2.2 Soil composition and structure

Soil composition and structure are crucial aspects of geotechnical science. They determine how soil behaves under different conditions, affecting everything from water movement to building foundations. Understanding these elements is key to predicting soil behavior in various engineering applications.

Soil is made up of solid particles, water, and air. The arrangement of these components creates different soil structures, which impact soil properties like strength and permeability. This knowledge is essential for engineers working on projects involving soil, from construction to environmental management.

Soil Components

Primary Soil Phases

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• Soil composed of three main phases determine soil properties
  • Solid particles
  • Water
  • Air
• Solid phase consists primarily of mineral particles classified by size and composition
  • Sand (2.0 - 0.05 mm)
  • Silt (0.05 - 0.002 mm)
  • Clay (< 0.002 mm)
• Organic matter plays crucial role in soil structure and fertility despite smaller quantities
  • Derived from decomposed plant and animal materials
  • Typically 1-5% of soil volume in mineral soils
• Soil water exists in various forms within soil matrix
  • Gravitational water moves freely under gravity
  • Capillary water held in small pores by surface tension
  • Hygroscopic water tightly bound to soil particles
• Soil air occupies pore spaces not filled by water
  • Essential for plant growth and microbial activity
  • Typically 15-35% of soil volume in well-drained soils

Soil Fabric and Particle Arrangement

• Soil fabric describes arrangement of solid particles and associated voids within soil mass
• Fabric elements include
  • Single-grain arrangements in sandy soils
  • Flocculated structures in clay-rich soils
  • Honeycomb structures in silty soils
• Particle orientation affects soil properties
  • Parallel orientation of clay particles can lead to anisotropic behavior
  • Random orientation often results in more isotropic properties
• Void ratio (e) and porosity (n) quantify pore space in soil
  • $e = \frac{V_v}{V_s}$ where $V_v$ is volume of voids and $V_s$ is volume of solids
  • $n = \frac{V_v}{V_t}$ where $V_t$ is total volume
• Particle size distribution influences packing and pore space characteristics
  • Well-graded soils have a wide range of particle sizes, leading to denser packing
  • Poorly-graded soils have uniform particle sizes, resulting in more void space

Soil Structure Types

Common Soil Structure Classifications

• Granular structure consists of small, rounded aggregates
  • Typically found in surface soils
  • Beneficial for water infiltration and root growth
  • Common in A horizons with high organic matter content
• Blocky structure features cube-like aggregates with sharp edges
  • Common in subsoils (B horizons)
  • Affects water movement and root penetration
  • Can be further classified as angular or subangular blocky
• Prismatic and columnar structures characterized by vertical columns of soil
  • Prismatic structures have flat tops
  • Columnar structures have rounded tops, often found in arid regions (sodium-affected soils)
  • Can impede horizontal water movement

Additional Structure Types and Characteristics

• Platy structure consists of thin, flat plates oriented horizontally
  • Can impede water movement and root growth if too pronounced
  • Often found in compacted soils or E horizons (eluvial layers)
• Single-grain structure lacks aggregation
  • Typical of sandy soils with low clay and organic matter content
  • High permeability but low water-holding capacity
• Massive structure has no visible aggregates
  • Often found in compacted soils or C horizons (parent material)
  • Poor for root growth and water movement
• Crumb structure similar to granular but with more porous aggregates
  • Ideal for agricultural soils
  • Promotes excellent water infiltration and aeration

Factors Influencing Soil Structure Stability

• Organic matter content acts as binding agent for soil particles
  • Increases aggregate stability
  • Improves resistance to erosion and compaction
• Clay mineralogy affects aggregation and structural development
  • 2:1 clay minerals (montmorillonite) more prone to swelling and dispersion
  • 1:1 clay minerals (kaolinite) tend to form more stable structures
• Environmental conditions impact structure formation and stability
  • Freeze-thaw cycles can break down aggregates
  • Wet-dry cycles can promote aggregate formation in some soils
• Biological activity contributes to structure development
  • Root growth creates channels and binds particles
  • Earthworms and other soil fauna enhance aggregation through burrowing and casting

Soil Structure in Engineering

Geotechnical Properties Influenced by Structure

• Soil structure significantly influences critical geotechnical parameters
  • Strength (shear strength, cohesion, friction angle)
  • Compressibility (consolidation characteristics, settlement potential)
  • Permeability (hydraulic conductivity, drainage properties)
• Arrangement of particles and pores affects soil's ability to transmit and retain water
  • Impacts drainage characteristics and groundwater flow
  • Influences effective stress distribution within soil mass
• Soil structure plays crucial role in determining soil's resistance to erosion
  • Well-aggregated soils are more resistant to water and wind erosion
  • Structure breakdown can lead to increased erodibility
• Susceptibility to compaction under applied loads affected by structural properties
  • Soils with stable structure resist compaction better
  • Compaction can alter soil structure, affecting engineering properties

Engineering Applications and Considerations

• Stability of soil structure affects bearing capacity of foundations
  • Well-structured soils generally provide better support for structures
  • Structural degradation can lead to reduced bearing capacity over time
• Performance of earth structures influenced by soil structure
  • Embankments require well-compacted, structured soils for stability
  • Retaining walls rely on soil structure for proper drainage and reduced lateral pressures
• Understanding soil structure essential for predicting and mitigating soil-related hazards
  • Liquefaction potential in loose, saturated sands
  • Landslides in soils with weak structural integrity
  • Differential settlement in soils with varying structural properties
• Soil structure influences effectiveness of ground improvement techniques
  • Compaction methods aim to modify soil structure for increased density and strength
  • Soil stabilization (chemical, mechanical) alters soil structure to enhance engineering properties
• Evolution of soil structure over time must be considered in long-term geotechnical performance assessments
  • Weathering can break down soil structure
  • Loading and unloading cycles may alter structural characteristics
  • Environmental factors (pH changes, chemical exposure) can modify soil structure

Composition vs Structure

Mineralogical Influences on Structure

• Mineralogical composition of soil particles directly influences formation and stability of soil aggregates
  • Clay minerals play crucial role due to their high surface area and reactivity
  • Quartz-dominated soils (sands) tend to have weaker structural development
• Particle size distribution affects packing arrangement of soil grains
  • Well-graded soils often develop more complex and stable structures
  • Uniformly graded soils may have simpler structures with less particle interlocking
• Clay content and type play crucial role in determining soil plasticity and cohesion
  • Higher clay content generally leads to stronger aggregate formation
  • Smectite clays (montmorillonite) cause significant shrink-swell behavior, affecting structural stability
• Chemical composition of soil influences flocculation and dispersion of clay particles
  • pH affects surface charge of clay particles, impacting aggregation
  • Cation exchange capacity influences binding of particles and structural stability

Organic Matter and Environmental Factors

• Presence and type of organic matter act as binding agents
  • Promotes formation of stable soil aggregates
  • Influences structural development through creation of micropores and macropores
• Interaction between soil composition and environmental factors determines formation and stability of different structural types
  • Moisture content affects cohesion between particles
  • Temperature influences chemical and biological processes that impact structure
• Anthropogenic activities can alter soil composition and consequently modify soil structure
  • Tillage practices disrupt natural soil structure
  • Addition of amendments (lime, gypsum) can improve structural stability
• Biological activity, closely linked to soil composition, affects structural development
  • Root exudates promote aggregation
  • Microbial activity produces binding agents that enhance structural stability