Soil Mechanics: Composition and Properties

• Soil mechanics is a civil engineering branch, addressing soil's engineering properties and behavior.
• It uses mechanics, hydraulics, and engineering geology to predict soil behavior under loads/environmental conditions.
• Soil mechanics is vital for designing foundations, retaining structures, embankments, and tunnels, and other infrastructure.

Soil Composition

• Soil contains solid particles, water, and air.
• Solid particles include mineral grains and organic matter.
• Mineral grains are sized as gravel, sand, silt, and clay.
• Water occupies voids between solids, with air in remaining voids.
• Proportions of solids, water, and air dictate soil's physical traits.

Soil Properties

• Essential soil properties:
  • Water content: water weight to solid particle weight ratio.
  • Void ratio: void volume to solid particle volume ratio.
  • Porosity: void volume to total volume ratio.
  • Saturation degree: water volume to void volume ratio.
  • Density: mass per unit volume.
  • Unit weight: weight per unit volume.
  • Specific gravity: density of soil solids to water density ratio.

Soil Classification

• Soil classification groups soil by similar properties/behavior.
• The Unified Soil Classification System (USCS) classifies soil by particle size and plasticity.
• USCS divides soil into coarse-grained (gravel/sand) and fine-grained (silt/clay).
• Coarse-grained soils are classified by particle size; fine-grained by plasticity (liquid/plastic limits).

Soil Compaction

• Soil compaction increases density by reducing air volume.
• It enhances soil strength and stability for construction.
• Compaction uses rolling, tamping, or vibration.
• Effectiveness relies on soil type, water content, and compactive effort.
• Optimum water content yields maximum dry density for given compactive effort.

Soil Permeability

• Soil permeability measures water flow ability.
• It depends on particle size/shape and water viscosity.
• Permeability is key for analyzing groundwater flow, drainage, and seepage.
• Darcy's law: v = ki (v = discharge velocity, k = permeability coefficient, i = hydraulic gradient).

Soil Strength

• Soil strength is a soil's resistance to deformation/failure under stress.
• It is important for designing foundations, retaining structures, and slopes.
• Characterized by cohesion (c) and internal friction angle (φ).
• Mohr-Coulomb failure criterion: τ = c + σ tan φ (τ = shear strength, σ = normal stress).

Soil Consolidation

• Soil consolidation is the gradual volume reduction in saturated soil from effective stress increase.
• It occurs mainly in fine-grained soils (clays) due to low permeability hindering pore water pressure dissipation.
• Consolidation is time-dependent.

Components of Consolidation

• Initial Consolidation: Immediate volume drop from air/water compression; often minor in saturated clays.
• Primary Consolidation: Major volume decrease as pore water expels, increasing effective stress.
• Secondary Consolidation (Creep): Slow volume decrease after primary consolidation due to soil structure adjustment; significant in organic soils.

Terzaghi's Theory of Consolidation

• Terzaghi's 1D consolidation theory is fundamental.
• Terzaghi's theory assumes:
  • Soil is homogeneous and saturated.
  • Soil particles and water are incompressible.
  • Water flow is vertical (one-dimensional).
  • Darcy's law applies.
  • Permeability and volume compressibility coefficients are constant.
• The theory predicts consolidation settlement rate/amount in saturated clay under constant load.
• Key parameters:
  • Coefficient of consolidation (cv): consolidation rate based on permeability/compressibility.
  • Time factor (T): dimensionless parameter relating consolidation degree to time.
  • Consolidation degree (U): percentage of total consolidation at a given time.
• Consolidation settlement (Sc): Sc = mv * Δσ' * H (mv = volume compressibility coefficient, Δσ' = effective stress change, H = clay layer thickness).

Determination of Consolidation Parameters

• Consolidation parameters are found via lab tests like the oedometer test.
• In the oedometer test, soil is confined and subjected to increasing vertical loads.
• Vertical deformation is measured over time to determine consolidation parameters.
• Key parameters from the oedometer test are:
  • Coefficient of consolidation (cv)
  • Coefficient of volume compressibility (mv)
  • Compression index (Cc)
  • Swelling index (Cs)
  • Preconsolidation pressure (pc)

Preconsolidation Pressure

• Preconsolidation pressure (pc) is the maximum past effective stress.
• It determines soil stress history and predicts consolidation.
• Soil types:
  • Normally consolidated: current stress equals preconsolidation pressure.
  • Overconsolidated: current stress is less than preconsolidation pressure.
  • Underconsolidated: current stress exceeds preconsolidation pressure.
• Overconsolidation is due to erosion, glacier removal, or desiccation.
• Overconsolidation ratio (OCR): preconsolidation pressure to current effective stress ratio.

Factors Affecting Consolidation

• Influences on consolidation:
  • Soil Type: Clays consolidate slower than silts/sands due to low permeability.
  • Soil Layer Thickness: Thicker layers take longer.
  • Drainage Path: Shorter paths (sand drains) accelerate it.
  • Applied Load: Higher loads increase consolidation.
  • Permeability: Lower permeability reduces consolidation rate.

Practical Implications of Consolidation

• Consolidation is critical in civil engineering.
• Excessive consolidation causes structural settlement, damaging buildings/infrastructure.
• Understanding/predicting consolidation is essential for suitable foundations/ground improvement.
• Ground improvement, like preloading and vertical drains, can accelerate consolidation and reduce settlement.