Structural Geology: Stress and Strain

• Structural geology studies the deformation of the Earth's crust
• Rock mechanics focuses on the mechanical behavior of rocks
• Understanding these fields helps in understanding of earthquakes, landslides, and resource extraction

Stress: Forces Within Rocks

• Confining pressure applies equal stress in all directions as occurs in situations of deep burial
• Tensional stress involves pulling apart from opposite directions
• Compressional stress involves squeezing together from opposite directions
• Shear stress involves sliding past each other, not from opposite directions

Strain: How Rocks Deform

• Elastic deformation is reversible which allows the rock to return to its original shape
• Ductile deformation is a permanent change in shape, like folding
• Brittle deformation is permanent breaking or fracturing of the rock

Rock Strength and Failure Criteria

• Rock strength is the stress a rock can withstand before failure
• Rock strength depends on composition, grain size, and temperature
• Mohr-Coulomb criterion is a linear equation that predicts when a rock will fracture based on the relationship between normal and shear stress
• Shear stress exceeding a rock's shear strength leads to failure, where shear strength depends on the cohesion and internal friction angle of the material
• The Mohr-Coulomb criterion is used to assess the stability of slopes, tunnels, and underground excavations in geotechnical engineering and geology
• Tensile strength measures the maximum stress a rock can withstand while being stretched or pulled apart before fracturing or breaking
• Rocks generally have much lower tensile strengths compared to compressive strengths
• Tensile strength evaluations are important for rock structures subjected to tensile forces, like landslides or surface mining operations

Fractures and Faults

• Fractures are cracks in rocks without notable movement, and these increase permeability
• Faults represent breaks in rocks where movement has occurred indicative of past tectonic activity

Types of Faults

• Normal faults occur when the hanging wall moves down relative to the footwall, often due to tensional stress
• Reverse faults occur when the hanging wall moves up relative to the footwall, usually due to compressional stress
• Strike-slip faults display blocks that move horizontally past each other, parallel to the strike of the fault plane

Folds: Bending of Rock Layers

• Anticlines form upward arching folds, where the oldest rocks are in the core
• Synclines form downward arching folds, where the youngest rocks are in the core
• Monoclines are single bends in otherwise horizontal layers

Engineering Applications

• Dam construction benefits from understanding stress and strain for building stable and safe dams
• Tunneling benefits from rock mechanics in designing tunnels that can withstand surrounding rock pressure
• Mining benefits from structural geology in locating resources and ensuring mine stability
• Petroleum geology benefits from understanding rock structures to help in oil and gas exploration and extraction

Conclusion

• Structural geology and rock mechanics help solve various engineering problems
• Stress and strain are key to understanding rock deformation
• Fractures, faults, and folds result from these stresses
• The principles apply to engineering and resource extraction