Introduction to Geotechnical Engineering

• Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials.

Soil Composition

• Soil is a complex material comprised of solid particles, water, and air.
• The solid particles are typically mineral grains or organic matter.
• The spaces between the solid particles are called voids, which are filled with water and/or air.

Soil Types

• Soils are broadly classified into two categories: coarse-grained and fine-grained.
• Coarse-grained soils: Gravel and sand, where individual particles are visible to the naked eye.
• Fine-grained soils: Silt and clay, particle sizes are much smaller.
• Organic soils: Contain significant amounts of organic matter.
• Peat: A type of organic soil consisting of partially decayed vegetation.

Soil Properties

• Physical properties: Grain size distribution, specific gravity, water content, density, void ratio, porosity, and degree of saturation.
• Index properties: Atterberg limits (liquid limit, plastic limit, and shrinkage limit) that indicate the plasticity of fine-grained soils.
• Hydraulic properties: Permeability (hydraulic conductivity) which describes the ease with which water flows through the soil.
• Mechanical properties: Shear strength, compressibility, and consolidation characteristics.

Grain Size Distribution

• Represents the proportions of different particle sizes in a soil sample.
• Determined through sieve analysis for coarse-grained soils and hydrometer analysis for fine-grained soils.
• The data is plotted on a grain size distribution curve, which shows the percentage of soil particles finer than a given size.
• Important parameters derived from the curve: Effective size (D10), uniformity coefficient (Cu), and coefficient of curvature (Cc).

Atterberg Limits

• Used to characterize the behavior of fine-grained soils with varying water content.
• Liquid limit (LL): Water content at which the soil transitions from a plastic to a liquid state, determined by the Casagrande cup test or cone penetrometer test.
• Plastic limit (PL): Water content at which the soil transitions from a semi-solid to a plastic state, determined by rolling the soil into a 3.2 mm diameter thread.
• Shrinkage limit (SL): Water content at which further reduction in water content does not cause a decrease in volume.
• Plasticity index (PI): Difference between the liquid limit and plastic limit (PI = LL - PL), indicates the range of water content over which the soil exhibits plastic behavior.
• Liquidity index (LI): Ratio of the difference between the natural water content and plastic limit to the plasticity index, indicates the in-situ consistency of the soil.

Soil Classification

• Standardized systems for grouping soils based on their properties and behavior.
• Unified Soil Classification System (USCS): Widely used system based on grain size distribution and plasticity characteristics. Soils are classified into groups and subgroups, each designated by a letter symbol.
• American Association of State Highway and Transportation Officials (AASHTO) classification system: Used for classifying soils for highway construction purposes.

Soil Compaction

• Process of increasing the density of soil by reducing the air voids.
• Achieved through mechanical methods such as rolling, tamping, or vibration.
• Compaction improves soil strength, reduces compressibility, and decreases permeability.
• Proctor test: Determines the optimum moisture content (OMC) and maximum dry density (MDD) for a given soil and compaction effort.
• Field compaction control: Monitoring the degree of compaction achieved in the field by comparing the in-situ density with the MDD obtained from the Proctor test.

Permeability

• Measure of how easily water flows through soil.
• Influenced by factors such as grain size, void ratio, and soil structure.
• Determined through laboratory tests such as constant head test (for coarse-grained soils) and falling head test (for fine-grained soils).
• Darcy's Law: Describes the relationship between flow rate, hydraulic gradient, and permeability.
• Seepage: Flow of water through soil, which can cause instability issues in geotechnical structures.

Effective Stress

• The stress carried by the soil solids.
• Total stress: The total force per unit area acting on a soil mass.
• Pore water pressure: The pressure exerted by the water within the soil voids.
• Effective stress principle: Effective stress is equal to the total stress minus the pore water pressure (σ' = σ - u).
• Effective stress controls the soil's strength and deformation behavior.

Consolidation

• Time-dependent process of volume reduction in saturated, fine-grained soils due to the dissipation of excess pore water pressure.
• Occurs when a load is applied to a saturated soil, increasing the pore water pressure.
• Water gradually flows out of the soil, leading to a decrease in volume and an increase in effective stress.
• Terzaghi's consolidation theory: Describes the rate and magnitude of consolidation.
• Consolidation parameters: Coefficient of consolidation (cv), compression index (Cc), and recompression index (Cr).

Shear Strength

• The soil's resistance to shearing stresses.
• Mohr-Coulomb failure criterion: Describes the relationship between shear strength, effective stress, and soil properties.
• Shear strength parameters: Cohesion (c') and angle of internal friction (ϕ').
• Factors affecting shear strength: Soil type, density, water content, and stress history.
• Shear strength tests: Direct shear test, triaxial test, and unconfined compression test.

Slope Stability

• Analysis of the stability of natural slopes and man-made slopes (e.g., embankments, dams, and excavations).
• Factor of safety (FS): Ratio of the resisting forces to the driving forces, indicates the stability of the slope.
• Methods of slope stability analysis: Limit equilibrium methods (e.g., method of slices, Bishop's simplified method, and Spencer's method) and finite element methods.
• Factors affecting slope stability: Soil properties, slope geometry, groundwater conditions, and external loads.

Soil Improvement Techniques

• Methods used to improve the engineering properties of soil.
• Compaction: Increases the density of soil.
• Drainage: Lowers the groundwater table and increases the effective stress.
• Ground reinforcement: Use of geosynthetics, soil nailing, or other techniques to increase the strength and stability of the soil.
• Chemical stabilization: Use of additives such as lime, cement, or polymers to improve soil properties.

Site Investigation

• Process of gathering information about the subsurface conditions at a construction site.
• Includes desk study, site reconnaissance, soil sampling, and laboratory testing.
• Purpose: Determine the soil profile, groundwater conditions, and engineering properties of the soil for design and construction purposes.
• Boreholes: Drilled to obtain soil samples at different depths.
• Cone penetration test (CPT): Measures the resistance of the soil to penetration by a cone-shaped probe.
• Geophysics: Use of techniques such as seismic refraction and ground penetrating radar to investigate subsurface conditions.

An overview of geotechnical engineering, focusing on soil composition, types, and properties. It covers coarse-grained soils like gravel and sand, fine-grained soils such as silt and clay, and organic soils like peat. Key physical and index properties of soil are also discussed.