Soil Mechanics: Key Soil Properties

• Soil mechanics is a branch of civil engineering that deals with the behavior of soil as a construction material.
• Principles of mechanics, hydraulics, and engineering geology are applied to analyze and predict soil behavior.
• Understanding soil mechanics is crucial for designing foundations, retaining walls, embankments, tunnels, and other earthworks.

Key Soil Properties

• Water content (w) is the ratio of the weight of water to the weight of solids in a soil mass, expressed as a percentage.
• Void ratio (e) is the ratio of the volume of voids to the volume of solids in a soil mass.
• Porosity (n) is the ratio of the volume of voids to the total volume of a soil mass, expressed as a percentage.
• Degree of saturation (S) is the ratio of the volume of water to the volume of voids in a soil mass, expressed as a percentage.
• Unit weight (γ) is the weight of soil per unit volume.
• Specific gravity (Gs) is the ratio of the density of soil solids to the density of water.
• Atterberg Limits define the consistency of fine-grained soils based on their water content.
• Liquid Limit (LL) is the water content at which the soil transitions from a plastic to a liquid state.
• Plastic Limit (PL) is the water content at which the soil transitions from a semi-solid to a plastic state.
• Shrinkage Limit (SL) is the water content at which further reduction in water content does not cause a decrease in volume.
• Plasticity Index (PI) is the difference between the Liquid Limit and the Plastic Limit (PI = LL - PL), indicating the range of water content over which the soil exhibits plastic behavior.
• Soil classification systems, such as the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO) system, are used to categorize soils based on their particle size distribution and plasticity characteristics.

Soil Composition

• Mineral composition influences soil properties.
• Sand and gravel are coarse-grained particles.
• Silt is a fine-grained soil with intermediate properties.
• Clay is very fine-grained and exhibits plasticity.
• Organic matter affects soil fertility and stability.

Soil Structure

• Soil structure refers to the arrangement of soil particles and aggregates.
• Granular structure is common in sandy soils.
• Blocky structure is found in cohesive soils.
• Prismatic and columnar structures are typical in subsoils.
• Structure affects permeability, drainage, and plant growth.

Soil Compaction

• Compaction is the process of increasing soil density by reducing air voids.
• Proctor test determines the optimum moisture content and maximum dry density for compaction.
• Compaction improves soil strength, reduces settlement, and increases stability.

Soil Permeability

• Permeability (k) is the measure of the soil's ability to transmit water.
• Darcy's Law describes the flow of water through porous media: v = ki, where v is the discharge velocity and i is the hydraulic gradient.
• Factors affecting permeability include particle size, void ratio, and soil structure.
• Permeability is crucial for groundwater flow analysis and drainage design.

Soil Consolidation

• Consolidation is the gradual reduction in volume of a soil under sustained loading.
• Primary consolidation is due to the expulsion of water from the voids.
• Secondary compression is due to the plastic deformation of soil particles.
• Terzaghi's consolidation theory describes the time-dependent settlement of saturated clay soils.
• Consolidation settlement is important for foundation design.

Shear Strength of Soils

• Shear strength (τ) is the soil's resistance to shearing stresses.
• Mohr-Coulomb failure criterion: τ = c + σ' tan φ, where c is the cohesion, σ' is the effective normal stress, and φ is the angle of internal friction.
• Cohesion (c) is the soil's ability to resist shear stress without any applied normal stress.
• Angle of internal friction (φ) represents the frictional resistance between soil particles.
• Factors affecting shear strength include soil type, density, and moisture content.
• Triaxial test, direct shear test, and vane shear test are common methods for determining shear strength.

Effective Stress Principle

• Effective stress (σ') is the stress carried by the soil solids.
• Total stress (σ) is the total load applied to the soil.
• Pore water pressure (u) is the pressure exerted by the water within the soil pores.
• Effective stress principle: σ' = σ - u.
• Effective stress controls soil behavior such as strength and deformation.

Earth Pressures

• Earth pressure refers to the lateral pressure exerted by soil on retaining structures.
• There are three main types of earth pressure: at-rest, active, and passive.

At-Rest Earth Pressure

• At-rest earth pressure (K0) is the lateral pressure when the soil is in its natural, undisturbed state.
• There is no wall movement in at-rest condition.
• K0 is the coefficient of at-rest earth pressure.
• Jaky's formula: K0 ≈ 1 - sin φ, where φ is the angle of internal friction.

Active Earth Pressure

• Active earth pressure (Ka) is the minimum lateral pressure when the retaining wall moves away from the soil.
• The soil mass expands, and shear strength is mobilized.
• Rankine's theory and Coulomb's theory are used to calculate active earth pressure.
• Rankine's active earth pressure coefficient: Ka = (1 - sin φ) / (1 + sin φ).

Passive Earth Pressure

• Passive earth pressure (Kp) is the maximum lateral pressure when the retaining wall moves towards the soil.
• The soil mass is compressed, and shear strength is fully mobilized.
• Rankine's theory and Coulomb's theory are used to calculate passive earth pressure.
• Rankine's passive earth pressure coefficient: Kp = (1 + sin φ) / (1 - sin φ).

Rankine's Earth Pressure Theory

• Assumes a smooth, vertical wall and a cohesionless soil.
• Considers a state of plastic equilibrium in the soil mass.
• Active earth pressure acts at an angle parallel to the backfill surface.
• Passive earth pressure acts at an angle parallel to the backfill surface.

Coulomb's Earth Pressure Theory

• Considers friction between the wall and the soil.
• Allows for inclined backfill and non-vertical walls.
• Assumes a planar failure surface.
• More general than Rankine's theory.

Factors Affecting Earth Pressure

• Soil type influences the magnitude and distribution of earth pressure.
• Wall movement affects the type of earth pressure (at-rest, active, or passive).
• Groundwater level increases pore water pressure and affects effective stress.
• Surcharge loads increase the lateral earth pressure on the retaining structure.
• Compaction of backfill can increase lateral earth pressure.

Retaining Wall Design Considerations

• Stability against overturning must be ensured.
• Sliding resistance must be adequate to prevent wall movement.
• Bearing capacity of the soil beneath the wall must be sufficient.
• Drainage systems are essential to reduce hydrostatic pressure.
• Proper selection of backfill material is important for long-term performance.