Term 1 - Compressional failure: faulting & shear fracturing of dry rocks Flashcards
1
Q
Normal & shear stresses
A
• Can resolve stress on surface, σ, into two components
– Normal stress (σn) acting perpendicular to the surface
– Shear stress (σs or τ) acting parallel to the surface
2
Q
Shear fractures (faults)
A
- Pairs of opposed-dipping (conjugate) faults with same type of displacement
- Acute angle between sets ~60º
- Active at same time (mutually cross-cutting)
- More complex (but predictable) relationship to principle stresses
3
Q
Compressional failure in dry rocks
A
- Stresses in lithosphere are compressional (σn +ve)
- Dry rocks fractured under compressional stress conditions always fail by shear fracture
- Triaxial testing rig: sample placed between pistons & sealed in fluid-filled vessel = confining pressure (=depth)
- Initial hydrostatic stress state: σ1 = σ2 = σ3
- Triaxial compression test
- Triaxial extension test
4
Q
Confining pressure & ultimate strength
A
- In compression rock strength increases at higher confining pressures
- θ values for fractures ~constant: define envelope of failure (ideally, a straight line)
- Use the failure envelope to define whether a state of stress is stable, critical (on point of failure) or unstable
5
Q
Coulomb-Navier failure criterion
A
- Coulomb-Navier failure criterion describes the stress state (i.e. the σn & τ on an incipient fracture plane) that lead to brittle failure under compressional conditions
- As S & µ are constants, τf = f{σn}
6
Q
Reality Check:
A
- ’Real’ failure envelope that describes critical stress states across a range of differential stresses - may or may not correspond to Coulomb-Navier or other predictive failure envelopes
- In general envelope flattens towards higher differential stresses/confining pressures (approaching ductile regime)
- Ultimately failure here occurs at constant τ & differential stress on planes sub-// to τ max planes (von Mises criterion)
7
Q
“Internal friction”
A
- Angle of internal friction, φ, is the total angular difference between the planes of maximum shear stress and the fault planes
- φ is controlled by the characteristic ratio of σn: τf
8
Q
World-stress map features:
A
• Good correlation between orientation of σH & plate motion-related stresses, e.g. – Slab pull – Ridge push – Basal drag – Collisional resistance • These first order controls are locally modified by second-order stresses related to: • Sediment loading • Glacial loading/unloading • Areas of thin crust & mantle upwelling • Ocean-continent transitions • Orogenic belts • Large weak faults
