad on Variable Fill

Background

A proposed warehouse requires a large shallow foundation pad. Preliminary investigation identifies natural dense sand in part of the site but loose, heterogeneous fill elsewhere. The fill contains sand, gravel, silt and fragments of construction debris.

Problem

Initial field density tests on the fill exceed 95 percent relative compaction at several locations, yet proof rolling reveals rutting and isolated soft zones. A design based only on the average density results would risk differential settlement.

Analysis

Further investigation shows that the laboratory reference curve was prepared from a relatively clean sample, whereas the field fill contains variable fines and oversized particles. Some tests were performed on the surface of lifts and did not represent the full layer. The fill also has pockets of high water content. Density alone therefore fails to capture variability, grading effects and local weak zones.

Approach and solution

The project team first removes debris and unsuitable material. The remaining fill is separated into reasonably consistent material types. Separate laboratory compaction curves are developed, and a trial pad establishes lift thickness, roller type and passes. Moisture is controlled within a specified range around the relevant OMC. Field tests are selected to suit each material, and proof rolling is retained as a complementary performance check. Soft zones are undercut and replaced or stabilised. Acceptance records identify the material group and laboratory reference used for every test.

Outcome

Subsequent testing shows more uniform density, and proof rolling produces no significant rutting. The foundation design proceeds with reduced uncertainty, while the project avoids relying on misleading isolated density results.

Learning points

Classification and compaction data are only meaningful when the tested material represents the placed material. Relative compaction is a control index, not a complete guarantee of performance. Spatial variability, moisture, lift geometry and field observation must be integrated into engineering decisions.

9. Limitations, Governance and Contemporary Practice

Classification boundaries are empirical and sometimes abrupt, whereas natural soil behaviour is continuous. Laboratory remoulding may destroy field fabric; sample disturbance can alter water content and density; and a single value of specific gravity may be inappropriate for mineralogically variable or organic soils. Engineers should report test methods, sample condition, uncertainty and deviations from standards.

Regulatory and contractual requirements commonly specify sampling frequency, compaction thresholds, acceptable moisture ranges, laboratory standards and record retention. Professional responsibility requires that failed tests are not concealed, that unsuitable materials are not reclassified without evidence, and that design assumptions are traceable to data.

Digital field systems increasingly link GPS-tagged density tests, moisture measurements, roller passes and drone or machine-control data. Intelligent compaction can estimate stiffness continuously, but sensor outputs are not automatically equivalent to conventional density. Machine learning may identify spatial patterns and predict likely nonconformance, provided datasets are representative and models are independently validated. AI should support—not replace—qualified engineering judgement, direct testing and accountability.

Sustainable practice includes minimising unnecessary excavation, using suitable recycled or locally available materials where environmental and performance requirements permit, optimising moisture use, and selecting low-energy stabilisation alternatives. Such decisions must consider leaching, contaminant mobility, long-term durability and whole-life performance.

Best-Practice Checklist

• Define the purpose of classification before selecting the system.
• Preserve representative samples and document disturbance and moisture condition.
• Check mass balance in particle-size analysis and use appropriate dispersants for hydrometer testing.
• Report both liquid limit and plastic limit methods, not only the resulting symbol.
• Distinguish compaction from consolidation in design and construction discussions.
• Match roller type, lift thickness and moisture to soil behaviour.
• Use the correct laboratory reference for relative compaction.
• Combine density testing with moisture, visual inspection and proof rolling where appropriate.
• Treat spatial variability as an engineering variable, not merely a statistical nuisance.
• Record assumptions, uncertainty, corrective action and acceptance decisions transparently.

Chapter Summary

Soil is a three-phase material whose engineering behaviour depends on solids, water, air and their arrangement. Phase relationships provide the basis for calculating water content, void ratio, porosity, saturation and unit weight. Particle-size analysis describes grading, while fabric and mineralogy explain why soils with similar grading may behave differently. Atterberg limits and plasticity indices support the interpretation and classification of fine-grained soils.

USCS and AASHTO are practical classification frameworks with different purposes; neither supplies design parameters by itself. Capillarity and pore-water pressure influence effective stress and the behaviour of unsaturated soils. Compaction mechanically densifies soil mainly by expelling air, and its performance is assessed through moisture–density relationships, maximum dry unit weight, optimum moisture content and relative compaction. Reliable field control requires compatible laboratory references, appropriate equipment, moisture and lift management, representative testing, documentation and independent assurance. Finally, classification and density results must be interpreted with awareness of variability, fabric, sampling limitations, regulatory obligations and professional ethics.

Key Terms

Phase relationship: Quantitative relationship among soil solids, water, air and total volume.

Water content: Ratio of mass of water to mass of dry soil solids.

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

Void ratio: Ratio of void volume to volume of soil solids.

Porosity: Ratio of void volume to total soil volume.

