Lecture GG303 28
8/19/05
1
JOINTS I Main Topics A Scientific method B Why are joints important? C Observations of joints D Formulation and testing of hypotheses: The origin of joints II Scientific method (See Fig. 1.1) III Why are fractures important? A Most common geologic structures on earth (joints in particular) B Exceedingly important in heat & fluid flow (all sorts of fluids) C Earthquake faults are fractures D Important in determining geologic & engineering rock strength E Main types of fractures: Joints, dikes, faults, and pressure solution surfaces (See Fig. 24.1 for some examples) IV Observations of joints A Geometry 1 Thin relative to their in-plane dimensions 2 Bounded in extent 3 Relatively planar (how do we know?) 4 Shape of an "isolated" joint a Massive rocks: joints are probably elliptical or circular b Layered rocks: joints are probably rectangular B Composition: Joints can be mineralized or unmineralized C Kinematics 1 Cut adjacent material; joints are younger than materials they cut 2 Relative (not absolute) displacement of originally neighboring points is perpendicular to the joint D Surface textures of joints (See Fig. 24.2) 1 Plumose structure 2 Hackles 3 Ribs 4 These are virtually identical to surface textures of opening mode fractures produced in engineering tests
Stephen Martel
28-1
University of Hawaii
GG303 Lecture 28
8/19/05
2
E Occurrences of joints 1 Sedimentary rocks (Bedded, porous) a Bedding-perpendicular joints b Mud cracks (Desiccation or shrinkage cracks) c Exfoliation joints d Stylolites as "anti-cracks" in carbonate rocks 2 Volcanic rocks (Discontinuous, bedded, highly variable) Cooling fractures/columnar joints 3 Plutonic rocks (low porosity, isotropic crystalline rock) a Exfoliation joints b Radiating joints in stocks c Cooling-related (??) joints in Sierra Nevada, California 4 Metamorphic rocks (low porosity, anisotropic crystalline rock) Fractures along cleavage planes (e.g. shales) F Joint sets (joints rarely occur alone) 1 Based on "similar" orientation and "similar" age 2 Field evidence for relative age of joints a T-intersections b Curved approaches c Mineralogy 3 Systematic joints (joint traces are straight) Interpretation: Difference in far-field principal stresses ( 1 - 2) is small relative to driving stress ( remote - joint)
n n
4 Nonsystematic joints (joint traces are irregular) Interpretation: Joints interact. Perturbing stresses associated with joints is large relative to difference in far-field principal stresses 5 Mechanical significance of joint sets: widespread fracture failure of rock mass V Formulation and testing of hypotheses: The origin of joints A Competing opening hypotheses: mode origin vs. shear mode origin B Field observations 1 Joints can form at depth (they are encountered in mines and deep drill holes, so they can't be entirely surficial phenomena) 2 Stresses at depth are characteristically large and compressive
Stephen Martel
28-2
University of Hawaii
GG303 Lecture 28
8/19/05
3
C Laboratory observations of shear fractures 1 Laboratory compression tests produce "conjugate shear fractures" 2 The conjugate shear fractures form at essentially the same time 3 Shear fractures form at ~30 to maximum principal stress 4 Aligned opening mode cracks precede shear fractures D Implications of detailed field and laboratory observations Observations/Evidence Relative displacement of joint walls Surface textures Single sets of joints are common Opening origin Favors Favors Favors Shear origin Disfavors Disfavors Disfavors
1 Opening-mode origin is favored over shear mode origin 2 Material heterogeneity (e.g., cockle shells in mud flats, fluid inclusions in grains, etc.) is important in the origin of joints 3 Body geometry (e.g., pre-existing "sharp points" [e.g., edges of older joints], bird footprint indentations in mudflats, etc.) is important in the origin of joints 4 Unresolved issue: how do joints open at depth where compressive stresses are large? ( n = effective stress = n - P) E Terminology 1 Opening mode (dilatant fractures) is preferred 2 "Extension fractures" ? (cooling volcanic rocks contract) 3 "Tension fractures" ? (joints can form in response to fluid pressure)
Key Additional References Atkinson , B.K...