Joint and Shear Fracture Analysis PDF

SECTION 3 1. Definitions and Importance of Joints and Shear Fractures Joint: A continuous, planar fracture with no significant movement; formed by brittle failure under low strain (1-3%). Shear Fracture: A planar fracture with imperceptible movement along the fracture p...

SECTION 3 1. Definitions and Importance of Joints and Shear Fractures Joint: A continuous, planar fracture with no significant movement; formed by brittle failure under low strain (1-3%). Shear Fracture: A planar fracture with imperceptible movement along the fracture plane. These structures are common, brittle features in rocks, especially critical in rock engineering, mining, and understanding rock strength near the surface. 2. Types of Fractures by Mode Mode I (Extensional): Joints that form perpendicular to the minimum principal stress (σ3). Mode II (Shear): Sliding fractures that form at an angle (~30°) to the maximum principal stress (σ1). Mode III (Tearing/Scissor): Fractures with tearing motion. 3. Kinematics and Orientation Conjugate Shear Fractures: Form at a ~30° angle to σ1 with opposite slip senses; the intersection of these fractures is parallel to σ2. Joint Development: Mode I joints form perpendicular to σ3, while stylolitic joints (pressure solution seams) form perpendicular to σ1. 4. Joint Surface Features Origin: The initiation point of the joint. Ribs: Circular/elliptical markings indicating variations in propagation speed. Hackles: Ridges and troughs showing joint propagation direction. Fringes: Rough textures at the joint edge due to energy dissipation. 5. Joints and Veins Vein Formation: Open joints allow fluid infiltration, leading to mineral precipitation (e.g., quartz, calcite). Vein Infill Types: o Massive or Fibrous: Blocky or prismatic structures from multiple crack-seal events. o Syntaxial: Grows from the wall mineral outwards. o Antitaxial: Grows towards the wall mineral. 6. Special Joint Types Tension Gashes: Short, elliptical Mode I joints, often in shear zones, arranged in an en echelon pattern. Stylolites: Irregular seams formed perpendicular to σ1 by volume loss, leaving insoluble residues (e.g., clay, iron oxide). 7. Joint and Fracture Analysis Spacing and Orientation: Controlled by rock properties; wider spacing in more competent and thicker layers. Challenges: Minimal displacement and reactivation complicate analysis; data from cross-cutting relationships, density, and spacing are needed. 8. Types of Joints by Origin Primary Joints: Form during rock formation (e.g., desiccation cracks in mud, columnar joints in cooling igneous rocks). Diastrophic Joints: Related to tectonic structures like folds or faults. Stress-Release Joints: Form as buried rock expands upon erosion. Impact Joints: Form from meteorite impacts (e.g., shatter cones). Anthropogenic Joints: Result from human activities like mining, blasting, or dam loading. 9. Significance of Joints and Fractures Structural Weakness: Can lead to rockfalls, slope instability, and mining hazards. Fluid Circulation: Allow fluid movement, essential for ore deposits, groundwater, hydrocarbon reservoirs, and geothermal energy. 10. Pore Fluid Pressure and Fracture Formation Effective Stress: Pore fluid pressure (Pf) reduces normal stress across fractures, lowering the stress needed for fracture formation. o Formula: σ∗=σN−Pf\sigma^* = \sigma_N - P_fσ∗=σN−Pf Fluid Pressure Ratios (λ): o Hydrostatic pressure ratio (0.37-0.47). o Abnormal (high) pressure ratios (0.5-0.9) at depths >3 km. Fracturing: Increased Pf leads to episodic fractures and faults, also used in fracking to extract trapped hydrocarbons. 11. Summary 1. Joints and Shear Fractures form at low strains and require statistical analysis. 2. Joint spacing depends on rock competence and layer thickness. 3. Primary, diastrophic, stress-release, and anthropogenic factors cause joints. 4. Pore fluid pressure significantly reduces fracture stress thresholds, important in natural and artificial hydrofracturing.