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

Particle-size distribution: Distribution of soil mass among particle-size ranges.

Coefficient of uniformity: Ratio \(D_{60}/D_{10}\), indicating breadth of grading.

Coefficient of curvature: Parameter \(D_{30}^2/(D_{10}D_{60})\) describing grading shape.

Soil fabric: Arrangement, orientation and contacts of soil particles.

Liquid limit: Water content boundary between liquid and plastic states.

Plastic limit: Water content boundary between plastic and semisolid states.

Plasticity index: Difference between liquid limit and plastic limit.

Liquidity index: Location of natural water content relative to the plastic range.

USCS: Unified Soil Classification System.

Capillarity: Rise or retention of water caused by surface tension and adhesion in pores.

Compaction: Mechanical densification of soil, principally through air expulsion.

Optimum moisture content: Water content corresponding to maximum dry density for a specified effort.

Relative compaction: Field dry density expressed as a percentage of laboratory maximum dry density.

Quality assurance: Independent verification that materials and processes meet specified requirements.

Review Questions

Short Answer Questions

1. Define void ratio, porosity and degree of saturation, and state the relationship between void ratio and porosity.

2. Why can two soils with similar particle-size distributions exhibit different engineering behaviour?

3. Distinguish compaction from consolidation.

4. What information is conveyed by the plasticity index?

5. State two limitations of using relative compaction as the sole earthwork acceptance criterion.

Long Answer Questions

1. Explain the three-phase soil model and derive the relationship \(S_re=wG_s\).

2. Describe sieve and hydrometer analyses, including their assumptions and principal sources of error.

3. Explain the USCS classification procedure for coarse- and fine-grained soils.

4. Discuss the mechanisms controlling a laboratory moisture–density curve and the factors affecting OMC and maximum dry density.

5. Develop a field compaction quality-control plan for a cohesive embankment, including testing, documentation and nonconformance procedures.

Analytical / Application Questions

1. A soil has \(G_s=2.68\), \(w=15\%\) and \(\gamma_d=16.5\,\text{kN/m}^3\). Calculate its void ratio, porosity, degree of saturation and moist unit weight using \(\gamma_w=9.81\,\text{kN/m}^3\). Comment on whether the calculated saturation is physically plausible.

2. A grading curve gives \(D_{10}=0.18\,\text{mm}\), \(D_{30}=0.42\,\text{mm}\) and \(D_{60}=1.20\,\text{mm}\). Calculate \(C_u\) and \(C_c\), then discuss whether the coarse fraction meets the numerical USCS criteria for well-graded sand, subject to fines requirements.

3. A fill specification requires 95 percent relative compaction based on a laboratory maximum dry unit weight of 19.2 kN/m³. A field test records moist unit weight 20.7 kN/m³ at water content 8 percent. Determine compliance and identify two additional checks required before acceptance.

Multiple Choice Questions with Answers

Question 1

A soil has a void ratio of 0.60. Its porosity is approximately:

A. 27.5%

B. 37.5%

C. 40.0%

D. 60.0%

Correct Answer: B. 37.5%

Question 2

For a soil with w = 0.20, Gs = 2.70 and e = 0.54, the degree of saturation is:

A. 54%

B. 75%

C. 100%

D. 135%

Correct Answer: C. 100%

Question 3

The plasticity index is defined as:

A. wP − wS

B. wL + wP

C. wL − wP

D. w − wL

Correct Answer: C. wL − wP

Question 4

In USCS classification, a soil with 50 percent or more passing the No. 200 sieve is generally treated as:

A. Coarse-grained

B. Fine-grained

C. Well-graded gravel

D. Clean sand

Correct Answer: B. Fine-grained

Question 5

The coefficient of uniformity is:

A. D10/D60

B. D30/D10

C. D60/D10

D. D30²/(D10D60)

Correct Answer: C. D60/D10

Question 6

Compaction primarily reduces soil volume by expelling:

A. Mineral solids

B. Air

C. Water under drained loading

D. Organic matter

Correct Answer: B. Air

Question 7

The zero-air-voids line represents a condition of:

A. Zero saturation

B. 50 percent saturation

C. Complete saturation

D. Constant water content

Correct Answer: C. Complete saturation

Question 8

A field dry unit weight is 18.0 kN/m³ and the laboratory maximum is 19.0 kN/m³. Relative compaction is approximately:

A. 94.7%

B. 95.0%

C. 100.0%

D. 105.6%

Correct Answer: A. 94.7%

Question 9

Which equipment is generally most effective for clean, coarse granular soil?

A. Vibratory roller

B. Sheep-foot roller only

C. Hand rammer only

D. Static scraper

Correct Answer: A. Vibratory roller

Question 10

For saturated soil under the conventional effective-stress framework, effective stress is:

A. σ + u

B. σ − u

C. u − σ

D. σu

Correct Answer: B. σ − u