Soil Compaction in Soil Mechanics

• Soil mechanics is a branch of civil engineering focused on the engineering properties and behavior of soil.
• It uses principles of mechanics, hydraulics, and engineering geology to analyze and predict soil behavior under various conditions.
• Crucial for designing and constructing foundations, retaining walls, embankments, tunnels, and other civil engineering structures.

Soil Compaction

• Soil compaction increases soil density by reducing air voids.
• Achieved through mechanical methods like rolling, tamping, or vibration.
• Improves soil strength, stiffness, and stability.
• Reduces settlement, permeability, and the potential for frost heave.
• Essential for constructing roads, embankments, and foundations.
• Different compaction equipment is used based on soil type, moisture content, and desired compaction level.
• Common methods include:
  • Smooth wheel rollers are suitable for proof rolling subgrades, finishing asphalt pavements, and compacting granular soils and asphalt mixes.
  • Sheepsfoot rollers are effective for compacting cohesive soils.
  • Vibratory rollers use vibration to compact granular soils.

Shear Strength

• Shear strength is the soil's ability to resist shear stress.
• Critical in geotechnical engineering for analyzing slope stability, bearing capacity, and lateral earth pressure.
• Described by the Mohr-Coulomb failure criterion, relating shear strength to effective stress and soil properties.
• The Mohr-Coulomb failure criterion is expressed as: τ = c' + σ' tan(φ'),
  • τ stands for shear strength
  • c' is the effective cohesion
  • σ' is the effective normal stress
  • φ' is the effective angle of internal friction.
• Cohesion is the inherent shear strength without applied normal stress.
• The angle of internal friction represents resistance to shearing due to friction between soil particles.
• Measured in laboratories using direct shear, triaxial shear, and unconfined compression tests.
• Field tests like vane shear and cone penetration tests estimate shear strength.

Soil Consolidation

• Soil consolidation involves the gradual volume reduction of fully saturated soil under sustained loading.
• Occurs as water is squeezed from soil pores, decreasing the void ratio and causing settlement.
• Particularly important for fine-grained soils like clay, which have low permeability.
• Terzaghi's consolidation theory describes the time-dependent settlement of saturated clay layers.
• The theory assumes homogeneous, saturated, and laterally confined soil with one-dimensional water flow.
• The rate of consolidation depends on the soil's permeability, compressibility, and the drainage path length.
• Consolidation settlement is divided into three stages:
  • Immediate settlement occurs upon load application.
  • Primary consolidation settlement is due to the dissipation of excess pore water pressure.
  • Secondary compression (creep) occurs gradually due to the plastic deformation of the soil skeleton.
• Preloading and vertical drains accelerate consolidation and reduce settlement time.

Permeability

• Permeability measures the soil's ability to transmit water.
• It is defined as the rate of water flow through a unit area of soil under a unit hydraulic gradient.
• Depends on the size and connectivity of soil pores, as well as the fluid's viscosity.
• Darcy's law describes water flow through soil: Q = k i A,
  • Q is the flow rate
  • k is the coefficient of permeability
  • i is the hydraulic gradient
  • A is the cross-sectional area of the soil.
• Measured in laboratories using constant head and falling head permeameter tests.
• Field tests, like pumping tests, determine the in-situ permeability of soil.
• The coefficient of permeability varies widely depending on the soil type, from high values for gravel to very low values for clay.
• Important for designing drainage systems, earth dams, and groundwater control.

Earth Pressure

• Earth pressure is the lateral pressure exerted by soil on retaining structures like retaining walls, basement walls, and sheet pile walls.
• The magnitude and distribution depend on soil properties, wall geometry, and wall movement type.
• Three main types of earth pressure:
  • At-rest earth pressure occurs when the wall cannot move.
  • Active earth pressure occurs when the wall moves away from the soil mass.
  • Passive earth pressure occurs when the wall moves towards the soil mass.
• Rankine's and Coulomb's theories are classical methods for calculating earth pressure.
• Rankine's theory assumes homogeneous, isotropic, and cohesionless soil with a planar failure surface.
• Coulomb's theory considers wall friction and soil cohesion, allowing for a curved failure surface.
• Active earth pressure is typically smaller than at-rest pressure, while passive pressure is significantly larger.
• Critical in designing retaining structures to ensure stability and prevent failure.