Different types of failure and sliding criteria

Failure in Structural Geology and Geotectonics: A Breakdown of Types and Sliding Criteria
In the realm of structural geology and geotectonics, understanding how rocks fail is crucial for comprehending the formation of geological structures, assessing earthquake risks, and ensuring safe construction practices. Here's a breakdown of different types of rock failure and the criteria used to predict them:
Types of Rock Failure:
Brittle Failure: Occurs when rocks fracture due to exceeding their stress capacity. This often happens in relatively shallow rocks under compressional forces. Common brittle failures include:
Tensile Failure: Rock pulls apart due to tensional stress (pulling forces). This can lead to the creation of joints and cracks.
Shear Failure: Rock slides along a plane of weakness under shear stress (forces acting parallel to a surface). This is responsible for faults and fractures.
Ductile Failure: Occurs when rocks deform plastically under high temperatures and confining pressures (deeper within the Earth). This type of failure involves continuous deformation rather than a sudden fracture. Ductile failures include:
Folding: Rocks bend and buckle under stress, forming folds of various shapes and sizes.
Shearing: Rocks deform and flow along planes of weakness under shear stress at high temperatures.
Sliding Criteria:
These criteria define the conditions under which rock failure occurs by relating stress to rock strength. Here are some commonly used sliding criteria:
Mohr-Coulomb Criterion: A widely used criterion for brittle failure that considers both normal stress (acting perpendicular to the surface) and shear stress. It takes into account factors like rock cohesion (resistance to separation) and internal friction angle (resistance to sliding).
Griffith Criterion: Focuses on tensile failure and crack propagation in brittle materials. It relates the critical stress required for a pre-existing crack to propagate to the material's surface energy and fracture toughness.
Drucker-Prager Criterion: A more general criterion applicable to both brittle and ductile failure. It defines a yield surface in stress space that separates elastic behavior (where the rock returns to its original shape after stress is removed) from plastic behavior (permanent deformation).
Choosing the Right Criterion:
The appropriate sliding criterion depends on the specific rock type, stress conditions (temperature, pressure), and the type of failure being investigated. Geologists and engineers consider these factors to select the most suitable criterion for analyzing rock behavior in a particular scenario.
Additional Considerations:
Anisotropy: Rocks can exhibit different strengths depending on the direction of stress. This anisotropy needs to be factored in for accurate failure predictions.
Strain Rate: The rate at which stress is applied can influence rock behavior. Rocks may be more brittle at high strain rates (rapid loading) and more ductile at low strain rates (slow loading).
Fluid Effects: The presence of water or other fluids in rock can influence its strength and failure mode.
By understanding the different types of rock failure and the various sliding criteria, geologists and geotechnical engineers can better predict how rocks will respond to stress, leading to safer and more sustainable practices in construction, resource exploration, and earthquake hazard mitigation.